Tire inflation vs rolling circumference

[Danh]

I had to replace a single tire on my AWD Honda CRV. While all tires are the same size, manufacturer and model, 3 tires have about 8 1/2/32nds of tread compared to 10-32nds on the new.

Honda seems to have no objective requirement for tire tread uniformity for its AWD vehicles, but as my difference is within Subaru and Audi standards, I think this is a non-issue. But it did get me to thinking about the effect of tire inflation on rolling circumference.

My question is, in my case could the rolling circumference difference between new and old tires be eliminated by varying tire pressures? For example, while Honda specs 33psi all around, would running the old tires at 36psi and the new tire at 33psi eliminate some/all of the difference?

Honda's TPMS system is based on comparing tire rotations, so lower psi in a tire must affect circumference. And, as rolling circumference seems to be a function of radius as measured from the axle center to the ground it seems obvious more pressure would increase the radius and the rolling circumference. But nothing I've been able to find supports this notion.

Again, I don't believe my situation is a problem, but in more extreme situations altering tire pressures (within limits) is sure more appealing than buying 4 new tires.

Thanks for any thoughts.

 

[09_GXP]

From chatting with a Tire Rack engineer a few years ago he gave me this advice when talking very generally about most tires. The difference in circumference comes more from how the sidewall deflects versus the actual "ballooning" of a tire. This only applies to radial tires not older bias ply tires that would balloon. TPMS works by counting revolutions since an under inflated tire will have a constant flat spot on the bottom. However, if you are comparing a properly inflated tire to one that is over inflated the different would be very minor. All of this comes from the fact that radial tires do a great job controlling the overall circumference of the tire and the air pressure acts to support the sidewall versus creating the tire's shape.

 

[turtlevette]

I've thought the same thing. Its just too difficult to predict the circumference vs pressure. And how that will change with heat.

The whole treadwear changing circumference enough to be a problem is bull in my opinion. The drive line has a viscous coupling that allows for differences in tire size and turning corners.

 

[LotI]

I would be more worried about handling problems vs driveline problems.

If you pay me enough, I could do the measurements for you to the 1/10 of an inch. But, it would only be applicable to that tire and size.

 

[Kawiguy454]

What weighs more a pound of feathers or a pound of lead?

The circumference of a tire is exactly the same no matter how much air is in the tire. The APPARENT DIAMETER with differeing air pressure does not change the actual CIRCUMFERENCE or LENGTH of the rubber tread.

If the tread circumference is 75" then for each rotation the tire rolls 75" Having 15PSI or 50PSI does not change CIRCUMFERENCE.

 

[eljefino]

Originally Posted By: Kawiguy454


If the tread circumference is 75" then for each rotation the tire rolls 75" Having 15PSI or 50PSI does not change CIRCUMFERENCE.


You're negating the effects of micro-scrubbing when the tread comes into contact with the road, and then again when it leaves.

Your heart is in the right place, though, as it matters so little, you'd never pin it down.

Best to put a scanner on the ABS and get an extremely exact MPH for each wheel.

 

[Danh]

Originally Posted By: Kawiguy454
What weighs more a pound of feathers or a pound of lead?

The circumference of a tire is exactly the same no matter how much air is in the tire. The APPARENT DIAMETER with differeing air pressure does not change the actual CIRCUMFERENCE or LENGTH of the rubber tread.

If the tread circumference is 75" then for each rotation the tire rolls 75" Having 15PSI or 50PSI does not change CIRCUMFERENCE.


OK, but then how does Honda's TPMS system detect low tire pressure?

 

[Traction]

Originally Posted By: Kawiguy454
What weighs more a pound of feathers or a pound of lead?

The circumference of a tire is exactly the same no matter how much air is in the tire. The APPARENT DIAMETER with differeing air pressure does not change the actual CIRCUMFERENCE or LENGTH of the rubber tread.

If the tread circumference is 75" then for each rotation the tire rolls 75" Having 15PSI or 50PSI does not change CIRCUMFERENCE.

The radius is what changes. That's how the early in-direct system worked, because the RPM changes. They do the same thing on some new Honda/BMW/VW/Mazdas, etc. The computers now can figure out which tire is low.

 

[Kuato]

Originally Posted By: Danh
I had to replace a single tire on my AWD Honda CRV. While all tires are the same size, manufacturer and model, 3 tires have about 8 1/2/32nds of tread compared to 10-32nds on the new.

Honda seems to have no objective requirement for tire tread uniformity for its AWD vehicles, but as my difference is within Subaru and Audi standards, I think this is a non-issue. But it did get me to thinking about the effect of tire inflation on rolling circumference.

My question is, in my case could the rolling circumference difference between new and old tires be eliminated by varying tire pressures? For example, while Honda specs 33psi all around, would running the old tires at 36psi and the new tire at 33psi eliminate some/all of the difference?

Honda's TPMS system is based on comparing tire rotations, so lower psi in a tire must affect circumference. And, as rolling circumference seems to be a function of radius as measured from the axle center to the ground it seems obvious more pressure would increase the radius and the rolling circumference. But nothing I've been able to find supports this notion.

Again, I don't believe my situation is a problem, but in more extreme situations altering tire pressures (within limits) is sure more appealing than buying 4 new tires.

Thanks for any thoughts.


Do the math for your size tire.....2/32nds difference in tread height is not enough difference to worry about in the rolling diameter.

 

[George Bynum]

Originally Posted By: Traction
The radius is what changes. That's how the early in-direct system worked, because the RPM changes. They do the same thing on some new Honda/BMW/VW/Mazdas, etc. The computers now can figure out which tire is low.
I had a long discussion with a tire design engineer friend with Michelin (I live in Greenville SC). I proposed that the circumference of the belt controlled the revs/mile, and that a new and worn tire rotated the same, there was more rubber squirm with the newer tire. I likened it to a gear driven track on off-road equipment. He told me I was wrong, but could not (intellectual property issue) give me any documentation to support it.

Now that vehicle manufacturers are effectively using that difference, and it must allow for different manufacturers tires, is there published data? Given that it works (?), it would seem that inflating a more work tire to a greater pressure MIGHT equalize the revolutions/mile.

To ask the same thing a different way, if my vehicle specification is 35 psig, would the TPMS sensing revs/mile be sensitive enough to see 20 psig?

 

[slacktide_bitog]

Originally Posted By: George Bynum
if my vehicle specification is 35 psig, would the TPMS sensing revs/mile be sensitive enough to see 20 psig?


They are required by law to sense 25% less than spec. So for your 35 psi spec, the TPMS would have to trigger when it senses 26 psi, so it will definitely see 20

 

[Barkleymut]

I use a tire pressure gauge, same as my grandpa used to do

 

[Danh]

Originally Posted By: Kuato
Originally Posted By: Danh
I had to replace a single tire on my AWD Honda CRV. While all tires are the same size, manufacturer and model, 3 tires have about 8 1/2/32nds of tread compared to 10-32nds on the new.

Honda seems to have no objective requirement for tire tread uniformity for its AWD vehicles, but as my difference is within Subaru and Audi standards, I think this is a non-issue. But it did get me to thinking about the effect of tire inflation on rolling circumference.

My question is, in my case could the rolling circumference difference between new and old tires be eliminated by varying tire pressures? For example, while Honda specs 33psi all around, would running the old tires at 36psi and the new tire at 33psi eliminate some/all of the difference?

Honda's TPMS system is based on comparing tire rotations, so lower psi in a tire must affect circumference. And, as rolling circumference seems to be a function of radius as measured from the axle center to the ground it seems obvious more pressure would increase the radius and the rolling circumference. But nothing I've been able to find supports this notion.

Again, I don't believe my situation is a problem, but in more extreme situations altering tire pressures (within limits) is sure more appealing than buying 4 new tires.

Thanks for any thoughts.


Do the math for your size tire.....2/32nds difference in tread height is not enough difference to worry about in the rolling diameter.


Yes, I can do the math. And I mentioned in my post that my situation wasn't a problem. The question was does tire pressure influence rolling circumference enough to compensate for differences that could be a problem.

 

[andyd]

I ran pairs of 2 different makes of tires on my Grand Wags. The viscous coupling in the NP 229 handled it without trouble.. I'm using pretty much the same set up in the Ranger. It is un drivable on the road in 4wd. If the surface has give, then they are fine.

 

[Kuato]

Originally Posted By: Danh
Originally Posted By: Kuato
Originally Posted By: Danh
I had to replace a single tire on my AWD Honda CRV. While all tires are the same size, manufacturer and model, 3 tires have about 8 1/2/32nds of tread compared to 10-32nds on the new.

Honda seems to have no objective requirement for tire tread uniformity for its AWD vehicles, but as my difference is within Subaru and Audi standards, I think this is a non-issue. But it did get me to thinking about the effect of tire inflation on rolling circumference.

My question is, in my case could the rolling circumference difference between new and old tires be eliminated by varying tire pressures? For example, while Honda specs 33psi all around, would running the old tires at 36psi and the new tire at 33psi eliminate some/all of the difference?

Honda's TPMS system is based on comparing tire rotations, so lower psi in a tire must affect circumference. And, as rolling circumference seems to be a function of radius as measured from the axle center to the ground it seems obvious more pressure would increase the radius and the rolling circumference. But nothing I've been able to find supports this notion.

Again, I don't believe my situation is a problem, but in more extreme situations altering tire pressures (within limits) is sure more appealing than buying 4 new tires.

Thanks for any thoughts.


Do the math for your size tire.....2/32nds difference in tread height is not enough difference to worry about in the rolling diameter.


Yes, I can do the math. And I mentioned in my post that my situation wasn't a problem. The question was does tire pressure influence rolling circumference enough to compensate for differences that could be a problem.


Sorry yeah I was answering on my phone and should have taken more time to say the following: I agree with the post that it would take a large difference in pressure to make it happen - way low for the bigger tire and super high on the smaller tire...probably unsafe to operate at both end.

 

[CapriRacer]

Some thoughts:

The usual concepts of diameter, circumference, and radius we apply to a solid objects don't work very well for tires. That's because tires are flexible and don't roll at any diameter/circumference that can be measured directly.

For example: If one takes a freestanding tire - one jacked off the ground - and measures its circumference, then lowers the vehicle to the ground, then measures how far one revolution moves the vehicle, that second measurement will be about 97% of the first measurement.

For this reason tires generally use revs per mile as an indicator.

HOWEVER, rolling diameter does change with inflation pressure. That's because the steel belts are 2 layers parallel wires one angled the opposite direction to the other. X-ray the tire and the belts form a diamond pattern.

Continuing that analogy, if one were to pull on opposing corners of a diamond (which is a form of a parallelogram), the diamond gets longer, but narrower. The opposite happens when the belt goes through the footprint - it gets shorter and wider! Not a lot, but you can measure it (with difficulty!) That is called pantographing.

One can actually measure the difference in rolling circumference due to inflation pressure. I did it once and published the results in some thread on some web site, but I can't seem to find it now.

It's that phenomenon that is used by some vehicle's ABS system to sense changes in inflation pressure, instead of directly measuring the inflation pressure.

And a comment on rolling diameter vs brand of tires: There is no law that says a tire of a given size MUST have a certain diameter (rolling or otherwise!) - and that means that it is possible to ruin an AWD unit by mixing makes/models of tires on one of those! That's why tire manufacturers do NOT recommend mixing makes/models on AWD and 4x4's. (Please note: It is also possible for different makes/models to have similar rolling circumferences (revs per mile) - and there would not be a problem, but unless you know for certain, it's not a good idea.)

 

[circuitsmith]

Perhaps one could measure for rolling circumference differences:
make a chalk mark at the 12:00 position on each tire.
Drive in a straight line for 100 yards or so.
Stop and check the chalk marks.

 

[supton]

Originally Posted By: Barkleymut
I use a tire pressure gauge, same as my grandpa used to do
I'd be quite impressed if you can do that at 60mph.

 

[jadatis]

As "pigheaded Dutch selfdeclared tirepressure-specialist" at the moment I am busy figuring out how roling circumference is calculated.
It is sertainly not the unloaded diameter x 2Pi.
And also not the loaded radius x 2PI. Something in between but same tire can have lower RC at more deflection.
What I found is that not only deflection is relevant but also the length of contactpach to the ground .
This can vary with tirekind, a same sise D-load tire has shorter surfacelength then a P-tire at the same deflection.
Found this youtube film about tractor tires,and the Galileo wheen that should keep the RC the same for different deflections of tire,or better diferent filled pressures.
For the moment my conclusion is that this Galileo wheel does not deflect that much at lower pressure.
Somehwat the same as most runflatt tires that bares its load on more on the construction of the tire.
then my question is if this Galileo wheel does what lowering pressure is meanth for for tractors, to lower the pressure on the ground.
Pressure ont the ground is higher then only the tirepressure,but also adds the construction pressure .

Galileo wheel