Im of to Morzine for the first time this summer, do I need to swap my fox rear air shock (rp2) to avoid this issue or is it not a real problem

Back in the day there was talk of it but I can't see it being an issue on modern shocks

It will be fine.

You're gonna die!

I think those uber-thin carbon chainstays would be more of a worry 😉

Well there is no denying that in theory it could be an issue – the damping properties of oil are related to the oils viscocity which does vary with temperature. I dunno exactly how the rp2 works but an air shock uses the compression of the air to act as spring and damper and again this is affected by temp.

In reality would you notice? I rode morzine for 2 years on my NRS before moving onto bigger suspended bikes, the NRS uses an air shock, I never noticed it and I am a bit of a suspension geek..

Thanks, It was going to be difficult to get an dhx shock onto my frame.

Simon- will not be taking that bike as I dont want to be to cheeky with my lifetime warranty.

sorry to interrupt ChunkyMTB, do you still want to sell the cotic frame?

I have no answer for the OP question, never had full sus.

Never seen a problem with a Fox shock, but had a guest on a Scott Genius last year whose bike was virtually locked-out at the bottom of long descents.

The boost valve of my DHX Coil also gets pretty hot.

That there will be an effect is simple physics – if you compress and then expand air, you will never do so 100% efficiently, there will always be losses. These losses will manifest themselves as heat, which will heat up the air in the shock. If you increase the temperature of air (or any gas) in a confined space then its pressure will rise accordingly. So if you double the air temperature, you will double the pressure and hence the spring rate of the shock. However, doubling the air temp doesn't mean going from 25°C to 50°C (which is pretty realistic). It means going from 298K to 596K, which isn't!

an air shock uses the compression of the air to act as spring and damper

Just as the spring, air shocks are still oil damped!

Plus when you compress air it heats up as a result, so that will prob heat up the damping oil gradually, although you should be fine.

Plus when you compress air it heats up as a result, so that will prob heat up the damping oil gradually, although you should be fine.

The damper itself will generate heat, that it how it dissipates energy.

Just as the spring, air shocks are still oil damped!

Like I said I dunno about the rp2 shock but my NRS shock used only air. I know I rebuilt it, and whilst there was some oil it was for lube only. I'm fairly sure that there are some air shocks that use only air. It really is a pretty good damper.

I've encountered this about three times in 10 years of riding.

All three times I've left the pro-pedal on my RP3 set in the middle setting while descending on the Mendips (so not exactly monster DH's). (after forgetting to open it up)

I'm heavy (14stone).
Ambient temps were high – middle of summer.

The shock starts a sort of 'quacking' like a dying, mournful duck, and damping goes all over the place.

Id worry more about your oil overheating than the air, sometimes after well long runs the damping can run away a bit on any shock cos the oil goes all thin. You can combat this by running high viscosity index oil like silkoline or redline or setting your shock a bit slow to begin with. Enduro MX boys only tune their shocks once their up to working temp, its a bit wierd with push bikes tho cos you take em from cold to hot throughout the run. If your going to morzine I would recomend a coil shocker any way. In spain we finally convinced a mate to swap his air shocker for a coil one and it was a complete revalation for him, air shockers in general have too much preload or ramp up to hard for downhill plushness. If you run a Ti spring then the weight aint that different. Diverse have just released some cheap shockers that come with Ti springs, I want a go on one.
K

The shock starts a sort of 'quacking' like a dying, mournful duck,

😆

Like I said I dunno about the rp2 shock but my NRS shock used only air

NRS shocks are oil dampened. The adjuster that sits at the top of the can adjusts the flow rate through a valve. You most likely did an air can service & didn't get to the damper, which on some Giant shocks is Nitrogen charged to keep the oil viscocity constant, the same as Fox shocks.
In reality I've never seen it cause a problem. I ran an air shock on my NRS, VT & Reign in the Alps & never had an issue.

Like I said I dunno about the rp2 shock but my NRS shock used only air. I know I rebuilt it, and whilst there was some oil it was for lube only. I'm fairly sure that there are some air shocks that use only air. It really is a pretty good damper.

Both the cheap and the expensive NRS shocks were oil damped and air charged. The bit with the oil in was the bit you can't get into (apparently, but I have) and is nitrogen charged at 200psi. Mine's due a stripdown, you just need to know how to get 200psi back into the oil can.

The only prob you'll have is increasing spring rate as you go down. That said, I barely noticed it on my NRS despite a full day on the mountain in the alps.

Sorry should have defined it better – my NRS shock was a rock shock SID it was air only, I'm fully 100% and I have more than one brain cell honest. Didnt have a damping adjustment, it had upper and lower air cyulinders

Ok I'm sure lots of modern air shocks use oil, once upon a time the theory was to use only air. Its not as controllable as an oil damper, I guess thats why they use oil dampers and air springs….

early dhx5 shocks used to start fort william with perfect feeling rebound and virtually throw you over hte bars at the bottom ….

solution – send for pushing at TF or start with the rebound extra slow ….

pretty sure its not as bad these days

I've had a Standard Fox Float R rear shock stop working due to heat build up after about 2200m of downhill. It was on a guests own bike, so when we got back to Kitzbuhel I gave it a simple aircan service, cleaned out about 2 spoonfuls of grime and it was fine for the rest of the week.

I don't think its really a problem; I guided all of last year with the same Fox rp2 and I never lost performance. The shock will heat up, but thats to be expected.