Safe Speed Forums

Following on from this topic: http://www.safespeed.org.uk/forum/viewtopic.php?t=7292 where overinflation is discussed...

I've long been interested in the claim that underinflated tyres wear at the edges, as per the left image and that overinflated tyres wear at the centre as per the right image:


from: http://www.carbibles.com/tyre_bible.html (compehensive and excellent reference source, but I think they are WRONG about these wear patterns...)

Well, it might have been true 50 years ago with crossply tyres, but I just don't believe it works like this with modern tyres. I've found that correctly inflated low profile REAR tyres ALWAYS wear out the centre of the tread pattern first. (The fronts are a little different because of understeer scrub).

So what's gong on? Does the wear pattern tell us something about tyre pressure in normal (i.e. moderate) cases? I don't believe it does...

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Paul Smith
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I've heard that misaligned tracking can also lead to higher wear on the outside of the tyre.

Perhaps your high wear in the center indicates a lot of high speed use? (centrifugal force)

Quite possibly.

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Paul Smith
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This would depend on whether the car is front or rear drive to start with, and how the tracking was knocked out; While I'd agree that 'kerbing' is most frequent and would tend to make a wheel toe in, potholes and some speed humps could knock so the wheel was toe-ing out instead, leading to wear on the inside edge.

I have a sneaking suspicion that the "correct" inflation figures quoted nowadays are a little on the high side. Several reasons, it gives a bit more leeway before serious underinflation occurs for the motorist who only checks his tyres once a year, and, probably more likely, it gives slightly better fuel consumption figures for the adverts.

When we raced Prodsports cars using (cough!) "Road Tyres", we were recommended by the tyre manufacturer to reduce the cold pressures by anything up to 30% so that they didn't overinflate due to the temperature they reached during racing. Interestingly, when the tyres were cold the cars drove and handled perfectly adequately.

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I suspect (but have nothing to back me up) that it still happens, but to a far less noticeable degree, due to far stiffer structures in all modern tyres (and even more so in low profiles).

There's a definite 'correct inflation' range, and it isn't very wide. Below the range the handling becomes 'squirmy'. As inflation pressures are increased the handling becomes 'crisper', but go too far (and it isn't much too far) and absolute grip falls. I'd say the max range of 'correct inflation' is certainly no more than 8psi and probably less. With ~6psi difference from cold to operating temperature, I'll tolerate a little bit of squirmyness when cold - but there's not much room to play with.

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Paul Smith
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This is fascinating.

I understood that, with "normal" tyres, ie, not low profile tyres, it is a kindness to put extra pressure in (when cold) for sustained high speed motoring. Low profile tyres do it themselves as they have less sidewall give and correspondingly less V increase so a greater P as it w3arms up (working on P1V1/T1 = P2V2/T2).

Dunno... I must say that that hasn't been my experience.

However, I have a theory: Tyre pressures are (correctly) checked with the tyres cold. But how cold is cold? If the manufacturers mean 20 degrees, for example, and you check the pressures on a cold winter morning when it's around freezing, your tyres are going to be overinflated when they warm up.

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Not necessarily.

The difference in temperature (20° in 300° approx) may be more-than-compensated for by increased rubber compliance and more sidewall ballooning, increasing the volume by more than this percentage. With low profiles, I think you're right. With standard tyres... not so sure.

I've always found my rear tyres wear evenly across the tread, but this is on FWD cars, the only RWD car I've had was a Reliant Regal Supervan III which ran on crossplys and as far as I can recall never needed new tyres.

The RWD vans I've driven had a tendancy to wear the centres first but this may have had more to do with the pressures being set for fully laden and the vans spending at least 50% of the time at half load or less.

[quote="smeggy"]I've heard that misaligned tracking can also lead to higher wear on the outside of the tyre.

Perhaps your high wear in the center indicates a lot of high speed use? (centrifugal force)[/quote]

I used to have a mondeo which is known for it's front tyre churning abilities. Mine was on 17" low profile tyres and I managed to make the front pair illegal in < 4k due to my tracking being out.

Thing is when I had the tyres fitted they checked the tracking which was slightly out and corrected it, so I assume that I had done less than 4k, but how much I don't know.

It's an immense subject with absolutely LOADS of variables! Funny you should mention it today Paul, I've just given our marketing manager his Peugeot 407 back yesterday after he'd borrowed my 807 for a few days. I wish I'd taken a photo of one of his front tyres! As he handed me the keys he said "oh, and keep an eye on the offside front tyre - it's got a slow puncture".

Well amongst the junk I always have in the 807 there is a reasonable quality tyre pressure gauge so I checked it. - He wasn't joking! about 20psi! The tread had worn EXACTLY as the classical description of an under-inflated tyre suggests - nearly bald at the inner and outer edges and perfectly legal (not even down to the tread wear indicators) in the middle. It was the best example I'd ever seen.

That said, I don't know of many cars that DO wear their tyres completely evenly. My wife's 156 always eats the inside edges of its rear tyres first. "They all do that sir!" is what I've been told. It also eats the inside edges of its front tyres but is supposed to run with a bit of toe-out. My car always wrecks the outside edge of it's front tyres first - particularly the nearside one - which gets all the pain on roundabouts. Cars that run a lot of negative camber tend to eat their inside edges first - obviously, but other suspension geometry characteristics also have an effect. The "bottom line" - as far as I can tell, is that all cars are different. Clearly, a tyre that has nearly full tread on one part of its circumference and is bald on another, probably suggests that there is a problem but it's also true to say that (especially with high performance cars) uneven trye wear is to be expected and is often an entirely deliberate decision by the manufacturer. Substantially different wear patterns on both tyres of the same axle does, however, probably suggest a problem (often tracking).

These days, worn suspension bushes are pretty common - largely because manufacturers use really squidgy ones to make the car feel refined, but also maybe in part due to the proliferation of speed humps. These can often allow the tracking and camber to change and exacerbate odd tyre wear.

Last thought, is that increasing the tyre pressure for high speed running might actually cause the tyre to run a bit cooler so the pressure inside it won't go up as much. Most of the heat in a rolling tyre is due to hysteresis in the rubber sidewall as it flexes each time it gets to the bottom and then straightens out again. Reducing the amount of flex by increasing the pressure tends to make them run cooler.

My recent experience bears out Paul's theory. My car is RWD and the back tyres are low profile 255/35 x 18. When new the tread is about 2mm deeper in the centre of the tyre than at the edges, yet after 10,000 miles the centre is down to about 2.5mm with the edges showing considerably more tread. This is running at the correct recommended setting.

I was running the tyres slightly harder as the car seemed to feel a bit tauter, but when the last set wore out in the middles I reverted to the lower pressure, yet still they exhibit the same wear pattern.

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And another point

Everyone is saying about tracking causing specific wear (toe-out wearing inside edge, toe-in wearing outside edge). I wonder... if the tracking is out, handling is bad - and it is like dragging the tyre sideways a bit - but surely this just increases wear all over - and it is camber/xcaster angle that determines wear pattern, not so much tracking?

If the tyre is being presented to the road slightly at a slight slip angle then I would expect it to deflect slightly and therefore one edge to wear very much more than the other. This will be worsened by the fact that the tyre being dragged sideways will take up any slight backlash in suspension / steering links, so the whole issue is "dragging along behind" ever so slightly. So I'd say that excessive toe-in or toe-out will cause uneven wear.

Thinking further about my previous post, re the "centre wearing" rear tyres, here's a possible theory:

When a tyre is driving the car, the drive is ultimately transferred via the sidewalls and thence onto the tread in contact with the road. Now the edges of the tyre are obviously closest to the sidewall and therefore most rigidly held, but the centre part of the tyre is further away from the point imparting the drive. Is is possible that the centre of the tread is flexing under torque loads and causing excessive wear, whereas the parts of the tread nearer the sidewalls wear less due to being supported better?

The really annoying thing with my car is that the fronts tend to wear the sides more than the centres. I don't think there's anything much wrong with the tracking, it's simply that they take more cornering loads and no drive. So I end up chucking out rear tyres with masses of tread on the edges and front tyres with masses of tread in the centres. If there were ever a classic case for rotating tyres this would be it, except that the rears are wider than the fronts so I can't! with tyres at £150 a corner this is exceedingly irritating!

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Well, exactly. I've been round that exact loop at least five times and never managed to change the wear pattern.

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Paul Smith
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OK, so who's driving a performance front wheel drive car with correctly inflated low profile rear tyres and experience of the wear pattern? That should give us a clue...

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Paul Smith
Our scrap speed cameras petition got over 28,000 sigs
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Thinking about this some more, I'm wondering if we're looking at inflation from the wrong direction... Perhaps we should be looking at the tyre from the side.

Low inflation is going to give a long contact patch and loads of flexing about near the road. High inflation will give a short contact patch with far less flexing.

Does that view help?

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Paul Smith
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I think the clue will be that the rear tyres are only there to stop the bodywork scraping on the ground, and tend to perish before they wear out!

...or they get rotated onto the front and scrubbed off.

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