2nd Gen Non-Engine/Transmission Low tire pressure with limited slip...

[wlowe]

A couple of friends of mine had a small disagreement regarding low tire pressures.



Friend number one says that low tire pressure will change the rolling radius, (distance from center of axle to ground), causing the tire to turn faster,(more revolutions), than the other properly inflated tires.



Friend number two says that the rolling radius doesn't matter, the total circumfrance of the tire determins how fast it turns, and with radial tires the total circumfrance is not affected by low pressure.



I found this arguement interesting in regards to how low tire pressure in one rear tire might affect/damage our limited slip differentials. If low tire pressure does cause a tire to turn faster than this would not be a good thing for our limited slip differentials. So what do you guys think, who is right and who is wrong.

 

[JLandry]

Friend number one is correct.



Low tire pressure changes the ERR or "Effective Rolling Radius" of a tire causing the tire to rotate faster than the same properly inflated tire. The apparent loss in tire circumference occurs by the tread bunching up before and after the contact patch.



It worth noting that severe enough underinflation can cause the rubber compounds in the tread to stretch beyond their elastic limits until tread tearing and separation from the belts occurs.



Here's an excerpt from an article on tirerack.com that mentions how the change in rotational speed from an underinflated tire was used in early tire pressure monitoring systems to sense low tire pressure:

Indirect Monitoring Systems



In the interest of providing a lower cost Original Equipment system, indirect tire pressure monitoring systems were developed by vehicle manufacturers wishing to comply with the law while minimizing development time and cost. Indirect systems use the vehicle's antilock braking system's wheel speed sensors to compare the rotational speed of one tire to that in another position on the vehicle. If one tire is low on pressure, its circumference changes enough to roll at a slightly different number of revolutions per mile than the other three tires. Reading the same signal used to support ABS systems, the vehicle manufacturers have programmed another function into the vehicle's onboard computer to warn the driver when a single tire is running at a reduced inflation pressure compared to the others.​

 

[Bob4x4]

Friend number one is correct.



Low tire pressure changes the ERR or "Effective Rolling Radius" of a tire causing the tire to rotate faster than the same properly inflated tire. The apparent loss in tire circumference occurs by the tread bunching up before and after the contact patch.



It worth noting that severe enough underinflation can cause the rubber compounds in the tread to stretch beyond their elastic limits until tread tearing and separation from the belts occurs.



Here's an excerpt from an article on tirerack.com that mentions how the change in rotational speed from an underinflated tire was used in early tire pressure monitoring systems to sense low tire pressure:

Indirect Monitoring Systems



In the interest of providing a lower cost Original Equipment system, indirect tire pressure monitoring systems were developed by vehicle manufacturers wishing to comply with the law while minimizing development time and cost. Indirect systems use the vehicle's antilock braking system's wheel speed sensors to compare the rotational speed of one tire to that in another position on the vehicle. If one tire is low on pressure, its circumference changes enough to roll at a slightly different number of revolutions per mile than the other three tires. Reading the same signal used to support ABS systems, the vehicle manufacturers have programmed another function into the vehicle's onboard computer to warn the driver when a single tire is running at a reduced inflation pressure compared to the others.​

Can you show me the math to support that ?



Bob

 

[JLandry]

Can you show me the math to support that ?

Bob

Google is your friend Bob.

Best regards,

John L.

 

[S.Morris]

Limited slip

Having worked on rearends for a living at one time in life, I can say from first hand knowledge that driving around with one low tire on a limited slip will waste the cluthes in a fairly short time period if being driven at road speeds.



A positive locking rear would bind up so hard you would have to constantly correct with steering to go in a straight line.



Having equal rolling dia. is very important to long life of a L/S diff.



My 2c

Steve

 

[EKlem]

Can you show me the math to support that ?



Bob

It has to do with the dynamics equation v=wXr where v is the tangential velocity at the edge of the tire, w is rotational velocity(measured in radians/s) and r is tire radius. There is actually a cross product between w and r but since they are perpendicular, you can multiply them for the sake of this argument. In this problem, v is a constant because your road speed is the same. r decreases due to underinflation so w must increase. You can think of w as a measure of rpm, just in different units while v is roadspeed. So for the same roadspeed, the smaller radius tire has a higher rpm.

 

[SnoKing]

Can you show me the math to support that ?



Bob

2 x pi x r



SNOKING

 

[WDixon27]

a quick and dirty confirmation would be drive any solid axle atv with a low rear tire and pop a wheely :-laf

 

[Bob4x4]

a quick and dirty confirmation would be drive any solid axle atv with a low rear tire and pop a wheely :-laf

No slip there:-laf I carried a very acurate pressure gauge with built in deflator in those days,

Launching a 60 hp 275lb trike in third gear wot will get your attention when things aren't right.



Bob

 

[SRadke]

The circumfrence of the average bicycle tire decreases by 1" when you're on the bike vs. walking next to it.

 

[Bob4x4]

The circumfrence of the average bicycle tire decreases by 1" when you're on the bike vs. walking next to it.

My michellin road bike tires are not average at 125 psi and less than 3/4" wide but they sure do stick in the corners. They do not change cicumferance with me on it either they just click off the miles at very nice pace... ... ... ... .



Bob