A few things. #1 mathmatically, terms like "diameter, radius, circumference" only apply to circles. Your tires are not circles. So all the mathmatical formulas you are used to from High School really don't apply. I think this is skewing some perceptions. #2 are you trying to say the size of the tire is different when rolling vs. "static". Ridiculous. #3 ABS "inferences" could only work when there is a gross under-pressure, and only when you apply the brakes. Again, a foolhearty system, experimented with perhaps, but not a good system, and not as reliable as stated. Sounds like a cheap "scam" for manufacturers to "technically" meet Federal requirements (barely) without investing additional $. #4 yes, you are missing something, and you are not alone, so don't feel bad. There is no such thing as "effective circumference". Only a "perimeter" around the tire, which has a definite length, that does not change by more than 1mm or 2mm of "stretching". EVER. #5 Quit the mind games and go out and try it. You will see everything I've stated is true.

For those who are visual learners, this should help. AutoCAD screenshot follows:
https://www.greenhybrid.com/share/fi.../8/5/tires.jpg

 

That guess would be correct. I installed an aftermarket double DIN in-dash NAV setup in my 05 FEH:

http://www.pbase.com/jmorton10/image/66287539


That setup needed a connection to the gray/black VSS wire to do turn by turn routing directions(the NAV maps worked without the wire connected, but the turn by turn voice didn't work)

The truck had not come with the factory NAV & the VSS wire did not extend all the way to the factory radio. I picked up a connection to that wire at the ABS brake module. It was slightly confusing as there was more than one black/gray wire at that plug, but after I figured out which was the correct one it worked fine.

~John

 

I haven't read this whole thread, but I can state that my wife used to have a Monte Carlo SS that had one of the early versions of a "low tire pressure" warning system.

I couldn't figure out how it worked at first(& it did work rather well), then I read the owners manual(that's always the last resort for a guy.......)

At least on that particular car, it was simply measuring how fast each tire was spinning. As a tire lost air, it spun faster because it wasn't as big around & a wheel sensor at each wheel would catch & report on the discrepancy.

~John

 

I know when my tire was at 29psi the TPM warned me.

 

OK, Apples to Apples.....a tire is not a tank tread. There is an "effective radius" and you measure it from the center of the wheel to the pavement, and THAT changes as you let air out of the tire.

You're making the assumption that if the wheel is moving at a constant speed, the outer edge of the tire is too, but look at your diagram and pick a "spot" on the tread and measure the speed as the tire rotates......it's not linear. As the spot passes between the rim and the pavement on a low tire, the speed will increase because the distance the spot on the outer edge of the tire travels is much greater for x degrees of wheel rotation than it is on the "high" side.

It's not a tank tread, and it's not round, and the wheel can move at a constant rate, but the rubber is not.

 

This is the fallacy in your thinking. There is no radius at the point the tire hits the road, because the tire is not circular then. This concept is not that hard!!!

If a train's locomotive is going 60 MPH then the caboose must also be going 60 MPH no matter how many curves there are in the track. And no matter how many curves there are in the track, the total length of the train does not change. A flat(er) tire just adds curves to the path of the rubber tire.

The tread on a rubber tire is very analogous to a tank tread.
I don't really care if you believe it or not, it won't change the facts!

"When the comparison of wheel RPM with this ABS wheel speed sensor is performed, because the deviation value is not stable, the tire abnormal air pressure cannot be judged. In short, due to noise, caused by variations in road conditions and vehicle running conditions, etc., as the prime factor, it becomes impossible to perform an accurate comparison.

With the manufacturing error of each vehicle tire as large as 0.3%, and the tire radius change (while moving) due to tire air pressure loss often less than 0.3%, a rather high degree of accuracy is needed for the tire air pressure judgment.

And, as for the aforementioned tire air pressure judgment, when the vehicle is on a low friction road surface or climbing up a hill, rounding a curve, etc., the amount of slip of the wheels gets larger, or the intermittent existence of gravel, snow covered roads, as well as other factors, can lead to the amount of drive wheel slip growing larger. Furthermore, when the slip of the drive wheels is large, the accuracy of the detected wheel velocity signal grows worse. Unless a truly valid signal is utilized for the detected wheel velocity, and the wheel velocity is determined with a high degree of accuracy, it will be difficult to improve the accuracy and dependability of the tire air pressure judgment.

Also, the tire air pressure judgment cannot be carried out when the car is accelerating or decelerating, and nothing is proposed regarding techniques to increase the accuracy or the reliability of the tire air pressure judgment taking into consideration the road conditions or the vehicle conditions.

According to the road conditions (road surface, bad roads, hills, canting road surfaces, etc.), the amount of drive wheel slip increases, the load on the 4 wheels becomes uneven, the wheel velocity of the 4 wheels also becomes uneven, and therefore the accuracy of the detected wheel velocity decreases.

Generally speaking, at low velocities, because the number of data detected from the wheel velocity sensor decreases, the accuracy of the detected wheel velocity decreases, furthermore, at high velocities, because the amount of slip of the drive wheels becomes larger, and because the accuracy of the detected wheel velocity decreases due to the load between the front wheels and the rear wheels fluctuating, the accuracy and reliability of the tire air pressure judgment decreases.

Particularly, as the drive wheel slip amount increases with the road surface friction coefficient, .mu., decreasing, the accuracy of the detected wheel velocity decreases, and the accuracy and reliability of the tire air pressure judgment decreases. When running on bad road surfaces, because the scattering of the wheel velocity of the free and the drive wheels grows larger, and the accuracy of the detected wheel velocity decreases, and the accuracy and reliability of the tire air pressure judgment decreases.

When climbing up a hill, the slip amount of the drive wheels increases, and, similarly to the condition when accelerating, the accuracy of the detected wheel velocity decreases, and the accuracy and reliability of the tire air pressure judgment decreases.

When the normal vehicle tire air pressure is 2 Kg/cm2, and when the air pressure falls to 1 Kg/cm2, the change in tire radius is approximately 0.3%. Because the tire manufacturing defect is also approximately 0.3%, in order to determine a decrease in tire air pressure using the 4-wheel wheel velocity technique, it is extremely difficult to discriminate between the tire manufacturing error and a decrease in the tire pressure.

Therefore a further goal of Mazda is to increase the accuracy and reliability of the tire air pressure judgment, by alternate means of measurement, such as a deviation in axle centerline height from the road surface."

-Source Mazda Motors ( Japan ) c2000

 

However, the tighter the curve, the closer the locomotive becomes to the caboose. With that, the effective length of the train gets smaller as the straight line distance between the locomotive and caboose lessens.

While the steel belt and rubber of a tire really doesn't change much, if any, due to air pressure, the effective circumference can change. When the effective circumference changes, the distance traveled per revolution changes.

Take it to an extreme, what happens with the lose of air pressure is that the tread of the tire starts to fold in on itself. With that, the road surface can "jump" over the fold and that "jumping" is what causes the effective circumference to change (reduce).

 

 

I am glad that we can agree that your train analogy has nothing to do with what we are writing about. I got your point, but that point was irrelevant.

You either misunderstood, or are misapplying, what Mazda said. What you quote from Mazda is for a tire in "free space", not in contact with anything, not supporting a load, nor in motion.

 

Hey John,

I am not trying to argue(I am just curious), & although I made a good living as an auto mechanic for years in the old days I have never claimed to be an engineer.

As I stated before, my wife had a 2000 Monte Carlo SS that had low tire warning system(& it did work rather well). It had no sensor of any kind on the wheels or inside the tires etc & according to the owners manual it was relying on tire speed to sense a low pressure condition.

If the tires do not change in outside diameter, how was the system telling when they where low on air???

~John