If you have a coil shock, finding the right spring rate can be tricky. In an ideal world, you'd experiment with a few different options until you find the right one. But with some springs running into three figures, that can be very expensive, not to mention time-consuming, so you want to have a very good idea of what spring rate you need before you order one.
For clarity, the spring rate, or stiffness, is the amount of force required to compress the spring by a certain distance. This is usually measured in pounds per inch (lb/in) or occasionally Newtons per millimeter (N/mm). So, for a 400 lb/in spring, it would take 400 lb of force to compress it one inch, or 800 lb to compress it two inches, and so on.
What spring rate you need will depend on your weight and on the suspension design of your bike. In particular, its leverage ratio, which is how far the axle moves for every millimeter the shock compresses. You can easily work out the average leverage ratio by dividing the wheel travel by the shock stroke. A higher leverage ratio means you'll need a stiffer spring to provide the same sag.
When it comes to estimating what spring rate you need, there are lots of online calculators to help you do this. Here are some links to spring rate calculators from
TFTuned,
MRP,
Fox and
The Suspension Lab.
These calculators are great - honestly, most people will be happy enough using any one of them. But they don't always agree with one another because they work in slightly different ways. The calculators from TF and The Suspension Lab rely on subjective inputs like riding style and trail speed, which maybe makes more sense to some people but it's hard to be precise with such terms. Meanwhile, the ones from Fox and MRP allow you more control over the objective input parameters like sag and, in particular, how much your weight is biased towards the rear wheel.
Experienced riders usually have a rough idea of how much sag they need based on their riding style. If you want something super comfortable, you might run as much as 35% sag; for a more responsive ride, you'll be nearer 25%. But for most bikes, 28-30% is the sweet spot. But how do you know what your weight bias is? And how do these calculators work anyway?
To calculate the spring rate you need to achieve a given sag, you can use the below equation. Unless you want to play around with different values of the leverage ratio (more on that below),
you don't actually need to use this equation because the online calculators from Fox and MRP do it for you. It's just there to show you how they work and what all the variables mean.
Spring rate in lb/in = W*B*L^2/(T*S), where:
W = Your riding weight in pounds. Make sure to add the weight of your riding kit.
B = The weight bias, or the percentage of your weight on the rear wheel when standing on the pedals without any pressure on the grips. This is given by the ratio of your bike's rear center divided by the wheelbase, but a good approximation for most bikes is 65%.
L = The bike's leverage ratio, which is usually just the travel divided by the shock stroke. Alternatively, you could find a
leverage ratio graph for your bike (from the manufacturer, a review, a
blog or
linkage software) and find the leverage ratio at the sag point. This will give you a more accurate estimate of the sag you'll get. But remember, with a more progressive bike you'll usually want to run more sag (and visa-versa); using the average ratio takes this into account to some extent.
T = Your bike's travel in inches
S = The percentage sag you want (e.g. 0.3 for 30% sag)
A note on preload:MRP and Fox's calculators allow you to see the effect preload will have on sag. But remember that preload does not affect spring rate - it's just a way to adjust the ride height (sag) independently of the spring rate. I'd recommend using zero preload, both in the real world for better sensitivity, and when calculating spring rate because sag is an indicator of the spring rate you need, not a goal in itself. Yes, you'll need a little preload to stop the spring from rattling and there is some breakaway force from the damper to consider too, but roughly speaking, these tend to be cancelled out by the weight of the bike, which is why bikes don't tend to sag much (if at all) under their own weight. That means, to a reasonable approximation, if you ignore the turn or so of preload needed to stop rattle you can ignore the weight of the bike too.
Here's an exampleI recently reviewed the
Starling Spur, which has 170 mm (6.69") of travel and a 65 mm stroke shock, giving it an average leverage ratio of 2.62. Because the
leverage ratio barely changes through the travel, we don't need to consider using the ratio at sag instead of the average ratio. In full riding kit, I weigh about 200 lbs, and by dividing the rear center by the wheelbase we get a weight bias of 64%.
Since it's a pretty linear bike with a very active suspension design, I wanted less than the standard 30% sag. If I put 28% (0.28 ) into the equation, along with all the numbers above, it suggests a 467 lb/in spring. If we plug in 26% sag for a firmer setup, the equation suggests a 503 lb/in spring. In the real world, I used a 502 lb/in spring, which gave me bang-on 26% sag.
I used the Fox calculator for my Capra (170mm /65mm/ 200lbs) and got 500lbs too - it was way too soft.
I had to adjust the calculator to DH settings to match the real world 550lbs spring.
With a more progressive rate the bike did sag into midstroke with the softer spring.
150/57 = 2.6
At the 30% sag point on the leverage curve, the ratio is actually nearer 2.8, so you need a stiffer spring, and I have weight balance data in my calculation too. To take another example, plugging these stats into the TF Tuned calculator linked in the story suggests between 457lb and 485lb spring for an 80kg rider. That rider actually need at least a 530lb spring for that bike according to my data and testing experience. Given that springs generally come in 450 or 500lb options, most are going to choose 450lb, which is going to need 4-5 turns of preload to get the ride height right. And then you're going to be coil binding the spring before bottom out on such a short shock. Less of an issue on more modern metric length shocks, but really you should never wind on more than 2 turns of preload. If you do, you're spring is too soft.
@emptybox resistance isn't force, it's the amount of energy the shock absorbs between the starting point in travel and bottom of travel. While a more progressive bike exerts greater force on the wheel at bottom of travel, it exerts less at top of travel. So as you move through top of travel it does less to resist that compression but it makes up for it by doing more to resist it deep in travel.
So you and your 6 followers need a box of tissues? Craig is probably the most knowledgeable person on the planet when it comes to suspension tuning for any discipline of riding... MX, sleds, mtb and anything else with a shock or fork. He has been at the core of the suspension evolution for the last 25+ years... Riding, racing, building, innovating and tuning wirh best riders and giving them the Avalanche advantage to perform better. That usually means when he speaks, what he has to say is very relevant and you should listen and je learning.
"noun
the act or power of resisting, opposing, or withstanding.
the opposition offered by one thing, force, etc., to another."
www.dictionary.com/browse/resistance
I tried LR at sag point with the formula in the article and it closely matched my current sag and spring rate. I had tried some of the spring calculators listed and the results were indeed all over the map.
I've got the 400 set up with just a tiny bit of preload, hit a perfect ~28% sag, and am just super happy with the performance of the shock. I for sure wouldn't want to be on a lighter spring.
Energy is what matter.
Alternative view is to see the suspension as a damped sprung system, and look at accelerations and speeds instead of energy.
If you remove damping, your spring will overshoot it's equilibrium position due to the inertia and the remaining kinetic energy. damping allow for a more controlled stroke that will overshoot less the equilibrium position. Force only tell you about the direction of acceleration, nothing more.
- no it's not a true damped mass-spring system. There is several independent masses (that aren't constant!) , several springs, several dampers. You can approximate when looking at small intervals.
- mass and acceleration tells you the force seen by the mass at a given time, providing the mass-spring system analogy is relevant. You can deduce the force given by the impact if you know the force at the spring and damper, but that isn't relevant to anything and once you've looked at accelerations and speeds you'll know about everything you need without looking at forces at all, and that's the goal of looking at it in this way.
What you can feel is a complex subject. But that's not relevant when looking at your suspension's action.
The "B" in the article's formula. It's not constant.
From the shock POV, the leverage rate variation also vary the "mass" seen by the shock thru the travel. Or if you look at wheel forces, it's the damping coefficients and spring rates that are varying.
But that's the easy take. Now you have legs, tires, wheels... everything flexes, with a "damped-spring-system" model. making things seen by the actual suspension element of your bike, not constant.
The wheels aren't fixed to the ground so even your references points aren't fix, and there is at least three inertial frames.
You can't feel an acceleration actually. You're good at feeling force or distributed force, you're good at feeling relative displacement of your body parts, but you don't feel anything from the bike. absolute movement isn't really felt at low frequencies. But I digress, how what you are feeling is relevant to your suspension action? Makes no sense to me.
B is not a constant, but that is not mass, neither is the force seen by the shock or the other things you mention. The masses are constant.
I think it's jerk you feel.
Perhaps consider that gravity itself is not a force, it's an acceleration, we experience the 'force' of gravity because we are a mass and an acceleration - gravity is acting upon us. In space for instance, we are still a mass, but there is no gravity/acceleration so there is no force. You need a mass and an acceleration to make a force.
So rather than 'cancelling out' the forces, implying to you they are not there, it's probably better to think of them being equal to each other. As the forces of mass and acceleration are the same in opposite directions there is no net velocity.
You could summarize more as:
1. Get the spring the manufacturer recommends for your weight. Round up if you like more sag, down if you like more. Jf you don't know, flip a coin.
2. See how it rides. If it feels too tall, go down one step in spring rate. If you get tons of pedal strikes or bottom out or etc, go up in spring rate.
3. Once settled, sell your old spring on pb buysell. If you also bought your replacement spring on pb buysell, you are probably not out much money.
4. Ride the bike and stop thinking about it. Unless you get fatter or less fat, then go back to step 1.
Progressive springs add yet another variable, but the process is still the same. It just means if you're in a weight range where the manufacturer recommends a 500lb spring, when you round up or down depending on where your actual weight falls within that range, you can round by 25 instead of 50.
If you really want to know actual sag you can always put a zip tie o the shock shaft temporarily but I've always found that with coils it doesn't really matter, better to just tune by feel.
EDIT: Dang it, left the page up for a few hours and forgot to click submit... I think we're all on the same page here.
I've measured a Sprindex 610-680 at 590-640lb/in.
Changed that to the factory dhx2 and that was completely different. Even with to high or soft spring rate it was better. Could ride further without falling off legs.
Same thing for my converted coil fork. The dive of the airspring on steeper stuff made me Sick. To compensate that the fork was always to stiff.
The smashpot inside with a adjustable hydraulic made it so much better. I don't even need to switch coils to make it stiffer. Mostly the same for me as the sprindex.
My bike weights probably 17 kg and everything is high-end except the pedals so I don't care if my coils make it on kg more heavy..
www.bikeradar.com/reviews/components/rear-shocks/sprindex-adjustable-coil-spring-review
Because it is possible to ride in the Alps for me I don't agree that air is good as a coil. Descending fast for an hour straight is something else. Same for brakes at this point.
Pedaling uphill is even faster then doing it with a 3kg lighter bike. All bikes with slack seat tubes are not good for me.
No problem to ride that thing for an entire day. This is build to last and mostly for low maintenance. No derailleur , only coils, burley tires and Trickstuff brakes won't need anything.
For my conditions anything is a waste below DD, at least for me IF I want to ride any MAXXIS off-road.
Anything less is just a flat tire.
@mikeetheviking a pimped out Privateer 161 that wasnt cheap overall because of the parts but it is a long term bike for me and the parts stay on it.
Thank you for highlighting our (MRP) spring calculator. It's quite robust, but we recognize it may be too complicated for many users. We are working on a new one, as well as more information and guidance about if and when to choose one of our Progressive Springs.
If anyone has any questions about how to use the calculator or wants a second opinion that takes into account their particular bike, please reach out to us via e-mail or the online chat function on our website.
It's one of the main things I want to change with our calculator. For now, just enter the same travel number three times.
All this talk about how the different calculators do things very slightly differently, and how it it could be done better and more precisely, and then you just throw down this approximation? It's one of the easiest things to calculate: as you say, chainstay over wheelbase, two of the easiest measurements to find or make. Except that number is going to be closer to 0.35, so it's really the inverse, front center over wheelbase, that will get you that ~65%
After using TFTuned, they suggested a drop of 150 lbs spring rate from 500 to 350. I conservatively dropped 100 lbs in my spring rate to 400, and am finally at 27%(ish) sag on a 180mm bike. Which i'm okay with, because I still pedal it.
Some manufacturers allow ample preload (for example rock shox), some virtually none (for example EXT).
It comes down to multiple factors like spring design, damper design (especially top out circuit) and a few other caveats I do not want to delve into right now.
When running bigger preload always make sure you have a spring that acutally supports it, so a spring with a 65mm stroke plus preload (up to 5mm for RS) needs to have at least 75mnm of acutal travel so you do not experience coil bind which basically, occuring once, pushes the spring past its design limits resulting in a damaged spring (shortened, not the same rate anymore).
I have dynoed / analysed a lot of springs from different manufacturers with some very peculiar results.
(EXT and SAR Springs are great, SLS only depending on length, others are utter shit).
I am one that thinks coil shocks typically outperform air so the inconvenience of set up is less significant. Except on bikes where its a PITA to remove the shock to swap the spring.
21' Shore 1, 165 lbs, 180mm travel, 70mm stroke DHX2, aiming for 28% sag
Norco Ride Aligned in 2021 recommended 500 lbs (15% sag, factory spring)
Norco Ride Aligned in 2022 now recommends 450 lbs
TFTuned recommends 358 lbs
MRP recommends appx 325 lbs
FOX recommends 300 lbs
Suspension Lab recommends 400 lbs for Balanced, or 370 for Plush.
I only knew about Fox and TFTuned, and used their calculators. I thought dropping from 500 to 300 or 325 was crazy talk, so I went with 400 using a Nukeproof spring and ended up with appx 27% sag. The wifes bike dropped from 450 to 350, and she is also around 27% now. Worst case, I could buy her a 300, and I use her 350 in the future if we want to go softer.
What did I learn? Leverage seems to have alot to do with it. FOX and MRP's calculators seem to be too light. Suspension lab, in my case, for my application was the ideal calculator. They were the only ones to factor in weight of bike. I just happened to nail their spring rate upon a guess, wishing I knew about it sooner.
It suggested 595lb (600lb) and it felt spot on
Even the damper settings for the DHX2 were 90% on perfect for me and I'm very in tune with setting up suspension
@mnguyen1224: I'd love to see bike manufacturers supply an array of springs - if it is going to ship coil. I just bought a Nukeproof coil for $30 CAD. You know their raw cost would be $5 or less per... When buying a $7000 bike, seems like the least they could do?
Of these four designs, single pivot and 4-bar are more alike while DW-Link and VPP are more alike. And why does the patent number on the linkage have any effect? Shouldn't we be looking at leverage curves and progressivity?
2nd step, buy a cheap RockShox coil, or any other steel coil for 20$
3rd Step, Ride it and you will know
4th step, Wonder if buying a light spring is really worth it since you love your bike with the heavy spring.
You _want_ to run the sag that feels best. A progressive frame _allows_ you to run more sag if that's what it takes to feel right.
For example at on the OEM link multiply my riding body weight 192# times the leverage rate at the sag point of the stock link, 2.85, and I get 547#s. With the Cascade at 3.25 LR I get a 624# spring. Both of these were very close to what I settled on, using the Springdex.
IME, the Springdex works really well on the first few turns of the adjuster, but when you get it nearly tightened all of the way down it's not working as well.
I jacked around with a high end coil for months. Never again. The future of MTB suspension is air, of this I am certain.
28% sag, 150mm/5.91" travel, 3.0 leverage ratio at sag point, 180 lbs weight rider+kit, 65% weight bias.
Your riding weight (body weight + gear) * 2 * (1+Sag%). and from there if your frame is more linear, think about a progressive coil or that adapter thingy or just jack up high speed compression.
Like for PaulWolf, with my quick and dirty formula, you get 460.8 lb coil. So I say 450 or 475 lb coil.
The cane creek was when I had my Santa Cruz nomad v4. MRP only fits rockshox.
Dvo and cane creek have the same ID. And fox and ohlins have the same ID. But you can use DVO and CC coils on the latter because it’s a 1mm delta.
I just used the MRP version and it put me on a 400# spring for a Spire with Cascade link, I run a 550# on that bike and it's perfect......
My wife and I both have 25 year old bikes. Hers is a Trek Y3, mine a Santa Cruz Heckler X. Obviously we are not killer riders ;-) Both bikes have 4" of rear travel, some flavor of a Fox Vanilla shock, and 300lb springs. Both also have way more than a "turn or so" of preload.
I've been slowly updating and tweaking them over the years. I stumbled here while trying to figure out the spring rate for her bike to give a comfortable ride. Her bike was a size too big when we got it so there have been some tweaks to get a really short stem, seat way low,.... I was thinking her spring was probably set for the typical 160lb male rider the manufactures think ride their bikes. Since she is ~120lbs I thought I might need a lighter spring.
Well.... I did the calculations for both bikes. She would need a 460lb spring for 30% sag, I would need 370lb. To make sure I was doing the math correct I used the example in the article and got close enough numbers.
With no preload I would sag 36% my wife 46%. So what to do here??? I don't want to get a heavier spring to make the ride smoother. Seems wrong. Is there any issue having a high preload to get the sag into the 30% range. I'm not sure where it is now.
Thanks
Jim M.
I like fast rebound, especially on a progressive bike I feel like the setup is really compromised with rebound closed enough to stop top out.
- why sag gives a good indication then, even for bike travels that can double across categories?
Natural frequency is consistent and is set by the whole mid-stroke.
And please don't preload shocks, they are already exploding without help.
Sensible preload isn't killing any shocks. Preload is a necessary part of coil suspension. With no preload your coil is loose and rattling.
Shocks blowing up is bad design and manufacturing. Also bike companies offsetting shocks to one side so the whole thing is flexed into a banana under bottom-out loads.
The first 20-30% of the suspension stroke is representative enough of the useful stroke for sag to be an efficient method. It doesn't give you a perfect rate, but you can either buy the closest spring and call it a day, or adjust air pressure in the future fore fine tuning. Bonus it gives you a coherent tuning for all the bikes, more or less the progression of the bike or spring/damping curve
Frequency tuning would require extra maths, would require extra basis of reference for each amount of suspension travel, for a result that would be less than 10% different.
You're probably the only one doing that in the mtb world.
Frequency tuning is helpful in some motor vehicles because you have some frequencies that matters, like around 1hz for seated human comfort, or your tire own natural frequency, and so on. On a mountain bike, I have a hard time seing any benefit from looking at frequencies.
Although on reflection it probably doesn't matter that a bicycle doesn't sag on its own because it also doesn't ride on its own!
Average response through the travel is what matters.
Frequency tuning by feel doesn't even require maths. Every rider has built in accelerometers which give the acceptable range.
Which is why it works far better than any other system. Including the bollocks that is sag.
172lb*.64*2.6^2/(6.49”*0.3%)
=172*.64*6.76/1.947
=744.14/1.947
=382lb/in
At that spring rate I would need about 5 turns of preload to get 30% sag on my 2018 Intense Tracer.
If I try using the leverage ratio at Sag (2) I get 226lb/in at sag. Is there something wrong with my calculations?
www.mtbr.com/threads/adjustable-rate-coil-spring-sprindex.1122499/post-15497121
450 = park.
475 = pedal.
500 = Porky.
Does it means the long of the chainstay length?
I swapped to the DHX2 on my canyon sender with a 700lb spring (the heaviest I could find), the sag was good but I bottomed it out so hard on medium drops and it blew the seals on the 2nd day out.