Nerding Out: Why a Lower Shock Position Doesn't Make a Bike More Stable

May 16, 2022 at 9:17
by Seb Stott  
Cannondale Jekyll


You occasionally hear bike manufacturers claiming they've placed the shock low down in the frame to give it a lower center of gravity, and it's often assumed that a lower center of gravity (COG) makes the bike more stable.

Probably the most striking example of this was when Specialized went to a bottom bracket concentric main pivot with their 2015 Demo, citing a lower COG as one of the main reasons. More recently, Cannondale (pictured above) nestled the shock inside the down tube to get it as low as possible.

Specialized Demo 2015

It might sound reasonable that putting heavier components lower in the bike would lower the COG and therefore make the bike more stable. After all, a car with its weight lower down rolls less and corners better, and if you were a sumo wrestler trying to avoid being knocked over, getting lower is a good idea. But with a bicycle, things are a little different.

Most obviously, the effect of lowering the shock on the center of gravity is pretty negligible. The heaviest coil shocks weigh about a kilogram. The total system weight - that's the bike plus the rider - weighs about a hundred kilograms, and the center of gravity of all that weight is typically about 1 m from the ground (we only need rough numbers here). The difference in height between a high shock position, just under the top tube, and a low shock position, just above the down tube, is about 20 centimeters.

2021 Nukeproof Mega
Nukeproof Giga
The difference between a high and low shock location is about 20 cm.

Moving that 1 kg shock 20 cm lower in the frame lowers the center of gravity of the entire 100 kg system by about 2 mm, or 0.2%. You could literally have a bigger effect on your center of gravity by wearing thinner socks.

Okay, but maybe it isn't only the centre of gravity of the system that matters. Maybe it's the centre of gravity of the bike itself. If you're trying to manoeuvre the bike left and right relative to your body, then it's the centre of gravity of the bike which determines how much effort that takes. A typical bike weighs about 15 kilograms, so if you are lowering the shock by 20 centimetres, the COG of the bike will be lowered by one-fifteenth of 20 cm, which is about 13 mm. The Center of Gravity location of a bike is in the region of 50 centimetres from the ground. So you're lowering its centre of gravity by about 2%.

But bikes are all about marginal gains at this point, so assuming you could lower the center of gravity by a noticeable amount, perhaps by putting an ebike motor right by the bottom bracket or by lowering the bottom bracket height (and therefore the mass of the rider) by a couple of centimetres, wouldn't that improve the stability of the bike, at least a tiny bit?

It depends on what you mean by stability.

In terms of pitching - that's how much the frame tilts forward when you apply the brakes, or squats backwards when you stamp on the pedals - lowering the COG will reduce this pitching, which could be described as an improvement to stability. Just like increasing the wheelbase, a lower center of gravity makes the bike less prone to "trip up" or pitch you over the bars - this is why taller riders need a longer wheelbase.

No doubt a lower COG makes it (slightly) easier to avoid going over the bars.

But here's the counter-intuitive part: when it comes to cornering and balancing from side to side, lowering the center of gravity doesn't improve stability.

That's because a bicycle is essentially an inverted pendulum, like balancing a baseball bat upright on your hand. In order to balance the bat, you continually move your hand so it remains under its center of gravity. Similarly, on a bicycle you're constantly steering so the wheels are directly underneath your COG. By the way, within a certain range of speeds, gyroscopic and caster forces will do this automatically (this is known as self-stability) but at lower or higher speeds, the rider has to make regular corrections to the steering to remain balanced.
Here's a video demonstration of a higher COG being easier to balance.

A baseball bat is easier to balance on your hand than a pencil because its higher COG means it takes longer to fall over and so you have more time to move your hand to correct the lean. Also, if your hand is knocked to the side by a given distance, the change in angle of the bat is less than it is for the pencil, so it is easier to regain balance. Similarly, if your bike wheels are knocked to the side - perhaps by a loose rock or sliding in a turn - with a taller COG the angle by which the bike will become off-balance will be less, and you'll have more time to correct for this by steering into the direction of lean.

Ramsay Dalgliesh carving his way through the berm section before the river gap
Lowering your center of gravity makes it easier to change the angle of lean quickly between turns.

So does this mean that we should all be riding bikes with the highest possible center of gravity to make them more stable? No, partly because of the pitching reason we discussed earlier, but partly because there's a trade-off between stability and maneuverability in the corners.

The higher the COG, the longer it takes to deliberately change the lean angle when initiating a turn or going from a left to a right turn (or visa-versa). Before you can turn left, you need to lean your weight to the left of your tires; in order to turn right, you first have to lean to the right. As you change the lean angle, the bike and rider pivot around the roll axis, or the line connecting the two contact patches. The distance between the roll axis and the COG is called the roll moment of inertia, and the longer the distance, the longer it takes to change the angle of lean, and so the longer it takes to change from turning left to turning right (or visa-versa). For this reason, you might want a low center of gravity for a series of tight turns, but a high center of gravity for a fast straight full of pinball rocks.

There's even an equation called the control authority which links COG height, along with wheelbase, handlebar width and head angle, to "twitchiness" - how much the bike responds to a given input. A lower COG height has the same effect on this measure of twitchiness as a shorter wheelbase, steeper head angle or narrower handlebar.

So, just like any other geometry measurement, the center of gravity height is a tradeoff between twitchy and slow handling responses. But counter-intuitively, a lower COG makes the bike more twitchy, not more stable.





276 Comments

  • 605 3
 I'm going to keep buying the bike that looks the best.
  • 168 10
 im going to buy the one with at least one bottle mount
  • 261 0
 I'm going to buy the one that's actually available
  • 83 5
 They all look like a session to me.
  • 57 1
 Same, that's why my next bike will be an Ellsworth
  • 3 0
 @bashhard: contact me for more Details.
  • 49 0
 I’m going to keep buying the bike that comes in aluminum.
  • 50 0
 I'm going to keep buying the bike I can afford.
  • 36 1
 I´m going to buy a Bonero
  • 28 0
 I'm going to keep riding my current bike because I have one less reason to "upgrade".
  • 11 8
 I'm going to get a bike that isn't priced for dentists
  • 10 1
 1. Availabity 2. Price 3. Bottle mount 4. Reliability 5. Look
  • 29 3
 I'm going to keep buying hardtails and no-one can stop me.
  • 51 0
 I’m going to buy the bike with the strongest bmx background.
  • 14 0
 I’m gonna keep buying the bike that is absolutely most difficult to perform basic service on. Luckily Theres a better one in this regard every year, it’s like they listen to what I want!
  • 2 1
 @bashhard: maybe instead of a bike you should invest in some glasses
  • 1 0
 I'm going to buy the bike that's available and fits my budget...
  • 4 1
 @cebolla:

I like this, except that it should just be

1. Look
2. Bottle cage

and skip the others.
  • 4 0
 ...max seatpost insertion; min seat tube length
  • 20 0
 It’s funny they mention “control authority”, I check with mine before I buy a bike, she usually says yes. As long as I sell the old one.
  • 3 1
 i go with ride feel, look doesn't matter too much when you're riding it
  • 4 2
 @NicolaZesty314: people will make fun of you if you ride an Ellsworth
  • 3 1
 What does that even mean? A bicycle at its best is a prime example of a "form follows function" machine.

20 years ago every bike had 71/73 geometry, and they sucked-but maybe they "looked good"??
  • 3 1
 @wyorider: isn’t that just a phrase industrial designers are taught ad nauseum to liberate them from the fact making swoops fancy shit is not the goal unless of course someone has said design is some swoopy fancy shit
  • 5 2
 @Compositepro: ... making swoopy fancy shit is not the goal ...

Are you new here? Razz
  • 2 0
 I’m going to buy the one with the cool blue sealant.
  • 1 0
 Buy for the color.
  • 2 0
 I'm going to keep fixing my bike
  • 5 0
 I’m going to keep buying the bike that christopher walken endorses.
  • 2 0
 Always depending on my budget = used
  • 1 0
 @stephenzkie: So many 10's that work in dentistry though
  • 2 0
 @Compositepro: the position of the bottle will change the COG as long as it is full
  • 2 0
 @sebc38: as a World Cup level nobody who never actually rides a bike much these days i thank you for pointing out this critical flaw in my plan.
  • 1 0
 That will be a hardtail then…
  • 1 0
 No water bottle mounts, moto shock placement, solda!
  • 1 0
 @sportstuff: I’m on the same plan!
  • 1 0
 @sportstuff: should be upvoted more!
  • 1 0
 @wyorider: Hey now One of mine is a 2000 Cannondale Raven. It’s got a 69 degree HA. All I did was put a 140mm Lefty Max on it. Fun!
  • 312 0
 mounting 1kg of shock as low as possible so I can mount my 1kg water bottle higher up the downtube
  • 12 9
 I need nothing of that, my water goes in my backpack! 2 full bottles loose in a backpack without waist straps, enough for 3 hour ride.
  • 5 1
 I have a GG with the bottle mount on the underside of the top-tube, and maybe my center of gravity is a bit higher, but it's MUCH easier to grab and drink while pedaling than the low bottle mounts, especially as a tall person. The bottle is right there, close to the handlebars, can grab it easily while pedaling without having to reach or bend. It's so much better than a bottle mounted down by the BB. I know some riders don't care because they stop to drink anyway, but I do a lot of 20-60minute climbs where I need to drink and also don't want to stop til I get to the top. Plus you drink the bottle empty at the top of the climb and then it doesn't effect CoG on the way down anyway.
  • 6 0
 @dthomp325: That's a good illustration of chassis engineering vs. designing for user experience. Maybe we could get the best of both worlds with a large water carrier under / in the down-tube as the master reservoir, and a small bottle located somewhere super convenient. Refill the little bottle when you pause at the top of those climbs.
  • 2 2
 @R-M-R: The Stumpy Evo has exactly that!
  • 3 1
 @dthomp325: The problem with underside of the top tube mounting is that most bottle cages suck and can't hold the water bottle in place
  • 1 0
 @hamncheez:

Stumpjumper Evo: I've been wanting to fill the frame with water for years. Cannondale did it with their motocross project (fuel, not water, and yes, I know they weren't the first to store fuel in the frame), but weren't quite crazy enough to try it with bike frames!

Weak cages: Yeah, but that's easily fixed with better cages or, in many cases, just a strap (credit to Seth's Bike Hacks). As I've mentioned before, I like the ⌀3.5" bottle standard (ex. Nalgene) because it carries so much more volume for a given length, and length is usually the most difficult constraint on bikes. A few tweaks to existing designs and we could carry more water, lower - not that the centre of mass location will make a big difference, but if the option exists to carry it low, we might as well.
  • 1 0
 @R-M-R: But a wider bottle is less aero!!

Seriously tho, have you tried the fidlock and other "alternative" retention systems? On my ti bike I have the bottle mount on the under side of the top tube, so I'm curious.
  • 10 0
 @hamncheez: No, I'm old-school ... or maybe just old ... or maybe just cheap.

If road wheels can be wider and more aero, and if Ribble can increase the frontal area of their handlebars in the name of aero benefits, surely we can market the ⌀3.5" bottles as being more aero. Turbulent boundary layers, vortex shedding, stall angles, trip surfaces, systems integration, CFD simulations of static riders ... fire up your 3D printer and I'll cut up some scraps of paper with buzzwords we can pull out of a hat - we're going to be rich.
  • 3 0
 @R-M-R: Brilliant. You could sell a urostomy bag with a straw leading back to your mouth (like a camel pak) with those kind of marketing skills. "Weight savings", "more aero", "eco-friendly"
  • 2 2
 @R-M-R: Are you referring to Cannondale's failed attempt to build a MX bike (and quad)? the one that bankrupted the company? I'll pass. (ftr, The Buell XBs were just as flawed).
  • 1 0
 @hamncheez: It's recycling at its finest, and, as the Super Wheel showed us, plenty of people are open to perpetual motion machines.
  • 5 0
 @o1inc: I am! The Cannondale project and Buell are full of both good and questionable ideas, combined with a healthy dose of dodgy execution. I'm glad they both existed (well, sort of existed, in Cannondale's case), and even more glad I don't own either.
  • 1 0
 @R-M-R: And rim stickers!
  • 2 0
 @R-M-R: Ah yes, I always wanted to drink water perpetually from a canister that I can never clean.
  • 5 1
 @grotesquesque: Then you can keep chugging from bottles while I enjoy aluminum-infused Future Juice.
  • 2 0
 Logical reasoning and physics don’t always win over marketing.
  • 2 0
 @hamncheez: I use Spec Zee cage. It holds great, even on full dh runs while being easy to insert/remove the bottle.
  • 1 0
 @hamncheez: Fidlock...

Side effect is the bike looks better when you don't have a bottle vs a cage.

And I've got a High Above hip pack that has a Fidlock on it so the full bottle gets transfered to there when I'm carrying two.
  • 86 10
 talking about "system weight" is a nearly pointless in this regard. The "system" we are talking about is not a uniform, unchanging mechanism. rider+bike are two seperate entities, both moving dynamically. You cannot assume anything about the two as a system, besides their mass. If lowering your shock makes far more difference if you take the rider's weight and where it is statically(a rider is literally never static in practice) out of the equation, the shock position becomes nearly an order of magnitude more important to the CoG. A bike with a lower CoG is going to behave a lot different(not getting into whether that is good or bad, because it is situationally dependant, which I agree with the author here) under the rider. That behavioural change is going to affect how much and what kind of input the rider needs to add. This will add up to either a bike that wears you the f*ck out over a day, or rides fast effortlessly.

this is why approaching this issues as a "system" is silly.
  • 36 3
 This is true...for some reason very few people in the industry and riders alike seem to be aware of just how often the riders weight becomes separate to the sprung mass of the bike...this is the whole reason why ebikes feel plusher....and it doesn't only apply to suspension it applies to the whole dynamics of the bike. And this is before you even consider the riders centre of gravity is changing all the time and to corner on a bicycle you tilt the bike under you as opposed to tilting with the bike like you do motorbikes that have much heavier mass to rider weight ratios...this stuff is all very complicated. It's nice to see Pinkbike make articles like this, more stuff like this is what we want as opposed to just superficial fluff and marketing but at the same time, some more in depth research, perhaps collaboration with engineers and suspension experts etc would go a long way as opposed to reading a bit of school physics on Wikipedia and putting an article together. This whole lack of communication between the engineers, moto and cycling industry etc is the whole reason moto has been ahead of bicycles in terms of suspension, geometry and tyres for decades....there seems to be no cross pollination of knowledge and bicycle designers are trying to figure all this stuff out from scratch when the info and research is all readily available.
  • 20 0
 Agreed, Seb talks about the bike and rider being an inverted pendulum - with input at the base. This applies for a rigid rider making stability corrections via minor steering adjustments. But it does ignore the inverted double pendulum, making stability corrections via weight shifts at the mid-point.
  • 24 3
 Exactly. That´s also the reason why the oversimplified argument that "bike weight doesn´t matter, because the rider is so heavy" is exactly that - oversimplified and far from helpful.
  • 10 3
 I never understood the system rider weight argument when it came to bike weight. A mountain bike isn't just ridden in straight line always sitting down. On mountain bikes you shift weight a lot, you jump and so on. Even if a light and heavy bike have the same tires, the heavier bike will still exhaust the rider much faster.
  • 24 3
 @Danzzz88: This was an interesting article but almost self-defeating. Yes shock placement isn't that critical; although the weight moved could well be more significant than just the shock itself, with the weight of mounting points and linkages etc also likely to be lower; so why write so much on it?
The real problem is BB height. Manufacturers have moved BB's lower and lower to get the undoubted advantages of a lower rider COG, but the trade-offs of this, pedal strikes and slowness of lifting the front end has perhaps gone too far?
Most riders aren't actually chasing KOM's and race results, they might want a bike that will clean tricky sections or that feels playful.
Long chain-stays, big wheels and a low BB's might be great for super tall riders who want to hammer through DH without pedalling, but they are the enemy of pedalling in ruts, pulling the front end up and not dropping your scrotum into the back tyre.
  • 2 3
 Riding a scooter vs a motorbike makes it clear the lower the cog the more stable the thing is at low speeds. Not sure about high speeds. I’m almost tempted to say a more central cog is the way to go when riding really fast.
  • 5 0
 @jaame: I've won multiple slow races over the years at motorcycle rallies using my 1970 Vespa Sprint 150. You can just pick your feet up and the thing will just stand there, my Lambretta was even better, but it was such a slow miserable POS that it was dangerous in traffic. (Good for slow races, bad for literally everything else.)
  • 5 1
 @FR33DOMdotCOM: Yes BMX riders figured this out ages ago but MTB riders usually seem to have one foot stuck in their roadie past.
  • 1 0
 @FR33DOMdotCOM: hallelujah brother
  • 2 0
 I understand none of this, but it sounds smart, so I buy it.
  • 1 0
 glitch
  • 17 4
 I could not disagree more with @seb-stott here. The 'inverted pendulum' doesn't apply to a bike sitting under you. This whole article can be disproven by simply riding a bike with a lower CoG. Even an enthusiast, non-pro rider like myself can feel the difference. Chirs Porter has done timed runs with various skill level athletes, putting lead weights around the BB area, with measurable benefits.

When riding through loose surfaces, like gravel, this is the most noticeable. Loose rocks deflect your tire left and right, and since the bike moves independently of the rider, the more mass there is lower in the bike, the less the bike will deflect for a given rock size/speed. Thats why heavy, DH tires do so well in loose rocks. Thats why ebikes feel so dang stable on gravel. For a given frame weight, having the weight higher means there is less mass near rock strikes, allowing for more left-right delfection.
  • 15 3
 ^^ this all day. I didn't have an opinion on shock position really but after this authors article with hand wavy math that concluded bike weight doesn't matter I don't trust him as an author and assumed there was oversimplification going on in this article as well.
  • 12 2
 @hbar314: ^^ also this all day. This article is pure pseudo-science and aims to simplify a hugely complex relationship between bike and rider. I would argue that kinematics far outweigh where the shock mass is placed on any frame.
Agree as well that the article on bike weight was pure tripe. Anyone who has ridden both ends of the spectrum is well aware of the differences that bike weight has on handling, acceleration, stability, liveliness. Trying to convince the masses that repeatable, empirical results are false is misguided at best.
  • 9 1
 Nerding out should be data based not Joe Rogan style shooting from the hip and finding loose fitting comparable evidence on Google to copy paste. Real world back to back testing and value are important
  • 3 0
 @hamncheez: just imagine an ebike with the motor and battery as high as possible.
  • 9 0
 @taskmgr: Both are valuable. Limitations of testing include the sensitivity of the tester(s), placebo effects, and expected results. Math doesn't lie, but sometimes it doesn't capture the full picture or fully account for the quirks of human perception. Neither approach is sufficient on its own and neither should be dismissed because of its limitations.
  • 2 3
 @R-M-R: the trouble with math is that you have to know what to put into in the first place...it's only a tool. It's fair enough saying I can prove this is correct because X and Y = Z but if there is another 5 formula that gives the whole picture then it is a mute point. And although Newtonian physics is the world we have come to appreciate and works well for us approximating things...if we are to be truly pedantic, it's all wrong as the true physics is at the quantum level and in this regard math we use everyday is in fact false, but so close percentage wise to the truth that it doesn't end up mattering.
  • 3 0
 @Danzzz88: Is your point to do away with calculations because of the probabilistic nature of quantum physics ... which was discovered via testing and calculations?
  • 4 0
 @R-M-R: no my point is that calculations are the tool..to get the correct data out you first have to put the correct and 'complete' data in first. If math worked for itself we could simply calculate the perfect bike for each application with math alone, but we need experience, testing and ideas to know what we put into those calculations in the first place. Hence why every mathematician isn't the next Einstein. I was trying to discredit what you were saying by the way I was just adding to it.
  • 1 0
 @R-M-R: wasn't*
  • 3 1
 @Danzzz88: Without talking about quantum bullshit, another way of thinking is, if you can find that a bike is 14 stability units , and another bike is 18 stability unit, without input from some rider's feeling, you wouldn't be able to know if these bike are "too much" or "not enough", or even rideable.
  • 1 0
 @faul: well that is why there is supposed to be baselines of peoples experiences...take suspension for example...it's remarkable the number of people that desire a retune with stock suspension even though stock is tuned around average rider weight and the most common rider weight is indeed average yet these average weight riders are often not happy with the performance. It's almost like these companies don't do anything but spit numbers through a machine.
  • 7 0
 @R-M-R: No measurements were done here. Just pseudoscience and the misapplication of physics equations. When timed runs are done, its clear that a lower CoG bike is faster. But since 99.99% of the time a mountain bike is being ridden its not being raced, handling, feedback, and enjoyment matter even more. I can say that most good riders can feel a lower CoG, and it is going to be a positive experience. Liike @KalkhoffKiller said- imagine an ebike with the battery on the top tube! It would handle terribly.
  • 4 0
 @Danzzz88: Fair enough, but math has served humanity pretty well. You're right it's a tool, and - like any tool - it's as powerful, impotent, or dangerous as the person wielding it. It's a poor craftsperson who blames their tools.

That said, many people make the mistake of trying to invent in a vacuum, i.e. to ignore the R&D of others and try to derive the secrets of the universe from the comfort of their desk, which is perhaps what you were getting at. It's an easy trap to fall into. That's why my design process is more data-driven than anyone else in the industry. I've compiled a huge database of many parameters and I use analytics to find commonalities between bikes with certain properties. Testing and novel calculations are still important, of course, but there have been probably billions of dollars and nearly as many hours spent on mountain biking and it would be foolish to ignore the consumer and professional learnings that have come from it.

So, maybe we agree after all. Testing is important and "garbage in, garbage out" will always be true ... but when you start talking about math, I'll always have math's back! Geek
  • 4 0
 Just keep finding errors. Bottom of the article he says "There's even an equation called the control authority which links COG height, along with wheelbase, handlebar width and head angle, to "twitchiness"..."

If you follow his link and find that equation on that paper it has nothing to do with the height of the CoG. That equation only cares about where the CoG is front/back. So relative to the wheelbase. Which makes sense because if all the weight of the bike is way forward or back the bike does indeed handle remarkably differently.
  • 2 0
 @hbar314: That equation mention CoG heigh in the "h" that is explicated in the drawing above. But as I said in another comment, this isn't a trustworthy equation, for several reasons.
  • 1 0
 @faul: Saw that right after I hit submit, doh.
  • 6 0
 @hamncheez: I'm not disagreeing with you, but I'd like to illustrate a point at which that wisdom breaks down. It's extreme and questionably applicable to mountain bikes, but it shows there are limits to this thinking.

I touched on it here, and I've written the full story many times, possibly including the Pinkbike forums.

For the first lowracer human-powered vehicles I designed, I went all-in on making it low for aerodynamics and assumed a lower centre of mass would be a perfectly good byproduct. The top of the rider's head was something like a half metre off the ground - it's been ages and I forget the numbers, but it was absurdly low, even for a competition lowracer. I couldn't fit the rear wheel under the rider, so I put it fully behind the rider. The length enough to worry the Ever Given crew, but I figured that would add to the stability - perfect for a speed comp racer.

Here's where the conventional wisdom broke down:

I've been describing this vehicle with the bat vs. pencil analogy since shortly after JNCO Jeans rose to popularity - and I'm pretty sure I've written . Like the pencil, the crazy low centre of mass allowed crazy fast roll axis rotation. It takes only a small amount of rotation to create a large angle between the line of contact (between the tires) and the centre of mass; this angle dictates the nature of the turn required to correct the wobble. Therefore, the rider quickly needs to make a hard turn, which is not easily done when the wheelbase is absurdly long.

Getting started was a nightmare and it was terrifying at moderate speeds. You can imagine how my riders felt when I told them yes, the bike is trying to kill you, and the solution is to go MUCH FASTER. The brave ones did, and it was serene - like gliding on a smooth lake ... until you tried to stop and returned to the death zone.

Anyway, just wanted to share some first-hand experience with exploring the outer limits of parameters that I once assumed would always enhance stability.
  • 2 0
 @hamncheez: Oops, pasted the link incorrectly in the previous message. Let's do it the old-fashioned way.
https://www.pinkbike.com/forum/listcomments/?threadid=119046&pagenum=436
  • 5 0
 I was reading the article specifically waiting for seb to mention “but it’s only a 1kg shock and your fat ass is 100x as heavy! Therefore if you strap 10kg directly to your valve stem you’ll only lose 0.3s of climbing time”

Leave this argument for road cycling, mtb moves the bike independently of the rider like 90% of the time, pretty much all attentive riders will notice an extra kg on their bike frame the second the wheels leave the ground
  • 1 0
 @R-M-R: I saw that comment in other thread, and I don't doubt your engineering experience or prowess. I don't doubt your experience you derived from the low racer. Yes, if the total CoG of the entire system is too low there are going to be problems. However, a mountain bike is not nearly as tightly coupled as the low racer, and the total CoG is much higher regardless of what is going on with the bike beneath the rider. On a recumbent you can't really move much on the bike, and the riders CoG is in pretty much the same place as the bikes CoG. This is completely different on a mountain bike, or any upright bike. This gets more true the more suspension you have and the more DH oriented the riding style is, since DH riding require more acrobatics with the bike than XC riding because the trail features become more extreme. The more you move around on the bike, the less coupled you are to it and the less the total CoG matters.
  • 3 0
 @hamncheez: Yep, I agree with those statements. Still, there are elements of stability to be gained from a high ride height. The bike doesn't require as sharp a turn to correct from an unintended lateral deflection, and it makes comparatively long, sweeping turns because of the greater lateral displacement when between the line of contact and the centre of mass when turning.

On the other hand, it takes more time to correct wobbles, which allows more time for things to go wrong before you can correct, and it feels like you're in a state of constantly making corrections - like how it's difficult for a performer on stilts to ever stand completely still.

So, the next question would be how these things work in the context of mountain bikes. A wheelbase can be only so long before the bike handles poorly on typical trails and at typical speeds. In this context - and especially when considering fore-aft pitch stability - I favour the lowest practical BB height, frame centre of mass, and rider centre of mass. The problem of roll axis instability vs. steered yaw rate isn't a limiting factor on mountain bikes, so we're within the region of "lower is better".
  • 1 0
 @hamncheez: "the less the total CoG matters"... for "stability".
Rider's CoG is really important in gravity riding, and even more important, is how much the rider can vary the height of the bike under it's CoG. By having a higher CoG in corners, You'll be able, when leaning, to reduce the distance traveled by your CoG (draw several corners seen from above and several CoG path if you're not sure), hence, by conserving your quantity of movement, you'll be able to go faster, everything being equal elsewhere. And if you are able to have a bigger difference in Cog high (having tall and/or strong legs), you'll be able to "push" more in the corner, hence increasing this effect.
This has limitations in longer, bigger radius corners tho.
  • 1 0
 @R-M-R: I agree with you on BB height- when I got my Enduro 29er back in 2014 and sold the DH bike I kept having OTB moments, something I hadn't experienced in years. Took me a while to figure out it was the 350mm+ bottom bracket height.
  • 3 0
 @hamncheez: Either that or you were too exhausted to support yourself after having to actually pedal Wink
  • 2 0
 @R-M-R: it did coincide with me graduating college and getting a desk job...
  • 1 0
 @hamncheez: a lot of dh bikes these days have higher bb's than enduro, at least static without rider weight...how much lower the bb ends up if at all compared to enduro due to increased sag and dynamically when riding I don't know...but I would have thought you were more likely to go over the bars them days due to steeper head angle, lower stack and shorter front centre rather than it being purely bb height but I may be wrong....
  • 2 0
 @Danzzz88: What really determines the OTB factor is the relation of BB height to front-center lengths (which is of course highly influenced by how slack the front end is). The 1st gen Enduro 29 combined XC steep HTA with a super high BB, which was made worse by the Monarch rear shock that required me to run only 15% sag to keep from bottoming out hard.
  • 1 0
 @hamncheez: True, but that was standard for early 29ers.
  • 2 0
 @R-M-R: Yes, and early 29ers all sucked balls
  • 1 1
 @g123: the whole mtb industry is determined to demonstrate that heavy bikes are better because it doesn’t take the same level of engineering skill to as more material compared to design and engineer it properly. The adhering podcast said they had deliberately made the bikes heavier than they needed to be
  • 7 0
 @chrismac70: which podcast?

The general movement from 26>27.5>29 has added mass that's difficult to overcome. You pay a substantial weight penalty to get a 29" wheeled bike with the same puncture resistance and wheel strength relative to a 26. But seeing a large portion of new trail bikes being paraded out with weights in the mid-30's (without pedals, a tool or water bottle) makes me cringe, especially when the trailer story is that 'bike weight doesn't matter'. If you're a 6ft+ diesel engine rider, maybe you don't care (thought maybe you should).

What I see on trail is a whole host of smaller/lighter riders working their ass off to climb an unnecessarily heavy sled in a 28/52 granny, just slugging out the vert. As the bike becomes a more significant percentage of your total bike/rider weight, the bike weight effects become so much more noticeable.
Tell me more about how bike weight doesn't matter.
  • 1 1
 sorry but this is wrong. Yes the system is not "uniform" but in terms of center of gravity the system weight matters. This is why bottom bracket height and top tube length affect bike stability so much (both lower riders COG) while having a lighter saddle and doesn't. .

Your assumptions would only make sense if the bike was unweighted most of the time. How often are you light on your pedals? in air mostly where bike COG is less of an issue. COG is most important for cornering and pitching so when you have all of your weight on the pedals.

Also "you cannot assume everything" but you are yourself assuming the shock becomes "an order of magnitude more important...". That's an assumption mate.
  • 2 4
 @FR33DOMdotCOM: It's a completely different argument. Bike weight matters. But in terms of COG calculations you calculate the mass of the system because that's what COG is. The bike doesn't turn on its own totally unweighted in the corners. You are on the pedals with your full mass on them. So the COG in question is for both of you. That's why your body movement affects how your bike behaves in corners. That's why lower BB = good for cornering. That's why longer TT (up to a point) = good for cornering (lower chest = lower cog)
  • 2 4
 @hbar314: This article doesn't say weight doesn't matter. it does matter. What doesn't matter is shock placement. This is simple physics. In situations where you want a low COG 100% of your weight is on the pedals so the COG you feel as a rider is your COG above the ground. The shock would affect it only if it was like 20kg.
  • 3 0
 @spaced: Same author is what i said. He wrote an article that bike weight didn't matter.

But on this subject, how did you calculate the 20kg number? You think a 15kg shock wouldn't effect bike handling? Do you think it would have normal decals or heavier decals on it?
  • 5 0
 What did readers expect? This is the same author I've seen reason in comments that it was okay to leave out gyroscopic calculations of the wheels because 'that math gets really complicated'. Dude is the Neil Degrasse Tyson of PB--he writes 'sorta science' articles for clicks, nothing else. If his calculations and understanding were really that good, he would be making a lot more money doing something other than writing articles for PB.
  • 1 0
 @jaame: It all depends on whether you're balancing by leaning the bike while the wheels track a straight line (picture balancing along a skinny), or balancing by letting the wheels track underneath to track the centre of gravity. It's certainly easier to pick a line at low speed if the centre of mass is lower, because you're moving the mass through less distance at the bars. But I wouldn't say that's the same thing as stability, which has a lot more to do with steering geometry, even at vespa speeds.
  • 53 1
 Kinematics over shock position, all of the time.

The wankerteering/marketing department will clasp at anything to get the sale.
  • 12 4
 I've been saying this since 2015, when Specialized introduced the low BB pivot. At the time I sarcastically predicted they would "improve" it in 2016 by lowering the pivot below the BB to even further lower the centre of gravity:

m.pinkbike.com/photo/11742494

Everybody mocked my criticism of it back then cause they just assumed Specialized knew what they were doing. They still don't, their frame designs cause shocks to blow up and their pivot is still too low for optimum suspension.
  • 6 0
 @Protour: To be fair, a super BB-area pivot can still produce high pedaling anti-squat, but yeah, I agree with the point you're making. Lowing an engine in a light motorcycle is the real deal; lowering a shock may be worthwhile for packaging (ex. fitting two water bottles or something useful like that), but the tiny change in chassis centre of mass is unlikely to be perceptible, let alone a major improvement in ride quality.
  • 9 0
 @Protour: He lives!
  • 1 0
 @Protour: I agree with you tho- more rearward travel is better than super low chaingrowth.
  • 3 4
 @hamncheez: the sole issue with discussions like these is most people are braindead "follow the trend" normies and will spit on you and negprop everything.

it's an enchanted planet i tellz ya
  • 38 1
 @seb-stott I really enjoy these articles.

One thing that I notice when riding is a significant difference in 'feel' when switching between a light (XC) wheel set vs. a heavy (DH) wheel set on the same bike. There is definitely a noticeable difference in the body english required to lever the bike over into turns.
Can you please do a similar article nerding out on the science involved in the rotational masses, centripetal and centrifugal forces at play in that situation?
For example: how much more/less force at the pedals and handle bars does it take to lean/turn a bike with say a 650B XC wheel set and tyres vs. 29er DH wheel set and tyres (maybe include tyre inserts for added rotational mass).
  • 22 3
 Thanks! That's a good suggestion, I'll do some thinking.
  • 8 6
 @seb-stott: I'm really disappointed you didn't acknowledge @R-M-R in this article, especially given you've borrowed a few phrases and concepts directly from his forum posts.
  • 3 0
 @seb-stott: most importantly, does countersteering in the air (like on a hip jump) change your direction because of the opposing force to yawing the gyroscopic front wheel, or due to precession in the inevitable table of the bike??

What's the optimum amount of table to maximise turning in the air if I'm under rotating a hip jump?

For reference I'm 95kg, 188cm, Leo.
  • 3 0
 Especially when the current trend seems to be to put the heaviest tyres you can find on and then stuff inserts in just make sure the whole wheel is as heavy as you can make it for your trail ride
  • 4 1
 One thing to take into consideration when switching from light to heavy wheels is rebound setting. I remember few year ago on my DH bike, I was waiting for my super light wheelset and used a very heavy wheelset from a friend to get to ride my new bike. The bike was setup with these wheels and I got used to it. When I finally put on my new light wheels, identical tires and pressure the bike was feeling way off and super twitchy. I had to slow down the rebound to have the bike working properly again.
But sure @seb-stott already told us unsprung mass doesn't do anything (trying to do a bike drop test and with a bike with a good ratio and a bad ratio and see which one bounce back more would be a decent test of concept), bike CoG is irrelevant too apparently so I can only imagine that he will come up to the conclusion that wheel weight is also irrelevant. Btw if wheel weight is significant because it is a rotational mass which multiplies its weight, then this weight should also be considered in your sprung/unsprung ratio. Blabbering some theory is great to generate hypothesis (Scott's opinions are just that), now back this up with experimental results would be more usefull.
  • 3 2
 @ratedgg13: Do you think that @R-M-R is the only one that has come up with this? I know of a number of people that have looked into this and come up with the same conclusion.
  • 7 1
 @Joecx: Just one example of where Seb uses his ideas almost verbatim:
www.pinkbike.com/forum/listcomments/?threadid=211801&pagenum=172
  • 10 2
 @ratedgg13: I appreciate the acknowledgement, but don't trouble yourself. It's been an ongoing issue and I would be surprised if this changes anything.
  • 1 2
 @seb-stott: I've been thinking about this problem for a while. Hopefully I have a Uni Grad researching the maths for me. Give me a shout and we can talk it through...
  • 26 0
 You must be wearing thick socks
  • 26 1
 That one sentence makes me suspicious of the entire article...
  • 11 6
 I live in Scotland. Sometimes I do. Okay, it was a bit tongue in cheek but a pair of thick socks could be in the region of a couple of mm.
  • 2 1
 @seb-stott: or ride through a puddle and saturate your socks and shoes
  • 6 0
 @seb-stott: Ohhh I see.. I thought you were referring to the mass of the socks and was trying to puzzle that out [facepalm]
  • 30 1
 @Mugen: Don't be too hard on @seb-stott, he's not the one who came up with the analogy.

R-M-R wrote:

I did a quick calculation. With a couple of roughly estimated weights for the bike and the added mass, it lowers the centre of mass for the combined bike & rider system by slightly over 1 mm. The effect is marginally greater than changing the thickness of the rider's socks.
  • 1 0
 Thicccc
  • 9 0
 @R-M-R: Nah, this was all Seb's work, the socks in your analogy are only just over 1mm thick, his are 2mm, obviously no connection between the statements. ;-)
  • 3 0
 @commental: Canadian hemp vs. Scottish wool.
  • 6 0
 @R-M-R: @seb-stott how is this not plagiarism lol
  • 23 1
 There is many misconceptions in this article.
Please try a cargo bike. You can Vary easily the position of the COG by placing a 20kg toolbox where you want. You'll find it more stable with a more centered, lower COG, but it's easier to ride without hands with a higher, more forward COG.
Because you are confusing "self stability", "rider input stability", and "maneuverability", and absolute value with relative values.
And no, a bike isn't an inverted pendulum. You explained the inverted pendulum and the bike stability with caster effects, That doesn't work the same, except for the COG height's effects being counter-intuitive.
You'll need 1kg socks to have a similar effect as a 1kg shock, that's a bit obvious
The Control authority's equation in your link has some bigger issues tho. Some first order terms are missing, like fork's offset. It has a value in squared distance. That's a bit weird.
  • 4 0
 'Because you are confusing "self stability", "rider input stability", and "maneuverability",'

THIS!
  • 1 0
 Why go to the trouble when you can vary the mass position with the push of a lever or a bending of the elbows or a repositioning of the feet. One can learn enough by doing low-speed U-turns with saddle at various heights
  • 20 0
 This article would not pass peer review.
  • 16 0
 The article on the control authority equation referenced in this story is a capstone project report from 4th year engineering students in California. They don't even reference where they get the equation from, they just present it as fact with no derivation or explanation. The dynamics of stability and control in the context of a bicycle are incredibly complex and not reduceable to a single equation as alluded to in this article. While I love that PB is trying to be more objective with their analyses of important questions that are constantly top of mind in the industry, I beg you Please stop half assing science.
  • 2 0
 Thanks for replying on this to provide some context.
  • 14 0
 Motocross has spent the last few years prioritizing lowering and centralizing the center of mass. Riding bikes is more like motocross than balancing a stick in your hand. If it works for moto, it’s got to be better for riding bikes fast in rough terrain. This opinion coming from someone with zero engineering knowledge.

I do enjoy the articles @seb-stott.
  • 2 0
 Moto is a tad different but I constantly agree if it works for moto it has some merit. That said, being significantly heavier and having different techniques makes the cog easier to manipulate in moto. Seated vs standing in moto moves your cog significantly as the weight goes from being high up at the top of the bike to through your pegs. (At least that's how I've always been taught). In mx I find it's a lot more instinctual as to what you should be doing. Street riding you seen tons of people pinning it in straight lines and then getting gun shy in the corners as they run out of lean sitting on the seat.

I have never though of centre of gravity to realistically have any impact on my ride and I think sprung vs unsprung weight ratios is far more interesting and that's why ebikes feel way more stable. My wheels, fork lowers, half my rear triangle combined are more than half the weight of my bike. It rides fine, but an ebike arguably has almost the same unsprung mass but significantly more sprung weight. To the point that your ratio instead of 1:1 is 1:2.3 or more. Which in itself is better. Add a rider to the equation and the ratio sky rockets but you still have that benefit out the gate. The day I see my bike moving in its travel while on a car rack is when I'll be happy.

@seb-stott I'd be interested to hear your take on that relationship you do a great job of laying it out for plain folks like me.
  • 2 0
 Bingo!

CycleWorld magazine studied this at length a while back (though in this case relating to sport bikes) and came to a similar conclusion.

In the case of a bicycle, the mass/weight of the chassis relative to the rider is far less which, might support a reasonable argument that lower CG takes a back seat to other more important considerations such as proper kinematics and suspension setup.
  • 3 0
 Road racing motorcycles have already been down this path, and discovered that the increased stability came at the cost of maneuverability. Kevin Cameron, national treasure that he is, makes these concepts clear and relatable.

www.cycleworld.com/story/bikes/motorcycle-center-of-gravity-motorhead-myths/?utm_campaign=trueanthem_trending-content&utm_medium=social&utm_source=facebook&fbclid=IwAR0sVUyl4UjgDF3LyYQTF216OX6mF-YNmX_tJf97q7YW4v60NtnBSs1t3No
  • 2 0
 The average dirt bike weighs roughly 200lbs. The average MTB weights roughly 30ish lbs. Compare that to a standard 170lbs adult the weight distribution of a dirt bike makes a huge difference vs a MTB. Doesn't take an engineering degree to realize that when talking about weight specifically, dirt bikes and mountain bikes are vastly different and shouldn't really be compared.
  • 1 0
 @freynolds: Ding! It's a spectrum!

And on a mountain bike, it could be argued that the ideal part of the spectrum should be closer to maneuverability, since sometimes, "stability" comes from maneuverability. Trail inputs are inevitably going to move the bike, and being able to maneuver the bike quickly and with (marginally, but margins add up) less effort to account for those external inputs is quite important for dynamic stability. And a low CG makes for easier (relative input effort) maneuvering, hence better dynamic stability.
  • 1 0
 @endoguru: Yeah, the inverted pendulum stuff kinda only applies to riding in a straight line on smooth(ish) level ground, perhaps even with no hands. I think moto has been working on lowering CG to reduce the energy needed to dynamically move the machine around. Leaning and un-leaning (and other maneuvers, of course) a 200 pound moto takes effort, and a reduction of that effort means more energy is available for making bigger moves, move moves, and more precise moves (stressed and/or tired muscles are less smooth/accurate/precise/etc).
  • 9 1
 That's all well and good, but we don't keep our balance on the bike like a baseball bat on the palm of our hand. And the rider+bike system is 'slightly' more complicated than a solid bat.
  • 9 1
 Not convinced by this one.
In the Rotational Inertia video, the bike is the inverse of that because the pivot point is the ground and you're moving your body (at the top) to keep balance.
  • 1 1
 If you're standing on your bike, 0 km/h, you are "moving your body to keep balance".
But when moving, imagine it the other way around : if you turn left, you "push" towards your right side (or if you're on ice, you don't push anything, you have no traction and you fall). The opposite reaction means that the ground is applying a force towards your left side.

That force is in some way similar to the guy pushing the bottom of the pole with his fingers, not only up (to support the weight), but on the necessary side to stay below the COG.

The main difference is : the ground is not the one 'thinking and correcting", you are.

It's a lot more complicated, but when assuming the bike+rider as a pretty rigid system (which is obviously not accurate at all! maybe when riding flat ground without obstacles?), the stick analogy makes some sense.
  • 1 0
 Oh btw : a friend of mine rides a "tall bike", his COG is like 1,8m above ground, and he also uses the stick analogy to explain why it is actually easy to ride (on flat ground without obstacles! obviously would be a nightmare in MTB)
  • 9 2
 Can you broscientists just take a very light, reasonably mid geeometry bike and ride some test loops with 2.5lbs strapped to the seatpost low and high and report back to us?
I will happily skim to the timed results and argue in the comments.
  • 5 0
 The important part is the third to last paragraph. When you lean your bike into a corner you separate your body weight from the bike's weight. The differences in mass distribution that are minuscule when looking at bike and rider as one static entity suddenly become more prominent when you are trying to move just the bike. I can't say that I would notice 20 cm of difference in shock placement but in other instances weight distribution is very obvious. With bikepacking for example it makes a huge difference if you put your luggage in a huge seat bag or the same weight in a frame bag. You notice the cornering and overall handling getting worse in both cases but the frame bag is much easier to get used to.
  • 5 1
 For sure with many kilograms of bikepacking luggage, it is important, not just the height of the mass but also the front-to-rear distribution. A shock is a much smaller mass and it has a much smaller range of available locations - you can only really move it up or down (not front to back) and only by a few cm.

The main point of the article, though, is that a lower COG isn't necessarily more "stable" as it would be with a four-wheeled vehicle. The inverted pendulum model of a bicycle implies that a higher COG is usually easier to keep balanced once up to speed.
  • 3 1
 Just to add something to the difference you point out between a frame bag and seat bag. The seat bag is not only higher but further back (I notice this effect even more with a child seat directly over the rear wheel). Either way, this brings the COG backwards towards the rear axle. This means any small steering correction at the front wheel is minimised by the ratio of the distance from the rear axle to the COG over the total wheelbase, so you have to do more exaggerated steering correction to remain balanced with a COG further back.

There's more here: en.wikipedia.org/wiki/Bicycle_and_motorcycle_dynamics#:~:text=The%20farther%20forward,wheel.%5B11%5D
  • 6 0
 This conflicts with what the bike companies have told me, and what seemed obvious when I'd given it no thought, and as such I am afraid and will now be getting very angry about it.
  • 8 4
 This is a fantastic article. More please. I think you may have missed a variable however @seb-stott. If you lower the COG of the bike, you are correct that the "upsidedown pendulum" effect makes it harder to balance. The baseball bat analogy is good. But lowering the COG also makes the wheels resist being deflected off line in the first place. The bat is harder to balance when the bottom is heavier, but its also harder to upset by deflecting the bottom.
  • 4 0
 A video showing the control point from the bottom (the finger), when the rider is on top of the bike....

The best thing about nerding is ensuring your inputs are correct Smile

Now, take a barbell, place it perpendicular to the ground, place a 20kg weight on the bottom of the barbell and control from the top to +- 45 degrees, now place the weight on the top and try the same experiment.

Nerded.
  • 8 0
 Pick a center of gravity and be dick about it.
  • 5 0
 A while back MotoGP bikes raised their engines toward the rider - they found that it moved the COG toward the roll centre and the bikes would change direction quicker.
  • 2 2
 Raising the COG moves it away from the roll centre (which is basically on the ground for a bicycle with narrow tyres, but slightly below the ground for a motorcycle with very large tyres), and this makes changing the lean angle slower. (there's a section on this in Tony Foale's Motorcycle Handling and Chassis Design, chapter 3, p25.)

I don't know much about MotoGP but I suspect that if they were deliberately moving the COG upwards for handling reasons it was to reduce twitchiness and make it easier to control a slide.
  • 3 0
 Check out the upside-down 1984 NSR500 that was a complete handling disaster with too low COG. I have always been suspicious of these mtb lower COG claims because of this.
  • 3 0
 @qldmtb: I just want to highlight that the NSR500 is an order of magnitude heavier than a mountain bike. I don't think this is an apples to apples comparison.
  • 1 0
 Cool lemme know how raising the COG on your mountain bike works out for you. -good luck.
  • 1 0
 To the doubters…
I have the same book and what a book. When I had it explained to me - the motorcycle does not solely change direction by pendulum over the top of the contact patch, but some pendulums under the COG and some pendulums above (the tyres are not fixed to the ground). Thus if you bring the rider and motorcycle mass closer together it will change direction quicker.
Bicycles will be the same and could be useful in DS?
  • 5 0
 1kg shock plus bearings, additional frame material around stress points, and a bunch of linkage could amount to much more than 2%
  • 3 0
 1 kg is already generous for a shock. You're correct those other factors should be considered, but it turns out the locations are often surprisingly similar. High shock mounts may have the link mounted below the shock, while low shocks may have the link above the shock. There are, of course, examples of a high shock with a linkage above the shock, but that's not always the case.

We could call it 3% or even 4% to be generous. It's still small, compared to the rider. It's also small when compared to adding or omitting a water bottle.

Your point is valid, but the conclusion is about the same.
  • 2 0
 And who on Earth is riding a 15kg trail bike
  • 3 0
 @chrismac70: That's 33 lbs, so I'm guessing a lot of people.
  • 1 0
 @pmhobson: that’s a heavy bike unless it has at least 170mm of travel
  • 3 0
 @chrismac70: Durable rims, tires, inserts, Fox 36, alloy frame, it adds up quick.
  • 3 0
 The balancing the inverted pendulum on your hand is instructive but not representative of the bike and rider. For the bike the contact patches are below the mass but the control from your hands is above the mass and from your feet close to the COG. This is far different than the video where the contact patch and the control are at the bottom of the stick and below the mass.
  • 1 0
 Ding ding! I think Seb's point had to do with steering causing the fulcrum (contact patches) moving like the hand in the pendulum video, but he kind of ignored that the main inputs are on the other end of the lever. Moving the CG lower definitely means a bike is (marginally, perhaps) easier to lean with handlebar inputs, but may make it "less stable" in that it may take more steering input (to move the contact patches under the CG) to maintain balance.

So, just looking at the inverted pendulum aspect, with respect to moving the contact patches via steering, ignores the simple 2nd class lever system of leaning the bike with handlebar inputs.

It also ignore bike-body separation when it assumes "you're constantly steering so the wheels are directly underneath your COG". That applies when attempting to go in straight line: there are constant small adjustments to steering to move the contact patches back and forth to get that invert pendulum effect. But in actual mountain bike riding, there are many many situations where you need to lean the bike (move the contact patches relative to CG) without any relation to the inverted pendulum aspect. For that movement, a lower CG means (marginally) less effort is needed to make the lean.
  • 2 0
 Also an issue of stability vs agility. A huge factor is what gives a rider the confidence to push a bike and ride it well. Some riders prefer a less stable more agile bike because it gives them the confidence that it will react quickly. Some riders prefer a more stable bike as its movements will stay more consistent over a range of conditions and it will react more slowly. So much can affect this. Caster angle, trail, front centre, rear centre, wheelbase, wheel radius, anti squat, anti rise, leverage ratio, spring rates, materials etc.

In the end, what matters is does it feel good to you as a rider and does it give you the confidence to ride it better and faster?
  • 4 0
 Lot's of scientific calculations to achive more stability, lightness and all that and in the end attach a bottle with 1 kg of water. Just sayin............
  • 1 0
 100%
  • 2 0
 Lowering your saddle on your 200mm dropper post effects your bike's CG and handling dynamics more than the difference in the shock position. The saddle is much further from the bike's CG and this makes the incremental movement of that mass even more significant. So, if you're worried about bike CG, maybe swap your 150mm dropper for a 200mm dropper...
  • 5 0
 If it's a pedal bike, it doesn't matter. Low shock= high water bottle. High shock=lower water bottle.
  • 2 0
 "That's because a bicycle is essentially an inverted pendulum, like balancing a baseball bat upright on your hand. In order to balance the bat, you continually move your hand so it remains under its center of gravity. Similarly, on a bicycle you're constantly steering so the wheels are directly underneath your COG"

^wrong wrong wrong^....you are constantly "LEANING" to keep the wheels under your COG. If you dont lean that bike over first it doesnt matter where your steering the bike aint going there.

More weight towards bottom of bike means easier to lean over/manuever = less fatigue. Lower COG = More stable in chunder, less deflection off rocks, more planted, holds better lines.

A better anology would be to stand a baseball bat upright resting on the ground then rock back n forth with hand on top of bat....do this both ways with barrel of bat facing upwards and downwards.
  • 2 0
 If this was true we’d all be raising our ankles instead of dropping them and standing as tall as we can. The easiest way to notice this is descending a trail with a heavy backpack vs no pack. Having no pack makes a huge difference on steep terrain.
  • 3 0
 I am sorry but this sounds like very poor "scientific evidence" to me: Especially when comparing a baseball bat to a pencil as a reference for demonstrating COG influence... not serious!
  • 2 0
 @seb-scott. If a higher cog was beneficial why don’t ebike have the battery integrated into the top tube and not the down tube. You could move a meaningful about of weight up which according to your article would help the bike handle better
  • 2 0
 This is not right physics wise. Or rather is only true at the balance point or equilibrium point. With the cg further away from the pivot, any deflection away from the equilibrium will cause larger forces to send you away from equilibrium. Therefore by the definition of Lyaunov stability, the system is more unstable. Ok simpler way to look at it. The most stable configuration is when the cg and pivot are at the same location. Hopefully that is obvious; the system won’t tip over under any deflection and therefore zero control effort is required to balance the system.. So if what the author says is true, the system suddenly gets more unstable by moving the cg infinitesimally higher and then increases in stability the further away from that stable point. I think he author is confusing moment of inertia with stability. Anyways, I fully expect this comment to be ignored.
  • 2 0
 This is not a good article PinkBike. Why do lower bottom bracket bikes corner so well? Having mass lower only increases a bicycle and rider's ability to change direction and stay planted through rough terrrain. Placing a rear shock lower in the bike might be a marginal improvement, but it is an improvement none the less. Bikes are quite refined these days and manufacturers are looking for a little edge where they can find it.
  • 2 0
 I think the most striking example was Banshee Legend like 6 years before the Demo having the shock a way lower.

Also I´d say the bike is balanced from above, while the video example shows us balancing from underside
  • 4 0
 Lots of people have no idea what they need until the marketing person tells them.
  • 2 1
 The old “system weight” argument about bike changes being negligible is irrelevant when it comes to ride feel. Your bike is your only connection to the trail so you’ll feel those changes quite a bit since it’s the only thing you’re acting on with your body. A lower COG is very noticeable in ride feel even though it is only a couple pounds moving a couple inches. Is it quantifiable through data? Maybe not. Does it make a perceived difference in how a bike handles thus changing the way you’ll ride it? Absolutely
  • 5 0
 WEIGHTED SHOES. THE NEXT BEST THING IN MTB !
  • 1 0
 Especially on carbon cranks!
  • 2 0
 It's why I never let my old five tens dry out.
  • 3 0
 Erik Buell spent a lot of time showing why centralized mass>low center of gravity.

But, MTB is determined to learn the hard way.
  • 3 1
 EB should not be referenced when it comes to doing anything correctly.
  • 2 0
 @JSTootell: HAHAAAAA, yeah Buell had some good ideas but also some issues lol. and i know from first hand experience ;-)
  • 1 0
 @shadethechangingman: Ex wife and I both had those POS's, I mean, Buell's.
  • 1 0
 The baseball bat to pencil comparison doesn't really show anything. The baseball bat is heavier, so it has a larger moment of inertia just by being more massive. A longer pencil that is hollow or thin, so that it's the same weight, would also be easier to balance than a short pencil

Better analogy would be a hammer with the head down versus with the head up. Head up is (perhaps counterintuitively) easier to balance as an inverted pendulum because the moment of inertia (relative to the fulcrum point of your hand is larger. Same system mass, but different inertia: much closer analogy to moving heavy bits of a bike up or down relative to the ground.
  • 2 0
 Dang it. It seems that I exceed “The total system weight - that's the bike plus the rider - weighs about a hundred kilograms” before I even get on my bike. Where did somebody say they got those 1mm socks from?
  • 5 2
 What I took from this article is that there's a market for thinner socks and people will pay a premium for the resulting COG reduction.
  • 1 0
 I think the small change to the system CG is indeed interesting, so I appreciated that.

But the rest of the article is a pedantic word game about what “stability” means and whether less or more is good. I don’t think riders actually misunderstand this. Less “stability” is roughly equivalent to less rotation inertia, i.e. more agility, which is indeed largely a good thing when leaning a bike over in a corner. This is analogous to smaller wheels being “less stable” than bigger ones. Riders make the that tradeoff for maneuverability in that case as well.
  • 1 0
 Why would I want stability? My bike moves because I create instability by turning the handlebars so it starts to fall over. If your bike falls over slower then it handles slower. I like my bikes to handle fast so they do what I want.
  • 1 0
 Better question :
Why do people who DONT MAKE BIKES always know better then those who do make bike how to build bikes ???
And why aren’t THEY make better bikes ?

Answer : cause it’s easy to wait till someone makes something and diss it then to actually do something yourself . Dissing Luther’s makes you feel better about yourself instead of realizing your all talk and not as good as others .
The word comes to mind - narcissism !
Basically why we are we’re we are today in every subject in society , the core of socialism - wait till someone makes money (something of value ) and then want to be part of it after it’s a success - not before - and when you get told no you say it’s shit or they are a*sholes or product bad or system bad - all but YOURSELF!
WANT HIGH SHOCKS ? Make them!
  • 1 1
 Any system must be analyze by its total, meaning bike (all components, including water bottles) and rider (with/out backpack).

The composed system (rider+bike) CoG, is determined not only by the individual CoG but also the weight of each.
Meaning that being a rider +80% of total system, the CoG of the system will be closer to the belly button of the rider.
The good news is:
1) we're flexible, and any rider should practice flexibility, and the hability to ride as low as possible (DH!)
2) there're dropper posts

Shocklocation as more to do with link location and suspension cinematic, than anything.

Like many things, people should LEARN how to ride, before spending on equipment that will have very marginal ganes.

PS: regarding people finding heavier bikes motorized bikes more stabble, has to do with the inercia of the bike, that will be less prone to deflect, but also more difficult to move the bike/motorized bike around.
  • 3 3
 I am glad someone is finally saying it. Low shocks look cool, but in the end do not do much. I also agree with everything written except for the last part. First, the momentum of inertia gets bigger the further away mass is from the rotation axis. Now, when you enter into a right hand corner, you do not really lean right, but your wheels go left first, which makes you lean. Which means, the close the mass is distributed to the COG, the easier it is to turn. That is well known in motorsports, where the balance is between a high center of gravity for good cornering, and low COG for braking and accelerating.
  • 3 0
 Good news: both concepts are correct! Smile

A high centre of mass creates a feeling of stability in two ways:

1. Slows the rate of rotation about the roll-axis
2. A given lateral deflection (ex. wheels knocked off-line after hitting a rock) produces a lesser lean angle, which we could think of as an unexpected turning event

These effects also delay the initiation of an intended turning event and increase fore-aft pitching, which tend to reduce the feeling of control.

I've designed and raced lowracer style recumbents for human-powered vehicle competitions. These can have an extremely long wheelbase and extremely low centre of mass, which taught me a lot about the extremes of centre of mass and turning rates. The conclusion - like most things - was that it's important to balance the parameters. As you pointed out, when all other handling parameters are held constant, any one parameter adjusted in isolation usually has an optimum value, rather than constantly improving as it approaches zero or infinity.
  • 3 0
 Aaron Gwin reading this: “call Intense. I’ve figured out why I’m slow again.”
  • 1 0
 Can someone help me understand why my Orbea Rise feels so stable in rutty straight aways, hitting corners, and jumping? Is it the overall weight of the bike, specifically at the BB?
  • 1 0
 Stable relative to what else? Without a comparison, it's impossible to say. Could be wheelbase, overall mass, handlebar width, stem length, front-center to rear-center balance. reach to rear-center balance, wheel mass, stack height, so many things.
  • 1 0
 The distance between the roll axis and the COG MULTIPLIED BY A FORCE (in this case mass of the rider + bike reduced to COG * centripetal acceleration) is called the roll moment of inertia.
  • 1 1
 Point of clarification (possibly correction) @seb-stott - the inverted pendulum analogy implies that the axis of the pendulum is the contact patch. It is, in fact, the bike CoG aka the center of rotation. Excluding mountain bikes turns where the bike is lifted and placed, and bicycle or motorcycle turns more like an airplane than a car. It is steered out from under the system CoG (countersteered) and then falls into the turn, which is a rotation of the bike to move the contact patch out of balance. So moving weight closer to the CoG reduces the moment of inertia and allows a bike to initiate a turn faster, and moving it further away increases the MoE and slows turn initation. Important because assuming the CoG (of the bike alone) is right around the top of the chainring, having 1kg centered there vs 30cm away at the top tube would have a significant effect on a 15-18kg system MoE - actually making it handle notably quicker, or with *less* roll stability.
  • 1 0
 "You could literally have a bigger effect on your center of gravity by wearing thinner socks." Hahahaha... Look for guys to start going sockless in pursuit of their Strava status.
  • 1 0
 In order to balance the higher cog he moves his hand too much. So having a a higher cog bike should be more difficult to balance because the wheels are in a given position. In my opinion lower cog is better
  • 3 2
 i am keep buying the bike, which have VPP, stiff rear end and MOST space for bottle/glovebox in front triangle! and those are Santas, New Pivots etc...
  • 3 2
 Read something before, when percentage of rider weight+ bike, applied to part or placement, was insignificant..until you ride it
  • 3 1
 For those who often ride muddy conditions I still believe a high shock location is better
  • 2 3
 Yes, Pinkbike. Moving 5% of bike weight a foot lower makes for more-better-game-changer bike and 180 pounds of flesh on top does not matter. Throw a hyperboost 162mm axle, turbotorque 2" headset and there as well and take my $10000.
  • 2 2
 Weight down low is great, don’t have to move it as much to lean. It feels nice. It also happens to look nice. Who wants their shock attached to the top tube? This has never looked nice. That’s about it.
  • 3 0
 Another great reason not to buy an e-bike
  • 4 0
 Nerd fight!
  • 3 0
 Bicycle dynamics are more complex than Formula 1 vehicle dynamics.
  • 1 0
 theres a clear difference in cornering a lower shock bike vs a mid or higher shock mounted bike. and no smarty pants science boi is gonna change my mind about that
  • 1 0
 We need dynamically unstable bikes with triply-redundant computer systems to keep them balanced:

en.wikipedia.org/wiki/Grumman_X-29
  • 2 0
 Great article and great comments. Now place the shock where I can reach the climb-switch while riding!
  • 1 0
 If you add 1KG to the bottom of the baseball bat, it will balance itself. Think of balancing a sledge hammer by the head vs the handle.
  • 1 0
 Statically, sure, it'll stand up on a flat non-moving surface. But now try to balance that bat with extra low weight on your hand. It'll be harder because the lower CG reduces the moment of inertia.
  • 3 0
 Good suspension kinematics >>>> CoG
  • 1 0
 What an enthralling article. Can wait for the top ten riding socks and the bike test based on a 1 minute flat trail. Nice one.
  • 3 1
 A baseball has a 30 times wither base than a pensil.
  • 5 0
 What side of the baseball?
  • 6 8
 Nope: "That's because a bicycle is essentially an inverted pendulum, like balancing a baseball bat upright on your hand."
this is plain wrong.
If a bicycle is similar to something, its more like you balancing on top of an upright baseball bat that's on the ground. I'm not sure of the effect of lowering the COG on a bike by a few mm by moving pivots here and there. Even a 1kg change in mass of the bike (frame) is hard to perceive on a flat section or downhill. What is percievable is a large change in mass like comparing an motorbike or e-bike to a pushbike. Also questionable is the term "stable". If stable means the ability to plough trough rough stuff, then a low COG is beneficial without doubt. Essentially the lower the COG, the less the bike want's to tip over the obstacle it needs to roll over. So it's certainly more stable in this sense.
  • 7 4
 At very low speeds (e.g. a track stand) a better model is a double-inverted pendulum, where the rider balances by pivoting her mass relative to the bike (I think this is what you're saying?) and so arguably the COG location of the bike itself is more relevant.

But at higher speeds (most of the time during non-trials mountain biking) the single inverted pendulum model is more appropriate.

See here for more: en.wikipedia.org/wiki/Bicycle_and_motorcycle_dynamics#:~:text=A%20bike%20is%20also,racks.%5B39%5D
  • 4 2
 @seb-stott: I am saying that a bike is nothing like balancing a baseball bat upright on your hand. It's just a very bad comparison.

Secondly, the concept of "stability" is not well explained in this article and I point out a situation where exactly a lower COG DOES improve perceived stability while riding a mtb.
  • 1 1
 Spoken like a very stable genius.
  • 2 1
 I thought the purpose was to make space available to carry enough water to cross the Sahara?
  • 1 3
 My old bike was a 27,5 2017 Cannondale Jekyll. That bike had the shock mounted "high" in the frame. My new bike,a 2020 29 Endur, the shock is very low. I do not think shock placement is key,suspension design overall made every bike a unique feeling.
My old Jekyll felt like like the bike rear end was moving up and down a lot in relation with the main triangle,like the bike was 2 separate pieces. The Enduro could not be more different,the bike does not fold so much we you compress the suspension,or at least is the feeling I get.
VPP bikes and almost the same thing,like the wheelbase is more constant over sigle pivot,4 bar suspension bikes. Same thing for the 6 bar suspension and Yeti suspension design (regular bikes with the shock guide).
  • 4 1
 REDALP WAS RIGHT!
  • 2 0
 Backpack vs fanny pack on the body.
  • 1 0
 Please, do more of those. They are, always and by far, the articles that excite me the most.
  • 1 0
 Basically, there is a reason Orange tested with lead weights at bottom bracket level and we never heard of that ever again.
  • 2 1
 Pretty shi#y article! Stop adding bikers weight to bike weight and then counting it as percentage! So annoying
  • 1 0
 I guess none of this really matters tho if your riding with a water bottle on the frame.
  • 1 0
 Word on the street is that they are in the works for some lead riding shoes to lower your center of gravity. Take my money.
  • 1 0
 The marketing department needs something to sink their teeth into.... all taken with a grain of salt.
  • 1 0
 This article goes against my unscientifically formed core biases. I don't think I'm allowed to read it.
  • 1 0
 Another example is an electric scooter (Bird, Lime etc). They have very low COG and they're twitchy as f*ck.
  • 1 0
 I think low shocks are mainly about supposedly improved leverage ratios (santa cruz claim) and bottle space.
  • 1 0
 Can't wait for Neko's experiments using a 100kg backpack... Ultimate stability status achieved!
  • 1 0
 Imagine how much lower the center of gravity will be when that cubby the shock is mounted to fills with mud
  • 1 0
 No Santa Cruz? That’s the first bike I think of that famously lowered its shock position.
  • 2 0
 Suspension design is way more important
  • 1 0
 Your article points out that manufacturers' marketing claims are dubious. Thanks, but it is about 100 years too late!
  • 1 0
 Great article- I learned a lot.
  • 2 1
 STOP MAKING SENSE, SEB!!!

(with apologies to David Byrne)
  • 1 1
 Wait till the 29r crowd are told their bikes are less leany, cause bigger spinny things.
  • 1 0
 Well, they literally are. Doesn't mean it's bad, it's just a trade-off. Big wheels have marginally better rollover, and small wheels are marginally easier to move around. Both good and both bad, depending on perspective.
  • 2 0
 @justinfoil: Fully understood. It was just an example to poke at the silliness of this article.
  • 1 0
 @cthorpe: yeah I was being a bit facetious, but trying to make the same point.

Moving the shock low has disadvantages and advantages, depending on situations and preferences, however marginal they may be. The article seemingly tried to show that a low CG doesn't affect "stability" but kind of ignored that there are many kinds of "stability" and that inverted pendulum stability only applies to a very narrow band of the riding experience. Seems like they just kind of wanted to shit on "low shock is good" marketing, and picked specific and limited arguments to support that.
  • 1 0
 I don't like mud n crap all over my shock.Sticking with my new stumpy Evo
  • 5 4
 And this ..is why ebikes suck.
  • 1 1
 Excellent, need to do one on the Mullet Bike fad and maybe we are getting somewhere!
  • 1 1
 I control my COG with adding or removing weed from SWAT storage…easy….and it works…
  • 3 0
 * It S-works
  • 1 0
 Time to roll out the TI shock hardware , anodised for the UK market.
  • 1 0
 This article is all COGs, no chain.
  • 1 0
 @seb-stott Another brilliant article!
  • 1 0
 This was kinda clickbait.
  • 1 0
 So yes, but also no. That's some good reading, Mr. B.
  • 11 12
 Ride an ebike with the extra weight close to the bottom bracket and tell me it isn't more stable..
  • 18 1
 that's more the weight, rather than where it is.
  • 7 0
 You’ll be feeling the effects of increased sprung mass not a lower centre of gravity on an E-bike.
  • 33 0
 That's because all the weight in the pocket (wallet) is removed
  • 8 3
 Owning an e-bike takes a lot of weight off your shoulders but it may replaced by that sinking feeling of punching below your weight on group rides.
  • 1 0
 looks like a Breezer
  • 1 0
 Nerds
  • 3 4
 The battery and motor on my ebike do wonders for lowering the CG.





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