Does rear shock air pressure drop when its cold??

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Does rear shock air pressure drop when its cold??

8 posts & 7 voices | Started 3 months ago by Chadders | Latest reply from bikewhisperer

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Chadders - Member

Got a new frame with a new air shock (never had an air shock before)..

Anyway I set the sag at pretty much room temperature. 100psi seemed to be just right for my weight.

First ride out (yesterday) and I was at a fairly high altitude most of the time and the temp was around freezing. I noticed that the shock was blasting through its travel a fair bit and was sagging at about 40%ish.

Got down the valley and my missis met me with my shock pump. The gauge measured 80psi, topped it back up to 100 and after a little bit more riding I checked it again and it was at about 90-95psi.

My concern is that the shock has a leak and will need to be sent back, but I have also read on a few forums that the pressure can be reduced when you ride in colder weather than when the pressure was first set.

Do this sound right to you?

I'm well open to give it a couple more tries before I send it back, but ive also been told that it wont be affected by temperature as its "a sealed untit"

any help mucho appreciated

chadders

Posted 3 months ago #
mintimperial - Member

You'll lose 5-10psi just by attaching and detaching the shock pump, stick a teeny bit more back in every time you measure it.

ive also been told that it wont be affected by temperature as its "a sealed untit"

That's not right, temperature definitely affects the gas in the shock, and it'll also affect the oil in the damper. I have no idea by how much though, I only have a Mickey Mouse degree in spelling, sorry...

Posted 3 months ago #
plumslikerocks - Member

altitude will increase the shock pressure relative to ambient. low temperature will decrease it proportionally. But temp is absolute (i.e. measured in kelvin) in thermodynamic terms. So from 20degC to zero you're talking about well under 10% difference. So I'd suggest mint's answer about loosing pressure to the shock pump is about right.

One reason you're getting more sag might be wearing full riding shoes, gear and camelbak. Or maybe your shock is better lubricated and has less stiction once its been on the move a bit.

Posted 3 months ago #
Surrounded By Zulus - Member

No, but it changes front shock pressure - honestly.

Posted 3 months ago #
ross980 - Member

PV=nRT
So for a constant volume/'amount' of gas, pressure is proportional to temperature (as above, temp is measured in Kelvin).
EDIT: To answer the original question, yes it will be affected by temperature, but a drop from 30 deg C to 0 deg C would only drop the pressure by about 10%

Posted 3 months ago #
Chadders - Member

Thanks for the replies. Yeah I realise I probably lost a bit in the pump, but its unlikely to be 20psi isnt it?

Thing is, when I reconnected the pump, it was down to 80psi, so although I was wearing a camelback etc, the actually pressure in the shock had definately decreased!

Its also been suggested I pump the shock right up and put it in the sink, looking for leaks. but I think my next step might just be to set the pressure in the same conditions i'm going to be riding in, and see if it drops then...?

Posted 3 months ago #
MarkyG82 - Member

My float was second hand when I got it. Bloke said it coukd do with a service. First Itook the can off and lubed the seal etc. Then before I pumped it back up I removed the core in the valve, cleaned it up an pressurised the chamber without it. Squirt a bit of fluid in the valve and replace the core. Was great after that.

and as above, go a tad over before removing pump.

Posted 3 months ago #
bikewhisperer - Member

Mostly the escaping air when you detach the pump is just the pressure left in the hose. The whole losing 5-10psi thing is a myth... except when the valve stem stands really proud, and then it's true.

Try this.. Pump the shock up to a known pressure, and then detach the hose. Then re-attach the hose and read the pressure. This is the drop in pressure to fill the hose and the gauge.
Now, pump the shock up to the same pressure, and detach the hose slowly.. Now re-attach it and see if the pressure drop is the same. If it is, then it's only the hose that's losing air.

Posted 3 months ago #

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