Does a tyre change its CIRCUMFERENCE when underinflated?

[Jeff]

Not convinced and that summary makes no sense either. Why would the tyre 'spread more' as it gets flatter and why would the circumference get shorter even if it did >

[Jeff]

They do in the sense that presumably the distance from the axle to the road dropped by rather more than the 0.4% that the ABS sensors recorded, so the effect must indeed be due to the circumference changing, not the dynamic rolling radius.

[T i m]

But the two are interlinked?

If you accept the circumference changes between the tyre carrying no load, the tyre carrying load and inflated correctly and the tyre running at a lower pressure, then in the first case there is a conventional radius and in the last two, because the tyre is no longer circular and because it shrinks under the footprint, we also have a rolling radius.

What we can't say with any certainty is the actual distance between the axle and the road is the same as the rolling radius (because by definition the centre of the axle will be low of centre of the effective circumference, simply because of how tyres work.

With iTPMS I believe some manufacturers shun (or shunned) it because it is much more critical in it's calibration than the direct reading TPMS.

A new tyre, tyre rotation, a managed change in pressure generally require a 'reset' or the system could become unreliable. This is considered 'too interactive by some manufacturers, having to rely on human input to keep the system reliable.

This would confirm the concept of iTPMS working with very small differences ... the sort you get when you change a tyre or get a puncture. ;-)

Cheers, T i m

[Tim Streater]

Well it does spread a bit, see Fig 5 on:

But the circumference can't get shorter except for a small amount, much less than the "rolling radius" people like Huge and T r o l l.

Fig 5 shows what *actually* happens when the tire is deflated (although fig 5 is about higher speed effects, supposedly). You will see it showing the bulge on each side of the flat part. That is where the extra circumference goes, the part that supposedly vanishes according to fig 6. AISB, fig 6 doesn't have anything to do with reality.

[T i m]

And so he's talking BS.

For different reasons that you obviously don't have the IQ or interest to understand.

Not to you it seems.

dennis quite clearly stated the scenario with a twin wheel lorry where the 'effective circumferences' can become substantially different if one tyre becomes deflated and *then* one tyre is likely to suffer further damage. It's more likely to be the low pressure tyre because it will be carrying less load and so more likely to scrub than the one carrying all the load.

Except all this is just basic mechanics and common sense, not characteristics generally found in 'artists' eh. ;-(

Cheers, T i m

[Andy Burns]

Aren't twin lorry tyres linked by a pipe between the tyres? So they're inflated as one?

[T i m]

Bwhahaha ... what's this, the penny staring to drop with Turnips goblin as well!!

I personally haven't stated any numbers so even more BS and lies from the goblin.

Ding! They all get there in the end ... even goblins!

Cheers, T i m

[Jeff]

Can't be by much given that you'd expect the axle to road distance to have dropped by more than 0.4%, although he didn't actually measure that.

39psi is quite high so it may be a pretty big solid wheel and tyre, so it would be interesting to have had that distance measured.

But it should be possible to reset it automatically when it is clear that a wheel has been changed, say because it had to be to pass the MOT or the tyre pressure had been changed at the service station

A slow leak too.

[T i m]

Then come up with a more credible one?

What part of 'pantographing' don't you get OOI?

Have you ever seen one of those expanding pop rivet guns ... of the joke boxing glove on an expanding arm?

When those things get wider they get shorter.

Imagine one bent round into a circle (like the internal structure of a tyre). What would you get if it was made out of flexible materials at the flat section at the bottom, where the thing spreads out?

I really can't think of any other way to make it any simpler for you?

Cheers, T i m

[Andy Burns]

I can drop the pressure back to 31 and measure the drop of the axle, certainly it was visibly more than 0.4%, of course I won't be inside the car adding weight while I measure it ...

[T i m]

I didn't watch whatever you are referring to as it doesn't matter. I don't need to because I know iTPMS does work and therefore the only way it can work. ;-)

Possibly.

Yup, possibly it could be automatic, especially if it could cross reference the iTPMS with GPS etc.

Quite.

Cheers, T i m

[T i m]

Some may be Andy but many I have seen have a long valve that brings the rear tyre and if they were linked there wouldn't be any redundancy would there (and that was part of the point).

A mate had a single wheel recovery truck and they banned them because of the consequences of the only wheel failing on one side.

I'm sure there are linked systems these days, especially with the introduction of TPMS etc?

Cheers, T i m

[T i m]

I don't think anyone questions the fact the most of the load is carried at the bottom of a tyre and so it will be there more than anywhere else that will change as the load increases or the pressure drops. Personally I'm not suggesting there is a direct relationship between the height of the axle over the road and the 'rolling radius' but there could be.

;-)

Cheers, T i m

[dennis]

Well that is the obvious bit, less pressure means more of the tyre has to be in contact with the road to take the load.

The circumference of a tyre is not a fixed length. Its flexible blocks (the tread) mounted on a flexible base mounted on a flexible core of wires/nylon (the plys).

As the tyre hits the road the blocks get closer together so the circumference decreases.

Even slicks compress because they usually use a soft rubber.

Shame TNP is so fixed he can't understand that this is too complex for his closed mind.

[dennis]

Why the sudden change? Now TNP has claimed that the circumference can change you also change to say it can.

You are a mug if you think TNP is right all the time, he isn't. And neither are you.

[dennis]

You are just hoping nobody bothers to think about what you claim.

TNP says if they rotate at different rates the treads fall off.. I say treads do fall of. You think it means I am agreeing with TNP.

Just think for a second instead of parroting TNP.. The tread changes length when it hits the road so that bit of the tyre is rotating at a different rate to the rest so yes tyres do rotate at different rates to the wheel some bits at one rate some at another and some at all the values in between. Now sod off if you can't be bothered to think for yourself.

[dennis]

Certainly not all, I have seen them with the tread and cassing stripped from one tyre and the other is still inflated.

[Jeff]

I prefer to measure, not speculate.

I've never been convinced by your pantograph line. The steel belts arent panto graphs.

Yes, but the steel belts in car tyres are nothing like that.

Trouble is that it nothing like a real steel belt in a car tyre and is encased in rubber too.

[Jeff]

Sounds like a relatively heavy vehicle given the 39 psi tyre pressure so that may not matter much.

[Jeff]

Nothing to watch, I was referring to Andy's report of what he saw reported by the ABS wheel sensors on the OBD2 system.

as it doesn't matter. I