How well will the Corset work with my frame? Posted on 02 Mar 00:53 , 1 comment

So we've had a ton of enquiries as to how the Corset will work on your specific bike. Basically, this is dictated by the interaction between the spring rate curve, and the leverage rate curve.

Let's discuss the effect of different leverage rates, and how the stock sleeve and the Corset each work with them. Let's consider that there are basically four main variants of leverage rate curve that exist on modern bikes:

LEVERAGE RATE CURVE TYPES:
1. Linear - no substantial change in leverage ratio throughout the travel. Example - Yeti SB66C.
2. Progressive only - leverage ratio drops throughout the travel. Example - Rocky Mountain Altitude; lots of DH bikes.
3. Progressive to linear/digressive - leverage ratio drops throughout the first 2/3 of the travel, then either flattens out or increases again towards the end of the travel. This is THE MOST COMMON leverage rate found on bikes designed around air springs. Example - Specialized Stumpy/Enduro, Banshee Rune, Pivot Mach 6, Ibis Mojo HD.
4. Digressive-linear-progressive - leverage ratio INCREASES (this is known as a digressive or falling rate because the spring rate at the wheel falls, not because the leverage rate falls), flattens out (linear) then decreases again (progressive). Examples - pretty much all VPP bikes where the shock is driven off the top link; Nomad/Tracer/Bronson/Carbine/Blur/etc.

Let's also consider the characteristics of an air spring in general. If we break the travel down into 3 segments (initial stroke, mid-stroke and end stroke) they are:

1. Initial stroke: Digressive. Spring rate starts high but drops off, meaning the feeling starts stiff and gets softer, relatively speaking.
2. Mid stroke: Linear. Meaning the spring rate stays constant but is already quite low, therefore feel stays soft for a while.
3. End stroke: Progressive. This means that not only does the spring force increase, but the RATE at which it increases, also increases.

That characteristic exists on all current air springs (DRCV being a semi-exception... long story for another day) to some degree. The Corset has a far less extreme version of this than the stock Fox sleeves, but it's still there as you can see from the spring rate curve graph here:



Anyway, let's look at how each of the previously mentioned leverage rates works with each of the Fox and the Vorsprung spring curves:

1. LINEAR leverage rate aka constant leverage ratio (note that the words rate and ratio are not interchangeable; ratio is a number at a given point in the travel, rate refers to the shape of the curve). Example graph below - looking at the blue line only. Note how it is nearly dead horizontal, ie negligible change in leverage ratio over the travel:

Credit: linkagedesign.blogspot.com, Antonio Osuna. Graph used only for the purposes of concept explanation.


Resultant wheel spring curve with Fox spring: same shape as the Fox spring curve, just multiplied by a constant number (scales it up/down) which is the leverage ratio. This means you end up with a falling rate (stiff but becoming soft) initial stroke, a low spring rate (soft) in the mid stroke, and an increasing spring rate (soft but becoming stiff) at the end of the stroke. Initial stroke is stiff, meaning low sag, which can be good for climbing geometry because it keeps the seat tube steeper and more weight on the front wheel. Mid stroke is soft, which usually means people want that to be stiffer for descending support, which in turn makes the initial stroke even stiffer and harsher. End stroke can be tuned by volume spacers to achieve whatever ramp you need to prevent frequent bottom out, but running suspension too soft and relying on sudden end stroke ramp creates harshness too because you end up just running into a wall of resistance. 
Resultant wheel spring curve with Corset: same shape as the Corset spring curve. Compared to the Fox spring curve, far softer initially and firmer in the middle. Once again, end stroke can be tuned with volume spacers to achieve whatever ramp-up you need. This means you run more sag (can be bad if you already have a bike with a slack seat tube angle (eg SB66) and climb a lot of steep stuff (the kind of stuff where you're just about breaking your shifter trying to find an easier gear that doesn't exist... pretty well all the climbs in Whistler!) because the front wheel will try to lift up more. On most bikes the difference isn't anywhere near that much though, because despite running more sag, the mid stroke spring rate is quite a bit stiffer, so the settle point when your weight is shifted backwards is closer to what you'd get with the Fox sleeve. Descending, more sag gives more traction, firmer mid stroke means more support on bigger hits and compressions as well as a more predictable and lively feel. 
Net result: Unquestionable improvement here, in my honest opinion.

2. PROGRESSIVE leverage rate:

Example graph (blue line) below. Note that the leverage ratio starts high, then decreases:

Credit: linkagedesign.blogspot.com, Antonio Osuna. Graph used only for the purposes of concept explanation.

Resultant wheel spring curve with Fox sleeve: high initial leverage helps somewhat to overcome the stiff initial air spring feeling, but unless it's progressive enough, the mid-stroke is still very soft - and if it IS that progressive then it ends up ramping like crazy at the end of the travel. Cross multiply the progressive end stroke of the leverage rate with the progressive spring curve of the air shock, and you're either going to have something that sits at excessively high sag or makes it far too difficult to use full travel. Basically, progressive end strokes don't mix well with air shocks, it's very hard to get them sorted in such a way that the mid stroke is sufficiently supported without crazy end stroke ramp, although they do feel reasonably good off the top of the travel usually. That said, no frame I've ever examined has been anywhere near as progressive in the early stroke as a Fox air spring is DIGRESSIVE in the same travel - to do so you'd have to have the leverage ratio drop by at least a factor of five in the first 15mm of the travel, which would mean your leverage ratio probably starts at about 15:1... this is also very bad (gen 1 V10 had something almost like that) and it causes a lot of play at the start of the travel due to the immense leverage on both pivots and shock.
Resultant wheel spring curve with Corset: High initial leverage ratio combined with relatively linear spring curve provides very plush feel initially, mid-stroke support somewhat better, end stroke still over-supported and hard to use full travel - Corset can't fix that.
Net result of the Corset: Better small bump absorption, marginal improvements in mid-stroke, but can't fix the excessive end stroke ramp. Overall, relatively minor gain - you'd have to design an air spring specifically for this application in order to make it work well, but fortunately most bikes don't use this configuration unless they're intended for use with coil sprung shocks. 

3. PROGRESSIVE-TO-LINEAR/DIGRESSIVE leverage rate:

Example graph below, looking at the pink and the green lines. The green line (Banshee Rune, progressive-linear-digressive) shows a decreasing leverage ratio for more than the first half of the travel, which then briefly holds fairly constant before reversing direction and increasing the leverage again. The pink line (Devinci Spartan) is a progressive-to-linear design, where the leverage rate decreases for the first 70% or so of the travel but doesn't change too substantially after that:

Credit: linkagedesign.blogspot.com, Antonio Osuna. Graph used only for the purposes of concept explanation.

Resultant wheel spring curve with Fox sleeve: high initial leverage helps somewhat to overcome the stiff initial air spring feeling, but unless it's progressive enough early enough, the mid-stroke is still relatively soft compared to the initial stroke. However, because the leverage rate flattens out (linear rate) or increases again (digressive rate) you do get better mid-stroke support with this, since you can run higher air pressures without causing crazy end-stroke ramp up. However, what we're really looking for overall, from start to finish, is an increase in wheel spring rate of about 55-65% overall (once shock spring rate curve has been multiplied through the leverage rate curve), which corresponds roughly to a coil-sprung bike with a leverage rate that starts 25-30% higher than it ends (progressive). This is slightly lower than what's been shown to work very well on both DH bikes and MX bikes, though most air-sprung bikes don't need quite that level of big-hit performance. In order for this to happen with existing Fox sleeves, we'd need to see frames with ~3-4x their existing initial progression, but only for the first 20mm or so of travel, before becoming comparatively linear throughout the rest of the stroke. This doesn't currently exist in anything I've seen.
Resultant wheel spring curve with Corset sleeve: much closer to the performance of a coil shock (on a bike that's well designed for a coil shock, not on the same frame!) because the leverage rate change can effectively cancel the slight falling initial rate of the Corset, the supportive mid-stroke of the leverage curve cuts the actual drop in spring rate at the wheel down to zero (or can even allow for a truly progressive spring rate in quite a controlled manner) without causing the excessive end stroke ramp. Once again, you can tune the end stroke with volume spacers.
Net result: Corset provides distinct improvements in bump absorption, support and liveliness. This combination, in my opinion, allows for the the highest performance currently available from an air shock and linkage curve, and is particularly good with the Corset.


4. DIGRESSIVE-LINEAR-PROGRESSIVE leverage rates:

Example graph below. Note how the leverage ratio increases substantially in the first 1/3 of the travel, holds pretty much constant in the middle, then decreases (becomes progressive) again at the end of the stroke:

Credit: linkagedesign.blogspot.com, Antonio Osuna. Graph used only for the purposes of concept explanation.

Resultant spring curve with Fox sleeve: Low initial leverage (stiff) combined with high initial spring rate (stiff) = very stiff initial stroke. High mid-stroke leverage (soft) combined with low mid-stroke spring rate (soft) = very soft initial stroke. Low end-stroke leverage (stiff) combined with high end-stroke spring rate (stiff) = very stiff end stroke. As a result, you get a very stiff feel at the start of the stroke, like there's tons of preload on the suspension, followed by an unsupportive mid-stroke that is prone to feeling wallowy, which eventually runs into a fairly abrupt wall of progression at the end of the stroke. Good in that you don't bottom out easily, bad in that if you run higher air pressures for mid stroke support, the early and late stroke segments are just much too stiff.
Resultant spring curve with Corset: Comparatively low initial spring rate reduces the effects of the low (stiff) initial leverage rate and makes the early stroke much softer feeling. You can now run higher pressures for more mid-stroke support without making the early stroke feel too stiff. End stroke is still somewhat progressive - once again, no air spring system out there can really fix an excessively progressive ending stroke of the leverage rate curve - so achieving full travel can still be difficult. 
Net result: much better overall in terms of support and bump compliance, but still the end stroke progression issues may remain - removal of any/all volume spacers in the shock is recommended in most cases. 


I hope this has helped clear up how the Corset can work for you. Basically, the summary of all of the above is that the Corset is better in pretty well all scenarios, except the following:
1. Frames that already struggle with steep climbs due to slack seat tube angles and/or short chainstays will be worse at climbing. No ifs or buts, it's just harder if you have more sag. Frames that have steeper seat tube angles, longer chainstays etc (basically the frames that already climb well) won't be affected to the same degree... I have plenty of personal experience with this!
2. Frames with excessive ending stroke ramp up already will still have those issues - that wasn't something that we could really address without requiring pressures of 400+psi for the average rider. That's not to say that the Corset will be any worse, just that it won't fix that issue. As a general rule, it'll still perform better overall, but it's not perfect. 
3. If your damper is improperly tuned for your frame (usually as a result of being switched out aftermarket, the ones that come with the bike are generally reasonable), in need of service/repair, this can't fix that. That'd be like changing your handlebars and hoping it somehow makes your tyres more grippy.

Is it the magic pill solution to every issue with every frame? Unfortunately nope. We've legitimately done our best to make this as good as it can be, and this is the ceiling of what we found to be possible. I can confidently say that on 99% of bikes it will make your suspension work distinctly better.