Subaru Forester Owners Forum banner
1 - 2 of 2 Posts

· Premium Member
2015 Highlander AWD XLE 6AT
Joined
·
4,276 Posts
I agree that tire pressure, load, and speed (angular velocity) will affect tire circumference, and therefore revs/mile. But, I don't think that static load radius (distance from center of wheel to ground of a stopped vehicle) is a particularly good metric.

There are many other variables, and even some almost constants. For example, what happens to the tire's "free radius" (distance from wheel center to top of tire) when the pressure is changed?

I also suspect that any change is not linear over a wide pressure range. For example, what's the real change in rolling circumference between 26 psi (TPMS warning pressure) and 36 psi (typical minimum pressure for max load) compared to rolling circumference at 20 psi?

Here's another perspective:


Regards,
Jim / crewzer
 

· Premium Member
2015 Highlander AWD XLE 6AT
Joined
·
4,276 Posts
...the "rolling radius" obviously does.
A tire's rolling (loaded) radius generally can't be used to accurately calculate overall diameter or rolling circumference. That's why I'm uncomfortable with your experiment's conclusion:

So.... a 2 psi change in tire pressure (above 26 psi) results in a 1/32" change in radius... and therefore a 2/32" change in diameter. 3.14 X 2/32" (pi x d) = a little over 3/16" change in circumference. Double that for pressures between 26 psi and 20 psi.

I agree that there will be some small change, just not 3/16" / 2 psi. For example, this formula suggests that dropping the tire pressure from 36 psi to 26 psi would result in an ~1" change in tire circumference. That's a lot!

Our BMW has the indirect flat tire monitoring (FTM) system. So far, it's only dinged at me one time, and that was when a single tire had dropped to ~20 psi (mechanic mistake). Normal pressures are in the 32F - 38R psi range, so the system does allow for some pressure and size differences.

On a related note, the ABS tone rings generate many pulses (~60?) per tire revolution. The ABS system can therefore easily keep track of absolute changes in rolling circumference and/or rate of change of difference, even if it takes several revolutions to detect a change.

HTH,
Jim / crewzer
 
1 - 2 of 2 Posts
Top