On Sat, 9 Nov 2019 20:15:13 -0500, Frank Krygowski
wrote:

On 11/9/2019 7:28 PM, John B. wrote:
On Sat, 9 Nov 2019 18:52:04 -0500, Frank Krygowski
wrote:

On 11/9/2019 6:20 PM, John B. wrote:

I suppose that I am a cynic but I can see no use at all for tire
pressure monitors, or for back up radar for that matter. Oh well, I
suppose it eliminates the need to turn one's head when walking by the
car, or backing up. Or maybe one might say, "to pay attention to what
one is going" :-)

My experience with the TPM system:

Again, mine monitors wheel rotation speed, which makes more sense to me.
The system is there for anti-lock brakes anyway, so why not use it?

Anyway, the Mazda 3 seems to have a problem with rear disc brakes. Front
were fine but rear were worn at maybe 40,000 miles. I replaced them,
after a little trouble getting pads. (Two brake sources, different pads,
mine the less common by far.)

The replacement pads seemed slightly tight going in. After some short
time, I got a "Tire Pressure" alert while driving, but pressures looked
fine. However, right rear disc was HOT. The pads were dragging enough to
slow that wheel and trigger a warning. I pulled the pads, ground the
edges that contacted the caliper, lubricated with anti-seize and all was
good. I later repeated for the other rear wheel.

Something seems funny here. The brake dragged so the wheel slowed
down? Was the tire skidding along the ground?

Not skidding in a noticeable way, certainly. The event happened at night
on dark country roads, so when I got the alert, I didn't stop. Instead I
felt for odd behavior when turning or when I swerved a bit as a test.
(In the past, I've been able to detect a low tire that way on other cars.)

I felt no difference, which I took to mean the low pressure was minor.
(I don't know the threshold for the warning light.) But when I came to a
business with a yard light I stopped to check it out. The problem was
intermittent, because the rotor was cool 15 miles later at home.

Tires actually "creep" a bit against a road surface, from the deflection
of the rubber as it applies force to the road. I think the braking
torque from the stuck pad generated enough creep to show up as a slower
rotation.

After doing a little reading I believe that the tire rotation speed is
the controlling factor. I read that the tire pressure system warns
when the tire pressure drops in pressure 25%, although this might well
vary from make to make or model to model and 25% of a tire's pressure
is a relatively significant value.From 30 psi to 22 psi.

Firstly a fully inflated has a larger diameter than a partially
inflated dire due to stretching or distorting of the tire, and
secondly a partially inflated tire will have a smaller effective
diameter as the tire will flatten more as it meets the ground, and
differences in diameter will, of course, effect the rpm of the tire.


The details of the system might interest Jeff and others. If you think
about it, differences in rotational speed must be ignored when driving
long sweeping curves. Maybe it looks for one out of three differences,
or maybe it looks at differences that remain over a certain window of time.

BTW, the system must be reset when tires are rotated, as I did two weeks
ago. To me, that says it's pretty sensitive to the differences in
rotational speed. Tire diameters aren't widely different when moderately
worn.
--

Cheers,

John B.