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Not really.

That would imply to me that higher pressure would reduce flexing at the edges of the tyre, where the sidewall meets the tread, but have little effect on the centre of the tread. Which would support the classic theory of higher pressure reducing wear at the edges (and therefore lower pressure reducing wear in the middle) which is the opposite of what we're finding...

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Yeah. It's a crime against decent engineering.



Oh, don't get me started on tyre rotation. I wouldn't DREAM of doing that. It's another crime. Tyres wear to suit the suspension characteristics. Moving them to another axle puts LESS RUBBER in intimate contact with the road because on the new axle the wear pattern is mismatched.

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Paul Smith
Our scrap speed cameras petition got over 28,000 sigs
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I'm not so sure - but let me sleep on it. I'm thinking that the long contact patch of underinflation might well leave the middle of the tread flapping about - it's furthest from any support.

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Paul Smith
Our scrap speed cameras petition got over 28,000 sigs
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Indeed. An early case of politics and spin getting in the way of engineering. How many people do you know that still believe everything they were told about FWD having better roadholding and steering, when what the manufacturer really meant was cheaper production?

I tell you, if we'd studied the way FWD was sold to the masses in greater detail we'd have seen Iraq coming a mile off!

There is the alternate "shelf" theory.

This is that rotating a tyre to another axle is like turning a sagging shelf the other way up, putting everything level again...

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I will look in daylight. My Accord 2.4 is shod with 17" Z-speed rated low profiles and has now done 35,000 miles or thereabouts.

Its history of tyres: Originally shod with Mich Pilot Directionals. Fronts wore down pretty quickly but also pretty evenly (a tad more on the outer edge of the front n/s - no surprise there). Backs moved to the front and a new pair fitted to the back at about 15,000.

At about 26,000 the fronts (originally from the back) had gone again. Couldn't get hold of Mich Pilot directionals, but did get Mich Pilot handed - which I put on the back - the 9,000 milers from the back being fitted to the front.

In all cases I never noticed anything other than even (and very limited) wear on the rear, with even (and fairly severe) wear on the front, plus the obligatory extra on the outer edge of the n/s to take account of spirited roundabout entries. I will critically appraise tomorrow and report back. (By "tomorrow" I mean in a few hours).

I washed the car this morning and was thinking about the tyres and inflation. The tyres on my car are fairly wide and low profile. Due to the rims of the wheel pushing down on the outside of the tyre (and if they where under inflated) I would expect the middle of the tyre to be forced up and thus causing the outside of the tyres to wear first. You would need a fine balance of pressure in the middle of the tyre to over come the effect of the down force from the rims. (If you get what I mean).

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Actually I expect the opposite is true due to 'centrifugal' forces at normal speeds.

In fact I'm slightly surprised that tyres don't stay round even if deflated at high speed. I'll have to do the sums...

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

Well as promised a report on my tyres. The rears have covered around 8 - 9000, all on the rear. they are 17" low profile Z rated and handed, but not directional. The outer edges have less tread than the remainder of the tyre in both cases. However, I'm fairly confident that was the new situation (but I'm open to correction here). The fronts are about 1/16" off the tread bars, but perfectly even across the whole tread area. They are also Z rated Mich, but these are directional. They have covered 9,000 on the front and beofre that did about ther same on the back. When moved to the front they were "as new" and even.

I think you'll find that these 'centifugal' forces, as well as sidewall strength etc, while they do exist, are tiny factors in the overall equation.

Air is very far from being the soft 'nothing' people take it to be - particularly when pressurised.

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I have 225/40R18's and FWD, but am still on my first set of tyres with these wheels. Previously it had the standard 215/50R16's which wore first on the centre of the front tyres at 34psi, during my honeymoon period with the car, with lots of hard acceleration. These lasted 21,000 miles.

The next pair of fronts spent 8,000 miles on the Monstertruck Mondeo suspension, followed by 6,000 miles on lowered suspension. These tyres wore out on the inside shoulder, due partly to increased negative camber, and mostly due to me not bothering to check the wheel alignment after fitting the new springs.

Next came the current 225/40R18's which have covered 6,000 miles so far, and are wearing slightly on the inside shoulders still, but much slower than before. I'm putting this down to negative camber.


My father's ZT runs 225/45R18's and it wears it's front tyres on the centre AND outside shoulder. The rear tyres wear only on the outside shoulder.

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I've been thinking of ways to actually 'see' tyre wear, or at least which bits are touching the ground.

Idea one: Have some circuitry inside the tyre (there's plenty of room, right? ) with some small pressure sensors across it's width pressing up against the outside off the tyre. Then when the monitored part hits the ground each pressure sensor would be pushed up a different amount. It may or may not be possible to use a scanning laser (think barcode scanner) if one can be used for accurate measurements at such a close range.

Idea two: Dig holes in a road or track. Cover the holes with toughened glass. Put a camera below the glass and take photos/video of the tyre at different angles from below as it passes.

Try these tests as different pressures and see what the differene is.

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Andrew.

Ziltro - even easier method: drive over a "pressure sensitive" paper or film. I first met it on the driving range (golf for those that don't like the game). You stick a bit of it onto the face of the golf club and hit some balls. As each hit happens the impact area goes dark showing where the contact point was. Lay some strips of this stuff on a flat surface and drive over with different tyre pressures... Example of possibly suitable stuff: http://iar-ira.nrc-cnrc.gc.ca/smpl/smpl_3d_e.html.

The tyre wear issue is not just pressure, it has a lot to do with suspension set-up, driving style and simple physics. My car which sits on 17" medium to low profile tyres has much more grip at the centre than at the edges. Both edges are evenly worn and the tyres are always maintained at the recommended pressures (the car handling is sensitive to incorrect pressures) after 22,000 miles on the original tyres (they have been rotated front to rear) I still have plenty of grip in the middle but a lot less at the edges.

According to the theory I have been running my tyres at too low a pressure, but I haven't! It is just that when cornering the edges wear more than the centre does. Think about it: if a tyre is say 20cm across the grip and you rotate it about the vertical axis, which parts of the tread will be worn most? The edges - as they have to slide furthest. This is what happens every time you go around a corner.

Very few cars (if any these days!) steer their tyres about the centre of the tread pattern. For a whole load of reasons, there is nearly always some "scrub" whereby the kingpin axis (about which the tyre steers) is slightly inboard or outboard of the centre of the contact patch.

Maybe not dead centre, but the effective centre (allowing for wheel rotation) has to be close to it, otherwise a lot of extra force is required to scrub the whole tread around. Either way, a correctly inflated tyre (one that has an even pressure to ground over the majority of the tread width) will always wear more at the edges than the centre. If you wear more in the centre, then it is over-inflated and significantly down on grip as the edges are not in full contact with the ground and are not contributing as much as they should during acceleration or braking.

I know that's the common perception - but is it really true (within a common range of pressures)?

It seems to me that the circumference of a modern tyre is remarkably inelastic in terms of rolling circumference, yet what you're suggesting would imply that the tyre is actually ballooning out in the centre of the tread. My guess is that you could vary the pressure from 10PSI to 100PSI and find no measurable degree of such ballooning.

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

Hmmm not sure about that. Especially as the tyres are brought up to high rpm too. There is a lot of centrifugal force there, coupled with a lot of pressure and the only means support for the centre of the tread is shear and bending of the rubber next to it.

The steel bands in radial tyres go a long way to help but it still expands/contracts a surprising amount depending on temperature. Obviously the wider the tyres the more pronounced the ballooning becomes -

Ever watched a drag racer spin it's wheels on the green light? Highly exaggerated compared to the effect on road cars, but the same forces are at play, although on a smaller scale.

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By the same argument, then under-inflation also has minimal impact on profile...

I expect that it varies depending on tyre profile - a low profile tyre will be more susceptible to ballooning (or caving-in in the middle) because there is less scope for the sidewalls to move, especially with the stiffened sidewalls that high-performance low profiles tend to have - for these tyres the only thing that can vary much is the ground contact area. I must admit that my comments are based on relatively low profile stiff sidewall tyres that are designed to take a lot of lateral load without flexing, which is why I also pointed out that my car is sensitive to tyre pressures - under or over inflation by only 10% will have a very noticeable impact on handling, especially if asymmetrically applied. I ought to think a little more about average vehicles...

A little browsing (and transatlantic spelling conversion) revealed this site that is rather dedicated to tyres, tyre technology and safety (well and selling them), but this Technical section is quite interesting:

http://www.tirerack.com/tires/tiretech/tiretech.jsp

You bet - but large drag racing tyres are very specialised - they even have 'wrinkle walls' to store energy and increase footprint. In other words - unlike most tyres - huge flex is part of the design.

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

I believe the opposite is true! If the kingpin axis hit the ground at the centre of thecontact patch, turning the wheel whilst stationary would cause only shear between the contact patch and the tarmac. Moving the intersection point inboard or outboard means that the tyre must roll a little (and correspondingly not shear as much). If you had (for a daft argument's sake) 6" wheel spacers fitted, the steering would get a lot LIGHTER when stationary because the wheel would just roll in an arc rather than "grind" against the tarmac!

I think dragster tyres "wind up" not so much to store energy (although I'm sure that helps a bit) but mainly to reduce gearing.