Tpms

[Clan]

Originally Posted by gaby

Possibly, but by then you may have a problem.
What I expect from a decent TPMS system is to inform me before a problem is actually there,
giving me more time to anticipate and prevent from having to stop the car at that moment.

That is exactly what it does , unless you have a fast puncture of course , no one can do anything about that .. It gives you plenty of time to deal with any slow loss of pressure .

[MidnightT4]

What system do they use.

I had a puncture repaired and when i seen inside the wheel there wasnt a sensor just standard rubber valve.

I presume mine works on rolling resistance of each well as the tpms system doesnt display actual pressure

[Clan]

Originally Posted by MidnightT4

What system do they use.

I had a puncture repaired and when i seen inside the wheel there wasnt a sensor just standard rubber valve.

I presume mine works on rolling resistance of each well as the tpms system doesnt display actual pressure

It uses the ABS sensors , there is one on each wheel and gets 48 pulses per wheel rev so resolution is very fine . It compares the 4 sets of signals counting them and comparing them to the "normal" readings which are remembered when you calibrated the system.. as the tyre pressure falls the pulses per rev drop for that particular wheel and vice versa .

[MidnightT4]

Makes it easier to get some different wheels then.

[Wizzpop]

Originally Posted by Clan

as the tyre pressure falls the pulses per rev drop for that particular wheel and vice versa .

Hmm, as the pressure drops the rolling radius reduces and the revolutions rise. So surely the pulses per rev rise.

[Fursty Ferret]

It's more complicated than that. The ABS sensors don't count wheel revolutions (if they did, it would be a very coarse system), they directly measures wheel speed. Because the tyre doesn't deform instantly, it vibrates slightly and the resonant frequency of this vibration varies with tyre pressure.

This is how the system can detect a flat tyre almost instantly, and why it can detect four simultaneously under-inflated tyres.

[Clan]

Originally Posted by Fursty Ferret

It's more complicated than that. The ABS sensors don't count wheel revolutions (if they did, it would be a very coarse system), they directly measures wheel speed. Because the tyre doesn't deform instantly, it vibrates slightly and the resonant frequency of this vibration varies with tyre pressure.

This is how the system can detect a flat tyre almost instantly, and why it can detect four simultaneously under-inflated tyres.

Of course they count revolutions, divided into steps of 48 pulses ... hence why i said it had a high resolution ... the car already knows the speed from the average of all 4 readings ...

As well as the number of pulses coming from each wheel The iTPMS system knows:

The correct number of pulses to expect when all the tyre pressures are correct from the information it gets when you calibrate the system .
The vehicle speed .
The position of the suspension arms .
Information on what the gearbox is doing.
The torque the engine is producing .
The steering wheel angle .
Whether a trailer is attached .
The ambient temperature .


So yes you could say it is rather more complex than just knowing how many revs per mile the wheels are doing :-)

[Clan]

Originally Posted by Wizzpop

Hmm, as the pressure drops the rolling radius reduces and the revolutions rise. So surely the pulses per rev rise.

yes of course

[gaby]

Originally Posted by Clan

That is exactly what it does , ...

I was aware of that.

[airforced]

Irrespective of how it works, it is a cheap system and far inferior to the system which has an individual sensor in each wheel. OK they work out more expensive for the manufacturer and owner but at least the owner knows which tyre is misbehaving rather than having to check all four corners.

For vehicles that purport to be 'luxury' they do not, in this respect, compare favourably with the likes of Mercedes. Even Citroen fitted proper sensors to their XM's all those years ago. Come on Volvo do the job properly.