Does a tyre change its CIRCUMFERENCE when underinflated?

I wonder if Turnip actually climbed out of his basement once in a while he's see all these things in action ITRW and realise just how stupid *he* is being?

Quite. I believe the regs for these tyre pressure sensors are that they should react before the pressure has changed by no more than 25%.

Many tyres are run at that sort of reduced pressure (slow punctures, no TPMS) and would have to be 'scrubbed away' in no time.

They aren't, so it's something else, something Turnip doesn't understand so it doesn't exist. ;-(

Cheers, T i m

The hight of the axle is reduced - therefore the effective radius is reduced and therefore its effective circumference. The part of the tire not in contact with the ground is irrelevant.

I DO get what TNP is saying. Consider a fully inflated tyre with a mark on the tread area. That mark will always be at the same relative angle to the wheel, otherwise the tyre would be sliding on the wheel rim.

The wheel rotates one revolution. Measure the distance between the first and second points where the mark touches the road.

Now partially deflate the tyre so that the distance between the axle and the road is much less. Repeat the measurement. The distance along the road will be almost exactly the same as before because the circumference of the tyre has not changed significantly due to all the steel wire in it and the tyre is not scrubbing against the road surface.

In both cases the wheel rotates exactly one revolution - the tyre is not slipping on the wheel rim - and the distance traversed in that rotation has hardly changed.

So the "effective radius" has hardly changed, even though the radial distance from axle to road has changed considerably. It really is the circumference (or perimeter) of the tyre that matters, not how much the axle has dropped relative to the road.

John

True ... so, it's either witchcraft, or your assumption that the

Because of the way the steel wire is laid up in the tyre they tend to cross and form a parallelogram. As the tyre deflects the shape of the tyre changes and therefore causes an effective change in circumference (more accurately, how far the vehicle travels per wheel revolution (down to pantographing)).

The smaller the distance the smaller the effective circumference (or effective rolling or load radius as it's known) and to the faster the wheel will spin for a fixed ground speed of the vehicle.

Cheers, T i m

Everyone seems very fixed in their ideas about this. However, a simple experiment should be informative.

Take a spare tyre. Wrap a tape measure around the circumference along the centre of the tread area. Measure the circumference. Place a plank through the tyre and stand on the ends. The tyre will deform in a similar way to a deflated tyre on a wheel. Re-measure the circumference. The air pressure will not have changed, so if "pantographing" has taken place the circumference will be significantly reduced and that change will be attributable purely to such shape factors. If on the other hand the circumference is not significantly altered then "pantographing" is not an important factor.

Does that experiment seem valid?

John

I think it all comes down to the distortion of the tyre and the reduced length of the flat chord BA vs the arc BPA that would exist if it wasn't loaded at all.

the more heavily loaded, or the less pressure it's inflated with, the greater z will be, and the smaller the effective rolling radius, reading through to the end of that page gives chapter and verse formulae ...

In short the flatter the tyre, the smaller the effective radius and the faster the wheel needs to rotate to keep up, which is what the sensors detect.

And yet if you consider a solid wheel, no tyre, it is the distance between the axle and the road that determines the rate at which it rotates at the same speed of the car over the ground. So when a wheel with tyre sees a reduction in the distance between the axle and the road due to a lower pressure in the tyre, you get the same change in the rotation rate of the wheel which is easy to measure with the ABS sensor on that wheel.

Bingo. ;-)

Pretty sure Turnip will still argue the case ... he's still arguing the that the world is flat. ;-)

Cheers, T i m

It's sorta what you do when you accurately calibrate a cycle speedo.

You place the valve at the bottom and make a mark on the ground at that point. Then with the tyre (of the wheel supporting the speedo sensor) and with the right pressure in that wheel and with the bike with it's typical load (so you and luggage etc), you wheel the bike forward one turn of said wheel (valve at the bottom again) and then measure the distance between those two points.

Now, presumably if it didn't matter about the load on the tyre they would tell you to just put a tape measure round the outside of the tyre?

When I built the kitcar I used the same process to determine the revs / m and so the speedo gearing, same on the electric motorbike I designed, built and raced (and got an award from the IEEE for 'Technical innovation'). ;-)

Something to so with the sophisticated speedo I designed and built ...

Cheers, T i m

No, you don't.

a tyre and wheel is not a sun and planet reduction gear, the tyre is not rotating relative to the wheel, ergo every revolution of the wheel one circumference of tyre must move along the road.

The ABS sensors do not detect a massive change in radius, but a small change in circumference due to the tread shrinking very slightly.

Yes you do, as you should be able to see with a rim with no tyre at all. As the rim diameter changes, the rotation rate will obviously change.

It is a rack and pinion tho.

the tyre is not

Correct.

ergo every revolution of the wheel one

Yes, but it is the distance between the axle and the road that determines the rotation rate.

They actually detect the substantial change in the rotation rate the is due to the substantial change in the distance between the axle and the road.

If the length of the perimeter of the tyre is say 1.5 metres, and the vehicle is travelling at 150 km/hr, that means the tyre rotates 100000 times per hour, or 1666.6 rpm, assuming no slippage between tyre and road or tyre and rim. There's no getting around that, and there's no mention of state of inflation, shape of the tyre or axle to road distance.

At what state of inflation?

At what state of inflation?

Ok.

See above. ;-)

But there should be as that's a key variable that affects the effective 'perimeter length. ;-)

No, but there should be as the shape of the tyre is instrumental in the calculation.

On a solid tyre it wouldn't be because you have removed that variable.

Do you also deny the change in tyre diameter on a dragster (and therefore rpm / mph)?

What about this. Imagine some lunar rover module where the 'tyre' is made up of many completely independent segments.

With the vehicle up on jacks you might measure the effective circumference of the wheel as the measurement made with a tape drawn around it.

But put it under load and with it standing on one (sprung) segment. the RW radius of that segment at that time under that load will be less. Imagine that reduction in radius being passed round the wheel segment by segment and you could then see (hopefully) that the effective circumference would be calculable from the loaded radius and it *would* be very different from the unloaded one.

Join those segments together on the outside by something plastic, like say thin balloon rubber and nothing really changes.

Join them with something heavier but with the similar ability to 'give' and you have a pneumatic car tyre. ;-)

Cheers, T i m

Yes, but it is the distance between the axle and the road that determines the rotation rate.

In message , Jeff writes

Why?

Long time since 'O' level maths but I suspect the relationship between the radius and the perimeter only works for a perfect circle.

I'll type this slowly for the hard of thinking.

The - Bits - Of - The - Tyre - Not - In - Contact - With - The - Road - Are - Irrelevant. The - Only - Thing - That - Matters - Is - The - Diameter - Of - The - Circle - Whose - Radius - Is - The - Distance - From - The - Axle - To. The - Road.

When considering this sort of thing I first consider the real world facts.

iTPMS systems obviously work.

I believe the regs for such systems state that they must warn the driver of a pressure lost *before* it passes 25% of the pre-set pressure.

This means that it must be accurately measurable so when a 32 psi tyre drops it's pressure to 24 psi (23?), it should raise the alarm.

The instructions for calibrating cycle speedos require you to measure the *loaded* rolling circumference of the wheel bearing the speed sensor (when it would be easier just to put a tape round the wheel).

So, a right brainer would take these sorts of things and then try to look for a scientific / mathematic solution as to why this is.

A left brainer would jump to a conclusion based on their lack of understanding and then look for information to support their denial (from other left brainers typically). ;-)

Cheers, T i m

I don't think it helps to introduce another myth[1] - let alone a mirror image of the usual one ;)

What they detect is a change in speed of the wheel.