On Mon, 25 Jun 2018 10:52:30 +0100, "NY" wrote:

"T i m" wrote in message
news
This means that it must be accurately measurable so when a 32 psi tyre
drops it's pressure to 24 psi (23?), it should raise the alarm.

I've found that it triggers the alarm at a much smaller reduction than 25%.
Our Honda developed a very slow puncture and when I checked the tyres, one
was about 0.2 bar (3 psi) lower than it should have been.

I was just suggesting what I understood to be the maximum pressure
drop threshold before such a system *must* indicate. It's good to see
RW systems are much better than that. ;-)

The instructions for calibrating cycle speedos require you to measure
the *loaded* rolling circumference of the wheel bearing the speed
sensor (when it would be easier just to put a tape round the wheel).

How much does the radius of a bike tyre (at the point of contact) decrease
when you sit on the bike.

I think that depends on the type of bike / tyre to some degree
(balloon / racing skin etc).

It's difficult to tell when I'm the one sitting on
the bike so I don't get a side-on view.

Video from your phone?

I think my speedo actually
recommends measuring the (unloaded) circumference by marking a point on the
tyre that is in contact with the ground and rolling the wheel along the
ground until the point is next in contact.

Agreed.

But I agree that *if possible*
you should try to measure the radius under load and *assume* that the whole
tyre is that radius.

You can do it yourself under load. Put the valve at the bottom and pus
something on the ground to align with the valve (screwdriver, tape and
pen mark etc). Then sit on the bike and walk it in a straight line
whilst keeping as much weight off your feet as possible. Stop when you
see the valve near the bottom, get off, adjust the wheel more
accurately if required (by moving the bike forward or backward
slightly) and make the second mark. Measure between said marks.

I've heard is suggested that there is *significant* error between a brand
new tyre and one with a worn tread, though I'd have thought that it was
negligible.

An extreme of that was replacing non-full profile 12" wheels on our
Escort based kitcar with full (80) profile tyres on 14" wheels. I
believe the rolling circumference increased by 30%.

https://www.halfordsautocentres.com/...pth-and-safety

says that a new tyre has about 8 mm tread. If you use it until the tread is
2 mm, and assuming the same pressure in both cases, then the radius has
reduced by 6 mm in a total radius of 635 (for my car's 215/65/15 tyres) so
about 1%. I wonder how much the effective radius varies for an
under-inflated tyre, assuming the trigger level for a sensor is 25% loss of
pressure.

(25% *maximum* pre triggering) ... On these iTPMS's I thought I heard
mention of 'calibrating' the system to what would be considered
'normal' (tyre pressure, tyre size, wear status) and so the alarm
would be a function of that? I know the prescribed pressures in the
rears (particularly) of our vehicles can vary quite a bit because of
load / speed etc.

Any sensor has to be able to distinguish between expected change in radius
due to tyre wear and unexpected due to loss of pressure.

Sure, but if it only triggers at say a 10% variation and tyre wear
would only ever cause a 1% variation you should be good to go. ;-)

Cheers, T i m