Low Tire = Smaller Circumference.......?

The tires air pressure in no way decreases it’s circumference.

The notion that because the diameter is less when a tire is low the circumference lessens is …hogwash!

Example: If one were to measure the diameter (distance form the ground to the top of the track) of the military’s main battle tank, the M1 Abrams, one would find that the circumference would be much greater than D*tt

Regardless of shape of the tanks track, the circumference would remain the same.

Not correct. The pressure is stretching the tire and the cords it’s made of. Ever tune a guitar? You’re stretching wires as you do it. And a tank tread, if laid out flat and pulled, will stretch. All materials stretch to some degree when put under tension. Not a lot, but they do.

p.s. - please keep the discussions in one place, it’s easier to follow them that way.

p.p.s. - I just did a rough calculation, and the tread on a typical tire is under 6,000 to 8,000 pounds of tension due to the inflation pressure (assumed 30 psi), so there’s plenty of force available to slightly stretch the tire.

Gee, I wonder how the abs sensors use tire rotation to determine change in tire pressure.

Magic and pixie dust :wink:

Thanks Shadow…for that enlightenment…;=\ I feel so much better now.

The OP statement is not correct. We used to use a band to squeeze a difficult tire to seal it to the rim. The band was broken several times by some technicians who did not remove it quickly enough after the initial seal. If the diameter did not increase then the band would not have snapped.

Increasing the pressure in a tire changes not only its circumferemce, it also changes it “cord” profile, that shape you’d get should you cut across the tire and look at the shape. Just as increasing the pressure in a balloon makes it get rounder, increasing the pressure in a tire causes the “cord profile” to get rounder. That increases the rolling circumference and reduces the width of the tread that’s in contact with the tarmac.

The question brought to bear is that if the rolling circumference (diameter) actually gets smaller with loss of air, why is it that the surface of the tire can still turn along with the wheel just as before. The answer is that it cannot and does not. The difference on a low ttire between the tread circumference and the rolling circumference is absorbed by squirming and scrubbing of the tread on the pavement. That’s why the rolling resistance of a tire with 10 pounds in it is so much higher than the rolling resistance of a tire with 30 pounds in it. It’s the stretching and squirming of the rubber both on the pavement and in the sidewalls that’s eating up that additional energy.

That squirming of the rubber also generates heat and can (and does) cause sudden tire failure. Try an experiment. Take three old spoons (new ones if you live alone). Bend the handle of the first back and forth about 5 degrees 10 times. You may feel some warmth at the bend point. Now take the second spoon and bend the handle back & forth 45 degrees 10 times. The bend point will be getting hot. Now do the same thing with the third spoon, only to 90 degrees. The bend point will get very hot and the handle may fail. Exactly the same process is happening inside your tires. And the more the rubber has to shtretch and flex, the hotter the tire gets. At some point it’ll fail.

As to the Tire Pressure Monitoring Systems that use the wheel speed sensors, the tire with the low pressure has a smaller rolling circumference and turne more rapidly for a given speed. The difference in speed trips the TPMS light.

flybyya wrote:
The tires air pressure in no way decreases it’s circumference.

Go out to your car, mark the top of both rear tires, let a lot of air out of one of them, drive a short distance in a straight line, examine the marks, and report back to us.

lion9car…utube has a demo of just such a test and it showed little if any measurable difference. But this is a gross test, not done at speed and over time where cumulative small changes become quite significant.
Auto engineers, tire manufactures and every one it seems with a degree and a real “need to know” that I have found seem to agree…there is a change.

Otherwise,as “Shadowfox” says…it’s “magic and pixie dust” The difference is routinely measured as an indicator of tire pressure change in indirect tire pressure monitoring devices as 'Same" and others have suggested.

The lack of observed gross measure difference is why some just can’t seem to “not believe their own eyes”, don’t trust car and tire makers and keep bringing this up.

In my 2007 Grand Cherokee, if I add pressure to one tire, I can observe the instrument panel display pressure indication rise without the car moving at all! This does not square with the differential speed theory.
I plan to check this out with Chrysler for their explanation. I am thinking there might be a pressure transducer somewhere on each wheel which transmits to the monitoring system.

red rolfe, yes you must have sensors in the wheel, which are more costly when the car is made, but a more accurate direct measure.

“Not correct. The pressure is stretching the tire and the cords it’s made of. Ever tune a guitar? You’re stretching wires as you do it.”

Steel probably does not stretch enough to matter at the stresses of a normal tire pressure. When you tune a guitar, you are probably distorting the wood more than you are stretching the steel.
It’s the steel e string on my fiddle that needs the most retuning, the non steel g, d, and a stings seem to hold their tune once they have been on the instrument a few weeks and the new string stretch has stabilized. That’s because it’s the steel string that doesn’t strech when the wood in the instrument changes dimensions due to humidity or temperature changes. This lack of stretch causes changes in string tension when the wood changes dimensions.

Also, a lot of guitars have been damaged by people who assume that the sharpest string is the one in tune and tune the rest of the stings to it. After repeated retunings, the pitch can be walked up to string breaking or bridge pulling off tension. Expanding wood can make a guitar go sharp as well as contracting wood can make a guitar go flat. It’s always best to tune to a reference pitch.

I didn’t say steel doesn’t stretch at all, if that was true, it couldn’t be used for springs. I just don’t thing your tape measure can measure the difference in a steel radial’s tire circumference between an underinflated and a overinflated tire.

BLE - not true, that’s how inflation monitors that depend on wheel speed work. And when you tune your guitar, the change in pitch results directly from stretching the wire. The flexing of the wood works against this, not for it.

As I said, there’s about 6000-8000 pounds of tension on the tire belt. That’ll stretch it (a small amount).

What keeps these inflation monitors from reporting a false low tire when you are taking a long radius turn on a highway?
When a string breaks on a guitar, the sudden release of that string’s tension makes the rest of the strings go sharp, so the wood does flex.
Also, when restringing a guitar, by the time you have the last sting up to pitch, you have to go back to the first string and tune it again because it’s now really flat even though you had it to pitch when you put it on, you often have to go through this several times before they are all in tune.
The wood compresses more than the steel stretches.

They probably average over several minutes, long enough for those variations to even out. But if you drive in a circle for that long, I guess it would fool them! Maybe that’s another reason the government required the in-wheel sensors, instead.

Well, I don’t know all the technical reasons that are involved in a low tire, but the circumference of a low tire is definatly smaller than a fully inflated one. And I have TPM on my van and I believe they monitor the pressure.

B.L.E. wrote:
What keeps these inflation monitors from reporting a false low tire when you are taking a long radius turn on a highway?

This is just a wild guess, but both outside tires would be turning faster than the inside tires in this case, so that information could be used to realize that’s what happening. Also, any car with a stability control system would have sensors indicating if the driver is steering straight or turning.

And here are the answers to all your questions- http://www.nhtsa.gov/cars/rules/rulings/tirepresfinal/tireprmonsys.html

This has nothing to do with the question at hand, but I am almost sure that i have this kind of TPM

B. Direct TPMSs

Direct TPMSs use pressure sensors, located in each wheel, to directly measure the pressure in each tire. These sensors broadcast pressure data via a wireless radio frequency transmitter to a central receiver. The data are then analyzed and the results sent to a display mounted inside the vehicle. The type of display varies from a simple telltale, which is how most vehicles are currently equipped, to a display showing the pressure in each tire, sometimes including the spare tire. Thus, direct TPMSs can be linked to a display that tells the driver which tire is under-inflated. An example of a vehicle equipped with a direct system is the Chevrolet Corvette.

Since direct TPMSs actually measure the pressure in each tire, they are able to detect when any tire or when each tire in any combination of tires is under-inflated, including when all four of the vehicle’'s tires are equally under-inflated. Direct TPMSs also can detect small pressure losses. Some systems can detect a drop in pressure as small as 1 psi.

The radius of the wheel between the axle and the ground determines the rotational speed.of the wheel. When the tire pressure is low the radius is shortened.

What the rest of the tire is doing that is not in contact with the ground is immaterial.