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2010 Forester 2.5x 4EAT
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Discussion Starter · #1 ·
We know that it is important to maintain tires with close to the same circumference on AWD cars, and pay a lot of attention to keeping identical tires with similar tread depths on the car. I have often seen it mentioned on the forum that maintaining proper tire pressures is also important as tire pressures affect tire circumference. I have not noticed any numbers regarding how much a drop in tire pressure in a tire affects circumference... so I did a little experiment.

I attached a bar clamp to a tire with a long nail taped to the end to act as a pointer. I taped a ruler to a board and positioned the board vertically next to the point of the nail. I inflated the tire (a cold 225/55 17 Geolander G95) to 36 psi and noted the pointer position on the ruler. Then I dropped the pressure in 2 psi increments down to 20 psi...noting the change in position every 2 psi. Then I reinflated back up to 36 psi in the same 2 psi increments.

The pointer dropped 1/32" every 2 psi from 36 psi to 26 psi, then dropped about 2/32" every 2 psi from 26 psi to 20 psi. The measurements were the same while reinflating.

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.

http://i1190.photobucket.com/albums/z450/SierraHotel058/IMG_0039_zpsb836a9bf.jpg
 

· Premium Member
2010 Forester 2.5x 4EAT
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566 Posts
Discussion Starter · #7 ·
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?
Great video! I have thought about the fact that the physical circumference of the tire does not change much (steel belts are not known for stretching/contracting!), but the "rolling radius" obviously does. I have seen people use a tank tread as an example: the revolutions per mile obviously has little to do with radius.

Here is another take on the issue. Here is Tire Rack's explanation of the two types of TPMS sensors. Notice that the "Indirect system" uses the difference in revolutions per mile to detect low tire pressure:

Direct Systems
attach a pressure sensor/transmitter to the vehicle’s wheels. An in-vehicle receiver warns the driver if the pressure in any tire falls below a predetermined level. Direct systems are typically more accurate and reliable and most are able to indicate which tire is underinflated.
Indirect Systems
use the vehicle’s antilock braking system’s wheel speed sensors to compare the rotational speed of one tire versus the others. If a tire is low on pressure, it will roll at a different number of revolutions per mile than the other three and alert the vehicle’s onboard computer. Indirect systems (except for the TPMS on several 2009+ Audi models and 2010+ Volkswagen models) are unable to generate accurate readings in cases where all four tires are losing pressure at the same rate, such as the effects of time and temperature.
 

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2010 Forester 2.5x 4EAT
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566 Posts
Discussion Starter · #14 · (Edited)
This is a complicated question (at least to me). In the case of a severely deflated tire, the rolling circumference is reduced so much that it is hard to get away from the fact that the distance the car should move forward with one rotation of the axle (based on 2r x pi) is not close to the distance that should be covered based on the circumference of the tire.

One possibility is that the circumference of the tire does change with changes in tire pressure. Has this been tested? It seems nonsensical because of the seeming rigidity of tire belts, but it could be a partial answer.

Another factor may be that the tire actually abrades the road surface at very low pressures... thus compensating for the difference.

The tire flat spot, or "footprint", obviously increases with reduction in air pressure, which in theory reduces circumference. But this would require some contraction of the tire belt.

I do believe that tire pressure affects the number of rotations a tire will make in a given distance, but I think that there are other factors involved that would make my calculations in my original post overly simplistic.... and therefore inaccurate. My head is starting to hurt!
 
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