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I have searched and didn't come up with much.

I know there is a lot of chatter on how air temp/altitude has an effect on air spring performance, but I would like to know what effect is has and if/how others compensate for it.

Reason I ask is my recently built Hightower felt like a blob last night during a colder then normal ride of 60 degrees and damp. RS Pike was brake diving, and felt like poo compared to the few other times I have had it out.

Is there an easy way to say that if the fork feels good at 70 degrees @ 90psi, at 60 degrees the pressure should be xxx, and at 90 degrees the pressure should be xxx?

I don't know if I was overly sensitive last night do to being rushed AND my wife calling me in the middle of my ride (which then messed with my new Jaybird Tahra headphones)...but it just had me thinking.
 

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accurate simple way involves math, but it the 'ideal gas law' [with correct values]

caveman way is carry shock pump and adjust pressure to sag at temp
 

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Use gay lussacs, its easier. p1/t2 = p2/t2, in absolute values. pressure1/temperture1 = etc etc. Use absolute temp and pressure, so psia (atmospheric pressure plus gauge pressure) and kelvin.
 

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To make it even easier, if you use 60psi in your fork at 90 degrees, at 60 degrees you need 56 psi.

Its not exact, but round and call it 1 psi for 10 degrees to keep you in the ball park of what works for average conditions around sea level.

Its a fairly small change, which is why suspension performance is fairly consistent if you start on a cold day and climb until its warm. Still works.
 

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Air pressure drops roughly 5psi from sea level up to 10,000ft at constant temperature. It also varies by just under 2psi in the extreme due to weather fluctuations. For slowly varying weather during a day and even on a big day of 6000ft descent, you can probably ignore both of these.

The ideal gas law (PV=nRT) can be used to answer your question about air temperature (along with a lot of other things that happen in an air-sprung suspension system). Treating the volume (V), number of moles of gas (n), and the ideal gas constant (R) as constants, then we just have a linear relationship between the change in pressure (P) and the change in temperature (T) in Kelvins. If the temperature changes from T1 to T2 then pressure will change from P1 to P2, where P2 = P1*T2/T1.

The highest air temperature ever recorded is generally accepted to be 54 C = 327 K, and the lowest is -89 C = 184K. That would produce a pressure drop of 184/327 about 1/2. It's also very unlikely to happen. Going from a super hot day (104 F = 40 C = 313 K) to freezing (0 C = 273 K) would drop your pressure by 273/313, which is about 13%, which for your 90psi fork is about 12psi.

Using your example, 70 F = 294 K, 60 F = 289 K, and 90 F = 305 K. So if it's at 90psi to start, then the pressure at 60F should be 90*289/294 = 88psi, and at 90F it should be 90*305/294 = 94psi.

Hope this helps,
 

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I've always wondered.....does elevation really affect shock/fork pressure? The fact that the air is contained in rigid, non-flexible chambers (unlike a tire) I wonder if there is an actually an impact?
 

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^^^The pressure is relative to ambient, which is also called gauge pressure. The absolute pressure doesn't change in the shock or fork as you bring it to higher elevation, but the ambient pressure decreases as you go higher, which means the gauge pressure in the fork/shock increases.

Think of it this way. If you inflate your fork to 75 psi it will be nice and springy. If you put it in a chamber and pressurize that chamber to 75s psi, the fork will be totally soft because the outside pressure counteracts the pressure in the fork. If you take your fork into the vacuum of space, it act like it has 90 psi as there is no outside pressure counteracting the internal pressure.
 

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I just came to this thread to make sure the ideal gas law was being respected. It was, so I am satisfied. I calculated long ago for my car tires a change of 0.6psi per 10 degrees F.
 

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Air pressure drops roughly 5psi from sea level up to 10,000ft at constant temperature. It also varies by just under 2psi in the extreme due to weather fluctuations. For slowly varying weather during a day and even on a big day of 6000ft descent, you can probably ignore both of these.

The ideal gas law (PV=nRT) can be used to answer your question about air temperature (along with a lot of other things that happen in an air-sprung suspension system). Treating the volume (V), number of moles of gas (n), and the ideal gas constant (R) as constants, then we just have a linear relationship between the change in pressure (P) and the change in temperature (T) in Kelvins. If the temperature changes from T1 to T2 then pressure will change from P1 to P2, where P2 = P1*T2/T1.

The highest air temperature ever recorded is generally accepted to be 54 C = 327 K, and the lowest is -89 C = 184K. That would produce a pressure drop of 184/327 about 1/2. It's also very unlikely to happen. Going from a super hot day (104 F = 40 C = 313 K) to freezing (0 C = 273 K) would drop your pressure by 273/313, which is about 13%, which for your 90psi fork is about 12psi.

Using your example, 70 F = 294 K, 60 F = 289 K, and 90 F = 305 K. So if it's at 90psi to start, then the pressure at 60F should be 90*289/294 = 88psi, and at 90F it should be 90*305/294 = 94psi.

Hope this helps,
C'mon man, stop glossing over things. We want details!!!!

Surely, there is some way we could work Bernoulli's Principle into this discussion! ;)
 

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I've always wondered.....does elevation really affect shock/fork pressure? The fact that the air is contained in rigid, non-flexible chambers (unlike a tire) I wonder if there is an actually an impact?
I've wondering about this too.

^^^The pressure is relative to ambient, which is also called gauge pressure. The absolute pressure doesn't change in the shock or fork as you bring it to higher elevation, but the ambient pressure decreases as you go higher, which means the gauge pressure in the fork/shock increases.

Think of it this way. If you inflate your fork to 75 psi it will be nice and springy. If you put it in a chamber and pressurize that chamber to 75s psi, the fork will be totally soft because the outside pressure counteracts the pressure in the fork. If you take your fork into the vacuum of space, it act like it has 90 psi as there is no outside pressure counteracting the internal pressure.
I don't quite follow. If we have a non-flexible air chamber, completely separated from the external environment, how would the external air pressure change the behavior of the internal/separate chamber? I understand that once you connect a pressure gauge,it will read as a different pressure gradient. But since pressure is a scaler quantity, it's not like it will exert a downward force on the fork to act against the forces in the air chamber?
 

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completely separated from the external environment?
It's not completely separated from the environment and its not non-flexible. It's got one degree of freedom (sliding in and out of the lowers) and external air pressure is pushing on all of the fork.

Importantly, it's pushing on the top cap in the direction that the fork can compress.
 

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I've always wondered.....does elevation really affect shock/fork pressure? The fact that the air is contained in rigid, non-flexible chambers (unlike a tire) I wonder if there is an actually an impact?
It's not completely separated from the environment and its not non-flexible. It's got one degree of freedom (sliding in and out of the lowers) and external air pressure is pushing on all of the fork.

Importantly, it's pushing on the top cap in the direction that the fork can compress.
Ahhh, roger that. Thanks!
 
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