"T i m" wrote in message
news
On Fri, 29 Jun 2018 06:48:50 +1000, "Jeff" wrote:



"Andy Burns" wrote in message
...
The Natural Philosopher wrote:

we don't disagree that the rate changes. Only by how much

As there was nothing on telly tonight, I used my OBD cable to log the
four
wheel speed sensors over a straight section of road with cruise control
set to 50mph, which was measured by VCDS as 77.6kph then let some
pressure
out of the right rear, turned around, rinse and repeat until the TPMS
triggered.

Starting at 39psi the right rear was on average 0.1kph slower than the
other three wheels, it triggered after letting it down to 31psi by which
time it was on average 0.2kph faster than the other three wheels.

So it triggered on a increased wheel speed of 0.3/77.6 = 0.4%

and why.

The numbers don't help with that.

They do in the sense that presumably the distance from the axle to the
road
dropped
by rather more than the 0.4% that the ABS sensors recorded, so the effect
must
indeed be due to the circumference changing, not the dynamic rolling
radius.

But the two are interlinked?

Can't be by much given that you'd expect the axle to road distance to have
dropped by more than 0.4%, although he didn't actually measure that.

39psi is quite high so it may be a pretty big solid wheel and tyre, so
it would be interesting to have had that distance measured.

If you accept the circumference changes between the tyre carrying no
load, the tyre carrying load and inflated correctly and the tyre
running at a lower pressure, then in the first case there is a
conventional radius and in the last two, because the tyre is no longer
circular and because it shrinks under the footprint, we also have a
rolling radius.

What we can't say with any certainty is the actual distance between
the axle and the road is the same as the rolling radius (because by
definition the centre of the axle will be low of centre of the
effective circumference, simply because of how tyres work.

With iTPMS I believe some manufacturers shun (or shunned) it because
it is much more critical in it's calibration than the direct reading
TPMS.

A new tyre, tyre rotation, a managed change in pressure generally
require a 'reset' or the system could become unreliable. This is
considered 'too interactive by some manufacturers, having to rely on
human input to keep the system reliable.

But it should be possible to reset it automatically when it is clear
that a wheel has been changed, say because it had to be to pass the
MOT or the tyre pressure had been changed at the service station

This would confirm the concept of iTPMS working with very small
differences ... the sort you get when you change a tyre or get a
puncture. ;-)

A slow leak too.