Did the Black Mountains killer loop yesterday. At the bottom of Rhiw Trumau I felt my RP23 air shock and it was really very warm. Any idea on how that effects the air pressure by being so hot? Or is the volume of air enough that it has an insignificant effect? Guess I'll have to attach my shock pump next time and see if there's any difference or not.

Spring rate will go up, (an increace of 30deg C will be ~10% firmer)

Damping will go down (this is harder to quantify)

Its basicaly becasue the damper is doing lots of work which generates heat, on a coil shock most of this heat is conducted away through the metal body and the threads etc provide adequate heat transfer area to the surrounding air. On an air shock you have the double whammy of insulating the damper inside the air chamber 50% of the time, and the polished smooth surface of the damper itself its rubbish at transfering heat to the air.

It's a factor with air shocks. Basic physics I'm afraid. It does make them stiffen somewhat, apparently.

yes it does happen, the affect varies between shock designs. Will be interesting to see how the new Vivid air performs as they are saying they have solved the problem.
Ohlins used a certain grade of alloy that expanded a specific amount when the damper got hot reducing the size of the 'hole' the damping oil traveled through offsetting the reduction in the viscosity in the oil caused by the heat build up.

Go down a mental enough decent and your coil shock will get pretty warm too, shouldn't affect spring rate on a coil but damping may go off a bit.

Ohlins used a certain grade of alloy that expanded a specific…

Now thats impressive attention to detail.

It was on an air shock of some kind from about 20 years ago from memory.
I wish I could afford some of their shocks for my motorbike.

How about a heatsink on the shock body/stanchions?

Space is the issue with rear shocks though, white brothers have a gas charged 'piggy back' on the back of the fork leg on their dh forks,

http://www.whitebrotherscycling.com/26dual.shtml

Fox used heat sinks for a while too,

Re the ohlins thing, presumably it was more to do with a bi metalic structure than a specific alloy as a hole in a plate when heated gets bigger, not smaller. Bi metalic shims maybe that would stiffen as the temperature increaced? Still not convinced though.

Like I said it was used on a shock used 20 years ago, can't fully recall the specifics.
It will be interesting to see how the new Vivid air works, as nobody seems to have cracked the dh air shock fully yet. Like you say not fully convinced yet.

So, basically the shock becomes more like a boingy, undamped spring. I'll have to start taking more notice of how the shock performs on a long bumpy downhill to see if it's something that bothers me. From memory it does alter the suspension, but my mind's on dodging rocks and stuff to be too bothered!

My shock heats up on the CLIMBS! 😡

For almost all air shocks, the damping issue for the air heating up is irrelevant – the air is not used for damping. It used to be a major issue on Cane Creek's shock that used air as both the spring and the damping – on a long DH, you'd finish with a stiff, very bouncy shock…

For DH though, the air can heat up enough that it starts to affect the oil temperature too – that's what Rockshox have tried to compensate for with the new Vivid:

http://www.bikeradar.com/news/article/rockshox-vivid-air-shock-first-look-25718

The air is the spring though, so surely as temp increases the pressure increases? So you get a stiff, not very bouncy shock.

If only we could harness that heat for something useful

Perhaps a little piggy-back pie warmer?

For almost all air shocks, the damping issue for the air heating up is irrelevant – the air is not used for damping. It used to be a major issue on Cane Creek's shock that used air as both the spring and the damping – on a long DH, you'd finish with a stiff, very bouncy shock…

The (oil) damper itself dissipates energy as heat though, doesn't it? On my shock it is the 'stanchion' containing the damper which gets hot, not the air can.

For DH though, the air can heat up enough that it starts to affect the oil temperature too – that's what Rockshox have tried to compensate for with the new Vivid:

http://www.bikeradar.com/news/article/rockshox-vivid-air-shock-first-look-25718

…and Fox with the DHX air

I had a demo Nomad a couple of months ago with a Monarch air shock on it. Managed to make that fade on Chapel Gate – it was rebounding really quick by the bottom, and even through gloves was really quite hot to the touch – like a mug of tea that's been standing for 5 minutes. Not impressed….

I've only had my float rear shock for a month and I've only noticed it warm on 15-30 minute rocky drop scary rough descents, by which time I have more important things to worry about. It also cools off very quickly and I didn't notice any fall off of performance, but I'm not sure I would unless it was really drastically bad.

get a coil shock

Er – strictly speaking, as regards the effect on the air spring, won't the spring rate go DOWN a little as the shock heats up?

Air heats up, and wants to expand.

So, for a given force on the bike through the shock, the shock will compress less. This means less sag when the bike's at rest – but the rate of increase in the force needed to compress the shock is a function of the change in volume of the air spring, not pressure at the sag point.

Less sag => greater air chamber volume => a more gradual ramp up in spring rate?

At the same time, whilst the sag is reduced as a result of the heated air, the load on the bike is unchanged and so the pressure inside the air chamber will also be unchanged. Just hotter and bigger.

I have no idea whether the effect is substantial in practice. One way to check would be to compare the sag at the end of a run with your normal set up.