"The Natural Philosopher" wrote in message
news
On 25/06/18 12:04, Andy Burns wrote:
Tim Streater wrote:

One revolution of the wheel has to equal one passage of the tire's
circumference along the road (since the tire does not slip around the
rim and assuming no slip on the road). How could it be otherwise?

Talk of "effective radius" is not relevant.

So are saying the circumference remains constant, hence the rpm remains
constant, hence iTPMS cannot work?


No, we are saying that te circumference remains *nearly* constant, and the
difference is due to belt shrinkahe under lower pressure NOT 'reduction in
radius' and is of the order of less than a percent for up to 10% reduction
in 'radius'

Has to be so or flat tyres would give *way* out speedo readings

I presume when you say "flat" you mean a tyre that is below the rated
pressure but still maintains roughly the same profile. I tend to think of
that as "low", and "flat" to mean a tyre that has bugger-all air in it, so
the wheel rim is rubbing on the inside of the tread surface, doing untold
damage to the tyre.

Having driven on a low tyre (to get me to a garage that I knew was only half
a mile away, to save the hassle of changing the wheel in heavy traffic) I
have no idea by how much the speedo may or may not have been overreading.
Short of checking it with a GPS, it's very difficult to tell.

I have once driven on a totally knackered tyre for about a hundred yards,
when a car pulled out and I had to swerve to avoid it, hitting the kerb
harder than I would have liked. The sidewall burst. As it was immediately
before a roundabout, with no way of traffic getting me round me, I decided
that since the tyre was buggered, I couldn't do any more damage. Once I'd
stopped safely beyond the roundabout, my fun really began because I
discovered that the wire cage which held the spare tyre had seized up: there
was a long bolt that went through the floor of the boot into a nut on the
cage: and that was rusted up. I actually had to call out the RAC for help
with changing a wheel, simply to get at the spare. The pillocks who made
that car had out a very broad semi-cylindrical nick in the head of the bolt
which you were supposed to turn with the flattened end of the wheelbrace -
utterly useless for getting any purchase if the bolt had seized. If only the
bolt head had been a proper hexagon the same size as the wheelnuts, it would
have been trivial to undo it.