Tire Pressurr

[deilenberger]

I'd add the caution to use your surface scan thermometer as soon as possible after stopping as the tread surface cools more rapidly than the rubber deeper in the tire. Do you have any data that correlates inner air temperature to rubber temperature? I'm sure that if you ride long enough they equilibrate, but how long does it take?

Wayne, don't know. I'll toss the IR thermometer in the tank bag.. should be doing some riding this weekend, at least a few hundred miles, so I'll try to check. It would be interesting data to have.

I think that it is interesting that 10-20 rule and the SportRider recommendation actually correlate pretty well for tires in the 30 - 40 lb inflation range.

A 20% rule comes closer to what I've observed with radials. The SportRider recommendation was close, but I wonder how it will hold up when the air and road-surface temperatures climb. I'll have to keep an eye on that also.

It would be really handy if I had a voice activated recorder so I could take readings of pressure/temperature while riding and save them. I'll have to look into that also..

[MTBeemer]

I am a bit confused.

Given: The recommended pressures are for tires at 68 F.

Given: The rear tire I am running has a max permitted pressure of 42 PSI.

I'm on a trip carrying a great deal of gear, so I'm running the max recommended pressure of 42 PSI. If I put 42 PSI in the tire at 68 F. will I not exceed the max allowable pressure when the tire hits much higher temperatures while running high speeds on a summer day?

[Jed]

I am a bit confused.

Given: The recommended pressures are for tires at 68 F.

Given: The rear tire I am running has a max permitted pressure of 42 PSI.

I'm on a trip carrying a great deal of gear, so I'm running the max recommended pressure of 42 PSI. If I put 42 PSI in the tire at 68 F. will I not exceed the max allowable pressure when the tire hits much higher temperatures while running high speeds on a summer day?

I believe the point is that the max pressure (cold) is defined at a certain temp only (68° in your case). As the temp goes up (from driving or ambient increase), so does the pressure - but now the tire is no longer cold but rather is hot . . . . see the difference ??

If you inflate the tire to 42 PSI while it is hot (over 68°F), the pressure at 68° would effectively be reduced resulting in less over-pressure at temp. If you inflate the tire to 42 PSI at 20°F and then (after taking the bike out of the freezer) run it at 100°F ambient - the pressure at 68° would effectively be increased resulting in a higher over-pressure at the higher temp.

Max pressure as listed on the tire is the max safe pressure at 68°F (to use your numbers). Max pressure at other temps would be different. You can solve for these other temp/pressure numbers/values by getting intimate with the Ideal Gas Law.

Pressure is proportional to temp - Gay-Lussac's Law, not 1:1 but still proportional (relative pressures based on the relative absolute temp ratios) - http://en.wikipedia.org/wiki/Gay-Lussac%27s_law

In my area it's not uncommon to have near freezing temps when I leave for work and 70°F or higher temps on my way home. My tires and the ride is noticeably "stiffer" on the way home if I fill to normal pressure in the morning. If I know the temp difference is going to me large that day, I'll often drop a couple of PSI in the morning or reset the pressures in the afternoon. There's a fair bit of acceptable / safe range to tire pressures.

cheers,

[red baron]

To end the air pressue saga from my side: Here are my observations from today:
Left this morning at 7:30 AM when the temperature in my garage was 79.2 degree, tire pressure at that time 36 Front and 40.5 rear. Returned at 1:25 PM. Temperaure at that time 86 degree and checked tire pressurer after taking off helment 40 Front 45 rear. My last stop was 125 miles before getting home. Drove 286 miles and according to the on board computer: average speed 61 miles/hr and average consumption 43 miles per gallon.
Goo day