Question - car technology

The circomference of a loaded tyre does change, that's how th echeap tyre pressure monitors work. They don't trigger reliably till you're 30% down though. Consider that a completely flat tyre is obvious.

Go and measure it. First measure across the tyre between 3 o'clock and 9 o'clock and then again between 12 o'clock and 6 (12 is the top of the tyre, 6 where it's touching the road). The second measurement should be less than the first because the tyre is squashed at the bottom.

The important measurement is the distance from the centre of the wheel to the ground straight downwards-- that's your rolling radius (call it R), but any bulging at the top is insignificant, so measuring diameter should be OK and might be easier than finding the centre of the wheel.

In one turn of the wheel you should travel 2*pi*R forwards.

Accompanied by the sound of a chisel on slate John, managed to produce the following words of wisdom

Steering wheel fairies. They use semafore to send the signals. Occasionally they get slightly dizzy.

The length of rubber doesn't change, there's dirty great big steel belts underneath it. The tyre doesn't rotate around the rim.

How does having 20 or 30psi in a tyre change the distance the tread travels per revolution?

John

Because the tread flexes more, that's partly why it gets hot.

Can I refer you to the post I made in this thread dated 27/01/2008?

There is a link there to a Wikipedia article about tyre pressure monitoring.

A snippet:

"Indirect TPMS measures the air pressure indirectly by monitoring individual wheel speeds and other signals available in the vehicle. Most indirect TPMS uses the fact that an under-inflated tire has a slightly smaller diameter than a correctly inflated tire and therefore has to rotate more times to cover a specific distance to detect under-inflation."

WRT the circumference of the tyre, think about a bicycle wheel with a a big dent in it that has forced a point on the rim nearer to the hub. The distance around the rim will still be the same, but because the wheel is no longer truly circular the effective circumference will have reduced.

Chris

Interesting question.

I think the answer is that the length of rubber does actually change. If you look above the contact patch there's a kind of "belly" where the sidewall bulges out to the side. That allows the treads to be pulled closer together under the contact patch without them stretching out anywhere else.

I think if your tread was a kind of checkerboard pattern and you measured the distance across squares, you'd find that distance got shorter under the contact patch but was about the same as normal everywhere else. If you draw a line all around the tyre, and measure the length of each square the line crosses, and add up those lengths, that's the distance travelled in one revolution. If some of the squares are shorter without others being correspondingly longer, then you've reduced the distance.

It's a bit like a balloon. If you get a balloon and draw a face on it and then blow the balloon up, the picture gets bigger with the balloon. Squash the balloon against a glass window and look at the bit you're squashing through the window and that bit of the face will get a bit smaller again.

The other side of the balloon will get a bit bigger, but that doesn't happen with the tyre, because the sidewall bulges out much more easily than the tread on the other side stretches.

"The effective tyre?s rolling radius is affected by tyre pressure only in a small degree, namely about 0.5mm per. 0.5bar of pressure change"

Anybody with access to the full report want to elaborate?

On 2008-01-30, Ben C wrote: [...]

Actually on second thoughts, whether the tread stretches on the far side or not is a red herring. It's only the checkerboard spacing distance at the contact patch that matters.

So I think you can just measure the distance from the wheel centre to the ground, and 2*pi*that distance is the distance travelled per revolution.

That sounds about the same as the numbers I recall. It's not a quick system for small deflations, 20miles to spot a 25% deflation IIRC.

=2E.because you're an idiot?

You're very special.

John