37 replies to this topic

[JDeRosa]

I have a question about road tyre wear, but before I ask, I'll explain why I'm curious.

A friend of a friend has recently bought himself a Subaru Impreza STi. He has since been caning the **** out of it. As you'd imagine it would only be a matter of time before an accident happened.... and it did. He has written off the scoob and has subsequently blamed the near bald tyres.

Now I know that it wasn't raining, so my question is this:
As the tyre wears and loses tread, the contact patch increases, will this rubber produce more grip? As we all know from F1, as the tyres near the end of their lives, the grip deteriotes, but as road tyres had far more tread, the contact patch increases, right?

Had it been raining, then yes, no tread, less water dispersal. But I suspect that the grip was fine and he just lost control.

Thoughts?
Thanks

[Dynojet]

Hi

I do not understand so much about tires, because it's a kind of black art, still.

As far I know, when a road tire wears, it scrapes the rubber layer that the tire is designed to work over. The subsequent layer has a harder rubber and have less friction coef. even that the contact patch becomes larger.

But i've heard once that some racers here in brazil uses scraped road tires on touring races , so my theory fails.

Anyone else?

[GBarclay]

Bald tires- I gotta call BS, 300HP AWD and a driver not up to the task perhaps?

Competitive autocross racers and certain road racing classes will actually shave rubber off tires so that the tread is "almost" bald. 4/32 of tread is common. This reduces "tread squirm" which is the blocks of tread squirming or deforming slightly under a load. So actually as the tires wear, especially a tire like the Bridgestone Potenza's which is stock on the STi, the tires actually offer marginally better grip.

Not something you are likely to notice under street conditions. And most drivers are not likely to notice the difference either. On a 1.7 mile road course, a 2003 Mini Cooper S, same driver, same tires, both sets heat cycled, One set full depth, one set shaved. Kumho Victoracers V700. The difference was in the region of 0.2 - 0.3 seconds per lap (1minute 22 sec lap time)

So yes a difference, not really noticeable though. And I doubt noticeable on the street.

I own an STi and can believe it can become too much car for a driver not experienced enough.

[desmo]

I wonder if "caning" a road car could cause enough heat cycles to cause chemical degradation of the tread compound? Probably not, even after hard road duty tires aren't really hot to the touch like racing tires get.

[Greg Locock]

I wrecked a set of road tyres on a 4wd turbo in about 300 km, so I'd say it is possible.

Dry grip will improve as the tyre wears, generally, but you will lose contact path width as you scrub the shoulders.

Wet grip is a direct function of the tread depth - so even light moisture will cause problems as the tyre approaches baldness.

Why is he 'blaming' the tyres? It is his responsibility to keep the car roadworthy and to drive within its limits.

HE FAILS ON BOTH COUNTS.

[JDeRosa]

I definitely think it was a case of 'losing control' - there were some valid points made here. From what I understand, even the hardest racing tyre is stil several times softer than the softest road tyre, so as you pointed out, a normal driver would not notice the difference in a change of grip level so small.

I suppose the biggest lesson to take away from here is to ease yourself into a car like that. Quite often you hear stories of people writing off supercars shortly after acquiring them - just goes to show how these same people have no respect for the power these cars have. Electronics cannot help a driver who thinks he's better than he actually is.

Anyway - thanks, I think we all know what caused this accident...

[Breadmaster]

Originally posted by desmo
even after hard road duty tires aren't really hot to the touch like racing tires get.

I had an '87 BMW M535i which after a caning would burn your fingers if you touched the tyres.....

I certainly found balding tyres were not a restriction to performance and grip.....

[JwS]

There is a point where you go beyond the tread rubber and get to the tire carcase, which will not tend to be very grippy. In any case, obviously your friend is at fault, new tires would have been cheaper, or driver training. So that is where all my insurance money is going, huh...
JwS

[JDeRosa]

update.... The accident investigator for the insurance company has actually deduced from the wreckage that the tyre actually blew out causing him to lose control and crash!! Huh.... thought I had him sussed, guess I was wrong. I suppose better driving could have prevented this, should still have bought a new set if they were that bad!!!

[milliepuppy]

whats an insurance companies stance on a car with less than legal tread depth .. surely its a ready made out for them paying up ......

.. if the tire blew because he wore through the canvas i reckon he's SOL.

[Greg Locock]

Yes, if the tyre is worn to less than the TWI (tread wear indicator) limit then the insurance company could walk away.

I've had some pretty bad blowouts, if one had occurred at the wrong time I don't think a normal driver (like me) could have caught it in time to prevent a wreck, even rear wheel blowouts can be quite hairy.

[random]

Originally posted by JDeRosa
update.... The accident investigator for the insurance company has actually deduced from the wreckage that the tyre actually blew out causing him to lose control and crash!! Huh.... thought I had him sussed, guess I was wrong. I suppose better driving could have prevented this, should still have bought a new set if they were that bad!!!

Oh it's still his fault, tires don't just blow.

With enough crazy driving, a set of tires can be totally compromised in minutes. Drifting will destroy tires very quickly. Just throwing around a 300hp Sti could certainly "cord" the tires in a very short time.

I suspect he misused the tires to such an extent that he wore them down to the cords. Then he thrashed it a bit more and caused a blowout. And if as I suspect, he was driving on worn out, unsafe tires, and didn't bother to check his equipment, the accident was entirely his fault.

But if the insurance adjuster wants to give him a break, who am I to argue.;)

[remmosffej]

Originally posted by random
But if the insurance adjuster wants to give him a break, who am I to argue.;)

We're the ones paying for this moron's claim.

[JDeRosa]

Originally posted by remmosffej


We're the ones paying for this moron's claim.

I doubt the investigator will overlook it if it was his fault. Remains to be seen what the measurements of the shredded tyre produce.

[Evo One]

Random - I take my Lancia Delta Evo on track quite often. It has about 290 BHP and is running Michelin Pilot Sports 225/35-17 tyres. I have done about 10 track days (average 30 miles track running each) and now after about 15k km. they are in need of replacement. Just in case you think I am pussy footing around - I usually have no trouble with the Scoobies or Mitsubishi Evos ;)

Back on topic - it is common practice for race cars that have to run road tyres for the tread to be scrubbed down to about 3mm. depth. My experience is that the balder they are the more dry grip you get.

[BRG]

Originally posted by JDeRosa
Remains to be seen what the measurements of the shredded tyre produce.

Well, if the tread-width turns out to be too wide, he may get all his previous results cancelled too...

[random]

Originally posted by Evo One
Random - I take my Lancia Delta Evo on track quite often. It has about 290 BHP and is running Michelin Pilot Sports 225/35-17 tyres. I have done about 10 track days (average 30 miles track running each) and now after about 15k km. they are in need of replacement. Just in case you think I am pussy footing around - I usually have no trouble with the Scoobies or Mitsubishi Evos ;)

Back on topic - it is common practice for race cars that have to run road tyres for the tread to be scrubbed down to about 3mm. depth. My experience is that the balder they are the more dry grip you get.

And I run the even softer Michelin Pilot Sport Cup. I get good wear on the track out of those as well. Michelin build very good tires, I like them quite a lot better than the other available products.

The problem I've run into with track tires isn't wear, but heat cycles. A quickly worn down set of track tires will stil run fine, even in the rain with only a few mills of tread. But if they've been through too many heat cycles they will harden. Then it doesn't matter how much tread they have left, heat cycled, hard tires are just awful.

Back to the point, I'm just saying that it's certainly possible to trash a set of tires in a single day. If you continually drift a car, do doughnuts in parking lots or a lot of burnouts, any tire will suffer a quick and painful death. JDeRosa seemed to suggest his friend may have been giving his tires just such punishment.

[Greg Locock]

I've just realised that it is unlikely that I wrecked the tyres in 300 km, the chances are they had had a reasonably hard life beforehand. Nonetheless, they were very unhappy, with chunks missing out of the shoulders, and a general air of decrepitude. The sipes had opened up and all the blocks were feathered.

[Evo One]

Thanks for the link to the Pilot Sport Cup page Random - unfortunately these tyres are a bit scarce here in the UK and it looks like Michelin do not make a size suitable for my car.

[JDeRosa]

that's incredible!! I had no idea that there were road tyres available with such little tread, practically slicks for the road. I've got michelin's on my car (not a sports car - Peugeot 306) and I've found that the wear has been a lot slower than my previous set of Continentals. I used to think that tyres were just tyres, but when I got my first 205gti I quickly realised that was rubbish.
Of all the tyres I've run on cars so far, Avon and Yokohamas have been the best. I find that the Michelins (ironically) are quite good on a very wet road but not so good on a damp road, on my car at least. The Yokos were good on the 205 in any condition as were the Avons. When I first went for the Avons, I was a bit unsure, but they proved to be very good. What about you guys? Tyres? Opinions?

[Greg Locock]

"I used to think that tyres were just tyres"

Yeah, the technical term is black round things.

According to the ride and handling guy I know best 70% of the improvement in handling is typically due to tyres. This is why all suspension books have a long, hard to understand, chapter on tyres.

Brands I've usually liked: Yokohamas, Michelin (but, make sure you get a performance tyre if you want performance), some Continentals, some Pirellis.

[Scoots]

All tires get harder with abuse or "heat-cycles". Bald, but not corded tires have usually been heat cycled enough to be rock hard and therefore less grippy.

Slicks don't like gravel any more than they like rain

random ... your assertion that "tires don't just blow" is provably false. Manufacturer defects happen. That said, I think the whole Ford/Firestone thing is BS ... people need to maintain the maintenance on their vehicles and underflated tires are so easy to see and so few people do. This is why the gub'ment is thinking of mandating tire pressure monitors on all cars sold in the US and why california has tire checks as part of the smog cert process now.

Dumbasses are out there

If your buddy gets another STi have him check out http://www.vishnutuning.com

[random]

Originally posted by Scoots
random ... your assertion that "tires don't just blow" is provably false. Manufacturer defects happen. That said, I think the whole Ford/Firestone thing is BS ... people need to maintain the maintenance on their vehicles and underflated tires are so easy to see and so few people do. This is why the gub'ment is thinking of mandating tire pressure monitors on all cars sold in the US and why california has tire checks as part of the smog cert process now.

Dumbasses are out there

If your buddy gets another STi have him check out http://www.vishnutuning.com

I should have qualified it. Manufacture defects do happen, but having driven tires with defects they are usually very evident and rarely cause blowouts. These days tires delivered on quality vehicles like Subaru Sti's are subject to very good quality control and will "almost never" fail without warning. For instance, my tire had a non-fatal defect that simply caused a bit of noise.

Sure tires will blow out, but my contention is that in most cases such blowouts could be easily prevented by proper maintenance and monitoring. And I strongly suspect such was neglected in the case that started this thread.

[Scoots]

Originally posted by random

I should have qualified it. Manufacture defects do happen, but having driven tires with defects they are usually very evident and rarely cause blowouts. These days tires delivered on quality vehicles like Subaru Sti's are subject to very good quality control and will "almost never" fail without warning. For instance, my tire had a non-fatal defect that simply caused a bit of noise.

Sure tires will blow out, but my contention is that in most cases such blowouts could be easily prevented by proper maintenance and monitoring. And I strongly suspect such was neglected in the case that started this thread.

Agreed.

I was just picking nits

[schuy]

Originally posted by desmo
I wonder if "caning" a road car could cause enough heat cycles to cause chemical degradation of the tread compound? Probably not, even after hard road duty tires aren't really hot to the touch like racing tires get.

How hot do race tyres get?

[DoS]

Originally posted by Scoots
All tires get harder with abuse or "heat-cycles". Bald, but not corded tires have usually been heat cycled enough to be rock hard and therefore less grippy.
[/url]

Exactly !!! What most ppl are missing here is that on a tire which is used for track-days the tread is usually scrubbed after a reasonable number of laps, in just one or two heat cycles. On the other hand, almost-bald tires which are exclusively used on road vehicles have 95% of the time gone through an excessive number of heat cycles, so even though the contact patch is actually larger the tire has gone hard and the grip is practically lost !!!! To ellaborate further on this i ll bring you up an example from my own personal experience. The previous set of tyres that i had in my car (Michelin Pilot's Exalto) lost suddendly and unexpectedly their grip after 15-16 thousand kilometers. I remeber i was shocked by the reduction in grip in just one month's time and at first i thought something was wrong with the car. It appeared that at around the 15k mark the tires were starting to get a bit hard and after some punishment from me they turned rock-hard in just 500 Ks. Note that the thread depth was fine (the car is a bit low on Bhp ~ 150). After that the tyres practically maintained the same low grip levels until i changed them with a set of Goodyear F1 D-GS3. By the way, IMHO these are the best overall perfomance road tyres money can buy, compared to Bridgestone S03 and Pilot Sport(old version)- with excellent grip in both wet and dry, low noise ride and good steering response although a bit poorer than Bridgestone's. But on the wet they are king

[Yelnats]

I foolishly ran a set of Michelin X-Ones down to a very low tread depth and experienced a massive failure of the belts on the rear tire. Driving at 50k afterwards and the car felt like it was doing the rumba on the way to the garage! I was surprised by this as I knew racing cars have run of semi-bald tires to get extra traction and assumed they would still be safe in this condition in the dry.

After some consideration I wondered if the very high road temps at this time (34C air temps) had penetrated deeper into the tire due to the lack of tread and heated the metal belts, causing them to separate from the carcase. I expect that a healthy tread depth would also act as cooling fins and further dissapate the heat before it reached the belts.

No more false economy for me!

[Yelnats]

Originally posted by schuy


How hot do race tyres get?

Surface temps on F1 tires exceed 100C.

[schuy]

Originally posted by Yelnats


Surface temps on F1 tires exceed 100C.

Respect

[DoS]

Originally posted by Yelnats


Surface temps on F1 tires exceed 100C.

Well, i think that the normal operating temp for F1 tires is in the range of 110-120 celcious but could go a lot higher if the tires are mis-used

[Scoots]

I like the Toyo T1-S for it's overall combination of wet and dry grip, and ride comfort, but it's a little slow in transition.

I've used R compound tires on the street and while fun, it does get expensive.

[Yelnats]

Originally posted by DoS


Well, i think that the normal operating temp for F1 tires is in the range of 110-120 celcious but could go a lot higher if the tires are mis-used

Specifically I understand that they operate at from 106C to 110C, 120C sounds like an overheating condition to me.

[crono33]

hi

perhaps i am totally wrong here, but what happens with the "fabric" of the tire with time and use?

is there any occurrence of fatigue, stress and so on that will change the shape of the tire or its behaviour?


i am asking this because in some countries it is possible to buy "rebuilt" tyres, which are old tires with a new thread installed on a old tire. they are cheap, but last little, probably due to a very soft compound used to rebuilt the tire, but have also usually crappy performances, perhaps due to wear and fatigue of the internal tire structure?


gm

[Greg Locock]

A tyre manufacturer told me that car tyres are not designed to be retreaded, unlike truck tyres. Well, he would say that. My particular concern would be the butt joint of the belt. It is stuck together with rubber but the steel cords just butt up against each other. I think this is a significant weak point.

Generally rubbers do age so I wouldn't be surprised if the sidewall also tended to lose flexibility, or crack over time, but that would be over ten years or so.

You are right about the short life, 30 or 40% of the new tyre's life would be good.

[black magic]

what pressures was he running as I've been advised to run them at higher pressures than manufacturer recommends

[gug]

tyres are often quoted as being 50% of the cars performance. all those little tweaks to the engine, changing the dash over for a carbon-fibre one and all the rest of the junk that rice-boys do, doesnt count for much compared to finding a good tyre.

tyre grip comes from two parts, some is from the friction acting between the tyre surface and the road, just like it would between your palm and a desk. the other part comes from a chemical interaction between the tyre and the asphalt. this chemical part is strongly related to the temperature of the tyre.

dont quite know how heat cycles effect the chemical factor, but i very much doubt it would be good. i know that a hard tyre will have less elasticity, meaning that it can not attain high slip angles, giving the driver much less feedback and a lower coefficient of friction.

[Scoots]

Originally posted by gug
friction acting between your palm and a d..."

Careful

Ideally a tires pressure should be set by adjusting up or down while driving how you usually drive until the temperature across the face of the tire is equal. Drive more aggressive and you'll want more air, but more air makes for a harsher ride, and too much air reduces grip, just like too little. Also balance air pressure front to back to balance the handling of the car, but the front to back differential shouldn't be more than a few pounds for most street cars/tires.

[MclarenF1]

see here for some more info http://forums.atlasf...&threadid=39295