2 Qs about tyre pressure

I had a friend who had a very old Rover that unfortunately had corroded alloys. His tyres continuously lost air, and required regular top-ups.

Two methods, either directly measured using wireless pressure sensors attached to the hubs, or (mine) the ABS supplies wheel speed information that enables the radius to be calculated - not an absolute figure but a warning if it changes during a period.

There are (at least) two types, one that has an actual pressure sensor as as part of the valve, which transmits via radio to the car, e.g.

And the other type measures wheel speed differences using the ABS sensors while cornering, as a proxy for an underinflated tyre.

By the time my Audi reached 5 years old one of the alloys corroded where the tyre bead fits and leaked all the time. I thought I had a slow puncture but when the local tyre/exhaust place investigated they found corrosion. They said they see it all the time and it took about 45mins for them to remove the corrosion with a powered wire brush and repaint the inside and remount the tyre. It was perfect after that. By the time I got rid of the car when it was 8.5 years old I'd needed the corrosion fixing on the 3 of the wheels. ISTR it was £22 per rim to get if sorted including rebalancing the wheel.

OP here: thanks Adrian, Andy and mm0fmf - very useful replies.

Part of Adrian's answer (echoed in Andy's) was:

Good god. 8-O

Thanks again! John

Clever stuff! Mine did once get triggered by a period slithering my caravan off a muddy field, but recovered its composure after a few miles on tarmac.

Chris

Er no, it enables the *circumference* to be calculated relative to the circumferences of the other tyres.

"Radius" is not a concept that applies to a non-circular shape. And tyres are never circular.

So called 'rolling radius' is simply the circumference divided by two pi, and is a total misnomer....

Tyres can loose pressure due to a faulty valve. I had to change the valve in a small tyre that was loosing pressure over a few months. I found the replacement valve outside a tyre place.

If not a new car, most likely reason is a poor seal at the rim. Caused by corrosion. Only real long lasting cure is to have the wheel cleaned and re-painted by a specialist. A tyre place will likely use a sealer which last for a short time.

Different temperature? Air loss while checking the pressure? Inaccurate test gauge?

What is "loose pressure", and how might it differ from "tight pressure"? Or do you mean that the valve was loose?

Damaged risn or leaky valves are the most common cause of pressure loss, followed by slow punctures. Actual loss from a well sealed tyre on a good rim is negligible

Just another victim of Blairs education policy 'education is too good for state school plebs, so why bother giving it to them: Just give them a degree instead'

Hmm?

I?ve just cut a deal to swap my 15 year old steel wheels for 15 year old Alloys. Now wondering if it?s such a good idea!

Hmm. I wonder if that's the cause of our slow puncture on one wheel, which happens even after a change of tyre (eg when the treads wear to the marks). It probably takes about 1-2 months after correcting the pressure and resetting the tyre-pressure sensor for the sensor to be triggered again. The pressure is typically 0.2 bar down (2.4 down to 2.2 bar). Garages have looked and have pronounced that the tyre is OK, but I wonder if they check for a leak at the bead. It is such a slow puncture/leak that I just keep pumping it up either when the sensor is triggered or when I remember.

I might get the garage to check next time the car is in for a service, since they do tyre replacements as well so they have the wheel-balancing kit that dedicated tyre garages do.

I had a slow leak on one of my 22-year old alloys about 4 years ago. That was also fixed by the tyre fitters cleaning up some corrosion and applying lots of "gunge". The other three wheels have been fine. I hardly ever clean the wheels and then only with a hose. Maybe that helps!

John

I had a car about 4 years old (had it from new). Had one flat tyre, then another, then another...

I thought someone was letting them down, but it turned out not to be corrosion, but flaking, rather thick varnish on the alloys.

Ended up getting the wheels cleaned and reground, one at a time.

Get the rims cleaned and sealed.

Bill