Tread depth and tyre grip.

[Fridgemagnet]

Wisdom of the ancients has it that when fitting a new pair of car tyres, the new ones should go on the rear axle to help prevent oversteer. Never having had much of a problem with oversteer, but wanting to lose excessive understeer and have the best boots for braking in an emergency, I always fit my newest tyres on the front.

This policy almost backfired a few days ago, when exploring the margins of my car* on a twisty, damp and autumn-leaf-slippy long uphill road at 5AM I caught a handful of oversteer and fishtailed violently about 4-5 times before regaining control. This got me thinking about standard road tyre tread, and about how much of the grip seems to be lost as soon as the tyre is only part worn. They never quite grip the same as when they're brand new, mould-release compound notwithstanding.

What I'd like to know, is: What mechanisms are at work to provide cornering grip on a road tyre (ignoring the extra complication of aquaplaning)? How much is down to contact patch area, and how much down to the scrubbing action of the tread? It's a common misconception that slick tyres provide the best grip on a dry surface. Yes, but only with very sticky racetrack rubber that is designed to work at high temperatures. Road tyres need the tread even in the dry, and when they're part worn they lose a lot of grip.

At what point (say as a percentage of original tread depth) should the tyre be replaced? At the moment I run them down close to the legal limit before replacing, but I may have to change my policy. For the record, all 4 tyres are Pirelli P6000, which is quite a soft compound for a road tyre, and as grippy as it is quick-wearing. Usually grippy, anyway...

*Something I do from time to time when there's no chance of hurting anyone or damaging property if it goes wrong. How else is one to know the operating limits of the car? The only times I've ever managed to get all 1.5 tons of my car to fishtail before has been on deep loose gravel and sheet ice.

[danceswithcats]

The most recent and thorough document which I've read is NTSB/HAR-05/02 dealing with a motorcoach median crossover and collision with SUV in Hewitt, TX. The report deals with tire groove depth on both driving and steering axles, and their combined influence on vehicle behavior when effecting evasive maneuvers on wet roads.

[commasense]

Originally Posted by Fridgemagnet

What I'd like to know, is: What mechanisms are at work to provide cornering grip on a road tyre (ignoring the extra complication of aquaplaning)? How much is down to contact patch area, and how much down to the scrubbing action of the tread? It's a common misconception that slick tyres provide the best grip on a dry surface. Yes, but only with very sticky racetrack rubber that is designed to work at high temperatures. Road tyres need the tread even in the dry, and when they're part worn they lose a lot of grip.

You seem to have answered your own question, but incorrectly IMHO. Why do you feel that the physics that applies to race tires doesn't apply to street tires? It's true that race compounds are stickier and wear much faster than street rubber, but it's my understanding that the only purpose of tread is to remove water. So a worn, nearly bald tire should give better cornering grip than a new one. Except in the rain, of course. I don't know what this scrubbing action you're talking about is.

If you're basing your new revelations on the fishtailing you did the other day, I think you've already provided the answer: the "damp and autumn-leaf-slippy" conditions. If there were wet leaves, or just a damp road surface (without standing or running water), it doesn't much matter what tires you had on. Slicks or treaded would have done the same (assuming the same compound).

At what point (say as a percentage of original tread depth) should the tyre be replaced? At the moment I run them down close to the legal limit before replacing, but I may have to change my policy. For the record, all 4 tyres are Pirelli P6000, which is quite a soft compound for a road tyre, and as grippy as it is quick-wearing. Usually grippy, anyway...

I see no reason to replace a tire before it comes close to the wear bars, with the proviso that you should be a little more cautious in wet conditions on worn tires. You didn't mention what kind of car you have, but FWIW, I don't care much for Pirellis. Not as grippy as Bridgestones. Michelins, or Yokohamas, in my experience.

[Fridgemagnet]

Originally Posted by commasense

Why do you feel that the physics that applies to race tires doesn't apply to street tires? It's true that race compounds are stickier and wear much faster than street rubber, but it's my understanding that the only purpose of tread is to remove water.

That's one of the purposes of tread, and is certainly the purpose of the big central groove down the centre of mine. Also the tread pattern is cunningly designed to throw water out sideways as well as provide a scrubbing action. But I can tell you for sure that a road tyre never grips as well as when it's new. This I have found from years of evil experimentation. Once the square edge has rounded off the tread they start to slide much more easily, and get progresively worse as the tread depth wears down. I've noticed the same thing with my aggressively-treaded winter mountain bike tyres.

Rally drivers know all about scrubbing action, or they'd use slicks on dry gravel stages. They don't! And track race slicks lose grip when they're worn because the thinner rubber starts to overheat, and pushes the tyre over its optimum temperature range. This isn't the mechanism with ordinary road tyres, as they run cool at road-legal speeds.

Originally Posted by commasense

I see no reason to replace a tire before it comes close to the wear bars, with the proviso that you should be a little more cautious in wet conditions on worn tires. You didn't mention what kind of car you have, but FWIW, I don't care much for Pirellis. Not as grippy as Bridgestones. Michelins, or Yokohamas, in my experience.

It's a Citroen XM, which looks a bit like a huge Xantia. It's not a sports car, it's an old executive cruiser, and weighs 1.5 tons. I had the suspension computer set to "normal" plush mode, when really it should have been flipped to "sports" for that section. Normally I am a cautious driver, but every now and then there's an opportunity to explore the outer margins of the car's handling without threat to life or limb, and this experience makes for safer normal driving. On this occasion I was deliberately pushing the sideways grip, but it normally drifts gently and predictably. Those autumn leaves are as slippy as graphite, just on a larger scale.

I'll look into your tyre recommendations commasense, thanks for those. I like tyres that are grippy and predictable! I used to run Uniroyal Rallyes because they're just about the best rain tyre out there, but I got fed up with the washy handling as they squished about when cornering, and switched to Pirellis. Maybe Michelins will go well with a French car, because those were the original fitted tyres, and Citroen will have done most of their suspension development using Michelins.

Thanks for the accident report link, danceswithcats. It may take me a while to plough through all 80 pages, but I think buried therein is the answer to my question. The tyre on that coach is as bald as a bastard - ain't no way that its slick state grips better even on a dry road.

[commasense]

As anyone who's watched Formula One or other pro racing series knows, tire construction and compound formulation is an incredibly complex mix of art and science. There are millions of variables and few easy answers.

However, I still believe that, all other things being equal, a slick tire will provide better grip on a smooth, dry, asphalt surface than a treaded tire. Tread will no doubt improve traction in gravel, rain, and snow, but otherwise I don't think it helps, because the gaps in the tread reduce the contact patch, and hence the friction that the tire can apply to the road surface. My cites: Wikipedia:

The gaps between the rubber sections are designed to allow water to be expelled from beneath the tire and prevent hydroplaning. The proportion of rubber to air space on the road surface directly affects its traction. Tires intended for dry weather use will be designed with minimal gaps to increase the contact patch. Tires without any tread patterns are known as slicks and are generally used for racing only.

And (more persuasively, IMO) Goodyear:

A racing slick, a tire without any tread, provides the ultimate in dry traction due to the maximum amount of rubber touching the road. A tread pattern, with its groove voids, actually impairs this adhesion capability on dry roads. Tire tread acts as a squeegee on wet roads to remove water from under the tire and channel it through the grooves for improved wet traction.

It's possible that as a tire ages its chemistry changes and its grip level decreases, so that an older tire (regardless of tread depth) might perform worse than a newer tire, but that's why I said "all other things being equal."

If you're basing your claims solely on your personal experiences, I suspect that other forces may be at work. Your reference to the square edge wearing off makes me think of an alignment/camber setting issue. If your tires are wearing unevenly because of suspension setup, that could cause traction to decline over time. Changes in ambient temperature and over- or under-inflating can also reduce grip, unrelated to tread depth.

Of course the performance of mountain bike tires will decline over time, but we were talking about running on road surfaces, not dirt. And on wet leaves, all bets are off. Nothing will save you. (The only time I ever broke a bone was when I drove over some wet leaves on my motorcycle at about 5 mph, fell over, and broke my collarbone.)

If you have a cite that supports your claim, I'd be interested in seeing it. I found this one that seems to agree with you, but I don't find it terribly persuasive or authoritative. It doesn't explain why tread would improve traction, just asserts that it does.

[Dog80]

I am the self-appointed tire expert here but I am too tired to reply now. So you will have to wait until tomorrow to be enlightened

[Fridgemagnet]

Cheers Dog80, I look forward to your post, bright-eyed and bushy-tailed tomorrow.

Having ploughed through danceswithcats excellent reference, here are some quotes from the accident report file (pages 43-44):

"Research and testing on passenger cars indicate that “friction forces at highway speeds are reduced to half or less of the new tire value if the tire wear exceeds about 50 percent, as it did on the drive axle tires. This research also indicated that the lateral friction of tires decreases well before hydroplaning is expected to occur. Further, when the worn tires are placed on the rear of passenger cars, the handling of the vehicle changes, since the rear tires have more tendency to slide."

and

"Research on passenger vehicles found that 'normal lane change maneuvers can lead to loss of control on a wet road if sufficient difference in tread depth exists front to rear, with the better treaded tires on the front axle of a passenger car.'"

Well, that pisses on my policy of putting the best tyres on the front for shorter emergency stopping distances. I may still carry on doing this, but change the rear tyres way before they approach legal minimum tread depth.

Originally Posted by commasense

However, I still believe that, all other things being equal, a slick tire will provide better grip on a smooth, dry, asphalt surface than a treaded tire.

I believe this is the commonly held view, and may even be correct. But it is my pet theory that the above statement is only true for rubber with a very high level of grip, and for the hard-wearing rubber of road tyres, the scrubbing effect of the square edges of the tread plays an important part in grip, even in the dry, and that a slick road compound tyre won't grip as well as the same road compound tyre with tread. But I could be wrong, which is why I was asking.

I certainly agree that nothing short of steel spikes grips on a certain sort of wet leaf. They're not so bad towards the end of winter when they've decomposed a bit, but the first leaf falls of autumn are super slippy. It's the only time of year I can't get any mountain bike tyres to grip in certain sections of the local forests, as when the ground is still hard from the summer winter knobbly tyres don't dig in, and slicker, stickier summer tyres just skate on the planes of leafy slime. Luckily this scary window only lasts a couple of weeks, and then the autumn tyres start to kick in nicely, to be replaced by winter tyres once the groundwater table reveals itself as permapuddles. I could write a monograph on MTB tyres, but they're a different beastie to a car tyre.

[Rick]

Working from memory I seem to recall that the guys running SCCA showroom stock who must run street tires, would have brand new tires shaved down to 6/32".
I did a search and it looks like if you buy Hankook tires [PDF!] their race tires come with 8/32" of tread. A little more than 1/2, but not the full 12/32 that most street tires have.
So I will go with max grip on a street tire at between 6/32"-8/32"

BTW with regard to wet leaves, they have the same friction co-eficent as greased owl shit.

[n0disguise]

Originally Posted by Rick

Working from memory I seem to recall that the guys running SCCA showroom stock who must run street tires, would have brand new tires shaved down to 6/32".
I did a search and it looks like if you buy Hankook tires [PDF!] their race tires come with 8/32" of tread. A little more than 1/2, but not the full 12/32 that most street tires have.
So I will go with max grip on a street tire at between 6/32"-8/32"

BTW with regard to wet leaves, they have the same friction co-eficent as greased owl shit.

This doesn't apply to real street tires, like you or I would get for our daily driving cars. No one shaves real street tires as far as I know. New competition-level (R-compound) tires are extremely soft, and the height of the tread blocks at factory tread depth makes them very squirmy, unresponsive, uncommunicative, inconsistent -- pretty much an all-round pain in the ass in track conditions. Some of these can legally be driven on the street. These are the ones people shave. I can't speak for all R-compound tires, but of the ones I have experience with, the only time that anyone runs them with full tread is when there is standing water. It would be pointless to shave street tires because for one, they are hard enough that they do not significantly deform under cornering loads, two, if you shaved some tread off of them, you wouldn't increase the contact patch since all of the grooves are perpendicular to the floor, and the manufacturer didn't hide some extra-sticky rubber under there.


Now for the OP.

Originally Posted by Fridgemagnet

What I'd like to know, is: What mechanisms are at work to provide cornering grip on a road tyre (ignoring the extra complication of aquaplaning)? How much is down to contact patch area, and how much down to the scrubbing action of the tread? It's a common misconception that slick tyres provide the best grip on a dry surface. Yes, but only with very sticky racetrack rubber that is designed to work at high temperatures. Road tyres need the tread even in the dry, and when they're part worn they lose a lot of grip.

I'm siding with commasense here on every point. I don't think it's a misconception that slick tyres proved the best grip on a dry surface. Tires take a lot of abuse -- heat cycling, expansion, aging, and all that rubbing around on the floor will take their toll on any tire. It is these things that cause a tire to lose their grip.

[Dog80]

I promised an answer, so here I am!

There are several factors that affect tire performance, but the most important is time (a tire is considered dead after 4-5 years from manufacture). The other important factor is how many warming up-cooling down cycles the tire has endured, what were the maximum temperatures it has reached, for how long and finally how gradual was the warming/cooling. No mention of tread so far!

The tread acomplises three things:
1. Removes water
2. Removes dust
3. Dissipates heat

A worn tread does not cause bad grip. It is only an indication that the tire is too old or has endured a few too many warm up/cool down cycles. Suppose we took a brand new tire and using a grinder we managed to remove most of the tread; the tire should have the same amounts of grip as one with full tread.

[chaparralv8]

One of the bigger factors here is that tire technology has advanced significantly in the past couple years. Any car will oversteer with Kumho V710s on the front and pre-V700 Victoracers in the back. The front tires you've got may just be flat-out better than the ones in the back.


And yes, on a dry surface, slicks are better by a clear mile. You're not wasting any precious contact patch area with voids in the tread.

[Fridgemagnet]

Thanks for all the replies, guys. It was possibly a bit of a mean question I posed, as there are so many variables to take into account it makes a broad definitive answer impossible.

Originally Posted by Dog80

A worn tread does not cause bad grip. It is only an indication that the tire is too old or has endured a few too many warm up/cool down cycles. Suppose we took a brand new tire and using a grinder we managed to remove most of the tread; the tire should have the same amounts of grip as one with full tread.

I take my trousers down to your greater driving skill and experience, Dog80, but I still contend that a part-worn tyre doesn't grip as well. It is my experience that it is the squareness of the tread edges that provides a large degree of the sideways grip, at least on a road tyre. When the tyres are part worn, the tread edges round off, and they just don't dig in as well. This is most noticable in the damp (ignoring aquaplaning on standing water, just a minimally damp surface) and is least noticable in the dry, though the effect is still present. Shaving a tyre to change the tread depth is a different thing, as you'll wind up with a tyre that still has nice square tread edges. You'd get a higher-pitched cornering squeal from the shallower treads as they deform and snap back at a faster rate, but I doubt this would affect grip too much.

I know I'm in the minority here in my views on tread, but after a little googling I feel somewhat vindicated by the sorry experiences of 8th Grade Kevin, who lost marks in a science test for eloquently stating the commonly held view that slicks always grip better. A little harsh, I feel, to penalise an 8th-grader on an issue that Dopers can't agree on.

This quote from my previous 8th-grader link:

"The traction in tires is different than the standard force of friction that most physics textbooks talk about. In automobile tires, the edges of the treads are a major factor in preventing sliding. They account for much more traction or friction than the coefficient of friction of the rubber on the pavement. Of course, a wider tire would usually have more treads than a thin tire.

With racing tires, there are no treads. They try to get the tires hot, so the rubber becomes somewhat sticky. Also, the tire pressure is lower, so there is more surface area on the track. The traction from the stickiness of the rubber is proportional to the area on the ground."

Grooves in F1 race car tyres were introduced a few years ago in a bid to slow the cars down in corners, and indeed they did reduce the grip. It's one of the reasons former champion Damon Hill quit in a fit of petulance - he complained the new grooved tyres ruined the feedback from the track. But upon closer inspection you'll see that the grooves are rounded, not square. I doubt that even square grooves in such a sticky compound as an F1 slick would improve grip, but the same wouldn't necessarily apply to harder road rubber.

For the record, I like all 4 tyres the same model, just for the balance. None of my tyres are more than 3 years old. The front tyres last about a year before they're changed, the rear tyres 2-3 years. They don't get the same temperature cycling as a race tyre as they're at least 130mph speed rated, and I don't cruise at more than 90. I'm very anal about my tyre pressures too, and check them about once a fortnight.

But after much thinking on the topic, I have made a major life decision. From now on, the new tyres are going on the rear. Upon reflection, I'd prefer to be dealing with understeer than oversteer. A big heavy car going into a spin is no joke, and I was very lucky to recover control from the sphincter-clenching moment outlined in the OP.

[eviladam, part 2]

Originally Posted by Dog80

I promised an answer, so here I am!

There are several factors that affect tire performance, but the most important is time (a tire is considered dead after 4-5 years from manufacture). The other important factor is how many warming up-cooling down cycles the tire has endured, what were the maximum temperatures it has reached, for how long and finally how gradual was the warming/cooling. No mention of tread so far!

The tread acomplises three things:
1. Removes water
2. Removes dust
3. Dissipates heat

A worn tread does not cause bad grip. It is only an indication that the tire is too old or has endured a few too many warm up/cool down cycles. Suppose we took a brand new tire and using a grinder we managed to remove most of the tread; the tire should have the same amounts of grip as one with full tread.

i think "heat cycling" is the answer that we're looking for. by the time a normal tire is worn down to a reduced tread level, it has been through thousands of heating/cooling cycles that have degraded the grip of the rubber itself.

if you look at high performance tires, you'll see large tread blocks, designed to put the largest solid patches of rubber on the pavement. tread design is a compromise between road noise, dry grip, wet grip, etc etc.

[brad_d]

Bridgestone/Firestone offers some tires that feature "Dual-Layer Tread." They describe it as higher-grip rubber that's exposed as the outer tread rubber wears away.

Scroll most of the way down this page (UNI-T AQ) to find this:

Our Dual-Layer Tread structure exposes high-grip rubber as wear progresses. We use a hybrid tread structure with two kinds of rubber in the tread.

The two kinds of rubber used in Dual-Layer Tread design both contain EPC. But we employ a higher-grip rubber for the base layer under the cap (shown in red in the diagrams). And we shape the base layer to mound up under each of the tread blocks. So as the tread wears down, the higher grip rubber underneath is exposed. The higher-grip rubber helps minimize the effects of wear. It also helps maintain more consistent wet braking and handling throughout the life of the tire. And it helps prevent the increase in noise levels that tends to occur as tire tread wears away.

They also have a page on UNI-T AQII which elaborates a bit more. Scroll down to the very bottom and see the section on "Enhanced Dual Layer Tread". The text is part of an image so I can't copy it all, but basically they describe two different tread compounds in the same tire - the outer "cap" rubber, and the inner "base" rubber: "The base compound is designed with higher content of Silica and better wet performance."

Those pages are orphans - this link allows one to browse around.

I checked them first since I remembered seeing ads for this dual-tread stuff in stores; I would not be surprised, though, to hear that other tire companies do something similar.

[tracer]

For those of us on the Left Side of the Pond, I'd like to translate some of the British terms used in the OP into American English (known around here as "Real" English):

tyre = tire
boots for braking = brake shoes or brake pads (I think; only the British know for sure)
exploring the margins of my car = testing the limits of my car
autumn-leaf-slippy = autumn-leaf-slippery
mould-release compound = mold-release compound
aquaplaning = hydroplaning
in the dry = when they're dry
grippy = Britney Spears

[n0disguise]

Originally Posted by Fridgemagnet

...
I take my trousers down to your greater driving skill and experience, Dog80, but I still contend that a part-worn tyre doesn't grip as well. It is my experience that it is the squareness of the tread edges that provides a large degree of the sideways grip, at least on a road tyre. When the tyres are part worn, the tread edges round off, and they just don't dig in as well. This is most noticable in the damp (ignoring aquaplaning on standing water, just a minimally damp surface) and is least noticable in the dry, though the effect is still present.
. . .
I know I'm in the minority here in my views on tread, but after a little googling I feel somewhat vindicated by the sorry experiences of 8th Grade Kevin, who lost marks in a science test for eloquently stating the commonly held view that slicks always grip better. A little harsh, I feel, to penalise an 8th-grader on an issue that Dopers can't agree on.
. . .

I don't want to sound like a dick, but . . .
First off, that guy in your link is totally full of shit. I'm utterly flabbergasted by his many blatantly false assertions.

Second, how could you tell if it was the rounding of the edges that led to the decrease in grip since the tire edges don't get rounded unless they are also getting heat cycled, and aging? Roads are hard. Tire edges don't "dig into" roads. There's also evidence for this in that race cars are set up to keep the tires perpendicular to the road. If the edge was what was giving the tires their grip, they would want the edges of the tire making contact. Sports cars', and even sporty cars' suspensions are designed to keep the tires perpendicular to the road and with as much of the contact patch making contact as possible.

The only time treads increase traction is when there's something in the way (rain/snow/dirt) that needs a way to get out of the way. Look at all of the tires out there. Generally, the more performance oriented they are, the less tread they have.

On a paved road with no standing water, no tread=faster than tread. This is something everyone in racing agrees on.

[commasense]

No disrespect, fridgemagnet, but I agree with n0disguise about tread edges. I think you may be letting your dirt bike instincts carry over to street driving. What's true in one isn't necessarily true in the other.