the-oneil

Thanks Alwyn Mike
I think there is a trade off here. The ABS sensors are indeed simple and has the advantage of no extra sensors to buy, replace or have seperate batteries for BUT it will warn you a bit late - don't you think? for the tire to lose circa 30% of its pressure before you are warned.

Any driver worth his salt would notice the misalignment before he gets the warning (for a deflated front tire). I would rather be warned as soon as the air pressure starts going down.

aybeesea

It can't be pressure dependent because different wheels have different profiles and run at different pressures and will reduce in rolling diameter differently for different tyre architectures.

The key factor is rolling diameter and as schoolboy physicists know, for circular motion, angular velocity (speed of rotation) is inversely proportional to radius of rotation (wheel diameter) for a given linear velocity (road speed).

The system measures changes in rotational speed of one wheel in comparison all the others others and when the threshold is reached (not a pressure threshold but a diameter threshold), the indication will be given on the iDrive screen.

ABC

the-oneil

My point exactly, and since the change in radius of rotation (wheel diameter) has to be significant enough to be sensed, the presure has to drop significantly before this change could be measured.

aybeesea

What is directly sensed is the change in angular velocity. We don't know the angular velocity change threshold and any preceding/causal percentage pressure change would vary from tyre type to tyre type.

ABC

brandsb

The system measures changes in rotational speed of one wheel in comparison all the others others and when the threshold is reached (not a pressure threshold but a diameter threshold), the indication will be given on the iDrive screen.

ABC
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Isn't this the same technology for the U.S. e.g. no sensors in the wheels? I'm sure this has been discussed extensively in the past but the reference to lack of sensors in EU E60 wheels raises the question. No special valve stems, etc. correct?

aybeesea

Yes and no.

MY 2007 US models have TPMs in each wheel in response to US legislation. Prior to those models, the rotational technology was used.

ABC

the-oneil

Tyre failures are usually preceded by long periods of running at lower-than-acceptable pressures - TPMS must be able to warn you about that. When the tyre pressure is low, the sidewall flexes a lot more, generating more heat - If sensors are installed in the rims, TPMS can tell you about that too.

With sensors in the rims transmitting data as soon as your vehicle starts moving. and every 6/7 seconds afterwards. If there is a change by more than 3 PSI/.21 BAR an alert is issued. Moreover, Rim sensors sense tyre temperature and this data is also normally transmitted alongside tyre pressure.

I feel the ABS sensor technoloy is out of date, but cheap & convenient. Personaly, I would have wanted to be warned ealy enough of any loss of pressure or increase in temperature as I maybe able to stop - call a roadside assistance and MAYBE able to save my tyre for repair. I would not feel comfortable repairing a RFT after driving for many miles with it deflated.

aybeesea

There's no doubt that TPMs wouldn't exist if they didn't do the job better than a preceding solution but how much better is down to marketing since there are no qualitative comparisons I can find.

Both systems work, but as you say, it's all down to cost and convenience. More expensive TPMs and arguably be able to repair a tyre or cheaper sensors and arguably have to buy a new tyre. Swings and roundabouts.

ABC

brandsb

Thanks for clearing up the confusion....