O.T. tyre pressure sensors - how do they work?

[Murmansk]

I understand some new cars can tell you when their tyres need inflating?

How do they work? Is there some kind of pressure sensor inside the tyre that communicates wirelessly with the car's systems?

[Andy Burns]

That's one type, a battery, sensor and transmitter in a lump inside the tyre, e.g.

The other type have no actual pressure sensor at all, but monitor the relative speeds of the wheels from the ABS sensors, and spot slow (or rapid) deflation based on that.

[alan_m]

On mine I believe it uses the ABS sensors to determine if one wheel (presumably over a period of time) is rotating faster than the others. A deflated tyre will be smaller in circumference.

[GB]

You could actually use google, you know ...

Anyway, there are two systems AFAIK. (I'd know more if I'd googled it, but if you couldn't be bothered ...)

System 1: Uses a pressure sensor.

System 2: Senses if one of the wheels is going round more often than the others => that wheel is flatter than the others. But if they are all going flat together, that doesn't work. Also, you'll get a false reading if you keep doing left turns and never turning right, like on the Indie 500.

[Tim Streater]

Last winter on a couple of frosty mornings the pressure sensor came on, went off later in the day. I wondered what it was at first.

[Andy Burns]

I've had about a dozen warnings from TMPS (the ABS type) over the last

7+ years, not one of them false.

[Tim Streater]

Thass yer own fault for driving in tight circles in front of yer gf's house smoking the tires.

[The Natural Philosopher]

Radius perhaps, but circumference - not much. The steel belts in the tread take care of that.

[FMurtz]

Some use bluetooth from sensors associated or near the filler nipple

[Brian Gaff]

I don't think its wireless. Brian

[Tim Watts]

2 ways:

1) Relative rotation - very hit and miss and tells you if one's heading towards being a flatty but not much more;

2) There is a pressure sensor built into the valve in each tyre - long life battery and radio transmission to one or more receivers.

[alan_m]

I assume that the tyre pressure check in the new MOT is actually checking that the system is not reporting an error because, say, the battery in the sensor has gone flat.

[Chris Green]

Does it *really* work like that? Tyres have steel or nylon bracing in them to *stop* them stretching much. If you run on a flat tyre it squishes somewhat but, unless it's actually skidding on the road surface, the same circumferance will roll along the road surface for each rotation.

The difference in revs/metre between a tyre at (say) 2.5bar and one at (say) 2.2bar must be really tiny.

Our tyre pressure warning goes off immediately you turn the ignition on whichsuggest to me some sort of pressure sensor in the rim or valve.

[Tim Watts]

I've had the light come on on the missus' Mini so it does work.

[Andy Burns]

Some absolutely do work that way, search "indirect TPMS"

Sounds like yours is the "direct TPMS" type with pressure sensors.

[Vir Campestris]

Some of that circumference must rotate while it's off the ground.

The _radius_ at the bit on the ground is reduced, so it must spin the tyre faster.

Andy

[Andy Burns]

Apparently 2nd generation indirect TPMS can use spectrum analysis instead of simple rotation speed

[Huge]

Does this look like a front end to Google to you?

[T i m]

It's not the circumference that determines the rotational speed but the 'rolling radius. If you had a solid wheel (nearer to a cart or train wheel) then the two would be pretty close but there would still be some slight compression (in the spokes / rim / tyre of a cart wheel and in the 'tyre' on a train wheel).

If you imagine the diameter of the tyre getting smaller as the tyre is getting compressed at the bottom then the effective circumference would change accordingly.

Yes, it probably is, but it might still be enough to be 'sensed' compared with the other wheel(s).

That sounds most likely.

I believe the most common is a unit that is part of the valve that contains a pressure sensor, battery and wireless transmitter and I don't think you can replace the battery.

I always wondered why it couldn't have a rechargeable battery with some sort of inertial charger but I guess that would make it too heavy / complicated.

Cheers, T i m

[Michael Chare]

How flat were the tyres? How much pressure had they lost? 12 flat tyres in 7+ years is much more than I would expect.