Safe Speed Forums

Mole, yes you are correct, but that is not what I am trying to say.

When a tyre turns a corner the inside and outside edges must move a different distance relative to the centre of grip. Whether it is rolling or not doesn't matter: if it is rolling, then the wear is spread around the grip, if it is stationary then it happens at one point on the tread, but it is the accumulation over time that actually shows the effect, as a single motion adds little wear. Relative to the centre of grip, the inside edge moves backwards, and the outside edge moves forwards, thus if grip is equal across the contact area the edges will always wear more than the centre because they move most relative to the surface.

I could explain it better, but I don't have time to draw the diagrams and do the maths.

Dunlop have a chamber of horrors showing trye wear, and they claim that under inflation causes excess wear at the edges while over inflation wears the middle, and who am I to argue with the manufacturer:

http://www.dunloptyres.co.uk/ourTyres/car/tyreCare/

Ahhhhh! comprendo! Sorry, I'd got it into my head that you were talking about steering effort when stationary! (don't know why!)

Anyway, yes, I see what you're saying now and agree (although I wouldn't mind betting that every corner puts the centre of the contact patch in a slightly different place depending on body roll, road surface camber, speed, lateral acceleration etc). The Dunlop site is giving the accepted theory but I'm wondering, in the light of other posts on this thread, whether that theory evolved when tyres were much narrower and had taller sidewalls than today's "big boots"!

As for tyre growth at speed, it's certainly a known phenomenon. I once ran a car up to 150 on the speedo while it was (effectively) on axle stands. to check that we'd got rid of the speed limiter in the ECU. The tyres (225/45s) grew visibly larger at that speed. Of course, what I don't know is whether that effect also happens at the bottom when the weight of the car is on them. I suspect that it wouldn't be significant.

As an aside, racing car pit crews often check that the suspension settings are putting as much rubber on to the road as possible by using one of those hand-held infra red thermometers. They scan it across the width of the tyre as soon as possible after the car comes into the pits to see if one edge or the other is running cooler or hotter than the other.

Since your talking tyres I thought I would mention my own expierence and see what you thought was going on here:

Its got 205/60 R13s on as standard which is an odd size nowadays and there is only a few manuafacturers who still make it. The standard pressure is 28psi all round. However before I set of for the South of France I had a look in the manual and it said "for sustained speeds of over 100mph inflate to 32psi". Well I thought I would be doing 80-90mph for 650 miles non-stop through France so I inftlated to 30psi.

Even though it was only 2psi differnce the car felt less "planted" and a little more skittish, seemingly worse as the miles rolled on. I thought about dropping them but as I felt them at a stop for fuel they were red hot and pulsating so I was concerned about overheating them. I probably should have stopped for a bit, but the ex Mrs Capri was getting somewhat annoyed by the fact we were still a long way off.

What was causing the skittishness? Surely not just by overinflating by 2psi? Or was it just an unavoidable aspect of a long distance in hot weather? I did it because I was concered about 13inch tyres being subjected to 650miles virtually non-stop through France in baking hot temperatures.

What? Tyres run colder with higher inflation pressures because they flex less. And I wouldn't expect a small pressure hike to make a vehicle skittish - quite the opposite actually; I'd expect it to be 'crisper'.

Any chance you used a faulty gauge?

And as for pulsating... we are talking about tyres, right?

_________________
Paul Smith
Our scrap speed cameras petition got over 28,000 sigs
The Safe Speed campaign demands a return to intelligent road safety

Yes indeedy. I interpreted his post in the sense that "I was thinking of dropping the tyre pressures but they were already hot and lower pressures would have made them hotter still".


For "skittish" you perhaps need to read "less languid"...

_________________
CSCP Latin for beginners...
Ticketo ergo sum : I scam therefore I am!

I suppose 30PSI hot would be about 24PSI cold. And 30PSI 'red hot' might even be 22PSI cold, which would be way under and possibly skittish.

But we all know to set tyre pressures cold don't we?



Yes, that's a very interesting translation into the subjective domain.

_________________
Paul Smith
Our scrap speed cameras petition got over 28,000 sigs
The Safe Speed campaign demands a return to intelligent road safety

Yeah, but I was only half joking.

Some sportier cars I've driven have had extremely direct and sensitive steering. I'd say they were "responsive" but I can imagine others would say "skittish"; and if you were to drop the tyre pressures I'd say they became "mushy" (or perhaps "languid") whereas others might say "less skittish".

It's nearly as subjective as the judgement of what constitutes hard or soft suspension...

_________________
CSCP Latin for beginners...
Ticketo ergo sum : I scam therefore I am!

Anything is possible I suppose but it was one of those "electronic" air machines at a petrol station in Dover the night before so I would rate it as doubtful. JT was right when he interpreted what I said, sorry didn't make myself clear. I inflated them to (in my theory) lessen the overheating, but this made the car "skittish" and so I was faced with a choice - should I deflate them a little to get the handling back but run the risk of them heating up still further? In the end I left them. By skittish I mean it seemed to wander about in anything that resembled a light breeze. Perhaps I'm using the wrong word. "Less planted" seems better perhaps.



honestly, it was like feeling your pulse. Quite strange. But I figured that it had done 500 miles without a problem, and so another 150 would be ok, which it was. My decision was partly due to the fact it would be starting to get dark and I really didn't fancy trying to get through Montpellier in the dark with no through roads or ring roads available. Plus I was risking a domestic.

Just to make clear the 30psi was set when cold after a long break in Dover. They wern't re-checked in France as I felt the reading would be completly misleading.

Well you've got me baffled, about the pressure, the heat, the skittishness and especially about the 'pulsating'.



That's just the stangest tyre story I've ever heard. Can anyone make any sense of it?

_________________
Paul Smith
Our scrap speed cameras petition got over 28,000 sigs
The Safe Speed campaign demands a return to intelligent road safety

Just taking a wild guess here, but is it possible that the tyres had water in them - which started boiling from the heat?

_________________
Only when ideology, prejudice and dogma are set aside does the truth emerge - Kepler

Pulsating is probably the best word for it, or maybe quivering. Somewhere between the two. It cetainly wasn't inert as you would expect.

Road tyres in excess of 100C? I don't think I've ever found road tyres over about 60C even in track use.

_________________
Paul Smith
Our scrap speed cameras petition got over 28,000 sigs
The Safe Speed campaign demands a return to intelligent road safety

In which case could it be something more volatile than water precipitated/sublimed from the rubber that was changing state? Ah - got it.... I wonder if the airline in Dover was contaminated with a LMP grease or a volatile oil in a bubbler filter that had been overfilled? That would do it.

The tyres won't, but the wheels could - at least, in places. Brake discs can reach several hundred degrees, and that heat has to go somewhere.

_________________
Only when ideology, prejudice and dogma are set aside does the truth emerge - Kepler

I'm starting to think that the pulsating must have been differential contraction - the same effect that might leave a performance exhaust ticking after a hot run.

_________________
Paul Smith
Our scrap speed cameras petition got over 28,000 sigs
The Safe Speed campaign demands a return to intelligent road safety

Yeah, that's probably the most plausible explanation.

_________________
Only when ideology, prejudice and dogma are set aside does the truth emerge - Kepler

The boiling point of water at ~30psi would be much higher than 100C

Very true

_________________
Only when ideology, prejudice and dogma are set aside does the truth emerge - Kepler

This may be a silly suggestion, but was it just hotter in France? Several hours of a black surface in the sun will heat it up - a lot.

Also garage pumps are notoriously inaccurate - yes they have to be calibrated, but they often go out almost immediately, and some are dangerously inaccurate:

http://www.aatrust.com/index.asp?PageID=31&Year=2005&NewsID=51

Very Long URL to Warwickshire Trading standards report

I have given up using them, and after a review in autoexpress I got a little digital gauge from halfords that the review said was 100% accurate (and the only one in the test that was) - it was less than a tenner. Added to this a portable inflator (tyres, boats, airbeds, etc) which runs from the cigarette lighter, and I now do my regular tyre checks from home.

The theory of increasing pressure for high-speed/high load conditions (as I understand it) it to limit side-wall deformation as the wheels rotate and thus over-heating

When I did increase mine by 2 or 3 psi for a long high speed run I did notice that grip was definitely down, with more of a tendency to under-steer and over-steer depending on conditions (and driver input). Actually the idea of running the rears a little over would help getting the car to drift...