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Author Topic: Cold and tire pressure??  (Read 1950 times)

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Offline Singletrack Samurai

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Cold and tire pressure??
« on: February 12, 2010, 08:34:19 PM »
Does cold air  have any effect on tire pressure????

Offline drb81

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Re: Cold and tire pressure??
« Reply #1 on: February 12, 2010, 08:51:17 PM »
Not as long as you put winter air in...





Seriously I don't think it affects bike tires at all, or atleast not enough to cause a problem. I've heard you can loose 1psi of air pressure for every 10 drop in temp.
« Last Edit: February 12, 2010, 08:57:51 PM by drb81 »
[/don]

Offline ClintonRH

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Re: Cold and tire pressure??
« Reply #2 on: February 12, 2010, 09:21:57 PM »
It should only matter if you check your pressure in a very warm area and ride in a very cold area (or the opposite). Even then the change will be minimal unless you're talking about a MASSIVE temperature difference. Probably only matters if you're running tubeless at a very low pressure. Could conceivably be a problem if you were riding in sub-zero conditions and pump your tires in your climate-controlled home.

I don't know if bike tires create enough heat through sidewall deflection and frictional drag to cause a noticeable amount of heating (and pressure increase) but I doubt it. I think it would reach an equilibrium with heat dissipation long before it became something to be concerned about or it would be a rule of thumb kind of thing by now to know about. Rim brakes can create a lot of heat and a pressure spike though.

If someone really wants me to do the math on the pressure change I will. Just name the high and low temperatures you're interested in and it would be relatively easy to figure (I reserve the right to check Google for a tool that will do it for me since I'd have to research a few things I don't remember of the top of my head anyways).

Offline Rocky Mountain OOTH

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Re: Cold and tire pressure??
« Reply #3 on: February 12, 2010, 09:23:33 PM »
pV= nRT

Offline ClintonRH

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Re: Cold and tire pressure??
« Reply #4 on: February 12, 2010, 09:25:32 PM »
pV= nRT


A bit more complicated than that actually since atmospheric air is not a simple gas. There are a bunch of partial pressures combined and since n is different for each of them the full equation is more complex.

For practical purposes though the Combined Gas Law works just fine for these things. http://en.wikipedia.org/wiki/Combined_gas_law

pV/T=k


edit : but if you knew pV=nRT then I'm probably not telling you anything new.
« Last Edit: February 12, 2010, 09:30:02 PM by ClintonRH »

Offline Darrinw2001

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Re: Cold and tire pressure??
« Reply #5 on: February 12, 2010, 09:41:58 PM »
My brain hurts... Stop it!!! ;D

Offline Singletrack Samurai

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Re: Cold and tire pressure??
« Reply #6 on: February 12, 2010, 09:57:42 PM »
at felasco, the normal tire pressure I used sure did seem alot harsher than it normally feels.  Thus the reason i asked.. Thought I read somewhere that in cold temps u should pump up ur tires 5 psi less than usual to obtain the similar feel and performance but I may have imagined that.

Offline ClintonRH

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Re: Cold and tire pressure??
« Reply #7 on: February 13, 2010, 07:36:18 AM »
My brain hurts... Stop it!!! ;D

Δp = p(T + ΔT) / T - p

Δp = change in pressure
 p  = initial pressure
 T  = initial temperature
ΔT = change in temperature

All T values must be measured in Kelvin (*C + 273.15). I think the unit of p doesn't matter but I could be wrong as measurements like psi are usually a relative measurement not an absolute. Probably best to convert to Pascals.

This of course doesn't take into effect the volume of the tire changing with pressure. I'm making the (possibly faulty) assumption that the pressure change were discussing would not significantly change the tire volume. I don't know how to model that.

The hardness of the sidewall rubber changing with temperature might all have an effect on ride quality that hasn't been discussed. This more than pressure might be the cause of what you felt at San Felasco. Modelling this due to the multiple compounds in a tire wall would be a massive PITA. Easier to do an empirical test.
« Last Edit: February 13, 2010, 07:40:15 AM by ClintonRH »

Offline Darrinw2001

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Re: Cold and tire pressure??
« Reply #8 on: February 13, 2010, 07:42:03 AM »
My brain hurts... Stop it!!! ;D

Δp = p(T + ΔT) / T - p

Δp = change in pressure
 p  = initial pressure
 T  = initial temperature
ΔT = change in temperature

All T values must be measured in Kelvin (*C + 273.15). I think the unit of p doesn't matter but I could be wrong as measurements like psi are usually a relative measurement not an absolute. Probably best to convert to Pascals.

This of course doesn't take into effect the volume of the tire changing with pressure. I'm making the (possibly faulty) assumption that the pressure change were discussing would not significantly change the tire volume. I don't know how to model that.

The hardness of the sidewall rubber changing with temperature might all have an effect on ride quality that hasn't been discussed. This more than pressure might be the cause of what you felt at San Felasco. Modelling this due to the multiple compounds in a tire wall would be a massive PITA. Easier to do an empirical test.


ZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZ

Offline Rocky Mountain OOTH

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Re: Cold and tire pressure??
« Reply #9 on: February 13, 2010, 08:14:24 AM »
A bit more complicated than that actually since atmospheric air is not a simple gas.

Just trying to illustrate that (obviously) temp affects tire pressure for my samurai buddy.  Don't make me bust out the fluid dynamics book!  ;) LOL

Offline Triple T

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Re: Cold and tire pressure??
« Reply #10 on: February 13, 2010, 08:36:55 AM »
Not as long as you put winter air in...

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Offline $irWattZZZZ

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Re: Cold and tire pressure??
« Reply #11 on: February 13, 2010, 09:07:42 AM »
Revenge of the NERDZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZ ;D

Offline rob_squared

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Re: Cold and tire pressure??
« Reply #12 on: February 13, 2010, 10:35:54 AM »
Finally a thread with some "meat". Geek zone. Love it!

Offline drb81

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Re: Cold and tire pressure??
« Reply #13 on: February 13, 2010, 10:59:27 AM »
Finally a thread with some "meat". Geek zone. Love it!


I'll order the chicken then.

Pump up tires, ride, repeat.
[/don]

Offline treadlight

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Re: Cold and tire pressure??
« Reply #14 on: February 13, 2010, 12:41:17 PM »
bwahahahaha.........there is a thermodynamics class going on here......what is next heat transfer?For simplicity go with drb8 1.0 psig per 10F change in temperature. Since you will be pumping inside your house (conditioned) account for temperature change when you are outside if there is big temp difference.Treat it as ideal gas no need to solve for partial pressure since both "gases" are the same, air in tire is same as air outside of tire or else you need to consider for each gases in the air and I don't think it is worth at my recreational fitness level ;D

 

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