Why air springs will never feel like coils

Whooooosh

If reliability is important

likely

Woosh.

See all the caveats? You dirty post modernist.

Dunno if mechanical engineers would do thermodynamics to that level

I did, and beyond. But that was over 20 years ago now and I’ve forgotten it all 🙂

I understand the basic physics. I wonder if the difference between a fast hit and a slow hit ( within the bounds of what is normally experienced in riding) is actually significant. In particular, I find it hard to believe that any hit can be slow enough for significant cooling of the gas to occur during the compression/ rebound.

Post modernism? Nah.
I simply prefer to have a coil spring in my suspension in certain situations whereas air is preferable for others. My preference. Not yours.
The sooner you get over yourself as a coil spring evangelist the better. For us all. 😉

Air is better in loads if respects. I missed the progression of air in the forks, when I originally went coil on my Pikes. Compared to standard coils they simply feel safer when jumping, on top of that they are easier to tune in terms of spring rate and they are lighter.

It’s just that I had problems with reliability and more frequent lower services being needed so that sensitivity didnt drop off. I find them to be more predictable in terms of chassis stability as well – when riding with wheels on the ground.

If there is a takeaway from all this, surely it’s that an air shock should be set up with a lower spring rate / static pressure / higher sag than a coil shock, to allow for the increase in temperature and pressure when it’s compressed.

Is this effect material? And if so, is it linear?

Coils feel better.

End of thread.

“Dual rate coils springs”

😆

Without getting into the physics of it, I’m pretty sure they must have run one of their coil shocks and air shocks back to back on a dyno without any damping and witnessed the phenomena?

In my mind what they are suggesting makes sense, but reckon it could be more than the thermodynamic theory.

But then what about the rebound after the compression? With the damper the return velocity will be limited, what if there were no damper?…….

Does altitude and ambient temperature / solar gain on a black can make a difference or does it move the entire system while not changing the relative positions?

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.

this is common sense is it not. anyone that’s done school chemistry/physics should grasp what is going on.

this is common sense is it not. anyone that’s done school chemistry/physics should grasp what is going on.

If this was “common sense” the first half of this thread wouldn’t have consisted of people repeatedly misunderstanding what I was saying! 😉

So, if anyone wants to run the numbers, the new FAST damper for a Yari has low, medium and high speed compression damping adjustment, and the thresholds are 150mm/s and 700mm/s. Vorsprung’s tuning videos mention an absolute max shaft speed of about 2500mm/s. Obviously, that’s for a fork which is 1:1 wheel vs fork rate. A shock could be running up to about a 3.5:1 wheel:shock rate, so we’re looking at in excess of 8000mm/s.

I have to do some actual work right now but may get around to doing the sums at some point…

“if you design a “perfect” air spring, whose positive and negative springs create a totally linear spring rate”

Impossible anyway regardless of any heating effect.

The curve of an air spring is C shaped and always well be. As you compress the piston the volume you are compressing gets smaller and smaller and therefore it becomes harder and harder to compress further.

You can “flatten” the C shape and make it “more” linear by increasing the volume of the air can but to make it completely linear would require an infinitely large air chamber…somewhat impractical.

a negative spring changes the characteristics but is only has any effect in the first 20%-30% of the stroke. Not only that, the negative spring is itself an air spring and therefore also a curve. So what you get is an S shape.

but as has been mentioned…. linear != Best …at least in my opinion.

Otherwise why does it feel better adding volume spacers left, right and centre?….Making our curves even curvier than they need to be.

Yes, it’s always an S-shape but within the main working range of the spring you can get it so close to linear that any non-linearity is imperceptible. Yes, you always get a ramp up at the very end but that’s actually a weak point of coil springs – and that doesn’t affect the general riding feel because it’s only on big hits that you get into the last 20% of travel.

So what we’re talking about is the difference in feel between 20% and 80% of travel.

Theres still a pretty strong curve from 65 percent of travel onwards, with the same bottom out resistance youre still going to get less support.

Not to mention that there arent actually any forks with negative springs that are that large on the market. And those that do have largish negative springs have issues with travel suck down.

Again, if you drew a linear line through this, there is still going to be a considerable difference vs the air springs that are commercially available? Isn’t there?

All of this shouting about air or coil being best, what about the poor old elastomer? Won’t somebody think of the elastomer? 😥

#prayforelastomer

I think the elastomer was meant to be a damper rather than a spring – My old RST Mojo’s had a coil spring

Please don’t lets get started on dampers 🙁

I was trying to be funny. Sorry, I won’t do it again.

Just go ride your f**king bikes, goddamnit.

Not to mention that there arent actually any forks with negative springs that are that large on the market. And those that do have largish negative springs have issues with travel suck down.

Isn’t the negative chamber size determined by the position of the transfer port on a self-balancing negative spring? If so, plenty of current forks have negative springs that large.

Also, we’re talking about the adiabatic effect causing up to 30% divergence in momentary spring rate – that’s much more than the non-dynamic non-linearity vs a coil.

Just go ride your f**king bikes, goddamnit.

That’s a brilliant idea! Close down the forum! And the magazine! And quit your jobs!

Haven’t we always known this!?….but like the marketing gullible freaks we are we’re obsessed with new tech, lighter weight etc and air suspension offered all of that….and for more money making us think we were getting a better product!

I still have some 2011 (’12) coil spring forks that outperform everything air sprung I’ve used so far…shame they’re 170mm and only useful in a handful of situations.

The transfer port is there to equalize the pressures, its not the actual negative spring – the negative side simply passes over it. Surely, if the size was determined by the port placement then vorsprungs luftkappe would not do anything. Or am I massively wrong?

I bet the negative spring size is constrained by packaging/a2c issues in single crown forks as well.

Haven’t we always known this!?….but like the marketing gullible freaks we are we’re obsessed with new tech, lighter weight etc and air suspension offered all of that…

NO. This thread is not saying that coil springs are better than air springs. It is saying they are DIFFERENT.

If coil springs were conclusively better in all situations then no-one would race DH or Enduro with air springs. But they’re not, they’re different. Air springs push back more at high shaft speeds – if that helps you clear a rock garden then they’re faster. But if that rock garden is mid-corner then you can’t air over it and so a coil is better because it absorbs high shaft speed bumps better.

The transfer port is there to equalize the pressures, its not the actual negative spring – the negative side simply passes over it. If tge size was determined by the port placement then vorsprungs luftkappe would not do anything. Or am I massively wrong?

Yes, you’re massively wrong. The Luftkappe changes the chamber sizes by keeping the piston seal in the same place whilst making the piston head heavily domed and hollow.

Also, we’re talking about the adiabatic effect causing up to 30% divergence in momentary spring rate

I agree… its interesting dynamic behaviour I hadnt considered perhaps overlooked since it is not modelled in standard spring curve graphs. I wonder if it has a similar effect to high speed compression damping?

I was trying to be funny. Sorry, I won’t do it again.

Im no comedian but im not sure that would register as funny even in the context of a relevent discussion about dampers? But quitting is also bad.

Just go ride your f**king bikes, goddamnit

I will just as soon as i clock off.

And isnt that essentiallly exactly what the new Rock Shox equivilent does? They didnt change the position of the transfer port did they?

Youre right, they moved the dimple but looks as though the physical size of the negative chamber increased as well.

http://forums.mtbr.com/shocks-suspension/2018-pike-1042978.html

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.

Surely this effect can be largely mitigated by designing less high speed damping into the damper? Likely to be a tricky calculation re shaft speeds and thermal sinking, but not impossible. Certainly if you use a computational model to design the damper and spring concurrently (which I would hope any serious manufacturer would), then I imagine you could match air – coil performance fairly closely (with different damper design of course).

Not sure about that TJ… HSC effects the rate at which the damper goes through its travel, not the amount (mostly). Otherwise we could start offloading damping to something like MRPs speed sensitive bottom out drop in kit thingy.