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[michaelbmenaker]

I'm riding from 5.300' (Denver) to eastern Nebraska (essentially, sea level)...tire pressure will drop with altitude. What's your best guess on how much to over inflate in the morning to end up with correct pressures at the end of the day? 2lbs? 3lbs? more?

As you know it's a major PITA to pull the saddle bag and adjust rear tire pressure at some gas station en route. Rather not have to do that.

[Twolanerider]

I have never been that anal on a bike ride....

[Tabatha]

In Denver, Colorado at an altitude of 1.6 km (1 mile altitude), the pressure is 0.9 atmosphere. Sea level is 1.0 atmosphere so tires loose about .1 of pressure.
ie--- 30 psi minus .1 = 3psi  ( really negligible)
.
Heat & cold will react the same way with regard to pressure-----------the moral here is to enjoy your ride and if the tires feel soft ; air them up !

[pokerpig]

Nitrogen does not vary in pressure like air does.  

Not true. Air is about 78% nitrogen anyway.
Its more important to make sure the air is dry - a lot of gas station air pumps dont have driers.

[Hugh Janis]

There's a reason the universal gas law (PV=nRT) is called the UNIVERSAL gas law. It applies to ALL gases and proves that the pressure is proportional to temperature in a fixed volume. I'd go as far to guess that you won't see ANY change in pressure since the tire would have to expand or contract from the change in altitude for the internal pressure to change. That's unlikely to be measurable in a properly inflated tire.

[grc]

There's a reason the universal gas law (PV=nRT) is called the UNIVERSAL gas law. It applies to ALL gases and proves that the pressure is proportional to temperature in a fixed volume. I'd go as far to guess that you won't see ANY change in pressure since the tire would have to expand or contract from the change in altitude for the internal pressure to change. That's unlikely to be measurable in a properly inflated tire.

      I always love it when someone tries to tell us that nitrogen won't change pressure when the temperature changes.  The UNIVERSAL gas laws obviously apply to everything BUT nitrogen, at least in the "advertising hype" world.

I never really thought about the altitude change causing a significant pressure change in a tire, because I just assumed the small change in atmospheric pressure wouldn't affect the shape of a heavy tire like it will with something like a flimsy balloon.  There might be a small difference, but I seriously doubt it would be enough to make me stay up at night worrying about it.  Set the air pressure at the max you normally run (probably 40 psi with a Harley / Dunflop tire) and don't worry about it.  Check the pressure again the next morning and verify it for yourself; assuming a similar air temperature on the two mornings, and no leaks, I'll bet you won't find more than a 1 psi difference.  And if there is a difference in temperature on the two days, a good rule of thumb is 1 psi for every 10 degrees F. (close enough for govt work).


Jerry

[Trey767]

I have never been that anal on a bike ride....

I agree I fly jets and temp. variations are much greater. 1 psi is the last thing on my mind, that's why engineers give you tolerances. Enjoy your bike and the ride. Do a per ride check in the morning, if it's that low add air

[Twolanerider]

 Nascar uses nitrogen in their tires just for that reason.

And every announcer on air talks about the tires "coming up" after fresh rubber and how it impacts the handling...

[grc]

ok guys...I didn't say if you us nitrogen the tire pressure would NOT change, it just doesn't VARY like air does. Nascar uses nitrogen in their tires just for that reason. (I hate being miss quoted)

What's the difference between "change" and "vary"?

One "advantage", assuming the tires are first flushed of any residual "regular" air, is that tires with pure nitrogen will contain less water vapor, which makes the calculated pressures a little more predictable.  That might be important in racing where they adjust in 1/2 psi increments, but I don't think the average Joe has gauges that are precise enough to worry about 1/4 or 1/2 psi differences on his street machine.  So predictability is improved, but nitrogen still expands and contracts just like regular old cheap air.  And it would have no real affect on the subject of this thread.  That is the point some of us are making.


Jerry

[Fireguy]

http://www.popularmechanics.com/cars/how-to/repair-questions/4302788

[Trey767]

I would love to read the topic on the invariant mass of the energy or mass-energy equivalence formula. I must not care too much, but it's just air in rubber to me

[Twolanerider]

I have never been that anal on a bike ride....

[Hugh Janis]

http://www.popularmechanics.com/cars/how-to/repair-questions/4302788

That article pretty much admits that the pressure fluctuations are caused by water vapor condensing in a cold tire and re-evaporating in a hot tire.  Zero air (moisture removed) would have exactly the same effect as pure nitrogen.

Also, Oxygen occurs in it's natural state as a dimer. So does Nitrogen.  The elements are right beside each other in the Periodic Table of Elements.  Therefore, the size of the molecules and their molecular weights are virtully IDENTICAL.  The theory that the difference in size between the molecules can possibly allow one to diffuse through rubber better than the other would be IMPOSSIBLE to prove under the strictest of laboratory conditions.

So, what's the last variable between Zero Air and purified Nitrogen?  Mainly, it's the presense of Oxygen and it's potential oxidative effect on metals (rims).  My 110 motor will fail LONG before corrosion on the inside of my rims causes any kind of problem.   

[iski]

I have never been that anal on a bike ride....

I have never been anal on a bike ride either. 



Helium would make the bike weigh less - it's the gassification of the future no doubt.

[Twolanerider]

Helium would make the bike weigh less - it's the gassification of the future no doubt.

Trouble is we're going to run out of helium.  I know this because I read it on the internet.  So then we'd all have flat tires and could not go nearly as fast as we do now.  Of course the solution is picking some place we don't like and nuking it.  Repeatedly.  And then some more. 

Multistage thermonuclear detonations (The H Bomb ! ) produce helium.  If you're willing to get close enough to capture the gas we can be riding again!