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SierraHotel
2010 Forester 2.5x 4EAT
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547 Posts
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
 

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2018 Crosstrek
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241 Posts
Clever test. Thanks for the info. If I read your results correctly you have proved that under-inflation is more problematic than over-inflation. I wonder what it would be like under 20.

Obviously, this is with a static wheel. Spinning, particularly at high speed, might be different.
 

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SierraHotel
2010 Forester 2.5x 4EAT
Joined
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547 Posts
Discussion Starter · #3 ·
Obviously, this is with a static wheel. Spinning, particularly at high speed, might be different.
Good point. I will leave that calculation to someone a bit smarter then me!
 

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Premium Member
2008 Forester X Premium 5MT
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8,064 Posts
This can also be different for different tires due to construction and air volume.
 

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Premium Member
2019 Crosstrek 2018 Forester XT
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15,019 Posts
I am surptised it is that much. It may or not equate to that difference of "Rolling Diameter" I may play around with this a bit. Thanks for the information.....good job.
 

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Premium Member
2015 Highlander AWD XLE 6AT
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4,255 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
 

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SierraHotel
2010 Forester 2.5x 4EAT
Joined
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547 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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Premium Member
2015 Highlander AWD XLE 6AT
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4,255 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
 

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2010 Forester Premium X AT
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1,000 Posts
I think this whole test is flawed. The chord length of the perimeter circle increases as you decrease pressure, so you cant simply take the measurements as radius and apply the pi*d calc.
 

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Premium Member
2019 Crosstrek 2018 Forester XT
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15,019 Posts
Exactly. The circumferential length of the tread does not change to any significant degree.
Well if you do the math....the fact that it rises and presuming it stays raised as the wheels turn the effective circumference will be effectively the calculated value done by the OP
 

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2019 Sport CVT
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820 Posts
My first thought was that this test was bogus as the circumference isn’t going to change. But then does it? If it doesn’t then indirect TPMS wouldn’t work…

"Indirect TPMS do not use physical pressure sensors but measure air pressures by monitoring individual wheel rotational speeds and other signals available outside of the tire itself. First generation iTPMS systems utilize the effect that an under-inflated tire has a slightly smaller diameter (and hence higher angular velocity) than a correctly inflated one. These differences are measurable through the wheel speed sensors of ABS/ESC systems."
 

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2010 Forester Premium X AT
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Well if you do the math....the fact that it rises and presuming it stays raised as the wheels turn the effective circumference will be effectively the calculated value done by the OP
That would hold true only if the contact area between the tire and the ground was curved. Its flat, so you have to calculate the circumference taking into account the chord length (flat area) increases as pressure decreases.
 

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SierraHotel
2010 Forester 2.5x 4EAT
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547 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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2011 Forester Limited Auto w/nav
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898 Posts
I would not accept the results of this test as being conclusive. :shake:

1. I would liked to have seen more precision in lining up the mark on the tire. Just eyeballing it as being verticle leaves a lot of wiggle room for error.
2. More importantly, I do not believe testing this over a length of one or two rotations is accurate enough. The variations over that short a distance will easily be undetected by the measurement methodology (two guys with a tape measure)

To be accurate, this test needs to be conducted over a much longer distance, but that would also cause more constraints on measurement.

Scientifically, two guys with a tape measure over 1 or 2 rotations is not a condusive test... Especially when it is done once.

Conduct this test over a measured and controlled mile and under controlled circumstances and repeat the test a few hundred times and then we might be getting close to a conclusive test.

Few theories are tested by doing them only once :icon_idea:
 

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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
So, if a Subaru tire is a little low in pressure, then the overall diameter is smaller. Will this affect the AWD? Is the Computer able to adjust for minor changes in wheel diameter? The differential adjusts from left to right, but what about front to back?
 
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