Frame design: how to select shock stroke & length?

Preface: armchair engineer, so take anything I write with a grain (or bowl) of salt. 

Regarding Cathro's bike I think the benefits of the link is not only that they could install a longer shock, but a link allows them to tune the leverage rate compared with what you can do with a single pivot. The more planted feel could be achieved by running a softer spring which would then not result in harsh bottom-outs due to the aforementioned tuning of the leverage rate.  

Regarding shock length, depending on the intended travel and linkage you can run into problems if you have a too short/long stroke length for a given travel range. For instance, a 160 bike using a 55mm stroke shock has a leverage ratio of approximately 3 to 1. For a heavy guy like myself I'd need a 700+ pound spring to achieve 30% sag (and may exceed maximum pressure of an air can), not ideal. 

Obviously, it's hard to say exactly what they were going for with that orange... especially so when you consider the oddities of boost valve etc that was built into the suspension products of the time.  

Generally, related and along with spring rate, has to do with shaft speeds at the shock and then the resulting oil displacement within the damper.  Higher leverage ratios mean higher shaft speeds, which means more oil displaced.  There is a delicate balance between the amount of oil being displaced, the compression/rebound piston ports and shim stacks and the base valve shim stacks.  Shocks back then, namely the dhx and the vanilla from fox were using very large diameter damper shafts meaning they were displacing a lot of fluid through the base valve.  Combine that with the significant amount of wheel travel and the small shock (relatively small compared to the longer option) and you are shooting for a very narrow window of shim stack tuning.  

This is something that you're seeing play out in real time with longer travel enduro bikes btw.  Back in 2019, when the boom of enduro bike and long legged trail bike development was in full swing, you were seeing 230x57.5 and 230x60 shocks being used for 160+ bikes.  You're now seeing the majority of the industry going to lower leverage ratios and 230x65 shocks for the same travel.  Neither approach is bad persay as your matching the damping/valving to the frame... however my assumption is it's easier manage a wider range of damping applications and spring rates with a more metered amount of oil flow.  

There are certainly cases of it being difficult to manage too little flow with smaller damper shafts.  I felt like the first dhx2 suffered from this after the transition from larger shafts being used in the final generation of dhx rc4.  This is obviously resolved now and there has been a lot of revisions to the dhx2 since then (poppet valves to more traditional hsc/hsr pre-load style architecture).

I digress... it's all a balance between leverage ratio, wheel forces, shock shaft speeds, damping as it relates to shaft speeds and oil displacement etc.  

You digress? I thought we are all supposed to hate digressive valving now! 
 

So the Orange is kind of a unique case - in stock single pivot form they typically had a very flat leverage ratio with little rising rate which meant the initial travel would have had quite a low leverage ratio relatively speaking. The linkage allow for more rising rate so while the average leverage ratio will have been lower, the initial ratio was probably higher. 

As for choosing shock length - its one of those things that is a sum of several factors - there will be a target rising rate at the wheel, which then needs to be mapped over to the shock and since there is a limited number of spring rates/air pressures available you will need to make sure those available rates can generate the force you need at the wheel. Mountain bike dampers are also quite delicate things - too much leverage will cause binding and friction which is really bad, so once you add in all these factors there is usually only narrow window of options left!

Using the longest shock possible will increase the control the damper has over the wheel travel, decrease forces, increase oil volume, decrease fluid pressures, reduce the sensation of any loose bushings/pivots. The biggest drawback is finding enough space to accommodate a long shock, especially on small frames.

The theoretical drawback of increased stiction seems to be something of a marketers excuse/justification to me. Forks have 1:1 travel ratios and two huge seals; and though fork stiction is a thing I don’t think people limit their fork travel choices to reduce stiction (though smaller stanchion diameters might better suit lighter riders).

Wait till you fit Bilstein shocks on your truck...

I wouldn't read much into the Orange comments. They could have done a million things with leverage ratio that gave it a "more planted" feel. That would not have been due to the shock change alone. From past experience in the industry these types of comments often get misinterpreted. Bigger shocks are nicer because they have more oil volume and can handle a bit more heat as a result. Especially in that era where shocks were not as refined this may have been a significant factor.

In terms of picking shock stroke it's exactly what previously people alluded to. You need to match it to a target leverage ratio + desired amount of travel. Generally I would start with an approximate shock length, get the leverage curve shape I want, and then pick a final size. There are no hard and fast rules for leverage rates but after experimenting with starting leverage ratios well under 3 (2.4-2.8) for a long time and then shifting to leverage ratios quite a bit above 3 (3.2-3.5) the industry seems to have largely moderated around about 3. 

This matters because leverage values have a big impact on shaft speeds and therefor damping force. Higher leverage ratios lower shaft speeds and require much firmer tunes as a result. Lower leverage ratios might require extremely light tunes. With how base tunes are on modern shocks leverages around 3 seem pretty close to the goldilocks zone. There are other knock on effects of leverage such as higher leverages overcoming stiction easier and requiring different spring rates but they are of a lower degree. 

You can retune shock and get custom OEM tunes but at the end of the day the shocks still have a optimum performance window. If you also want to maintain a good range of effective adjustment that window is narrower than people probably think. 

As an aside, the above is also why something like a Cascade Components link works really well for some people on some bikes and other people have more mixed responses. On a bike with a lower leverage ratio (SBG era Transitions) the increase in starting leverage could drastically improve the shocks and bikes performance. On a bike that already has a very high starting leverage (Spec. Enduro) you might be staring to compromise shock support unless you get a retune. There are too many variables to say absolutely that it is a good or bad thing for a rider but Cascade is open about it being a tool for experimenting and about possibly needing a retune. Rider size will also play a big part in that equation. 

There was a while where DH frame manufacturers were playing around with longer shocks. Foes famously had their ridiculously long (and heavy) 2:1 Curnutt shocks, so an 8" travel frame has a 4" travel shock. But Devinci, Morewood, YT, and a few others (can't remember) employed 3.25" stroke shocks as well. Pretty much no one uses those extra long shocks anymore, and the reason I heard is that they overheated a lot. Longer stroke/lower leverage rate actually means faster shaft speeds on the shock, which means everything (oil, seals, etc) is moving faster and moving more. I had a bunch of friends on the original Split Pivot 26" long shock Devinci Wilson and they were roasting RC4 shocks pretty fast, whereas I never blew up an RC4 over several years. [Editor's note: I recognize my anecdote has an "n" value of n=5. Take it with plenty of salt]

That's interesting. When you say "roasting" were the failures fading, loss of oil, or something else? Could it have been correlation rather than causation - maybe there was more side loading/displacement because of their length, maybe you or your bike was better? Some seal materials are only rated up to certain shaft speeds, perhaps they needed to be different for different stroke lengths? Did these riders experience similar problems in their forks, forks dampers have that 1:1 motion ratio and are kinda insulated within the fork tubes (thermal insulation bad, structural insulation good)?

It seems counterintuitive since the no matter what size the damper, it's function will be to convert a given amount of kinetic energy to heat. The higher shaft speed should be met with correspondingly lower piston forces, for equal power. A larger damper will have a greater surface area to dissipate the heat, and a higher oil volume with a greater thermal capacity. I'm not arguing a position here, I'm interested to know if I should revisit my assumptions. 

Follow up thought - a long stroke shock might need a larger IFP chamber. Without that the pressure and temperature of the oil might suffer.

I spent a lot of time on the Enduro and had it set up with an Ohlins coil shock. It worked really well. I then figured I'd throw an Ohlins on my Crestline DH bike. It didn't go so well. The shock seems to have a fairly stiff off the shelf tune. With the Enduro being a higher leverage bike, it makes sense that it worked really well on that. The link for the Enduro shares the same starting leverage ratio as the link for the V4 Nomad, which is 3.7, and a fairly close final leverage ratio (just under 2.5). This was very much so intentional. For my weight (~150 lbs) and what I ride, the V4 Nomad is still one of the better feeling suspensions I've messed with so I was trying to get something similar out of the Enduro. That worked out quite well and I rode the Enduro for a pretty long time.  If I weighed 75 lbs more I'm not sure I'd feel the same way. This is more or less how the turbo levo long shock kit came to be.

When we were developing the turbo levo link, a coworker who does weigh about 75 lbs more than me was the one primarily riding the bike. Between that bike being heavy and being ridden by a heavier rider who can ride hard, the little 210x55 was easily getting overwhelmed. Came up with the long shock kit so that shock pressure would be lower and shaft speeds would be higher to allow for a higher damping force felt at the wheel for a typical shock tune. This all worked out quite well. Even weighing less I'd take the long shock over the stock one. 

On the other hand, there is an upper limit to how long of a shock you can go to if you want off the shelf tunes to be suitable. I view off the shelf tunes being suitable as an important thing because most people aren't going to want to change their base tune. Tweaking dials is fine, but shims are left alone by most. We did a link for the Crestline DH bike that reduced travel to 180mm with the same 250x75 as before. This necessitated a much lighter tune to feel good. 

Looking at how damping is felt at the wheel for a given leverage ratio and wheel speed input it all makes sense. You end up finding that damping force as felt at the wheel is inversely proportional to leverage ratio squared. In another thread full of fun rhetoric someone who's sensible (not the other guy) was saying he felt that on a Dreadnought between HBO and the more progressive link he was unable to access bottom of travel. This example here isn't a Dreadnought, but it makes it easy to see how HBO might be too much for some people.

Yeah from a conservation of energy standpoint, if you are absorbing the same amount of kinetic energy through damping then heat generated would be the same but the larger oil volume would be advantageous. If running a tune that's more suitable for lower shaft speeds I can see too much heat being a thing.

Yeah, I don't know why the longer shocks didn't work, I'm only passing on hearsay.

The long shocks stand out in my memory because around the same time I was riding Santa Cruz for downhill and SC was messing around with really short shocks on the V10, originally on the '06-'10 frames and then again in 2015 with the new 27.5 bike. I had a lot of trouble ever getting enough low speed damping out of the 27.5 bike with the short RC4. I got custom tuned shocks from Fox with stiffer compression stacks that were still very undamped and open feeling. After that, it seems like the whole industry had converged on a shock stroke/frame travel ratio somewhere in the middle (around 2.75:1) until recently. And it seems like even those recent outliers (2020 Enduro, 2020 Megatower, etc) have all iterated back to longer shocks in their newest versions.

I'd love to know why the manufacturers have gravitated towards that "golden ratio" of 2.75:1. I know in theory you can always make a shock of any dimensions more or less damped with different shims, bores, valves, etc, but it seems like frames with longer shocks have always felt a little over damped and frames with shorter shocks have always felt a little under damped. And both long short and short shocks seem to have reliability problems, based on my observations.

My guess is that that 2.75:1 ratio was gravitated towards simply as a result of human sized packaging limitations. Vertical shocks need to fit under the top tube of the smallest frame size, horizontal shocks need to fit behind the head tube of the smallest frame size. The new Specialized and SC bikes align their shocks with the down tube and cut holes through the seat tube, which allows them to be longer.

There might’ve also been shock length/stroke configurations which just worked better or were used when developing stock shim stacks. Perhaps they had a better IFP length, oil volume or bushing overlap; IIRC Metric shock sizes were supposed to eliminate the worst builds regarding these aspects. 

I will forever remember the day when Cascade Components called me "sensible."

I do wonder how much of the current trend of HBOs in shocks is affecting this whole discussion as well. It seems to me like we're moving more towards position sensitive damping in general, which would probably facilitate a longer stroke shock as well. 

I've always been under the impression that the longer the shock stroke, the more "tune-ability" you could muster out of it due to the length of fluid displacement. But I guess you could probably achieve the same thing with a short stubby shock with a huge internal piston. 

I guess, in general, is there a preferred level of fluid displacement/shaft speed that is needed for shocks to actually function?

So theres a few other things that compound here - 

The thing about damper tunes that gets overlooked is that each damper really only has a limited window that it can be revalved to. The piston design, shaft side, damper bore and base valve dimensions all add up to work in a certain range of speeds and forces. It's not that straightforward to revalve a shock as soft or firm as you like. Once you are outside that window the pressures and forces inside the shock will no longer be balanced properly and it won't perform anywhere near what it should. This is the kind of thing Vorsprung deals with in their tractive tunes - it isn't really about "higher flow" its goal is creating pressure balance with the main piston so it can be revalved better than the stock damper could.

The other part of that is if you have a digressive compression tune (which is pretty common) then the break point from low to high is roughly around the same speed (not always, but thats another story) but with different leverage ratios, low speed at the wheel translates to a different speed at the damper. So if you can't shift that opening point to the appropriate speed as well as force then it won't work as well as you intend. 

Lastly the thing to realise about leverage ratios isn't just the overall change in rate but also the rate of change and also the total travel used. You can have 2 frames with 20% progression that feel completely different, especially in a longer travel bike. A high rate of change will feel much more progressive than something with a more consistent rising rate even if they have similar beginning and ending ratios. They will also use wildly different spring rates! 

One thing I’ve always struggled to wrap my head around is what the right cut off for high speed and low speed should be. I’ve certainly ran into issues with impacts and where wheel speed is right around the crossovers, but it’s few and far between. Interestingly it’s with the fork and 90% of the time it seems to be when the front gets stuffed in a hole under during heavy braking. What it feels like it’s the impact is just fast enough to be in the high speed range and with the tune being digressive not enough damping force is generated. It feels like this could be a good probability problem where the goal is to minimize the likelihood of an impact in right around crossover where something undesirable happens. 

Yeah I don't think you will get much agreement on what is "correct", forks and shocks both compromise here but probably for slightly different reasons! First thing is your low speed adjuster has a massive effect on the whole range of speeds so sometimes the shims don't open until way over 1m/s, but then they don't want the overall damping to be too firm and why most companies use a digressive design so you can add damping at lower speeds without increasing too much at high speed. Problem is a lot of people need that extra high speed damping so you end up compromised at some point in the speed range. I could be wrong here but what you describe is the kind of thing Push were targeting when they created the HC97 - there's a point where its beneficial to add some substantial damping between 0.5 and 1m/s but its tricky to do without being too firm at higher or lower speeds in most dampers out there.

The issue is you basically can't have a low speed adjuster that only works at low speeds without also having a carefully matched high speed valve. Also if your LSC only works at low speeds it tends to feel like its doing nothing at all so people complain! So the result is a struggle to tune for different speeds independently. 

With shocks you have a little bit of the same problem where the low speed dials dominate the adjustment, but it gets compounded by the different leverage ratios. If the wheel rate is very soft in the first part of the stroke it will spend more time at higher speeds (at the wheel) and if the leverage ratio is quite low the damper will also be at much higher speeds so while the damping rate needs to be softer, that opening point will need to be later to keep it properly controlled or again you will get that weird underdamped feeling in bumps.