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

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?
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Murmansk wrote:

That's one type, a battery, sensor and transmitter in a lump inside the tyre, e.g.
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https://www.eurofrance.pl/userdata/gfx/0f53520cb6b93802b9dd618dcafedc1b.jpg

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.
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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.
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Tim Streater wrote:

I've had about a dozen warnings from TMPS (the ABS type) over the last 7+ years, not one of them false.
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Thass yer own fault for driving in tight circles in front of yer gf's house smoking the tires.
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On 21/06/2018 21:51, Andy Burns wrote:

How flat were the tyres? How much pressure had they lost? 12 flat tyres in 7+ years is much more than I would expect.
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On Sat, 23 Jun 2018 00:54:54 +0100, Michael Chare wrote:

I had a spate of flattening tyres a few years ago. Turned out to be peeling varnish on the allow wheels, spoiling the seal.
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On 23/06/2018 01:03, Bob Eager wrote:

I do wonder about the benefits of alloy wheels. There was a recent thread about the tyres on them sometimes not staying inflated and they are quite easy to damage.
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harry brought next idea :

Actually alloy does neither. The weight is about the same and alloy is sometimes even heavier than steel. The use of alloy is more about design and fashion.
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On 24/06/2018 07:05, Harry Bloomfield wrote:

I agree although I think the original alloy wheels 50-60 years ago on rally cars were to reduce the unsprung weight. Also it does appear that reducing the unsprung mass does increase cabin noise and vibration which is not what you want on a normal road car.
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On 24/06/18 11:35, Michael Chare wrote:

I agree - alloys are a pointless gimmick. Nickable so need locking nuts, easily damaged by kerb grinding, expensive.
Why they couldn't just get the "look" by careful design of a full sized "hub cap" beats me.
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On Sunday, June 24, 2018 at 5:02:56 PM UTC+1, Tim Watts wrote:

h

I used to have a Rover 416 which had alloy-look wheel trims. One went missi ng and I went to the dealer's for a replacement. "What type is it?" asked t he chap behind the spares counter. "One like those", I said, pointing out t he door to my car, which was parked so that the two fully-shod wheels were visible. "But those are alloys" he insisted, and wouldn't change his opinio n until he'd gone outside to see the bare steel wheel.
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I had a Stag with "full sized hub caps". They looked OK but vibrated and thus generated unwanted noise.
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On 24/06/2018 06:19, harry wrote:

The unsprung mass of a fat tyre is only the mass of the bit of tyre on the road - at least for small bumps. Modern skinny tyres have almost no give, so the suspension has to do all the work. And the result is a harsher ride, poorer handling, and vulnerability to potholes.
Some think they look better. Which overrides the other three.
Andy
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Doesn’t need to be flat, just under inflated. I've had lots of those where when checking the tyre pressure at the service station does see some more air needed in a particular tyre.
And my Getz is a real bugger with a true flat tyre, very hard to even notice its gone flat and that means most of the flats result in a buggered tyre since you drive on it flat long enough to fuck the tyre so that fixing the leak wont work.
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Michael Chare wrote:

Varied ...
Several (yellow warning) times when tyres had just gradually lost pressure without a puncture, maybe 43 down to 38psi, which would be detected at about the same level as when I started to notice going over speed humps was a bit softer than usual.
Several normal slow punctures from nails/screws (also yellow warning)
Once (red warning) where it picked up a fast puncture on the motorway before I had noticed it, and by the time I had come to rest on the hard shoulder the tyre was completely flat.

Overall I think 5 were punctures (all except one were repairable) two were from kerbs, the rest were just reminders to top-up pressures.
Several of the punctures occurred in the first year of buying the car, I was half convinced there were nail-magnets fitted.
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On 23/06/2018 06:38, Andy Burns wrote:

I thought I was prone to getting punctures but not as bad as that. I had one wheel which vibrated a bit so I fitted it to the rear. Even then I noticed a vibration at about 75mph. Eventually I had to stop for a flat tyre and found that the tyre had developed a bulge say a couple of inches in diameter which had then worn completely through. I wonder if the pressure sensors in the tyres of my new car would have noticed that before the tyre became completely flat.
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On 21/06/2018 20:54, Murmansk wrote:

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.
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On 21/06/18 21:02, alan_m wrote:

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