Tire pressure thread

[HARDTRAILZ]

But at the lower PSI, the tire is not "round" and the effective diameter would change.

[bartonmd]

I can see where you both are coming from, but I would tend to agree with Kyle. I base this solely on the fact that from 2002-2006, a lot of the TPMS systems simply used the difference in wheel speed from the ABS sensors to tell if you had a tire getting low. They could see as little as 3-4psi difference in pressure causing the wheels to turn different speeds. If James were correct, that type of system wouldn't work at all, because the wheel speed would be the same with the tire full or empty. Knowing that, I can only guess that part of the heat that comes from running tires at lower pressure is caused by the tread compressing length-wise (circumferential) as it contacts the ground, therefore changing the effective circumference (as it hits the ground) as the tire changes diameter.

Mike

[DirtyBacon04]

But at the lower PSI, the tire is not "round" and the effective diameter would change.

I agree, but a tire rolls on it's circumfrance to determine distance, not diameter. The only other thing would be if the tire was folding underneath itself when it came in contact with the ground, effectivley "tailoring" itself. I'm trying to relate it to a tank tracks, but i cant quite get it into words.
REGARDLESS, i dont think a 4 psi difference would be enough to cause extra wear/damage on 4wd train, especially at low speeds in soft surfaces.

[HARDTRAILZ]

I dont think 4 psi matters either, but at 30 psi difference, as i asked James about with his theory, I believe it would.

The circumferance is based upon the diameter...C= PI x D or C = 2 PI x R.

With a lower pressure the tires center point will no longer be the center of the wheel and thus the radius and diameter will change from that of the fully inflated tire and cause a different circumference to be calculated.

A flat tire with vehicle weight on it will roll a shorter distance than a fully inflated one in one rotation.

[DirtyBacon04]

Uh oh, he busted out the formulas. I can't compete. Your last sentence is what made it all make sense to me. But a flat tire is

folding underneath itself when it came in contact with the ground, effectivley "tailoring" itself

So I'd say, as long as you have enough psi for the structure of the tire to remain intact; the difference is minimal at best.

[HARDTRAILZ]

10 PSI is flat to most, but is fine offroad and the tire has a completely different diameter and therefore circumference than a street pressure 40 PSI tire.

[HARDTRAILZ]

A tire aired down like this



Is going to be a few inches shorter and therefore have a differnt radius, diameter,and circumference than that same tire fully inflated.

[DirtyBacon04]

At that point, the circumference formula no longer applies, as the tire is no longer a circle.

(Note: I'm suprised a "Tire Pressure Thread" has gone on this long. Who'd a thunk)

[boog2006]

The diameter is never in question...

The radius is the driving factor. The distance from the center of the axle to the ground, I'm calling the effective radius, determines the rpm's.

So a flat tire or a smaller tire or a larger tire would show some issue in the drivetrain.

I've seen this first hand only once when using a small spare on the rear with AWD on. The truck was constantly trying to transfer power. To resolve it I turned the traction control off and returned to 2wd. The truck felt like it was driving on soft ground while on pavement.

And this was before I new of AWD issues and my wife liked to use it in the rain.

[HARDTRAILZ]

I am not surprised at all. It is a big issue both on and off road. I truly think that something will break if you run too different of pressure front/rear in 4x4 in any situation. At 5 psi difference. probably 100s of thousands of miles for the wear to matter. At 20 or more psi different, I think you would be potentially driving a time bomb for something popping.

I can tell when my PSI is off by more than a couple PSI side to side with my locker via the clicking as it slips...but since the circumference of the tires never changes it should not matter, correct?

[HARDTRAILZ]

The diameter is never in question...

The radius is the driving factor. The distance from the center of the axle to the ground, I'm calling the effective radius, determines the rpm's.

So a flat tire or a smaller tire or a larger tire would show some issue in the drivetrain.

I've seen this first hand only once when using a small spare on the rear with AWD on. The truck was constantly trying to transfer power. To resolve it I turned the traction control off and returned to 2wd. The truck felt like it was driving on soft ground while on pavement.

And this was before I new of AWD issues and my wife liked to use it in the rain.

The diameter is in question, because, it and the radius, are tied hand in hand. The diameter is double the radius. You can not change one without changing the other.

[navigator]

interesting debate, If you think about it like this, the length of the tread never changes.
I think we can test this.
I think if you aired down one rear tire on one side and put a chalk mark on the tire and pavement and drove one complete revolution, make another mark on pavement and measure the distance.
You could then repeat the process with a full pressure tire and see if there is any difference in the distance.
I would think you would want to at least do the same axle to make sure the weight is similar. You might even be able to do the opposite side at the same time.

Another test might be to do all 4 at one time. Air down tires on 2 different corners. Compare front to back and tires on the same axle. Maybe do 40lbs in one tire and 15lbs in the aired down version.

[DirtyBacon04]

I thnk another variable we're forgetting about is the circumferance formula is for a 2D circle. We have a third dimension to take into account. But as Chris just stated, the length of the tread doesn't change. If you have a tire at 10 PSI and one at 40 PSI, the only thing thats going to change any measurment is weight. If both of those tires are off the truck and laying next to each other, they'll be identical.

This is way too much thinking for tire pressure...

[bartonmd]

interesting debate, If you think about it like this, the length of the tread never changes.
I think we can test this.
I think if you aired down one rear tire on one side and put a chalk mark on the tire and pavement and drove one complete revolution, make another mark on pavement and measure the distance.
You could then repeat the process with a full pressure tire and see if there is any difference in the distance.
I would think you would want to at least do the same axle to make sure the weight is similar. You might even be able to do the opposite side at the same time.

Another test might be to do all 4 at one time. Air down tires on 2 different corners. Compare front to back and tires on the same axle. Maybe do 40lbs in one tire and 15lbs in the aired down version.

Or, you could read my post above, and test it by driving a 2002-2005 Impala (or any of the other vehicles that were like this) around after letting some air out of one of the tires, and waiting for the tire pressure monitor system to kick a low tire pressure light, because THE DIFFERENCE IN ROTATIONS PER MILE CHANGES WHEN YOU LET AIR OUT OF A TIRE, AND THAT'S HOW THESE SYSTEMS WORK! If that wasn't the case, I seriously doubt that there would be several years of cars out there that reliably use this system to monitor for low tire pressure. End of thread! Regardless of how or why it happens, it happens, or these systems would not work.

Mike

[DirtyBacon04]

perhaps the difference in speed can be due to increase in resistance... not necessarily change in tire diameter/radius/circumferance.

[bartonmd]

is the difference in speed? Or can the system detect a change in resistance?

It is speed. It uses the ABS wheel speed sensors, like I said above, which can only measure speed.

Mike

[Trail X]

Here's a source (post #2 has some good information): http://www.bobistheoilguy.com/forums/ub ... ber=851563

Yes, the circumference can change with psi. But it only changes so much as it stretches the steel belts. This circumference change is not directly related to the "loaded radius" of the tire.

So while there is a relationship between circumference (and therefore rolling distance) and tire pressure, the effect is overall minimal, and much less than a calculated rolling distance based on a loaded radius between a hub and the ground.

[HARDTRAILZ]

I can assure you that on the same axle...they turn different enough speeds at couple PSI difference to engage my locker. I would not even try to drop one to 10 and leave the other at 40 due to what it would do to the locker. At least not for long

Is one revolution enough of a test? I think it could offer some info, but not sure it will be exact enough. You may only have a difference od tenths of an inch, but that could be washed by the width of your chalk line and thus rendering the test worthless. Also you would be dismissing the cornering effects of the different psi. We already know that tight turns in 4x4 and bring on crow hop...this may happen on much less of a turn w a 25 psi difference front to back, but a one revolution test would never show it.

[bartonmd]

Here's a source (post #2 has some good information): http://www.bobistheoilguy.com/forums/ub ... ber=851563

Yes, the circumference can change with psi. But it only changes so much as it stretches the steel belts. This circumference change is not directly related to the "loaded radius" of the tire.

So while there is a relationship between circumference (and therefore rolling distance) and tire pressure, the effect is overall minimal, and much less than a calculated rolling distance based on a loaded radius between a hub and the ground.

Yes, it won't be a direct calculation to either circumference or radius, as both are pulled to "center" of the difference by the other. It will be somewhere in between.

Mike

[navigator]

post 3 from that link brought images of tires from a top fuel dragster to mind