Does a tyre change its CIRCUMFERENCE when underinflated?

[Andy Burns]

I bought a car like that once ... never again. I think we have to accept that manufacturers are going to fit space savers, but the gunge really isn't worth having if you do motorway miles

[Andy Burns]

Neither have I got a section of glass road with an array of high speed cameras underneath it to record what happens to an underinflated tyre "rolling" along it

[Dave Plowman (News)]

Just wondering how often you've used the spare? Actually *had* to?

At a spares day with my Rover car club, I bought a beautiful spare from a car which had been written off. Obviously for show, as the lettering on the tyre had been painted in using white. And the tyre perfect - although not the same make as mine. I think in the mid 90s. Never been used. ;-)

I have had slow punctures, but never so bad it couldn't be pumped up and driven to have it fixed.

[Andy Burns]

Three or four times in 7 years, other times I could top-up the tyre and carry on, the one time the gunge let me down the tyre was completely destroyed on a motorway, so that turned into a wasted half a day getting recovered to a garage that had a suitable tyre in stock.

Most tyre places seem to have cut down what they keep in stock to just "popular" sizes, so you have to order tyres, fair enough they can get them next day from the wholesaler, but people really hate you sticking to 50 on a motorway.

Some you'd get away with pumping it up, driving from home to a local garage to get it fixed, but you wouldn't want to do a long journey on it.

[T i m]

No?!

Yes, to make that easier for some to understand let's call that 'the height of the tire wall (you left brained artists do have a strange way of looking at mechanical things).

Where has anyone 'implied' such a thing, apart from in your head?

What are you babbling on about now ffs?

Brilliant. The people who actually understand what is going on *know* that it is in fact exactly what does happen ... and for reasons you would never be able to understand. ;-(

Cheers, T i m

[T i m]

I'm not sure it will make any difference Tomas as 'some people' don't believe the facts, imperial evidence (millions of cars with iTPMS) and the 'experts'.

They would prefer (if ever) to come to the exact same conclusion themselves but shroud it very carefully in a desperate attempt to make it look like they understood it all along and thinking they can hide the *fact* they are indeed 'stupid and arrogant tw*ts'. ;-)

So, that's because of the pantographing effect (like the dragster drive wheel I linked to ages ago) as it is spun up, opposite to that of a deflation (or course).

;-)

The left brainers will 'get' the direct stuff but won't get the stuff that requires some imagination.

Thanks for trying though. ;-)

Cheers, T i m

[Jeff]

As I said, not possible, because while my car has ABS, the wheel rotation rate of the individual wheels isnt displayed on the OBD2 data.

But they arent joined where they cross like those pop rivet setters.

When you load a tyre that causes it

Never been convinced about that either. Yes, when the tyre pressure is reduced a bit, there is more tyre in contact with the road, and the walls clearly bulge more, but there isnt any evidence that there is more tyre in contact with the road across the tyre itself.

As I said, I prefer to measure and then, when that measurement does show that the wheel rotation rate does vary with the circumference and not with the distance between the axle and the road, look for an explanation of why it happens like that.

There is no pivot at each crossing point like there is with the pop rivet setter

[T i m]

Ok, please let us know when you come up with the answer. ;-)

Cheers, T i m

[Jeff]

Interesting, and a lot smaller than I would have thought.

So now you have blown up the proof that it must be the circumference that is changing, because that change is less than the 0.4% you saw the ABS wheel rotation rate change by. You bugger :-(

[Andy Burns]

I make it a 1.5% change in "radius"

[The Natural Philosopher]

Is that what we would call a 'crossply'?

a radially belted tire 1% more.

Which is more than low/high pressure changes, so that's why they do wheel to wheel comparisons..

Which is in effect what I have been saying all along

and a

[The Natural Philosopher]

No, in fact he has proved it can't be the radius change because 5 mm in 320mm is *more* than 0.4%. It's around 1.5%.

Ergo it has to be the circumference change due to elasticity

Denial is fascinating to watch

[The Natural Philosopher]

Ah but you had an open mind to start with...

[Andy Burns]

Just a normal radial ...

[Andy Burns]

Famous for their hard ride, Audi generally recommend high pressures, so

42F/39R for this one, which is a bit teeth-rattling over potholes.

I noticed Michelin for this car and tyres recommend 33F/29R which is a big step down, I briefly tried them at those pressures this afternoon and it felt like riding on balloons, so I'll try it on 38F/35R which is a softer "Audi allowed" setting if you're not in autobahn mode.

[dennis]

Watching TNP going from denying that the circumference changes and quoting his engineering background as proof and now saying it does change is fascinating. It makes you wonder how he expects anyone not to notice.

[T i m]

He's one confused puppy that's for sure.

I think they do, they just know what he's like and choose to ignore it (basically because he can't help it).

I don't think most people would mind him just being a bit slow (he is old after all) but it's all the 'attitude' that comes with it that makes it more difficult to take him seriously. But hey, he told us that is just his 'online persona' so I guess he knows what that's going to ern him.

He's good to his few goblins though so that's something. ;-)

Cheers, T i m

[Jeff]

He actually said originally that it changes by less than 1% and it turns out that he was correct about that and it is now clear that the distance between the axle and the road changes by much more than that, 3 times more in fact, at least with that particular car and those tyre pressures.

and

[Richard]

empirical

[The Natural Philosopher]

I see T i m is in complete denial and is in fact projecting everything that he is and has done on this thread onto the people who were in fact right all along.

Pore ole T i m.

Like Plowperson, he really needs medical trreament or therapy to help with his mental condition.