Mathematicians, Help me Formulate my tire PSI!

[JanGeo]

I think it was on the Tire Rack website - they didn't say how much it increased but they did say it did increase along with wall stiffness and increased weight. It didn't make a lot of sense to me since bicycle tires that were larger usually rolled much easier but that could also be from bearing drag differences at lower RPM for larger wheels.

[Lug_Nut]

Higher tire pressures reduce the length of the contact patch chord, the flat section of the tire on the ground. This changes the rolling "circumference" distance (and rolling resistance) without changing the tire "diameter".
A loaded tire is shaped like a "D" on its side. The straight distance is less than the curved distance. At an absurd extreme, with a flat tire, no wheel rim, sitting on the axle centerline, the distance around the outside periphery would be (1/2 Pi * D)+ D. The resulting 2.57*D is less than the perfectly round unladen tire's 3.14*D.
More air pressure will increase the distances traveled for one tire revolution without necessarily increasing the tire size, just by reducing the size/length of the flat spot.
By enough to make a difference in miles recorded? Not likely.
Odometers and speedometers are not calibrated and certified to some degree of accuracy for most of us. Use a GPS to compare one set of data to another and use that empirical information to adjust your accuracy to the degree that you want.
The tire 'size' is probably no more accurate. My 100 "miles" before be different than my 100 "miles" after because of nothing more than my new 195/60-15 tires are a different brand than my old 195/60-15 tires. Never mind the circumference change from tread wear...

[bobc455]

Originally Posted by JanGeo

I think it was on the Tire Rack website

I looked around the website but couldn't find anything. If you happen to find a link, could you post it? It sure seems counter-intuitive.

-Bob C.

[JanGeo]

4th paragraph down on this article:
http://www.tirerack.com/tires/tirete...jsp?techid=175

Larger tires require more rubber and longer reinforcing cords than smaller tires. Therefore within a single tire model line, there is typically a relationship between tire size, weight and the resulting rolling resistance force where larger tires have more rolling resistance than smaller tires.

They really go over a lot of info in the tech section of Tire Rack - makes for very good reading.

LugNut . . . even with the D shapped tire the CIRCUMFERENCE doesn't change (steel belts do not shrink or stretch) and it still makes ONE revolution and that means the same distance traveled - the side walls take a beating with an under inflated tire and that's all.

[kamesama980]

the info on higher RR for smaller tires makes sense in a way. Though the difference in circumference usually makes up for it by far so you don't notice.

If you're worried about trip odometer accuracy, get a GPS and drive 100 miles and compare to your trip odometer. I've seen speedometers off by over 5% and odometers by slightly less from the factory