Originally Posted by
Phantoj
For what it's worth (???) I believe some auto tire pressure monitors work by measuring wheel rotational speed -- an under-inflated tire can be found because it will be spinning a little faster than the others.
That's true. They use the ABS system and its wheelspeed sensors to figure out which one is spinning more quickly than the others. A tire with low pressure gets squashed to the ground further and is effectively "smaller", so it has to spin faster to keep up the same speed as the other three tires.
Other TPS systems use electronic sensors inside the wheels, which transmit their pressure readings by radio to the car computer.
I'm still a little cloudy on the mechanics of why this all works.
Think of the extreme example I alluded to -- a flat car tire. It's still a whole tire, and it looks the right size if you hold it up or lay it on its side on the ground, but when you load it, it deforms a lot.
What people are forgetting is that the radius we're looking at is ALWAYS straight from the hub to the ground. Nothing else matters.
FWIW, on the flat car tire tangent, this is one reason why car manufacturers recommend putting the "donut" compact spare wheel on an undriven axle -- if you flatted a front tire on a front-wheel-drive car, you need to replace that wheel with one of the good ones from the rear and put the donut in back. Having two different circumferences on the driven wheels makes the differential work harder than necessary (among other things). If it's an all-wheel-drive car, the best thing is to put it on a flatbed (or carry a can of fix-a-flat, or use a full-size spare).