Does anyone know SPECIFICALLY how the tire pressure monitor works.

Gary in Sandy Eggo

Hello,
I started this post thinking I had the answer...I had a whole paragraph typed up. Then I realized, "that doesn't make sense" I always figured it worked by noticing differences in tire rotation...when the tire gets low on air, the diameter changes..the sensors pick up the difference in rotation. But, after pondering a bit, I said..wait, the circumference doesn't change, so the rotation should remain the same. So...This is a great question. Hope someone has an answer. This whole BB is getting really slow.
Later now,

I'm pretty sure you were on the right path but convinced yourself otherwise. Yes, the diameter will change. Circumference is 4*pi*D. So the circumference will change. So, for every 10 feet of travel, the wheel with lower tire pressure will rotate more than the wheel with higher pressure.

The ABS system measures wheel speed, and will notice this difference, and show that you have low pressure.

This does not work when all 4 wheels have low pressure, because they would be rotating at the same speed.

Your formula proves itself, but, the formula is for a true circle. As the tire deflates, the circle becomes more "untrue".
If you put a mesuring device on the tire fully inflated, and the tire w/ only 10lbs in it, won't the circumference still be the same?

Good point. I really don't know how to explain it now.

I have read articles about NHTSA making some tire pressure measurement system standard (after the Ford/Firestone debacle) and the two systems that have been proposed are: 1) pressure transducers that transmit pressure via radio signal and 2) use of ABS wheel speed sensors

The car companies like 2) because it requires the least amount of new hardware (cheaper) but it won't work in situations I described above, where all 4 tires are deflated the same amount.

Like I said, I can't explain it, but apparently it works, somehow.

As the tire deflates, the distance from the center of the wheel to the road surface(the actual radius of the circle) is reduced, thus creating a new "true" circle. The tire will be(and always is) a different shape when weight is applied to it, but that is immaterial, so the statement in the earlier post is correct. The ABS measures wheel speed and the tire with lower pressure will have a greater wheel speed than the other one. There is a range of allowable difference so that slight variations wouldn't keep flashing the warning. From what I have experienced, the tire has to be mighty low to trigger this warning.

Doug
'99 Black GP GTP Coupe

Hello,
I still have a hard time buying this.....Unless the tire actually expands when properly inflated, therefore changing the circumference, I don't understand.
The radius definately changes when the tire deflates, but, the circle becomes "untrue" at that point.
The ground covered is still the same, unless the tire actually expands when properly inflated.
There is a larger portion of the circumference making contact when the tire loses air, but still, the same amount of contact (per revolution) when the tire is full of air.
Jeese....I'm just a Plumber, don't hate me for thinking out loud....
Later now,
Clint

I understand the confusion. Please do not be insulted if I have oversimplified the following explanation. I wish I could draw a picture, but try this, forget the tire is there at all. The car is suspended from a rope at that corner. Assume the road is a flat piece of glass. With the corner at a height level with the other three corners, the center of that hub is an exact, measurable, unchanging distance from the surface of the road below it right? That is the radius of our circle. Lets say for the sake of argument that it is exactly 12" from the center of the axle to the surface of the road. Now, cut a piece of carboard in a perfect circle with a radius of 12". It will be 24" wide--it's diameter. If you mounted this cardboard circle on the hub, it would just touch the surface of our glass road. Now lay that aside. Lower that corner of the car by 6". Now our measurable distance is 6" to the surface of the road--our radius. Construct another cardboard circle with this radius and mount it on the hub. It too will just touch our glass road. Set it aside. Now we have a 24" diameter cardboard disk and a 12" diameter cardboard disk. Now just for the hell of it raise the car back to it's original 12" from the road then raise it another 6" to be 18" from the surface of the road. Go through the same process of creating a carboard disk, this time 36" in diameter. Now we have 3 disks of three different diameters. Now put your wheel back on so you don't forget and drive off without it )
Now lets make a jig with a straight dowel serving as an axle. Place all three disks on this axle and pin them together. Notice we have quite a difference in circumfrense in our three disks. Mark off an arc of twelve inches on the largest and smallest circles. This represents the car moving one foot. With the size difference of those two disks it it easy to see that the larger circumfrence disk with turn the assembly less to cover that 12", then the smaller disk. Another example, if we rolled each of the two disks one mile seperately, the smaller disk would make many more revolutions than the large one. This equates to revolutions per mile. If you travel the mile at the same speed both times then the wheel with the greater revolutions per mile will also have a greater wheel speed. This is what the ABS system measures. Now this all equates to tires because as air is added, the car is lifted higher off the road(to a point) just like our example suspended by a rope. As more air is let out, the distance to the road decreases and our wheel begins to spin faster to cover the same distance as the other tire on that end thus creating a measurable difference in wheel speed between the two.
Damn that was long winded!

Hope it makes some sense! Been out sniffing carb cleaner and I may be just a bit addled. )

Doug
'99 Black GTP Coupe

Hello,
Maybe we should move this to the "Anything goes" section....heheh.
I'm totally with you on the cardboard cutouts. I've always understood the fact that a smaller diameter is going to turn more RPM's to make the same ground.
The difference between the cardboard and a tire is the fact that the card board is a solid circle, the circumference clearly changes when you change the diameter. When changing the diameter, (or radius if you want,) you are adding or taking away material from the true circle. Therefore changing the Diameter, and the Circumference. With a tire, the tread remains the same, leaving the circumference the same as when it's full of air. Now, I can understand how changing one tire will be noticed by the ABS system, because of the other tires having lost tread, (material missing) the circumference is different.
Let's say a tire has a circumference of 24". with 5lbs, or 35lbs, the circumference remains constant. While the radius changes, it shouldn't have an affect on the revolutions, seeing how the circumference remains the same, so should the RPM's. With something like cardboard, when you change the radius, you change the circumference, therefore, changing the RPM's.
Man...I haven't typed this much since WebTV.
Later now,
Clint