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Great read on tire pressure............(7 posts)

Great read on tire pressure............Len J
Jun 24, 2003 12:20 PM
Clincher rims and downhill breaking.

Found this on Velonews.

Great read on the interaction of Lightweight rims, clincher rims, tire pressure and the effects of repeated braking on rim temps and tire pressure. I never thought of this in this light.

Learn something everyday.

Jun 24, 2003 1:01 PM
Yup, pressure goes up as the temperature does-simple thermodynamics. A few other things they consider (and don't mention in the article) are the effects of constant hot/cold temperature cycles and what that does to the material of the rim(a.k.a. "fatigue"). Another consideartion with clinchers is the thinness of the wall under the braking surface. Wear that down, and your rim is even weaker still.
More generally, PV = nRTHumma Hah
Jun 24, 2003 2:54 PM
... which also takes into account why you want to let the air out of the tires before shipping on a plane, or even driving over a high mountain pass.
Jun 25, 2003 4:25 AM
What are you smoking?? The difference of pressure from atmosheric pressure to the absolute vaccuum of space is only a mere 14.7 psi. Even the highest mountain pass will differ less than .5 psi atmosperic pressure from top to bottom. Planes don't matter more than 1 or 2 psi, because they are pressurised.

Since you brought it up, BTW, the equation I gave IS the same as yours, being as how V,n, and R are nearly constant. Pressures and Temperatures are, of course, given in absolute units.
OK, a poll ... how many here have had a tire blow ...Humma Hah
Jun 25, 2003 6:03 AM
... in transport?

I've known maybe half a dozen racers who have reported this happening. They prep the bike the night before a race, including taking the tires up nice and hard (usually well beyond the recommended max). This is, of course, in a cold basement or garage. The next day, driving to the event, with sun on the tires and while going over a mountain pass .... POW! The tire blows right off the rim!

It is at this point that they:

A) Discover it makes more sense to pump the tires up just before a race.

B) Realize that 140 psi is maybe a little over-the-top.

C) Decide maybe its time to move up to glue-ons.
Some pretty funky mathKerry
Jun 24, 2003 4:27 PM
If you take them at their word, they've got the math wrong. 10 degrees F increase at 70 F is a 1.9 % pressure increase. The same increase at 300F is 1.3 % pressure increase. When you factor in the very poor heat transfer from the rim to the tube, who knows what the actual air temp inside the tube is - this is the only pressure that counts. Also, when they say a "conservative" 300F rim surface, I'm thinking they're wacky too. I once descended 22 miles on rough switchbacks with a fully loaded touring bike. Stopping halfway down (to rest my braking hands!) I touched the rim and it was very hot. However, it didn't burn my fingers, so it was likely much less than 200F, probably closer to 150F. Even if the tire had heated to 150F (not likely due to cooling effects of the road and the wind) the pressure would have only gone up 15 psi (from 100 psi). If the air in the tire actually got to 300F, that would mean a pressure of 143 (starting at 100). I think Zipp is either math challenged or they are making some hidden assumptions about the heat transfer from rim to tube to air in the tire.
The Zipp analysis looks pretty good to meContinental
Jun 25, 2003 2:50 AM
The Zipp example has tire pressure increasing from 120 psig to 143 psig. The gas law is applied to absolute pressures. The absolute pressure in the tire is the guage pressure plus atmospheric pressure, or 120 +14.7, or about 135 psia. An increase of 23 psia is an increase of 17%. An increase in the absolute temperature of 17% will cause the increase in absolute pressure of 17% since the ideal gas assumption is very good for air at these conditions. 70 F is an absolute temp of 530 R. A 17% increase in absolute temp gives an absolute temp of about 620 R or 160 F. I think that it's reasonable to estimate that the air in the tire can reach 160 F from braking.

Now, how to get from that 300 F rim to 160 F air temp? For a long decent it's a steady state problem with part of the tube directly in contact with inner surface of tire air and part directly in contact with the hot rim.
Assume that 1/4 the area of the tube is contacting the rim and 3/4 is contacting the tire. At steady state the heat transfer out of the tube to the tire is equal to the heat transfer into the tube from the rim. Heat transfer is directly proportional to temperature difference. So the temperature difference between the air and the tube exposed to the rim must be 3 times as big as the temperature difference between the air and the tube exposed to the inner surface of the tire. At steady state if the rim is 300 F and the air in the tube is 160 F, then the inner surface of the tire is 160 - (140/3) or 113 F. If the ambient temperature is 70 F, then the inner surface of the tire is 43 F warmer than ambient and 47 F cooler than the air in the tube. Thats about waht you could reasonably expect. This is a quick analysis full of assumptions, but it confirms to me that the Zipp axample is reasonable.

I believe that the rim can reach 300 F, but I haven't really thought it through to be sure.