# Is suspension air pressure affected by elevation change?



## PeytonP (Aug 29, 2011)

I'm leaving for Angel Fire this week and had a quick question for you bike park pros.

Will the air suspension on the two bikes I'm taking be affected by the 7-9,000 ft elevation change? There is atmospheric pressure and it's easy to see bags of chips and water bottles expand, but does this affect metal-contained suspension? How about tires?

I guess I'm wondering if I should lower air pressures before loading up the bikes and making the 14 hour drive.

Any advice is greatly appreciated!


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## motochick (Jun 22, 2010)

It has for my stuff, tires as well. I ALWAYS bring a tire and sus pump and check before I ride in a new place. I don't adjust them until I am there.


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## PeytonP (Aug 29, 2011)

Good to know. Thank you :thumbsup:

Have you found your usual preferred susp/tire pressures to still be good once you've adjusted for any change during the trip there?

Sorry to be a bother. I've been to the mountains dozens of times, but never with my bike! You don't have to think about this sort of stuff snowboarding.


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## ustemuf (Oct 12, 2009)

Elevation never affected my suspension pressure, but it has changed my tire pressures.


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## motochick (Jun 22, 2010)

Once it has been set to where I normally run it, no other changes are necessary. That place is fun, hope they have had a ton of rain! It was ubber dry when I went.


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## jurichar (Aug 19, 2009)

Internal pressure of a sealed system can only change when the size of said system is altered. Read: if the body holding the pressure is rigid (fork/shock) then no it does not change. If it is not rigid (tire) then it can change. But tires should be checked every ride anyway


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## lelebebbel (Jan 31, 2005)

jurichar said:


> Internal pressure of a sealed system can only change when the size of said system is altered. Read: if the body holding the pressure is rigid (fork/shock) then no it does not change. If it is not rigid (tire) then it can change. But tires should be checked every ride anyway


.... However, relative pressure does change, which does _theoretically_ have an effect on the suspension.

Example: 
Pressure inside shock: 150psi
Ambient air pressure: ~15psi at sea level
Difference: 135psi

At 9,000ft:
Pressure inside shock: still 150psi
Ambient air pressure: ~10psi
Difference: 140psi

Ambient air pressure does matter, because it tries to compress the shock from the outside. The shock works on relative pressure. To help understand this, try to imagine extremely high ambient pressure of, say, 151psi. The shock would be compressed before you even sit on the bike.

The difference at altitude would be marginal and probably not noticeable at all, but in theory it is there.


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## Mr.Magura (Aug 11, 2010)

lelebebbel said:


> .... However, relative pressure does change, which does _theoretically_ have an effect on the suspension.
> 
> Example:
> Pressure inside shock: 150psi
> ...


Oh no, another one of those guys whom does math, and stuff like that.

Since when has facts had anything to do with this? 

Magura


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## lelebebbel (Jan 31, 2005)

Mr.Magura said:


> Oh no, another one of those guys whom does math, and stuff like that.
> 
> Since when has facts had anything to do with this?
> 
> Magura


Ha! And now, lets discuss how the elevation affects the pressure gauge on your shock pump!
If you inflate the shock to 150psi at sea level using a standard pump, will the gauge still read 150psi at 9,000ft?


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## RTM (Sep 17, 2005)

ultimately USTEMUF closed it out from a biking perspective. set your shock at home, give your tires a squeeze in the parking lot and go have fun.

But the concept is interesting. Good stuff. I thought this was kind of cool.

SCIENCE!

http://www.altitude.org/air_pressure.php

- Rob


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## msimsiraq06 (Jul 30, 2010)

As a general rule air pressure changes with elevation.


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## ruppguts (Sep 7, 2007)

I've seen bigger pressure changes with temperature change. When I go from Phoenix at 110 degrees, up into mountains where it may be 60-70 degrees, pressure drops but I'm pretty sure it's due more to temp change than elevation change. Especially with the shock.


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## Uncle Six Pack (Aug 29, 2004)

jurichar said:


> Internal pressure of a sealed system can only change when the size of said system is altered. Read: if the body holding the pressure is rigid (fork/shock) then no it does not change.


But your suspension ISN'T rigid, is it? This is a common misconception. An air spring acts on relative pressure, which means that just like tires, it is the difference between internal and external pressure.

Now for all practical purposes, if you are running 30 psi in the tires and 150 in the shock, you are gonna notice a loss of 5 psi in the tires way more than the shock. Other than that, can't add anything to what lelebebbel already posted.


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## charging_rhinos (Jul 29, 2008)

ruppguts said:


> I've seen bigger pressure changes with temperature change. When I go from Phoenix at 110 degrees, up into mountains where it may be 60-70 degrees, pressure drops but I'm pretty sure it's due more to temp change than elevation change. Especially with the shock.


^ this. Just bring your pump and check it at the trailhead and all will be well.


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## derby (Jan 12, 2004)

lelebebbel said:


> Ha! And now, lets discuss how the elevation affects the pressure gauge on your shock pump!
> If you inflate the shock to 150psi at sea level using a standard pump, will the gauge still read 150psi at 9,000ft?


If the same guage used at lower altitude didn't loose pressure while attaching, it would show higher psi at higher altitude.

I've noticed low pressure stuff, tires and forks with about 30 psi become noticeably firmer going from sea level to 6000 ft elevation. High pressure stuff, like an air shock with 180 psi, is not noticeably firmer although is slightly.

I'm sure there is some percent factor of rate change with altitude. Roughly 1% increase per 2000 or 3000 feet of altitude.

Barometers in the inverse and mechanical altimeters work similarly when changing altitude.


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## Gemini2k05 (Apr 19, 2005)

uncle six pack said:


> but your suspension isn't rigid, is it? This is a common misconception. An air spring acts on relative pressure, which means that just like tires, it is the difference between internal and external pressure.
> 
> Now for all practical purposes, if you are running 30 psi in the tires and 150 in the shock, you are gonna notice a loss of 5 psi in the tires way more than the shock. Other than that, can't add anything to what lelebebbel already posted.


bingo!


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## howardyudoing (Mar 31, 2010)

Coil shock ftw.

..but what about oil pressure!!! and brake fluid dynamics at different temperatures!!


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## Ithnu (Feb 16, 2007)

lelebebbel said:


> .... However, relative pressure does change, which does _theoretically_ have an effect on the suspension.
> 
> Example:
> Pressure inside shock: 150psi
> ...


Tires lose pressure at altitude.

Let's say your hands are the atmosphere and they could compress a balloon evenly. You squeeze and it represents a lower altitude. You release and it is parallel to you going up in altitude. When you squeeze the balloon and it decreases in size the air pressure increases right? Now when you go "up in altitude" and release the balloon what happens?

Now replace the balloon with something more durable, say an empty Nalgene water bottle. Do the same thing with your hands. Think the pressure inside changed as you pressed on it?

One reason you hear an empty water bottle make noise after you open it in the mountains is that you sealed it at higher pressure down at a lower altitude. The bottle is strong enough not to change shape. There are intermediate cases where a bottle may expand a bit, in that case it is losing some pressure.

But an aluminum canister in bike suspension isn't going to expand at any level where you could measure it.


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## Ithnu (Feb 16, 2007)

Uncle Six Pack said:


> But your suspension ISN'T rigid, is it? This is a common misconception. An air spring acts on relative pressure, which means that just like tires, it is the difference between internal and external pressure.


I thought about this after my last post. The air pressure in the canister of the fork/shock will not change with the bike just sitting there. Knowing that I was considering an unchanging volume; but a rider applying a force changes the volume. But this change is not air pressure, it's gravity.

So then I was thinking about how the air canister compresses. Will a lower atmospheric pressure assist in that the spring has less to "push back" against? I'm thinking no; because the canister is sealed off from the atmosphere and it is pushing back against the gravity load the rider caused.

Anyway, my thoughts. Others are can chime in, I'm not going to get butt hurt from a discussion if some one else had a better idea.


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