The Tuesday Tune Ep 12: Leverage Rates
Mountain bike frames have various forms of what we broadly refer to as suspension design, but more specifically could be termed kinematic design. Kinematic design refers to the ways which each member of the suspension linkage move, particularly relative to one another. In other words, the suspension kinematics are the geometric motions of the linkage, before we start introducing force or mass to our considerations.
Anyone who's ridden more than a couple of different mountain bikes is well aware of how different the rear end of the bike can feel, even with the same travel and when using the same shock. Variations in leverage rate, among other things, are a very large factor in determining how the suspension performs, so this week in the Vorsprung Suspension workshop, we're looking at the ways in which variations in the leverage ratio can affect the feel of the bike, and why small changes can result in fairly large differences.
Leverage ratio is typically defined as the ratio of mechanical advantage that the axle has over the shock. This can be an instantaneous leverage ratio at a single point in the travel, or an average leverage ratio obtained by dividing wheel travel by shock stroke. Plotting the instantaneous leverage ratio against travel gives us a leverage rate curve. The difference between ratio and rate is that the rate describes how the ratio is changing throughout the travel. In the motorsports world it is more common to use the term motion ratio, which is the same thing but inverse - it is obtained by dividing shock stroke by wheel travel.
When considering the overall wheel rates - that is, the spring rate and the damping rate when measured at the wheel - it is necessary to understand the spring and damping characteristics of your shock as well as the leverage rate characteristic of your linkage. Some frames work really well with the linearity of coil shocks and relatively poorly with progressive air shocks, and vice versa, and it is this interaction between the leverage rate and the shock's spring characteristics that is primarily responsible for this. An in-depth discussion of how air springs, particularly those of our own products, interact with various leverage rate curves can be found here.
Please note a few things here:
1. As usual, the devil is absolutely in the details. Generalisations, including some of the things we refer to in the video, are not applicable to every variant of any particular linkage design, and this is a long long way from being a comprehensive breakdown of leverage rates.
2. A certain acronym or name for a linkage type does not mean all frames of that type perform or feel the same. They can vary hugely.
3. We're not here to tell you what system is "best". There is really no objective way to assess that - everyone's preferences and priorities vary, and what works well for someone who likes a super firm feel doesn't necessarily work well for someone who wants the plushest thing out there.
4. If you'd like to see what your own bike's leverage rate is like, download a program called Linkage and measure up your own bike to see how it stacks up. Note on using this program: there are a great many existing files out there and a lot of them are a fair way off the mark, particularly on bikes with very short links. Don't rely on clicking on points on a photo for accurate inputs - that can be accurate enough for singlepivot bikes with no linkage, but on anything else it's more than likely going to be misleading. Use a measuring tape to verify distances between points on your suspension if you want accurate results.
Terminology worth knowing:
Linear leverage rate: this would be better termed "constant leverage ratio", as this is what delivers a linear force vs displacement curve at the wheel (if using a linear spring).
Progressive leverage rate: also known as a "rising rate", this creates a force vs displacement curve at the wheel that is progressive, ie an increasing wheel spring rate even if using a linear spring.
Digressive leverage rate: the opposite of the progressive leverage rate.
MENTIONS: @VorsprungSuspension
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Cool, got it.
This is all great stuff, but please FFS! one thing at a time... I'm still getting to grips with the whole spring/bouncy theory
Seems like even 63mm would have been a better option and that's not even all that much bigger.
The industry can't optimise leverage rates as such because there is no optimal rate - everything is a compromise to some degree, and the industry as a whole is constantly learning and refining. There are advantages and disadvantages to everything, and a lot of the time it's more useful to minimise a disadvantage than to maximise an advantage, if that makes sense.
The i-Track website is solid, Hugh knows what he's talking about. That page is one of if not the the best text you'll find.
To be honest, I like Santa Cruz, they make good looking bikes and know how to build quality carbon frames. I just wish they could build a shorter travel version of their V10
I have a Knolly Delirium at the moment with the wrong coil on it for me, before i source a new one, do you know if the Delirium is better suited to a coil or an air shock?
Question -- in theory, I can get very similar results at the end of travel either with a strictly progressive rate and a coil spring, or a progressive/digressive rate and an air spring. The damping rates (given the same damper) would be quite different. Will this difference be obviously noticeable?
so theoretically, on my Giant Glory 2016 which has a slighlty progressive ratio the first 2/3 of the travel and then a regressive ratio I should more likely use an air shock? You probably got some experience with this pretty popular bike, any advice?
Thanks for those vids they're super instructive. Waiting for them each week.
I did find the curves on Youtube indeed... and I honestly don't have the level to need a lot of bottoming resistance (27% to 30% SAG) ha ha!
I was hesitating cause of what I read here and there and also interested in a little weight loss (a SAR spring could be a solution) but bike works well I was just curious to hear from someone who knows better. Make things clearer.
Rémy? Giant? Good old days or...? @sillyseason
I have way too many non-engineer friends who spend hours on Wikipedia soaking up information about this stuff just so they can have the proper words to justify why they're selling Bike A and buying Bike X, even though they've thoroughly enjoyed Bike A for 10 years.
For reference, look up the nocebo effect (it's related to placebo).
In short, it means that once you think you have an ailment, your body/mind will in turn actually give you that ailment.
Thus, if your buddy suddenly reads about "Bike A's suspension is regressive," then he'll show up the next day, alarmed, and say, "Braaah, dude, my linkage is 2.3 to 1, not 2.29 to 1; no wonder I've been going OTB on that one g-out by our house! I need to get Bike X, because its ratio is 2.31 to 1, which is waaaay better, braaah!"
See, a monster was just unnecessarily created.
I 100% support the notion of riding stuff and deciding for yourself whether it works for you - theory has to fit reality, not the other way around. But if it happens that you decide you don't like it or that something could be better, understanding how it works helps you work out what the best course of action might be to improve things. For example, if you have Bike A and there's one particular characteristic about it that annoys you, chances are you won't be too stoked if you end up replacing it with Bike B that actually still has that same annoying characteristic.
At about 9:00 you draw attention to the similar curves
Of a VPP system (which are similar to meastro and Dw, or somwhat anyhow yes?) and an air shock can "exacerbate" or double up the "stiff-soft-stiff" quality of the curve.
Is this why a coil shock seems transforms the ride so significantly beyond the inherent linear nature of the coil shock itself?
By that I mean it does a linear shock let the bike express it's own mechanical leverage curve design more clearly?
Thanks again for another great Tuesday night brain bleed!
However, yes, linearising the first 2/3 of a shock's spring curve (either with a coil spring or a more linear air sleeve such as our Corsets) makes a very noticeable improvement on any frames with that leverage rate curve type basically because you're no longer doubling up on the spring rate variations to the same degree, so you don't get such an extreme swing between firm/soft/firm.
Thank you much for the reply,
More to learn about the various suspension types apparently!
At least on the right track with latter, thankfully.
I can see the merit in the architecture of the vorsprung corset more clearly now. It seems another rather large suspension company has since evolved their air cans as well since the corset's release.
I have a question about my bike setup: I have a capra which is very progressive with a float X2, and I find that it is hard to hit full travel. Would there be any disadvantages of removing spacer to decrease the shock ramp up? Another way of putting it would be: will the float X2 with few volume spacers behave like a coil shox?
Also, what is the impact of a very progressive linkage like this on riding? Obviously you have little damping at the beginning of the stroke and a lot at the end of the stroke, but I have trouble guessing whether it is actually a good thing or a bad thing. (I guess it is good for small bumps absorption but the rebound will be slower at the end of the stroke so it will maybe have trouble to recover from big repeated hits. But other than that…?)
The X2 has a wide range of comp/reb adjustments so that you are likely to find someyhing that fits you.
As I understand highly progressive linkage can lead to either too much sensitivity at the beginning of the stroke or difficulties for using full travel.
If you take the initial progression too far, you can end up with something that sags too far and/or runs into a wall of force at some point, but initial progression is not necessarily incompatible with coil shocks. How suitable a coil shock is for any given bike is, in our experience, more determined by the ending stroke ramp up of the leverage rate rather than the first half of the travel. Lacking ending stroke ramp up in the leverage rate means you need either a suitably progressive spring (and/or damper) curve from the shock, or you need to basically overspring the bike to prevent it bouncing off the bottom out bumper all the time.
Since you mentioned the Yeti SB series, I have a nagging question about that bike's suspension design (and perhaps others).
The idea, as I understand the marketing material
I have batted this idea around with an engineer friend of mine and neither of us can decide from thought experiment alone whether this "chain as a stiff spring" concept is actually correct. For example there are a few degrees of play in the ratcheting mechanism of the rear hub which could allow slack in the chain, and tension in the chain can be relieved by accelerating the rear wheel's rotation.
I would love to hear your thoughts on this design feature and whether it really works as intended.
Their analysis, at first glance, seems to take place on a bike that isn't moving. Since chain tension can be relieved by rotating the rear wheel, I'm still not clear on how effective a design paradigm it is in practice.
Sounds like magic or some kind of lifting-yourself-up-by-your-ears, I realise, but it is possible because it's a purely transient dynamic effect opposed by the rider's mass, not a static force balance. Funnily enough this is one of the major tenets of the DW-link design, and is outlined in the patents.
www.canecreek.com/resources/DBair_IL/Reliability_graphics/LinEair_Airsping_Comparisson.jpg
Now if I could just figure our why Scott decided to go with a digressive - linear - digressive leverage ratio on my Genius 720. I've been battling this since I got the bike and would not have bought it originally if I had known about it. I've maxed out the recommended number of volume spacers in two different air shocks I've tried (the stock fox float nude 2 and a DB Inline) and I'm still bottoming more than I like unless I run less than 20% sag or take my chances with more volume spacers than recommended.
2:1 leverage ratio with a 200lbs/in spring rate at the shock would mean that to compress the spring halfway through its travel (total travel, given the 2:1 overall leverage ratio, is 4"/100mm), you need to compress it 2" which is 400lbs at the spring, or 200lbs at the wheel. The wheel has moved 4" for that 200lbs, which means (200/4) = 50lbs/in wheel spring rate.
Let's say the leverage ratio now becomes 4:1 but you want the same stiffness at the wheel (50lbs/in), we reverse the order of calculations:
50lbs/in at the wheel means 200lbs to move the wheel 4" again, but now the shock is only moving 1" (4:1 leverage ratio). In addition to only moving 1", it has half as much leverage over the swingarm, so it needs to be able to generate 4x the force that the wheel is seeing (instead of 2x - ie twice as much force as before). So now we have twice the units of force, over half the distance, meaning a 4x higher spring rate.