Does a tyre change its CIRCUMFERENCE when underinflated?

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Answer. not by very much, if at all.

This seems to be a perpetual urban myth.

It tyre pressure sensors are using this, it has to be a very very complicated bit of software to detect - say - less than 1% change in RPM relative to the other wheels.

Reply to
The Natural Philosopher
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But the radius clearly does vary significantly.

It can't be given that Andy and others have had a number of warnings that have turned out to be accurate every time.

Yes, but that isn't hard to measure when its relative to other wheels.

Reply to
Jeff

Does it? The perimeter certainly changes shape from being near-circular to having a significant flat side against the ground, but if the radius changed significantly, that would also mean a circumference change, and the read-outs from the various gauges such as speedometer and odometer would no longer be accurate. There are also steel reinforcing belts beneath the tread that would have to stretch if the circumference increased.

Reply to
Chris Hogg

I wasn't impressed by a bloke pushing a van who shows no signs of understanding the meaning of "resolution". OTOH put "tyre rolling radius pressure" in a well-known search engine[1] and the first hit is a paper from University Politechnica of Bucharest giving the variation in rolling radius for a range of pressures and speeds on a dynamometer[2].

Includes:

"When the inflation pressure is changed by ± 30 % with respect to the nominal value, the increase of dynamic rolling radius has relative values ranging between 0.88 % and 0.95 %, therefore close to 1 %. It can be asserted that, for all the considered values of rolling speed, the inflation pressure has an important influence on the tyre dynamic rolling radius."

[1] I looked for it after reading the thread in u.r.c.maintenance about pressure monitors, not in response to your "fake myth" [2] G Anghelache and R Moisescu 2017 IOP Conf. Ser.: Mater. Sci. Eng. 252 012014

iopscience.iop.org/article/10.1088/1757-899X/252/1/012014/pdf

Reply to
Robin

looked to be half the width of a piece of insulating tape per revolution, what was even the point of only measuring the rolling distance once?

And iTPMS systems *are* using it

Little extra complication within the ABS unit, how many pulses per revolution does it process from an ABS reluctor ring? 40 to 60 "teeth" seems common. If the ABS unit can detect wheel slowing (i.e. a skid starting) then it can detect a wheel speeding up (e.g. from a puncture)

Nobody is suggesting it decides if it's a puncture within a single revolution, it detects over a period of time, 1% isn't that tiny.

I noticed the actual TPMS trigger often tended to be when you put your foot down going round a corner/roundabout.

Reply to
Andy Burns

A piece of online research

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showed that relative wheel rotation was pretty crap at detecting low tyre pressures especially in all 4 wheels or 2 wheels on the same side (= I am going round and round in circles!)

Hence the move to in wheel sensors.

I am notr syoing it doesn't work, just that it relies on some pretty iffy interpreation of very small differences in road speed.

Tyre tread belts do strech under inflation, but not by much. The radius is completely irrelevant as a road weheel is not, in use, round and does not have a 'radius.'

Reply to
The Natural Philosopher

Well that is what I said.

Around 1% *at best* circumference change, Though why they have to call it a 'dynamic rolling radius' is beyond me. There is no such thing as a constant radius on a tyre. It isn't round.

1% will be totally swamped by wheelspin, slip under traction or bråking or cornering itself.

If you are lucky in a straight bit of road it might spot a wheel 30% underinflated. Not two though.

But I can tell if a wheel is 10% underinflated by the way the car handles

In fact your data supports my position. And the video. There is bugger all (

Reply to
The Natural Philosopher

It clearly does with the distance between the axle and the ground.

The perimeter certainly changes shape from being

No it does not. What happens is that the distance between the rim and the outer flat surface of the tyre changes a lot. That is very clearly visible with a flat tyre still on the car.

and the read-outs from the various gauges such

That inaccuracy wouldn't be noticed even if it was done from the wheel which has the flat tyre.

There are

But the circumference does not increase when the radius where the wheel touches the ground reduces significantly and visibly.

Reply to
Jeff

Of course it does at the only place that matters, between the axle and the road.

Reply to
Jeff

Robin quoted:

A car with e.g. 35R18 tyres/wheels progresses just under 2m per revolution, with the ABS sensor receiving e.g. 48 pulses per revolution, at 30mph that's about 320Hz.

Even if the distance travelled per revolution is reduced by just 1% due to low pressure, a shift on one wheel of +3Hz wouldn't be hard to detect, especially when it has other clues such as steering wheel angle sensor so it knows when the differential(s) is/are doing their thing in a corner.

Reply to
Andy Burns

Because that is the distance that matters.

There is no such thing as a

They don?t say it is constant around the wheel. The radius at other than where the tyre is on the road is irrelavant and clearly visibly different to where the tyre is on the road.

But when the rotation rate is ignored in those situations it is still possible to detect a flat tyre when it is observed that the rate of rotation of that wheel is different to the others.

But it isnt two, it is one out of four.

But not all drivers can do that. >

And yet Andy has seen it do a lot better than that.

Reply to
Jeff

Not looked at this, but seeing as an over inflated tyre is not as grippy as the designed inflation and also given that hot air adds pressure I have also often wondered about the detection. Of course a couple of ideas do come to mind. Monitor the tyres and compare them. If one is not behaving like the rest then something is definitely not right. The other thought is that if the car is still and one tyre is changing, say over night, then you got a leak in that tyre. Its not so much a pressure sensor issue, but a comparison one. Brian

Reply to
Brian Gaff

That ruling in 2001 was in the context of what could and couldn't meet a statutory requirement in the USA for pressure monitoring in new cars that also alerted if more than one tyre - including all 4 tyres equally

- are 25% or more below pressure. It's no surprise that indirect methods couldn't meet that requirement.

OTOH

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has

"Since factory installation of TPMS became mandatory in November 2014 for all new passenger vehicles in the EU, various iTPMS [that "i"'s for indirect] have been type-approved according to UN Regulation R64. Examples for this are most of the VW group models, but also numerous Volvo, Opel, Ford, Mazda, PSA, FIAT and Renault models. iTPMS are quickly gaining market shares in the EU and are expected to become the dominating TPMS technology in the near future.

iTPMS are regarded as inaccurate by some due to their nature, but given that simple ambient temperature variations can lead to pressure variations of the same magnitude as the legal detection thresholds, many vehicle manufacturers and customers value the ease of use and tire/wheel change higher than the theoretical accuracy of dTPMS."

Reply to
Robin

In message , Jeff writes

I've been thinking that but... unless the effective circumference is changed, the tread in contact with the road surface will not alter. Steel bracing etc. as mentioned above.

Maybe there is some perceptible scrubbing going on.

>
Reply to
Tim Lamb

What's the rolling value of Pi for a wheel and tyre?

/me runs away.

Reply to
Andy Burns

I'm not convinced that it is the circumference that determines the rotation rate of the wheel. And those who actually measured it talk about the rolling radius, for a reason and must have done the most basic tests of watching the rotation rate as the tyre pressure varies, and see that it does vary by enough to measure.

Why should there be any significant scrubbing with say a 20% lower pressure in the tyre ?

Reply to
Jeff

Yes but there seems to be a lot of work which supports changes in dynamic rolling radius separate from slip. Eg this paper reports the results of modelling it in terms of tread and belt displacement, and claims the predicted variation of radius with pressure agrees well with measurements.

Though my gut is more attracted to Andy's "variation in the value of Pi" thesis: sort of fits with my feeling that BMW drivers come from a different universe.

Reply to
Robin

It seems to work fine on my Audi. It tells me if there is a problem with a single tyre or to check all tyres. It detects tyre pressures which are different to what was saved, you need to tell the car the tyres are at the correct pressures.

I've seen it detect something is wrong with all tyres. This was after rotating the tyres front to back and failing to reset the pressures monitor. The fronts are run at 38-40PSI and the rears at 32-34PSI. It chimed after about 1/2mile that I should check all 4 tyres.

It detects rotational rate not expected due to pressure change, either under or over inflation. I had a persistent slow. It turns out the alloy's painted surface where the bead meets the wheel was corroded and leaking. My local tyre place cleaned and painted this and the tyre was fine since. However, they pumped up the tyre hard during checking and didn't set the correct pressure. About 2miles after driving the check tyre chime went off. The screen said check pressure and it shows a flat tyre but it was over inflated in this case. The system detected it was rotating at the wrong rate for what was set. It doesn't have an over inflated graphic!

A pressure sender would give an instant out of range indication but this system has never failed to warn me of issues I'd not notice from driving immediately.

Reply to
mm0fmf

In any rolling contact that is transmitting tangential load there will be some "slip". Not much, maybe, but some. I could believe that the slip might vary with inflation pressure and this could be enough for the brake sensors to detect, given suitably clever logging and processing.

(Ref. Bowden & Tabor Volume 2, can't quote the chapter because I don't have a copy to hand).

Reply to
newshound

The problem with Turnip is that he is a left brainer and once he can't comprehend something, it must be wrong or simply can't happen and he will find other (bogus of course) examples to try to support his case. ;-)

It's *obvious* such things work because they just do ... and the fact that he can't understand how or why, doesn't stop that from being a fact! ;-)

If you want to calculate the rpm of a wheel at a particular mph you use the 'effective rolling radius' to do so, not the nominal circumference of the tyre (with conventional tyres [1]).

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I did this when designing an electronic speedometer for the electric racing motorbike I built.

I measured the 'rolling radius' with the tyre at the optimum (high) pressure and with all the normal loads on the bike (me, batteries, fairing's etc). I then used that to determine the rpm/mph and used a f to V converter to take my rotational sensor output and feed it to a DVM, calibrated to represent the mph (all done on the bench using an oscillator and DVM). On the track a RADAR gun determined I was doing

37mph and my display said I was doing 37.4. ;-)

At no time did I measure or use the circumference of the wheel directly. ;-)

Cheers, T i m

[1] With dragsters the driven tyres expand quite a lot to effectively increase the gearing > speed. ;-)
Reply to
T i m

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