tribulation138

Well as long as i keep the bfgs up front its perfect.

But when i rotate them in fall ill deflate them alittle. Or just buy new ones. Trying to avoid that

jedg

Nope... that's wrong. Inflate one of your tires to 35psi and measure the diameter then deflate to 28 psi and measure. Circumference is a function of radius which is 1/2 diameter... so yeah... changing diameter does change the EFFECTIVE circumference. If the tire didn't flex (or expand) based on tire pressure then sure, what you said would be right.

mr72

I don't think that is correct. The circumference is the only thing that matters, and of course if you could accurately measure the diameter you would find that it doesn't change in any meaningful way if you measure it across the non-loaded dimension (parallel with the ground) which is the only place that it would be accurate.

A steel-belted radial tire will not stretch along the circumference of the tire due to air pressure within the limits of air pressure that the tire can tolerate and still be used on the vehicle. The tire is just not elastic in the dimension of the circumference of the tire. The entire circumference of the tire must roll along the ground each time the wheel completes a revolution, unless the tire is so flat that the tread buckles under the wheel.

Basically on a steel-belted radial tire (i.e. all modern automobile tires), the tire does not stretch radially to any meaningful extent based on usable tire pressure changes.

Bias play tires may have been different. Certainly non-automotive tires are different, such as bicycle tires.

jedg

Absolutely makes a difference. The effective circumference is a result of the radius measured from the center of the axle to the ground. Reduce tire pressure and the tire bulges at the sidewall and lowers the vehicle and so the radius is reduced. Increase the tire pressure and the radius is increased. The actual circumference of the tire doesn't change (as you stated) but the MEASURED distance from the center of the axle to the ground changes. This measured distance is what the vehicle sees as a radius. So the vehicle (which is really using circumference) will see a change based on that distance.

Now, can you make up the 1/2 " of diameter which really means 1/4" radius.. maybe... might have to run one set higher pressure and one set lower pressure. Of course doing this might cause bad tire wear patterns

mr72

Forget it. I posted a math/physics lesson but it doesn't matter. Believe what you like.

jedg

Actually, you're correct. I sat down later and worked it out on a piece of paper. If I choose a point on the tone ring and then line that up with a point on the outside of the tire, no matter how high or low I make the pressure of the tire, the entire circumference of the tire has to be travelled to get back to the same point on the tone ring (which is what the computer is counting).

Thanks much for the conversation.

Jahamm88

Yay science

mr72

Extra credit for using the term "tone ring"

BTW the reason the 2∏r formula won't work for a loaded tire is because the tire is no longer a circle, so the perimeter cannot be calculated using a the formula for a circle's circumference.

Experts suggest that a very, very small difference in tire circumference (and therefore rate of travel) can be caused by large differences in tire pressure but for the purposes of the traction control light or attempting to correct for differences among different tires, it is insignificant. Rounds down to zero. But this is how early TPMS would detect a low tire... they would detect very small differences in wheel speed between tires, such that if one was significantly different than the other three the TMPS light would come on indicating one (very) low tire. At speed centripetal force essentially causes the tire to deform such that this really doesn't make any difference.