Does a tyre change its CIRCUMFERENCE when underinflated?

Another benighted idiot.

Really catching em out tonight.

No. The effective radious first of all is almost bollocks. But if you mean te circumferenmce divided by 2 PI then that will only change with pressure, as the belts strech, Not with loading.

Otherwise p[eole would be travvling 30% slower when their tyres were flat, for the same speedo reading

Yes.

No. Once yopu put a non circular tyre on, you vannot talk about radius.

What counts is the distance round whatever shape the tread is. Al;l of that has to be in contact with the road during one wheel revolution unelss the tyre slips on the rim

They *will* be travelling slower - you've got that right. But not by anywhere near as much as 30%.

Thats becausd in that case the wehel is in fact round. Ther is a perfect relatinship between circumgerencve and radious

Once iuy has a non round tyre on, your assumptions are invalid.

No it isn't.

it is NOT a sun and planet gear. It is a squashy tyre on a wheel

Look at the video 8:45 in and see what YOU are describing happening.

If you think thats how a flat tyre behaves god help you.

There are no substantial changes. Ther is a very small change due to elsatyicity of the tread belt.

You know this is true otherwise speedos would be massively variable with tyre pressure and wheel loading They are not.

No it is not.

Go to the top of the class.

What is the radius of an egg?

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 - Circumference-of-The-Non-Round-Tyre

But if you were to label part of the tread with a line continuing radially up the sidewall to the centre, that line would not remain a straight line; it would curve one way as the labelled part approaches the road surface and then the other way as it leaves it; this happens because the plies of the steel belt move towards or away from each other as the tyre rotates. Not by a noticeable amount; probably by an amount that is hard even to measure. The tyre is no longer a perfect circle but a circle with a flat on the side which is contact with the road at this precise instant. The distortion of the sidewalls, both rotationally and as the bulge "travels" around the tyre as the tyre rotates, is the reason that tyres get hot. The lower the pressure, the more distortion and the more heating - to a point that the tyre softens, melts and bursts.

No, it doesn't.

I think Huge, you are probably too stupid to understand

Nope, also changes *because* of loading.

That would be advised, or possibly 100% slower!

Well, probably not 30% but slower for sure.

Higher pressures will (also 'of course') change the *profile* of the tyre (as can be seen by the tyre wear) and so the effective circumference (and the actual (unloaded) circumference).

Not because of any steel belts *stretching* but because the tyre changes shape (from wide and small to narrow and tall) ... hey, we could call it 'pantographing'. ;-)

The outcome is similar to a racing motorbike where when the bike is upright the driven wheel is maximum diameter but when leant over the diameter changes (smaller) and to the revs would go up for the same road speed. Same tyre, different results under different circumstances.

I wonder if Turnip will ever get it and how he will get out of the position he is currently in? Should be fun ... ;-)

Cheers, T i m

Obviously APB is greater than AQB, but so what?

That is an idealised diagram which does not reflect reality. In actuality, near A and B there will be extra material that has been squidged out of position.

Well Huge, I am afraid you are the one at kindergarten level

I guess it goes with remaoning.

What I am saying is 100% correct and is borne out by tshe papers on the subject and by the you tube video I linked to.

Even that doesn't work.

Huges simple minded O level fizzix cant cope with the reality of an elastically coupled non-circular tyre attached in a non slip fashion to a wheel.

This is the sort of thinking that creates climate Beleivers - people who THINK they know a little bit of science ....

Because it is not simple. Only you are simple. It is in fact fiendishly complicated. And the short cut is to forget radius and think only of circumgference If you think in terms of radius you will get it all wrong

Because the rim of the wheel is moving slower than you think, because the tyre is flexing as it goes round.

Yes - the guy in the video did that and there was sod all difference.

Its a very small difference due to belt elasticity thats all.

No, I accept that you simply dont have a scientific brain.

You are like te Afrikaaaner I spoke to ocne 'Black people don't have souls' he said' It was as obvious to him as....this is to you.

No, we are saying that te circumference remains *nearly* constant, and the difference is due to belt shrinkahe under lower pressure NOT 'reduction in radius' and is of the order of less than a percent for up to 10% reduction in 'radius'

Has to be so or flat tyres would give *way* out speedo readings

I am not sure that is so on a modern radial. Thats ounds like a cross ply.

Oh. SA quck google sdsyas that some tyres are slightly angled tread belt wise and some are truly circumferential.