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      09-01-2009, 08:28 PM   #1
swamp2
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Traction vs. tread depth

My OEM PS2s are currently at 1/8th inch tread depth (one is actually down to just above the wear bar to 1/16th inch). In day to day driving when getting a bit aggressive I feel the rear tires have less traction than when they had more tread depth. However, I also know most track or race tires have very little to no (slicks) tread depth. Does anyone know about the variation of ultimate grip with tread depth? I am heading to a track day soon and do not want to replace my tires before my new wheels come. Please refrain from rumors and speculation, I am looking for a real solid answer here. Thanks.
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      09-02-2009, 05:09 PM   #2
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In dry weather, the closer to a slick, the better. More rubber on the ground and less void area = more grip. So all else equal, low tread depth is actually good in the dry. In wet, of course, it is a whole different story.

The other factor is heat cycles. The more heat cycles on a soft compound tire (warming up while driving and then cooling off) the harder the rubber becomes. Time spent out in the weather also hardens the compound. The hardening of the rubber decreases grip.

Lower tread depth gives more responsive handling. I have had customers complain that their new tires feel sloppy in steering, because they were used to the sharper response of a nearly-slick tire.

My guess is that in your particular case, the loss of stickiness has more than offset the small gains in contact patch, and thus you are feeling a little bit of traction loss. On the other hand, brand new PS2's will be a little deep in tread depth to be optimal for the track. The tread may squirm a bit more and heat up excessively.

If you are headed to the track soon, assuming this is for fun and not competitive, I would probably take the old PS2's, drive within your limits, and send them out in style. If you are driving competitively....well, what the heck are you doing on street tires??? Get some R compound immediately
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      09-02-2009, 06:10 PM   #3
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I agree with Ben. Just use up your PS2s and retire them after the track day as long as they are not worn out to the point where they might cord, but it doesn't sound like it. Perfect way to say good bye. I've done it. If they have hardened a bit, they have hardened a bit. So what? It's not a big deal. You'll just start sliding around a bit earlier, and you'll drive accordingly. Having a little more grip only means that you'll start sliding around later and at a little higher speed. So, as long as you are not trying to post lap times, it doesn't matter. And, you won't beat up on your brand new replacement tires on the track that way. Totally different story if it rains though...
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      09-21-2009, 05:31 PM   #4
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I tried my extremely worn PS2's at the track this weekend with absolutely terrible results. My experiences with tires up to this point told me the same thing. With street tires, when they get worn down to the wear bars (and even a bit before that point) the traction begin to substantially decrease. You can simply feel more wheel spin in hard acceleration and more slip in corners with really worn street tires. Anyway mine were worn enough that it significantly screwed up my track days. My rear end was really loose. I had to baby the car through the course. Early on I even got caught with a very surprise series of tail outs that almost turned into a full spin. Anyway, I appreciate the advice but it turned out to be completely incorrect. I really should have gone with my gut based on much prior experience on the street.
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      09-21-2009, 05:48 PM   #5
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Sorry to hear that. It was probably too many heat cycles that messed up the tires.??
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      09-21-2009, 06:50 PM   #6
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Sorry to hear that swamp. Unfortunately less tread doesn't always equate to more grip. Conditon of the rubber compound through use/heat cycles can easily break down the adhesion to the point where it more that offsets the increase in contact patch. Look at any racing slicks and you will see the same thing, they lose grip after a period of time depending on a number of factors. To be fair to lucid, I think he was thinking "why not" take them to the track since you were going to trash them anyway, although sounds like they were too bad to even be fun.
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      09-21-2009, 07:13 PM   #7
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Quote:
Originally Posted by swamp2 View Post
I tried my extremely worn PS2's at the track this weekend with absolutely terrible results. My experiences with tires up to this point told me the same thing. With street tires, when they get worn down to the wear bars (and even a bit before that point) the traction begin to substantially decrease. You can simply feel more wheel spin in hard acceleration and more slip in corners with really worn street tires. Anyway mine were worn enough that it significantly screwed up my track days. My rear end was really loose. I had to baby the car through the course. Early on I even got caught with a very surprise series of tail outs that almost turned into a full spin. Anyway, I appreciate the advice but it turned out to be completely incorrect. I really should have gone with my gut based on much prior experience on the street.
That has not been my experience at all, but my tires were worn out within 5 months of my purchase, so they didn't really get a chance to harden due to weather. But, that still does not add up since if yours lost traction due to external conditions, you would think that effect would be present on all 4 corners, and not just the rears. So, why you experienced increased oversteer as opposed to less overall grip--if that is what you mean--is not clear to me.

Otherwise, as Ben stated above, the observation that responsiveness actually increases with decreased tread depth is well established (and the recommendation to shave new r-compounds for competition), so I am not sure about why you experienced what you experienced. One possible explanation is that you had poor traction on all 4 corners due to hardening of the rubber over time, but what you really felt was the oversteer condition since this car does oversteer quickly if you get on it. (As I said in my previous post, you might have simply ended up sliding around earlier and at lower speeds, and that's not a big deal IMO, but appearently it was a big deal to you). Another explanation is that this was your first time tracking the car, so maybe you weren't used to/familiar with the oversteer behaviour of this car. I guess a third explanation is that the tire compound is different below 2/32" depth, but I just don't see how that makes sense from a manufacturing point of view.

I went through this type of experience with two types of tires: PS2s and PSCs. Both had considerable grip when at 2/32" and 3/32" tread depth compared to new. I still have the PSCs, and I will take them to the track again. I plan to drive them until the tread is almost completely gone. Sure they won't be like new because of hardening/heat cycles, and I might slide around somewhat earlier, but they will still be fine. I'll just be slower, that's all.
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      09-21-2009, 07:48 PM   #8
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Most of the experienced track nuts I spoke to over the weekend suggested that street compound and r-compound tires will behave opposite as far as extreme depth wear vs. traction. It might be an interesting experiment to compare the traction of a good street tire such as a PS2 that was work very quickly vs. one worn slowly. However, such a test is almost for sure a luxury we won't be able to get access to.

lucid: I don't think your advise to simply use them and get every last mm out of them was necessarily bad advice. However, the degree to which the traction was different on the well worn fronts vs. extremely worn rears was huge and unfortunately really hampered my ability to push the car very hard at all. Oh well, it was still fun as track time always is.

As far as the oversteer thing goes, I am fairly used to the understeer/oversteer balance in the car. I'm also used to not being able to get anywhere close to the grip limit of the PS2s during street cornering without pushing really hard (or show boating...). The experience this weekend at the track was really different.

In the end my advise for anyone heading to a track day with well worn street tires will be to seriously consider the tradeoffs of limited traction when you really want it vs. cost of new tires.
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      09-21-2009, 07:52 PM   #9
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Yeah, such a test would be a luxury for sure. I would be interested in finding out more about it though as I wonder what differentiates the r-compounds from street tires in that respect.

Ben, do you have any info that might explain that?

Also, the downside with getting "addicted" to prime traction that new tires offer is massive $. I say addicted because once you experience what new slicks offer for instance, you keep on wanting that level of performance as their grip starts to fall after a couple of days. You somehow remember how sticky they were/can be, and yearn for that although what you've got is still OK for your average track day, but you can't keep on buying new tires every 2-3 track days (well, that's how I feel anyway). I understand you are saying something somewhat different as you seem to have experienced an uneven fall in traction front to rear and that might have messed up the balance of your car.
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      09-21-2009, 09:20 PM   #10
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Here is my 2 cents on this subject...

I have written a brief summary of what I know about high performance tires, and the track vs. street debate.


The only reason 'street' tires have actual thread pattern at all...is because DOT mandates it. (it's the law) They do this as a public safety measure. Radial tires without a grooved tread pattern, are technically illegal to drive on public roads. Tread-less tires are dangerous to drive on the street in WET conditions.

The grooves (tread pattern) that are molded into the tire, will allow water to evacuate out the sides of the tires footprint. A tire without a grooved tread pattern cannot disperse water very effectively, so it actually traps some of this water under the tire. This dramatically increases the chances of hydroplaning. (even at low speeds). You are literally riding on a patch of water instead of the concrete or asphalt.

This is obviously a bad thing for a street car. (causing you to lose control of the car)

Racing tires typically have no grooves, because it reduces their DRY grip. Since many racing organizations race in dry conditions, the need for a tire that disperses water is largely unnecessary.

Remember, a street tire is by it's very nature...a compromise.

There have to be provisions made for wet weather performance. (for everyday driving conditions)

The 'ultimate' grip for a street tire is not determined by the tread vs. no tread comparison. That is more directly tied to the hardness (durometer) of the rubber compound used, the side wall reinforcement methodology, the wear resistance of the rubber compound used, the resistance to heat-build-up, and the strain resistance over time.

Reinforcing materials such as fabric and steel belts help a tire hold it's shape when new. These construction aspects are all worn down through out the life of that tire. (as the miles pile up) Heat cycling will eventually take it's toll, and the performance will degrade with receptive heating and cooling of the harder rubber compound typically used in high performance street tires today.

Adhering to proper tire inflation pressures is also very important. (track vs. street) These tire pressures are not universal across the board. The tire pressures used on the street, are not condusive to achieving the best lap times on the track in most cases. This is a very important and often overlooked detail by the end user.

A high-performance street tire is designed to make maximum dry/wet traction with the top layer of rubber used in the actual tread. Modern radial tires use many different types of natural and synthetic rubber compounds, silica, polyester cords, steel belts, aramid fibers like Kevlar and yes...even carbon fiber.

For example, the PS2's use a very soft rubber compound that provides excellent wet/dry "grip", but the soft sidewalls (which is the major complaint with PS2's) makes the turn in a little less precise. (not quite as sharp as other brands)

The soft rubber compound also hurts the treadwear performance (lifespan) of the PS2's as well. (they wear pretty fast on coarse or 'grooved' concrete roads)

Manufactures are always trying to find the perfect balance between performance and lifespan.

This is a general rule of thumb about high performance tires...

A tire compound that is soft (low durometer), may outperform one that has a harder rubber compound...but that edge (and the tire) won't last very long, as the tire's performance falls off quickly. (relatively speaking)

A tire compound that is hard (high durometer) will last longer, but the performance will suffer. The harder tire cannot maintain the same grip levels of the softer more playable rubber compound used. The harder compound tires will lose contact with the racing surface (in 'spirited cornering maneuvers'), due to the smaller contact patch that is actually working against you. This results in less grip, but you will generally get better turn in performance and longevity as a trade-off. (allowing you to race longer without making a pit stop for tires)
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      09-22-2009, 02:53 AM   #11
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And the relevance of your post ^ to this topic is where exactly....
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      09-22-2009, 06:19 AM   #12
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Quote:
Originally Posted by swamp2 View Post
I tried my extremely worn PS2's at the track this weekend with absolutely terrible results. My experiences with tires up to this point told me the same thing. With street tires, when they get worn down to the wear bars (and even a bit before that point) the traction begin to substantially decrease. You can simply feel more wheel spin in hard acceleration and more slip in corners with really worn street tires. Anyway mine were worn enough that it significantly screwed up my track days. My rear end was really loose. I had to baby the car through the course. Early on I even got caught with a very surprise series of tail outs that almost turned into a full spin. Anyway, I appreciate the advice but it turned out to be completely incorrect. I really should have gone with my gut based on much prior experience on the street.
My Nitto INVO's are about at the wear indicators for the rear tires. I honestly feel the traction is the same. As for the fronts, they are at 50% and turn in feels much better than when it was newer. Still a shitty tire but at least I feel an improvement.
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      09-22-2009, 10:05 AM   #13
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Some of the track guys on a 6speed will only use new tires for a couple sessions and then take the whees/tires off for 24 hours or more. then Then they'll use them regularly. It's suppose to allow the tire to cure so they'll provide good grip longer. Other wise they'll go bad even with a lot of tread left. Maybe that's what happened with your street tires?

This just gives you a good excuse to buy a dedicated track wheel/tire set-up!!
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      09-22-2009, 07:00 PM   #14
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Quote:
Originally Posted by swamp2 View Post
Most of the experienced track nuts I spoke to over the weekend suggested that street compound and r-compound tires will behave opposite as far as extreme depth wear vs. traction. It might be an interesting experiment to compare the traction of a good street tire such as a PS2 that was work very quickly vs. one worn slowly. However, such a test is almost for sure a luxury we won't be able to get access to.

In the end my advise for anyone heading to a track day with well worn street tires will be to seriously consider the tradeoffs of limited traction when you really want it vs. cost of new tires.
With regard to the street compound vs r compound tires performance at low tread depth:

Most street tires are not built with different compound layers within their usable tread depth.*

The big difference is probably age/heat cycles. For a street tire to wear down to 2/32nds or so will take much longer due to its comparatively hard compound. So it spends a long time (often years) out in the weather, and undergoes hundreds of heat cycles.

A track tire is in use for maybe 2 years at most. Most of that time is spent in a garage, and it undergoes only a fraction of the number of heat cycles. It wears down from a maximum depth of 6/32nds at most, and often 4/32nds rather than a max depth of 10/32nds or more on a street tire. So even though the race compound is much more sensitive to heat cycles and weather, they could still be relatively more effective at 2/32nds than a street tire.

Anyway, sorry that your street tires worked out so poorly on the track. It is unusual to see the balance of handling thrown off that much, although the lower overall grip level would not be so unusual.


*Footnote for the sake of completeness:
They sometimes lay different compounds next to each other, and Bridgestone tires with UniT AQ II technology expose a greater amount of a HIGHER grip compound as they wear down but there is nobody I know of who builds a performance tire with a lesser traction compound at lower depth.
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      09-23-2009, 09:05 PM   #15
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Quote:
Originally Posted by ben@tirerack View Post
With regard to the street compound vs r compound tires performance at low tread depth:

Most street tires are not built with different compound layers within their usable tread depth.*

The big difference is probably age/heat cycles. For a street tire to wear down to 2/32nds or so will take much longer due to its comparatively hard compound. So it spends a long time (often years) out in the weather, and undergoes hundreds of heat cycles.

A track tire is in use for maybe 2 years at most. Most of that time is spent in a garage, and it undergoes only a fraction of the number of heat cycles. It wears down from a maximum depth of 6/32nds at most, and often 4/32nds rather than a max depth of 10/32nds or more on a street tire. So even though the race compound is much more sensitive to heat cycles and weather, they could still be relatively more effective at 2/32nds than a street tire.

Anyway, sorry that your street tires worked out so poorly on the track. It is unusual to see the balance of handling thrown off that much, although the lower overall grip level would not be so unusual.


*Footnote for the sake of completeness:
They sometimes lay different compounds next to each other, and Bridgestone tires with UniT AQ II technology expose a greater amount of a HIGHER grip compound as they wear down but there is nobody I know of who builds a performance tire with a lesser traction compound at lower depth.

It sounds like you also believe that a normal ultra high performance street tire that is worn into the wear bars but worn down to that level fairly quickly (say a couple of months) will have superior traction to a very lightly worn identical tire (again worn in the same amount of time). Could be possible but I doubt it.

Also, how can it be surprising that with 2/3rds to 3/4s of the traction the front tires had in the rear the balance of the car was toast?
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      09-25-2009, 01:14 PM   #16
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Another point of reference from the track on PS2s. I just got back from WGI. I ran into a guy I know who was driving his E36 M3. He had some worn out PS2s, which he seemed to have driven to the track on (he was swapping track wheels on). The PS2s were on or close to the wear bars, so ~2/32-3/32" tread. We started talking about their grip although he wasn't driving on them on the track yesterday. He said he used them on the track several years ago and tossed them aside (probably because they were worn). Then he bought a 911S and a 997 GT3, so it sounded like he hasn't driven the M3 much--at least not on the worn PS2s. Then he said he recently drove on them at LRP with the worn out tread, and turned faster laps on them than he did years ago. When I asked about traction, he said they were fine, and whatever driving skill he picked up over that time made up for whatever loss of traction that might have occurred.

So, I still don't really get how your F/R traction balance was altered so drastically because the fronts were slightly less worn (since he has not reported a drastic loss of grip, and I haven't experienced it myself with my own worn out/destroyed PS2s). I still think of the following 2 potential explanations:

1. Tread compound somehow changes significantly beyond 2/32", but we don't have any evidence and it's not clear why a manufacturer would do this. I can only assume it would cost more to build tread with varying compound.

2. You are just getting used to how the car behaves when pushed in a track environment. You mentioned you have a good sense of the oversteer behavior of the car from street driving, but I don't see how you can really unless you are driving it recklessly. I corner nowhere near any kind of traction limit at 60mph+ on the street even when I'm having fun, and that is pretty much what you have to do to learn about the handling of the car at the limit. I guess you'll find out if this is the case or not when you go back to the track on newer and more evenly worn (F/R) tires.
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      09-25-2009, 02:06 PM   #17
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^Well it sounds like he and I had radically different experiences. Again my simple and repeated observation that well worn tires begin to spin easier and easier on hard acceleration (I've experienced this on many sets of street tires on multiple M3s) gives me plenty enough evidence to believe that traction is substantially reduced under extreme wear conditions on most UHP street tires.

-One of my rears was actually into the wear bar on one side and a couple 32nds away from the wear bar on the other when beginning the event. The wear was severe enough that the tread pattern did not even look like a PS2. However the fronts were something like 1/4" away from the wear bars. Worn but legal and clear tread pattern left everywhere. I did not notice any difference in the traction available from the fronts during cornering nor braking compared to my expectations vs. on the street.

-Although it is difficult to get to the grip limit on the street in our cars, in isolated cases of no traffic and a turn you know well, along with some judicious throttle, getting to oversteer and probing the traction limit can be accomplished in what I believe to be a sufficiently safe fashion. Similarly getting to mild understeer can be done as well. These are not things I do often on the street but I think many of us do ocassionally. Everyone has their own definition of safe/reckless. Some call any breaking of any speed law reckless, regardless of the car or driver.

-The amount of lateral g forces required to get the rear end to slip (throttle induced or not) was absolutely and plainly less than what I was accustomed to on a typical hard freeway entrance/transisiton when the tires were in better shape.

-The temperature was extreme, over 100. Everyone complained about traction overall. However, a high temp does not pick which end of the car to mess with.

-Both of my instructors (serious modded GT3 owners) felt the lack of traction available in my rear tires relative to the fronts and in comparison to other E92 M3s on the circuit.

It is going to be next to impossible to convince me to the contrary of my conclusion on this one. Extremely worn street rubber has worse traction than just broken in or well worn or any other state of wear well outside the wear bars.
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      09-25-2009, 02:27 PM   #18
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Swamp, you can sure make the car oversteer or understeer on the street at low speed without being reckless. That's not what I meant. If you are testing the car at the limit at 60mph+ though (track speeds), which I really doubt you are, I'd say that is being reckless. Obviously, the dynamics of the car are rather different when cornering at high speed (plus doing the other things you do on a track like braking and constantly changing direction in conjunction), and doing that at the limit on the street is not a very likely scenario.

The other thing is that it sounds like one of your rear tires wore out even further prior to the event. In your original post, you said the least amount of tread on your rears was just above 2/32", and in your recent post, you said it was into the wear bar, which means somewhere between 1/32" and 2/32". That's not much tread at all, so did you actually get really close to cording the tire? If so, that would clearly be detrimental in terms of traction. And, I am assuming you were monitoring your hot pressures to make sure you weren't heating up the rears more and over inflating them.

The issue is not that you will not have less traction. It is more about if the loss in traction will be drastic to the degree you experienced (to the extent that it significantly changes the balance of the car), and we are disagreeing on the potential explanations (I haven't ruled out the tread compound possibly being different beyond 2/32"), and so be it.

I guess you'll be able to report back on how the car behaves once you put on the rear tires I sent you that have about 4/32" tread. If you track those, and your car regains its balance, then it was surely the tires, so that would be a simple experiment.
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      09-26-2009, 02:29 AM   #19
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Just above the wear bars, at the wear bar or even into them. All about the same, we are talking +/- a couple 32nds. There may also be some discrepancy in my measurements because some might have been total tread depth and some might have been from the wear bar. Again, either way, the tires were extremely worn. However, I checked carefully before and after the event and there is no cording whatsoever.

I did monitor cold and hot pressures and wear patterns. Nothing exciting nor out of the norm there. I did not get perfect wear but it was acceptable. I'm not absolutely anal about my pressures.

Unfortunately, I doubt I will be back to the track with the used PS2s I picked up from you. It would be a good experiment. I can however still do some decent testing on the street through both straight line and cornering grip, just before and just after the tire swap. I know the answer already though... I've experienced it enough times in the past. I'm just kicking myself I did not listen to that voice or personal empiricism a bit more before this event.
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      09-26-2009, 07:35 AM   #20
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Yes, a couple 32nds should not matter as much, and that is actually what is odd since it is not as if you had 7/32" fronts and 2/32" rears or anything. You had a difference of just 2/32" between F/R as far as I can tell from your OP (you said your OEM PS2s were at 1/8" with one at 1/16"), so how such a small difference results in such unbalanced F/R grip is the interesting part. It is possible that something does change drastically beyond 2/32" with the PS2s and there is a significant loss of grip. I didn't experience that, but my case was somewhat different in that sense that my fronts were worn out (and eventually destroyed) well before my rears, so I should have experienced severe understeer.

But then, you said in post 17 that the fronts are 1/4" away from the wear bars. That would put the fronts at 10/32", which is literally brand new, so I am confused here. How did you wear the rears down to the bars while keeping the fronts at full tread? Did you install new fronts?

Well, let us know what you find out with my PS2s either way. I have a set of PSC+s that have slightly differrent wear F/R. The fronts are at 2/32" and the rears are at 3/32"-4/32". I will eventually take them back to the track on the stock wheels and drive them until they are dead. I will let you know how they behave although they are not PS2s, but they are not full out r-compound tires either.
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      09-30-2009, 01:51 AM   #21
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^OK you have me on precision/consistency and actual measurements. I've been far less precise and more careless than I am typically. However, you don't have me on the concept.

Perhaps some prior numbers were guesses from a quick visual or perhaps some measurements were from the wear bars vs. from the channel bottoms. Fronts are now at 5/32" total depth (i.e. from the channel bottom) as measured with a caliper. Of course there is a 32nd or so variation L to R and around the tires and in different positions L to R across one tire. So extrapolating from there I very likely did have 7/32" front and 2/32" rear a short time ago, precisely what you said I must not have had. This comes from the obvious; more fun accelerating in a straight line and on the freeway driving as opposed to time in the canyons or on the track (oh woe is me...). As you know before the wear bars the bulk appearance of the tread is fairly constant with wear. However at some point the tire really begins to change drastically in appearance (near, at and into the wear bars). I was in this spot on the rears only before hitting the track and was not at all in this regime on the fronts.

Again, not sure how many times I am going to have to repeat it, all of my experience on the street to date, over many cars and many tires, along with this recent track experience, tells me the exact same thing. And until confronted with some very strong scientific evidence to the contrary, I will hold my opinion. Traction falls off strongly at the end of a UHP street tires life. This could very well be some combination of effects, bulk tread pattern change, heat cycling effects, the much lower overall rubber thickness (mass), compound variation with depth. My best guess is that these factors are listed in order of decreasing effect size and decreasing probability.
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      09-30-2009, 03:11 AM   #22
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I've certainly noticed a reduction in (dry street) rear traction as the tyres approached the wear bars, although it does seem counterintuitive....however it is matched with a marked reduction in tyre wear rate which may give some indication of the reason.
A while back I called Michelin tech support about another matter and mentioned this phenomenon and asked if the tyre compound varied with tread depth but he said not...I think he put it down to the tyre compound hardening due to heat cycling and the tyres reduced ability to remain within its optimum heat range due to the reduced amount of tread. I would also imagine that reduced tread would also decrease the tyres flexibility and thus its ability to contour to uneven road surfaces.
I have always found a tyres performance peaks at about 4mm of tread depth.
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