REVIEWS DEALS BIKESHOPS TRAILS

Rising temps = rising pressure in shocks?

1576 Views 26 Replies 11 Participants Last post by Blue Shorts, Apr 8, 2005

simian23

Discussion Starter · Apr 4, 2005

The last few days I could have sworn the plushness had disappeared from my Blur. So I checked sag front and rear and sure enough, sag was about half what I had originally set.

Temps here have been 7 - 10 C higher than when I first set the bike last week...is it possible that pressures in the shock have increased due to the rising temp? I can't think of any other reason for pressures to increase without having added air.

At least my seals aren't leaking!

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006_007

· #2 · Apr 4, 2005

simian23 said:
The last few days I could have sworn the plushness had disappeared from my Blur. So I checked sag front and rear and sure enough, sag was about half what I had originally set.

Temps here have been 7 - 10 C higher than when I first set the bike last week...is it possible that pressures in the shock have increased due to the rising temp? I can't think of any other reason for pressures to increase without having added air.

At least my seals aren't leaking!

Basic laws happening there. For sure higher temps will cause the air/gas inside the shock to expand.

Ever leave a tire (attached to a rim an inflated of course) inside a car on a hot sunny day with the windows rolled up. Makes one helluva bang when it goes off!

This is why it is important to set your pressures before every ride.

Ole

· #3 · Apr 4, 2005

Air shock blues...

006_007 said:
This is why it is important to set your pressures before every ride.

And if you store your bike in a temperature controlled environment (i.e. your house), you need to either leave the bike outside for 30 mins prior to the ride, or check the pressure 30 minutes into the ride. I've got a 30 min ride to the trailhed, and it's the perfect opportunity to check the pressure.

On that note, agressive riding on rocky/rooty trails can raise the temp in the oil enough that the rear shock gets noticably harder. Not a good thing when the oil also becomes thinner. Less rebound damping and stiffer spring - not a good combo. My 4-Way Air on my 5.5 sometimes gets lukewarm to the touch even when it's nippy out. Forks usually have the damper in one leg and the air spring in the other, so they only get thinner oil, not firmer spring. I have a 4-Way Coil as well, but it just isn't as nice as the Air for some reason.Plus it's a over a pound heavier...

Ole.

simian23

Discussion Starter · #4 · Apr 4, 2005

Ole said:
And if you store your bike in a temperature controlled environment (i.e. your house), you need to either leave the bike outside for 30 mins prior to the ride, or check the pressure 30 minutes into the ride.

Ahhhh, that could be it! Usually I keep the bike locked in the bike room of my building, but lately it has been in my flat while I do maintenance.

I'll leave it in the bike room for a while and then check the pressure before my next ride. Bet the problem goes away. Thanks for the advice all.

Blue Shorts

· #5 · Apr 5, 2005

simian23 said:
Ahhhh, that could be it! Usually I keep the bike locked in the bike room of my building, but lately it has been in my flat while I do maintenance.

I'll leave it in the bike room for a while and then check the pressure before my next ride. Bet the problem goes away. Thanks for the advice all.

A 10 degree C rise only equates to a 3.5% difference in pressure. It's not large, but it may be noticable.

LyNx

· #6 · Apr 5, 2005

006_007 said:
Basic laws happening there. For sure higher temps will cause the air/gas inside the shock to expand.

Ever leave a tire (attached to a rim an inflated of course) inside a car on a hot sunny day with the windows rolled up. Makes one helluva bang when it goes off!

This is why it is important to set your pressures before every ride.

Was not wondering exactly this, but kind of. I wknow about air and temps, but also know that nitrogen doesn't get effected by temps like air. I was thinking that once I got my shock/fork dialed in I would inflate them using nitrogen, my only concern is will they keep the pressure or will I have to keep topping them up /checking them as nitrogen isn't free or easily available.

Dougal

· #7 · Apr 5, 2005

LyNx said:
Was not wondering exactly this, but kind of. I wknow about air and temps, but also know that nitrogen doesn't get effected by temps like air. I was thinking that once I got my shock/fork dialed in I would inflate them using nitrogen, my only concern is will they keep the pressure or will I have to keep topping them up /checking them as nitrogen isn't free or easily available.

Nitrogen expands in pressure less mainly because it's dry.

I find the best solution is a coil spring.

SCUBAPRO

· #8 · Apr 5, 2005

LyNx said:
I wknow about air and temps, but also know that nitrogen doesn't get effected by temps like air.

Hmmm!!! Nitrogen is a gas and the pressure in a confined cylinder (i.e., constant volume) will be affected by temperature just like any gas including air (Ideal Gas Law: PV=nRT). Though the change in pressure is directly proportional to n (molecular weight of the gas), so a heavier gas will exert more pressure at a given temperature and volume relative to a lighter gas.

Thus, the pressure of a gas (air, nitrogen, argon or any gas) in a bike shock will change relative to ambient temperature.

jkish

· #9 · Apr 5, 2005

That's right. PV=nRT, so the amount of gas

didn't change, the volume (V) didn't change and R (gas constant) didn't change, that leaves P=T. If the temperature goes up so does the pressure.

Dougal

· #10 · Apr 6, 2005

SCUBAPRO said:
Hmmm!!! Nitrogen is a gas and the pressure in a confined cylinder (i.e., constant volume) will be affected by temperature just like any gas including air (Ideal Gas Law: PV=nRT). Though the change in pressure is directly proportional to n (molecular weight of the gas), so a heavier gas will exert more pressure at a given temperature and volume relative to a lighter gas.

Thus, the pressure of a gas (air, nitrogen, argon or any gas) in a bike shock will change relative to ambient temperature.

Very true, but don't forget about the presence of moisture in normal air exascerbating the pressure changes compared to an ideal gas.

Bikezilla

· #11 · Apr 6, 2005

It's been more than a decade since I did this kind of stuff on a scientific level but at the risk of showing my ignorance: While the formula is quite correct, I vaguely recall it takes some pretty extreme numbers to make a significant change in the real world...but hey run some numbers if you feel like it, just use compatible units...and work in Kelvins

Just on a lark I occasionally do rinky-dink sci projects for my 6-yr old. (baking soda and vinegar kind of stuff) and recently I tried to demonstrate this very principal by putting a fully inflated ballon into the freezer for a few hours. It came out almost exactly the same size it went in. I'm guessing: The reduction in tempature change didn't change the pressure much so the volume didn't change much either.(at least I couldn't tell)

Maybe, just maybe since the shock has such a small volume, and high pressure, it might be more sensitive to tempature changes but I really don't think +-10C is going to do it. There are plenty of folks who live way the heck up north and they never mention problems about shock pressures changing in their chilly environments or making special allowences for going from inside to outside, other than the oil gets thicker. I'm pretty sure you'll see a greater change in oil viscosity than you will in shock pressure for the temp differences we're discussing.

But like I said feel free to run some numbers, let me know if I'm wrong.

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Blue Shorts

· #12 · Apr 6, 2005

I did the math. A 10 degree C rise is only a 3.5% change. That's 7 psi for a 200 PSI shock. Maybe it's noticable...maybe not.

Bikezilla

· #13 · Apr 6, 2005

Tnx Blue. Good luck getting a pump accurate to 7PSI much less feeling it over the thicker oil, stiffer seals and flatter tires.

Dougal

· #14 · Apr 6, 2005

Blue Shorts said:
I did the math. A 10 degree C rise is only a 3.5% change. That's 7 psi for a 200 PSI shock. Maybe it's noticable...maybe not.

To put that 7psi into perspective, my watch has a barometer/altimeter. I've seen a 20 millibar (roughly 20% of STP) change on a ride with a lot of vertical, which is almost 3psi.

It's hard for most people to feel a change of less than 10% in anything.

SCUBAPRO

· #15 · Apr 6, 2005

Good point! The changes are most likely negligible.

simian23

Discussion Starter · #16 · Apr 6, 2005

Hmm, so any other theories?

SCUBAPRO said:
Good point! The changes are most likely negligible.

I checked the shock today. It is significantly more firm than the last time I rode it a couple days ago. As if I added 10 psi or more to the platform.

I'm sure it will be fine if I just go back to my previous settings, but isn't this a bit weird?

SCUBAPRO

· #17 · Apr 6, 2005

simian23 said:
I checked the shock today. It is significantly more firm than the last time I rode it a couple days ago. As if I added 10 psi or more to the platform.

I'm sure it will be fine if I just go back to my previous settings, but isn't this a bit weird?

Did the air pressure in the platform change?

Blue Shorts

· #18 · Apr 6, 2005

simian23 said:
I checked the shock today. It is significantly more firm than the last time I rode it a couple days ago. As if I added 10 psi or more to the platform.

I'm sure it will be fine if I just go back to my previous settings, but isn't this a bit weird?

What is the typical pressure that you set your shock at? As I mentioned earlier....if you're running 200psi, then you will see an increase of 7psi (or 3.5%) for a 10 degree C rise.

When I was testing an RP3, I could notice a difference of 5 psi. That was between 140 and 145 psi...which is a 3.6% change. I can't definitively say that it wasn't all in my head. I wasn't about to do a double-blind study just to add a bit of air to a shock. The bike did give a firmer ride....imo.

Did you do anything to see what pressure actually is in the shock. That's the best way to determine how much the pressure is increasing.

Here's how to test the air pressure.

First...Hook up the shock pump to the shock. Make sure that the handle of the pump is fully depressed. Take note of the pressure (P1). While still making sure that the pump handle is depressed, remove the pump from the shock.

Repeat. and measure the pressure again (P2). The original pressure in the shock is:

P2+(P1-P2) This system works well as long as you have a good pump. I tested mine for repeatability and it's quite accurate AS LONG as the handle of the pump is always in the same position for all measurements.

You can use this method to see for your self what the effect temperature has on the pressure.

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Bikezilla

· #19 · Apr 7, 2005

Apologies for diving way off the deep end on this.

Blue, a quick academic question...(goodness knows why I would care)
on your calculation did you use the entire equasion (PV=nRT) or just the ratio (PV=T)? I'm curious because I would think even 3.5% is quite a swing for such a small change in tempature. I was under the impression the ratio is good for illustrating the behaviour but when plugging in real values into the full equasion, the changes are pretty small unless tempature changes in the 100's of Kelvins or the pressure changes by great numbers of Pascals.

I realize the rest of the equasion deals with constants but I seem to recall if one plugs in real volumes, gas constants, and molar quantities, the size of those constants would dwarf the volume tempature and even the pressure changes in the shock. (I'm clearly thinking way beyond anything useful in terms of this thread.)

BTW if anyone wants a really useful units conversion site check this out

jkish

· #20 · Apr 7, 2005

This has caught my interest so I did the full calculation with all the decimals in the conversions to get to the bottom of this. Because the temp change is not great and a different gas was not introduced, the gas in the shock will obey the ideal gas law very closely. I assumed a starting temp of 50C and 200psi with a 2" travel shock. If you increase the tmp to 60C then the pressure rises to 206.17 psi for an increase of 3.09 % which is negligible. Let's say you set your shock at 200psi @ 50C and the sag at 15% for a total of .3 inch. Because the pressure doubles every time the volume is cut in half the pressure obeys the formula pf=(2*pi)/d (pf=final pressure; pi=initial pressure; d=remaining shock travel) For example: if the shock has 2" travel remaining 400/2=200psi; 1" remaining 400/1=400psi; .5" remaining 400/.5=800psi. Therefore the final shock pressure has to equal 235.29psi to maintain .3 inches of sag. If the temp increased to 60C for a pressure of 206.17psi we can use the above formula to find out how much travel will remain. (2*206.17)/d= 235.29 therefore d=1.75" the shock will sag (2-1.75)=.24" or 3.09% decrease in sag, just as expected.

As far as a stiffer ride, according to this table oil viscosity changes quite a bit due to wide temperature changes, and I'm guessing even more so if it is dirty. I I don't know anything about hydraulics (yet) or if viscosity/tmp has a linear relationship. Regardless, viscosity won't effect sag, just ride quality.

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