Why air springs will never feel like coils

So I listened to an interesting podcast over here:

http://www.vitalmtb.com/features/TECH-TALK-Air-Shocks-Versus-Coil-Shocks-Whats-Better,2065

And once they mentioned adiabatic issues, I had to start googling because it’s a while since I did my engineering degree and the field of loudspeakers I work in doesn’t really suffer from adiabatic problems (unlikel in very high output PA compression drivers, where it’s one of the main sources of distortion).

Basically, when you compress a gas it gets hot. You’ll know that if you’ve ever used a bike pump – or in the reverse, used a CO2 cartridge. If the heat can escape immediately then you have an isothermal situation and this heat does not cause a temperature change in the gas which would change the pressure.

In an air spring which is moving quickly there is no time for the heat to escape, so this heat causes an additional pressure change (other than that caused by the volume change). There are some equations to describe this but in short, very high shaft speeds will result in a adiabatic behaviour whilst very low shaft speeds will result in isothermal behaviour.

100% adiabatic behaviour of air increases the spring rate by about 32% according to my quick sums. That is a lot! In reality what’s going to be happening is a mix of adiabatic and isothermal behaviour, which is called polytropic behaviour.

So if you design a “perfect” air spring, whose positive and negative springs create a totally linear spring rate, it will still not feel like a coil, because the coil’s spring rate will be always the same, regardless of shaft speed, whilst the air spring will ramp up by as much as 30% when moving fast.

That’s why, whatever you do with the suspension design, air springs feel more “poppy” and coil springs feel more plush and stuck to the ground.

True, but in the field you’ll only really notice the heat build up in long long descents…and more so in the rear shock than a fork..

I remember in the alps the air can on my meta 5 got really hot and I could clearly feel the spring had firmed up significantly…

Can’t say I’ve ever noticed it on a fork, mind.

I now ride a hard tail. The rear end is always firm, regardless of temp 😉

DrP

True, but in the field you’ll only really notice the heat build up in long long descents…and more so in the rear shock than a fork..

No, you misunderstood. And you’re a Dr and everything! 😉

It isn’t about the air gradually getting hotter and firming up the shock down a long descent. It’s about the fact that on every single stroke of the shock the temperature goes up on compression which firms up the rate, and then down on expansion (thus reverting the rate back to normal). And the quicker the spring moves, the more it firms up under compression.

Get two identical bikes one with the latest air shocks and one with the matching coil shocks and bunnyhop them or ride at a kerb fast and you’ll feel a clear difference, even if the air shock is tuned to be perfectly linear like a coil. They’ll feel the same under low (shaft) speed compression but under high speed compression the air spring will always push back harder, the faster it’s being compressed.

Like you, I thought the big reason for choosing coil over air was to solve the problem of the shock getting hot on long descents (hence choosing them for big EWS races where they’re descending as much as 1200m in one stage). But actually they feel different, even when cold.

On that Podcast, the two guys from SRAM (who are clearly much better riders than me – one’s an ex WC DHer), both agree that they prefer coils when they’re riding blind, because the coil deals with unexpected bumps and rough better than the air shock, whilst on trails that they know the extra pop of the air shock allows them to work the terrain more. Hence we may continue to see the rise of coils in EWS vs air shocks in WC DH.

There is another article on there now also about coil vs air. It’s annoying that once again all the discussion is about performance. I’m more interested in how much more reliable and how often compared to air, coil needs servicing. The comparison should be between two shocks with 6 months riding on them rather than brand new.

The comparison should be between two shocks with 6 months riding on them rather than brand new.

This is so true! Fortunately I definitely need the air spring ‘pop’ otherwise I’d be wishing I had a coil.

I’m more interested in how much more reliable and how often compared to air, coil needs servicing

Basically the same. The damper is the same so that doesn’t change. Air obviously has extra seals to look after but that can be done as a (do at home) air can service

Man listens to experts, learns something new. Shocker.

I would imagine that to some extent that can be compensated for by using a linkage driven shock with a regressive rate as it moves through travel?

Man listens to experts, learns something new. Shocker.

And your point is? Or did you just want to use a lame pun? 😉

I would imagine that to some extent that can be compensated for by using a linkage driven shock with a regressive rate as it moves through travel?

No, it absolutely can’t, because that change in spring rate happens regardless of where you are in the travel or how much the spring is moving on each impact.

BPW this weekend just gone, I used the CCDB Coil for a day and a half, then popped the Air CS on for the last half-day. The coil felt like it soaked everything up but was a little ‘dead’ when jumping (not that I’m very good at it, mind). With the AirCS on, there was more ‘pop’ off jumps which helped my poor technique.

Both feel pretty much identical at low speed and are set up as close to the same as I think I can get them. Spitfire MY2016.

Agree that air will never feel like a coil. But tbh, by far the best suspension I’ve ever had, is my remedy- it’s got all sorts of sci-fi happening in the can and a good suspension design and it kicks arse despite being air. I don’t think it’d gain anything by being more coiley

This thread could also be titled:

“Why coil springs will never feel like air”.

It isn’t that one is better, it’s that they’re fundamentally different and the laws of physics make it impossible for one to behave like the other, whatever you do with the air spring design or the damping. I don’t think coils would work as well for my riding style as a good air spring.

I’ve got one of these on my Cannondale

https://www.pinkbike.com/news/To-The-Point-Cannondales-Dyad-Pull-shocks-2013.html

Fox designed it to address some of these issues however the added complexity means they binned it and now just run an air shock again. Shame as it seems to perform pretty well AFAICT.

No, you misunderstood. And you’re a Dr and everything!

It isn’t about the air gradually getting hotter and firming up the shock down a long descent. It’s about the fact that on every single stroke of the shock the temperature goes up on compression which firms up the rate, and then down on expansion (thus reverting the rate back to normal). And the quicker the spring moves, the more it firms up under compression.

Actually I think you misunderstand. Yes gases get hotter if you compress them….but the process is reversible and they also get cooler when you expand them…so in a suspension system the gas gets hotter as it compresses then cooler as the suspension extends – that is why you get ice forming on the valve when you use a CO2 cartridge to inflate your tyre. The heat build up in the gas (and therefore increases its pressure) is partly to do with conduction from the heat into the shock body and friction with all the seals that you get in an air spring which you don’t get in a coil spring which is why the heat builds up over a longer period of time – I’ve never noticed a deterioration in shock performance over the short DH runs I’ve ever done. So the air is not heating up as a result of the compression and expansion, because it’s both heated and cooled as the suspension works.

The peppiness issue is a whole other thing altogether.

https://www.pinkbike.com/news/To-The-Point-Cannondales-Dyad-Pull-shocks-2013.html

Fox designed it to address some of these issues however the added complexity means they binned it and now just run an air shock again. Shame as it seems to perform pretty well AFAICT.

No, it doesn’t address this issue. Let me repeat myself, again.

1. You can now buy air shocks and forks which are so linear that its near impossible to detect the difference between their low speed spring curve and that of a coil (although it can be measured). This is achieved by careful design of the positive and negative springs.
2. When an air spring compresses or extends quickly, its spring rate diverges from the low speed curve – it gets firmer, and the faster it moves, the more the rate increases.

The Dyad shock does have a linear rate compared to other shocks of its era but many modern shocks are similar – large volume positive springs combined with big negative springs achieves this. But it’s still constrained by the gas laws.

Actually I think you misunderstand. Yes gases get hotter if you compress them….but the process is reversible and they also get cooler when you expand them…so in a suspension system the gas gets hotter as it compresses then cooler as the suspension extends – that is why you get ice forming on the valve when you use a CO2 cartridge to inflate your tyre. The heat build up in the gas (and therefore increases its pressure) is partly to do with conduction from the heat into the shock body and friction with all the seals that you get in an air spring which you don’t get in a coil spring which is why the heat builds up over a longer period of time – I’ve never noticed a deterioration in shock performance over the short DH runs I’ve ever done. So the air is not heating up as a result of the compression and expansion, because it’s both heated and cooled as the suspension works.

OMFG, I’m not sure why I started this thread now.

Is there anyone here who is also a mechanical engineer with an inclination towards resonant systems who will confirm what I’m talking about?

Here’s some of the papers/info I read before I posted this:

https://www.degruyter.com/downloadpdf/j/ijame.2015.20.issue-1/ijame-2015-0009/ijame-2015-0009.pdf
http://stumejournals.com/mtm/Archive/2010/4-5/1.Mashini/75_gavriloski.mtm10.pdf
https://www.mrostop.com/pdfs/Goodyear-air-spring-catalog.pdf
http://www.fst.tu-darmstadt.de/media/fachgebiet_fst/dokumente/forschung_1/verffentlichungen_1/040922_pelz_buttenbender_the_dynamic_stiffness_of_an_air_spring.pdf

As soon as heat transfers from the air to the shock body it is no longer an adiabatic process and the pressure change is diminished. It isn’t about a change in performance over time. An air spring behaves differently in an isolated movement, not just over time, due to the adiabatic behaviour of gases under compression or rarefaction.

Is there anyone here who is also a mechanical engineer with an inclination towards resonant systems who will confirm what I’m talking about?

dont be daft, armchair experts only round here..

i get what you’re saying..
i fill my shock at 90 psi and slowly compress it, and it behaves like a 90psi shock.

But..If i quickly compress it, the gas heats quicker, expands, and behaves like a 110psi shock (on that compression)….
right?

DrP

OMFG, I’m not sure why I started this thread now.

I know why you started it brainbox 😆

what if the shock is osscilating on a treadmill?

DrP

at all related to how a diesel engine compresses to ignite the mixture? Or is that total unrelated BS?

Is there anyone here who is also a mechanical engineer with an inclination towards resonant systems who will confirm what I’m talking about?

*Puts hand up*

Nah, only messin’ with ya.

I heard this podcast too. Interesting but frustratingly short. I wanted to hear them address more aspects of riding and perhaps talk about the differences between single and twin-tube dampers. My experience is that twin-tube air shocks can feel pretty close to coil, but still lack that last bit of composure. (RS don’t offer twin-tube dampers of course, so they’d have probably said they’re shit anyway)

It’s interesting they put the “poppy” feel we usually attribute to air shocks down to this obscure process, saying that air shocks are not inherently more progressive.

I’d have wanted to ask whether said process actually makes air shocks more progressive. It seems like two different ways of cutting the same cake, no?

Is there anyone here who is also a mechanical engineer with an inclination towards resonant systems who will confirm what I’m talking about?

Chemical Engineer and Chemist do you? Dunno if mechanical engineers would do thermodynamics to that level (and it’s not a resonant system problem).

What you’re talking about is the ratio of specific heats, Cp/Cv, gamma, Poisson constant. That’s what controls how much a gas heats up when you compress it.

pv^Cp/Cv=constant*, so if you halve the volume pressure doesn’t double as A-level physics would tell you to expect. It actually depends on the degree;s of freedom of rotation of the gas molecules, so nitrogen is very high (low degrees of freedom due to the triple bond), Chlorine is very low (single bond). You can actually initiate an explosion when starting up a chemical plant if you start filling it with explosive gasses and compress the nitrogen you purged it with as the nitrogen pockets left at the far end of the system get very hot.

It’s independent of rate though. The only rate dependence is in how much entropic work the system does (i.e. energy transferred to the outside as heat). A long slow compression would allow the heat to escape into the shock body and outside, a fast compression wouldn’t. But I doubt any suspension action is that slow.

*heat isn’t part of that equation, but if you plug the numbers back into PV=nRT then you can see that T has to change as n and R** are constant.

**this is also a lie but that’s another story.

Basically, when you compress a gas it gets hot. You’ll know that if you’ve ever used a bike pump – or in the reverse, used a CO2 cartridge. If the heat can escape immediately then you have an isothermal situation and this heat does not cause a temperature change in the gas which would change the pressure.

Yeaaaa, no.

That’s the Joule Thomson (or Joule Kelvin) effect, and is a different phenomenon again. Hydrogen for example would get hotter if you released it from a cartridge.

You’re way into some quite tricky to understand thermodynamics here.

Basically when doing reversible work with (theoretically) no heat transfer (an air shock) the gas get’s cold as it expands because it does mechanical work outside the system (pushes your suspension) or gets hot as it compresses (because work is done on it).

When undergoing free expansion (air rushing through a throttling valve) the temperature of an ideal gas wouldn’t change, but a real gas does, except when above the JT inversion point (which H2 is at atmospheric conditions).

To be honest though, I think the guy on the podcast might be wrong (if anyone has any research to prove him right I’d be interested to read it).

I’ve never heard of time dependency for an adiabatic compression unless the speed is above that of the speed of sound at which point the same gas laws apply, but you no longer have the luxury of the gas being homogeneous.

What I suspect is maybe happening, is kinetic friction of wetted surfaces does have a dependence of velocity (dry surfaces don’t until you get to the speed of sound again). So the lubricated seals produce more friction and generate more heat the faster they move, and an air shock has bigger seals than a coil shock. The heat produced is a red-herring in this case, it’s the increase in friction that’s slowing the shock down.

Don’t coil forks compress air as well?

This isn’t news.
A decent rider can adapt to the shortfallings of either spring type. So doesn’t really care*.

*except Minnaar

Don’t coil forks compress air as well?

They’re not pressurised normally though.

As an aside, I had a Marz coil fork that you could pressurise, but it made it feel worse.

They’re not pressurised normally though.

Yea, but thermodynamically atmospheric pressure throws up just as many weird anomalies as any other pressure, they (historical scientists) just wrote the gas laws in such a way that air tended to be considered normal at those temperature/pressures.

As an aside, I had a Marz coil fork that you could pressurise, but it made it feel worse.

Yea, that was a horrible system, as soon as you put air in it the stiction made them almost unusable.

Fair play to op for creating this thread here, a good read for sure.

Oh and try cheer up scienceofficer, life’s too short to be tetchy as I’ve noticed from your posts.

Strain dependency; Same as a rubber. The stiffness depends on the rate of loading.

Didn’t know that gases do this…

You know I work with compression all day long (Chemical Engineer like TINAS but I’m much longer in the tooth) and all this back of fag packet stuff is subject to so much error that its frankly pointless. The biggest error is that folk seldom remember to convert temperature to the absolute scale.

Strain dependency; Same as a rubber. The stiffness depends on the rate of loading.

Could it ever apply to a gas at significantly less than the speed of sound?

i get what you’re saying..
i fill my shock at 90 psi and slowly compress it, and it behaves like a 90psi shock.

But..If i quickly compress it, the gas heats quicker, expands, and behaves like a 110psi shock (on that compression)….
right?

Yes, that’s it!

Regarding spring progression, a simple air spring is strongly progressive but if you combine positive and negative springs and get their characteristics right you can effectively linearise the spring.

TINAS et al – read some of the papers I linked, it’s all there!

In short air is a newtonian fluid.

You can now buy air shocks and forks which are so linear that its near impossible to detect the difference between their low speed spring curve and that of a coil (although it can be measured)

I don’t buy this, for the same given sag – my coil 2018 36 simply feels stiffer than with an airspring. The major differnce though, is that it has a good deal leas stiction – you can physically feel it. What that equates to is being able to run less sag and still have enough grip.

This brings me to some other points, springs actually feel more poppy to me when you run correct sag…. most people do not though as they try and get the softer midstroke of the air fork or shock they used to have….for comfort. Then, with say the ACS3 spring kit – you get a dual stage spring that is both more supple, more supportive in the midstroke and progressive. I reckon it will be reliable as well as you shouldnt find yourself using the last third of travel that is supported by an airspring.

I’ve had way too many issues with air suspension, since going coil…. I have had none….when I had coils years ago… I also had no issues.

There was some guy at BPW commenting on why spoil a nice bike (mine) by using a primitive coil spring. I laughed.

I should add, you wont be using the last third of travel in the acs3 kit as much – so it should be reliable.

Having coil springs in your dandy horse makes you no better than other men. You’ve simply made a choice based on what you perceive to be “better”.
This perception doesn’t actually make it so.

http://forums.mtbr.com/santa-cruz/santacruz-nomad-3-vs-yt-capra-linkage-analysis-1005560.html

Theres a graph in here somewhere, although Im not sure of its accuracy – but even with say.. an EVOL spring…. eyeballing the graph it appears for a given sag you are still going to get a bit more midstroke support and with the same given bottom out resistance… you are going to get considerably more.

This perception doesn’t actually make it so.

Less seals are simply always going to be more reliable. If reliability is important, then that isn’t just perception – it is likely to be grounded in reality.