Sometimes I get a flat rear tyre and drive a few miles on it without realising. Of course, then it cannot be safely re-used.

So I think that the ABS system could be put to good use:

With some ABS systems, each wheel has a set of teeth that a sensor checks for wheel rotation. When a tooth goes past the sensor it is detected. A rotating wheel sends a stream of pulses to the ECU, and if the wheel slows down faster than is normally possible, then a 'locked' wheel is detected and the braking force is released from that wheel.
Every ABS system that I have seen depends on the pulses coming from the wheel. Perhaps the implementation of the detection is slightly different in some cases, but that isn't important for this idea.

For the flat tyre indication implementation, the pulse rates from each wheel are compared. They should, on average, be about the same rate. Differences will occur when cornering, of course.
But if one wheel is consistently showing a faster rotation, then the radius could be said to be smaller, which would indicate a flat tyre.
Finally it's just a bit of extra software in the ECU.
-- Ling, Nov 12 2004

see [phoenix]'s 28/6/01 anno http://www.halfbake...m/idea/worm_20train
[hippo, Nov 12 2004]

(?) Different TPM technologies www.atmel.com/dyn/r...l_TPM_2003_fsec.pdf
[RayfordSteele, Nov 14 2004]

"Distance traveled is somewhere in between what the free radius and the loaded radius predict." http://eng-tips.com...hread.cfm?qid=94153
Search for "NormPeterson" just shy of half way down the page. [half, Nov 14 2004]

(?) Anyone have a copy handy? http://www.alibris....20pneumatic%20tires
"Mechanics of Pneumatic Tires" [half, Nov 14 2004]

As a general rule, your ABS should be able to tell you if one wheel is consistently grippier than the others - a slightly flat tyre would have slightly more gription (q.v. see link) than other tyres.
-- hippo, Nov 12 2004


Actually, a flat tyre doesn't change the rotation rate of the wheel, unless you're actually running hub-to-tread, in which case it's already too late and you're doing some serious sidewall damage. All that happens is that the contact patch gets larger and the tread distortion is higher as the tire rolls along. You're still pushing just as much tread circumference along at each rotation, so the rotation rate of the tire doesn't change. Once you're running on the rim, however, there's gonna be some serious stress and slipping between the tyre and rim, and you can say bye-bye to the tyre.

Some other method that senses axle forces or suspension response might work, but that would probably be more complicated than a simple pressure transducer.
-- Freefall, Nov 12 2004


[Freefall], interesting point. If that is the case, then changing the pressure in the tyres has no effect on speedometer reading.
Easily tested if anyone has the inclination. I wonder if anyone would be so kind...?
-- Ling, Nov 12 2004


could a sensor detect the distance from wheel arch to ground?
-- po, Nov 12 2004


Yes, but it would blip every time you braked, accelerated or hit a bump.
-- angel, Nov 12 2004


you could ignore it unless you were parked.
-- po, Nov 12 2004


If tires are not stretched in the course of rotation, their circumfrence wouldn't be affected by an increase or decrease in the contect batch. But I would think that a properly-inflated tire would be stretched as a consequence of inflation. Not sure it'd be enough to be very well measurable, but I think it would be somewhat at least.
-- supercat, Nov 12 2004


Modern radial tire treadwalls stretch very little, thanks to the steel belts.
-- Freefall, Nov 12 2004


Would the slip change if the contact area is different?
Anyway, my idea was intended for those times when the tyre deflates reasonably quickly (not a blow out), so I didn't mean those cases where the tyre was slightly under-inflated (where I could spot it before driving).
I want a system that tells me when the tyre is going to be trashed if I don't pull over.
-- Ling, Nov 13 2004


I just tested my bicycle tyre.
It's a 26 inch x 1.95 mountain bike tyre.
Inflated at normal pressure, one rotation was 80 inches.
After letting some air out (nearly flat, but not on the rim), one rotation was 75.25 inches.
Both were done with someone on the bicycle. It's easy to do if the valve is used as a reference.
The difference was much more than I would expect from measurement error, therefore I am reasonably confident that a car wheel would behave as I expected. Although I cannot explain exactly why it happens.
-- Ling, Nov 14 2004


That works out to about a 3/4" lesser radius, which seems reasonable.
-- FarmerJohn, Nov 14 2004


Active tire pressure monitoring sensors are baked on incoming 2005 models of Jeeps. Not certain of the type of technology used.
-- RayfordSteele, Nov 14 2004


[tahuyahick], thanks for your links. They show that the idea is correct and baked. Why there is discussion about safety of either direct or indirect systems, I cannot understand. Surely it is better than having no indication?
Why isn't it widely available? If the ABS indicator on the dash is used, then no extra hardware is required.
One of the links suggests that the indirect (via ABS system) cannot detect which tyre has a fault. I don't agree with that, but who cares anyway - it's enough to know that *one* of them is flat.
-- Ling, Nov 15 2004



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