Thinking about tires - input appreciated

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Thinking about tires - input appreciated

Postby rigby » Wed Nov 14, 2012 10:24 am

My MX5 has 16x6.5 wheels. I want to keep it on the stock wheels so that I can run in our stock/tire class and RTR with other clubs. That leaves a conundrum regarding tire selection. This year I was on 225/50-16 Star Specs. I was more or less happy with the tires' grip, but unhappy with their effect on gearing and thus acceleration. I'm trying to decide where to go for next year.

The stock tires are 205/50-16 Advan A11A's. They make 862 revolutions per mile for gearing reference. Essentially all of the 225/50-16 Extreme Performance tires make fewer revs. Some offer 205's that make similar (or more) revs than the stock tires. Only the R1R is available in a "perfect" size but I'm R1R skeptical

Fundamentally, the question is: would you buy 205's over 225's for my car if it meant better gearing? Would you buy 225/45-16 R1R's if it mean perfect gearing on 225's, but you had to drive on R1R's?
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Re: Thinking about tires - input appreciated

Postby Zoomy » Wed Nov 14, 2012 10:36 am

ref: gearing...it would depend on the venue/event and course layout IMO...fewer revs=larger diameter which may be better on a course that has a spacious layout and higher speed sweepers (i.e. Loring or perhaps Devens). More revs = smaller diameter which could be an advantage on a course where you have less opportunity to run at higher speeds but need the gearing to "shoot you out of the hole" to sort of speak. I have never been on R1Rs so can't offer any opinion on that.
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Re: Thinking about tires - input appreciated

Postby SoloSE-R » Wed Nov 14, 2012 11:10 am

I would wait a few months before making a tire selection. The ST tire class is poised for a shakeup soon (new model from Dunlop is coming, star specs already discontinued, there are rumblings about other new hot tires as well).

The problem with R1R is not the compound, it's the construction. One size (195/50/15) has the good compound all the way through, the rest have it down to 2/32 or so, so they run out of good rubber just as they start to get their fastest. Given their high pricing, they are not worth using in sizes other than 195/50/15 for autox.
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Re: Thinking about tires - input appreciated

Postby rigby » Wed Nov 14, 2012 11:17 am

And the 195 is now banned, so the R1R may be done for competitive use....

Fair points, Carl - thanks
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Re: Thinking about tires - input appreciated

Postby Rocwandrer » Wed Nov 14, 2012 1:28 pm

Long-winded preface to long-winded post: My perspective on issues like this often does not match with the majority. Sometimes I come out with something that is contrary to the conventional wisdom, and it is based in engineering facts and I believe it to be truth, not opinion. On the entirely of the following, I'm just giving my perspective as a racer.

New tires coming: There is a new bridgestone coming called RE-11A. There is a new kumho coming called Ecsta 720 (a street tire tread). There is the new dunlop star spec II. My speculation is that the dunlop and bridgestone will both trade some wet performance for more dry performance, and the kumho will go the other way, but I know nothing for certain. Both bridgestone and dunlop are claiming north of 1.5% better lap times in an autocross-like environment, which would be a game changer for the segment, if true. I am waiting to buy new tires to see what plays out. My default plan is to buy more rs3's because i can live with their faults, and I love the low cost per run and everyday driveability.

Rim width and tire width conundrum: A wider tire mounted to a narrow rim is a losing proposition. "But so and so ran 275/35R15 A6's on a 6.5" rim width and won at nationals!" So what? Excellent driver, bad at setup decision making. A common occurance for sure.

So what if you are stuck with a narrow-ish rim? What size tire do you mount up? The one that comes in the best compound for your wish list of attributes. Seriously! And if that tire is available in two tread widths, I'd run the reasonable one, rather than the shoe-horn-fit one. My trials with 205 and 225 on a 6" rim: My average place at events was 3-6 places higher with the 205 on a 6" wide rim compared to the 225. I think the reason was mostly consistency, with the wide tire feeling floppy and inconsistent on a narrow rim. Most street tires (with tread squirm being a real issue, unlike the hoosier I did my trials with) are even more sensitive to losing raw grip from too narrow a rim.

Diameter: There aren't enough choices. The differences are not huge among the choices. I'd pick based on compound. Given equal compound or equal compound compromise, and a narrow range of potential diameters, I'd take 1st: a slightly taller tire to preclude the annoyance of having to regularly decide between the shift to 3rd or bumping the rev limiter, and second: the smallest diameter possible. I would and have reversed the above priorities if the size range available is substantial, which for the OP, it isn't.

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Re: Thinking about tires - input appreciated

Postby Rocwandrer » Wed Nov 14, 2012 1:37 pm

SoloSE-R wrote:I would wait a few months before making a tire selection. The ST tire class is poised for a shakeup soon (new model from Dunlop is coming, star specs already discontinued, there are rumblings about other new hot tires as well).

The problem with R1R is not the compound, it's the construction. One size (195/50/15) has the good compound all the way through, the rest have it down to 2/32 or so, so they run out of good rubber just as they start to get their fastest. Given their high pricing, they are not worth using in sizes other than 195/50/15 for autox.


The star spec is discontinued? [checks tirerack.com] Holy crap.... When is the last time you've seen them mark UP a tire that is discontinued?!?

The R1R is good to 2/32nds? I've heard they go "off" after 50 or so runs regardless of shaved or full tread, and have (junk) casing-only rubber from 2/32nds. For those that don't know, the star spec rubber compound isn't as good near the casing either. I would guess wet (but no standing water) performance starts to drop off somewhere around 4/32nds strata, and the compound near the cords up to maybe 2/32nds or so (?) is not as sticky in the dry. This might be rubber compound or it might be heat cycles. I've been assuming rubber compound, since the wet performance transition seems dramatic, and happened with my rears before my fronts.

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Re: Thinking about tires - input appreciated

Postby rigby » Wed Nov 14, 2012 2:00 pm

If we cut apart my (old) tires can we find the compound strata? I can confirm that wet performance tanked at about the point Richard suggests, which is why I was the only person to come close to binning my car at the wet event at Crooker, depsite my lecture to everyone else about not doing so...
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Re: Thinking about tires - input appreciated

Postby SoloSE-R » Wed Nov 14, 2012 3:29 pm

Justin - The R1R is far from done as a competition tire, but it is done as an RTR tire. The STC and STS cars will still use them, at least until something faster comes along.


Richard -

I am also waiting to buy tires. I figure I have at least half a season's worth of RS3s left, so I will wait for people with bigger budgets than me to figure out which of the new tires are faster.

I haven't heard that the R1R goes off due to number of cycles, just that they wear much faster than other street tires. The 195s are fast until they cord, others are slow once they get into the second compund (which may indeed only take 50 runs on a bigger/heavier car).

I have no evidence, but I don't believe the dunlops have multiple compounds. Some people seem to be fast on them straight to cord, but most people probably cycle them out before they wear them out.

Over-tiring an A6 will almost always work better than over-tiring a typical street tire. I honestly think they design for that capability because they know so many people in the Stock classes oversize on stock rims. I think it's a little arrogant to call that a "bad setup decision". It's faster for some people and it isn't for others, and it does have a major feel component. Rest assured most of the people competing for national titles actually do the testing to find out for themselves.

I do agree that compound should be the main factor in tire choice over width or gearing though, it's pretty clear that most of the top drivers in STC and STS nationally are running a less than optimal size just to get the right compound all the way down to the cords.
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Re: Thinking about tires - input appreciated

Postby Rocwandrer » Wed Nov 14, 2012 7:12 pm

SoloSE-R wrote:Justin - The R1R is far from done as a competition tire, but it is done as an RTR tire. The STC and STS cars will still use them, at least until something faster comes along.


Richard -

I am also waiting to buy tires. I figure I have at least half a season's worth of RS3s left, so I will wait for people with bigger budgets than me to figure out which of the new tires are faster.

I haven't heard that the R1R goes off due to number of cycles, just that they wear much faster than other street tires. The 195s are fast until they cord, others are slow once they get into the second compund (which may indeed only take 50 runs on a bigger/heavier car).

I have no evidence, but I don't believe the dunlops have multiple compounds. Some people seem to be fast on them straight to cord, but most people probably cycle them out before they wear them out.

Over-tiring an A6 will almost always work better than over-tiring a typical street tire. I honestly think they design for that capability because they know so many people in the Stock classes oversize on stock rims. I think it's a little arrogant to call that a "bad setup decision". It's faster for some people and it isn't for others, and it does have a major feel component. Rest assured most of the people competing for national titles actually do the testing to find out for themselves.

I do agree that compound should be the main factor in tire choice over width or gearing though, it's pretty clear that most of the top drivers in STC and STS nationally are running a less than optimal size just to get the right compound all the way down to the cords.


Dunlops: Justin, I expect the compounding is in some way functional grading, rather than true strata like a recap. I am not in the tire business, and only have a rudimentary understanding of how tires are constructed. I can tell you that all tires have multiple "layers" and that almost all tires use a durable, relatively thick protective coating of some sort on the inside; a different rubber for the cord region than the tread, and a temporary uv protective/ozone protective/"mold release" layer on the outside.

Josh, my experience was that the wet performance (not hydroplaning resistance) went away at a specific tread depth, with 2 tires losing it well before the other two, corresponding to the same tread depth, not the same cycle history. The dry performance was still good, but took a step-wise drop around the same time as the wet performance dropped off a cliff (even when bad it was still a lot better than the RS3).

I do realize correlation is not causation, but to me, that combined with similar experiences of others is fairly compelling, as tire test data goes. I'll add to that the fact the Dunlop themselves advertise their shifting tread compound with tread depth as a feature for other tires they make. The data point I'd be looking for is someone who has had them shaved and run them past 4/32nds. I would also bet small money that not all tires even close to exactly the same in this regard. Batch to batch, size to size, and so on, there will be variations. The old RT-615 had a much higher wear resistance, slightly lower grip layer somewhere around 2/32 tread. I've confirmed this by talking with people who had them shaved. The wear slows down and so do the lap times around that point, regardless of the wear/thermal/shaving/time history.

Wide tires on narrow rims: It takes greater and greater skill to take advantage of a tire that is inconsistent but offers more grip. I am sure the limit of that is well beyond my skill as a driver. But for even the best of drivers, at some point, a narrower same compound tire would be faster even without consistency differences, given a narrow rim width. I proclaim that the point at which it is actually not faster to go to a wider tire given a fixed rim width is substantially narrower than conventional wisdom would suggest. My basis for this proclamation comes from my own testing with input from Hoosier Tire (a great company, I wish I liked their product), testing done by others, and my own engineering judgement. I put a lot of stake in that last bit, which you are welcome to call arrogance :D

On more than one occasion I've been lucky enough to get a window into the process fast people use to test tires. In some cases, the data is less than we'd like, but enough to draw some reasonable conclusions. In other cases the tests are so unscientific as to be, in my mind, throw away data, yet the person doing the testing makes a decision based on it and considers it a definitive result. I think I'm a pretty good judge of this sort of thing. The plaque on my door at work says "Research and Development Engineer" for a reason. Of course, you are free to argue that the reason is that I told the woman who makes the plaques to put that on it, and you wouldn't really be completely wrong :D

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Re: Thinking about tires - input appreciated

Postby SoloSE-R » Wed Nov 14, 2012 10:05 pm

Richard, I know it's literally your job to declare a right and wrong answer to every question, but it gets pretty tiresome to listen to. This is a sport where a top driver's entire season boils down to 3 very short performances, which entail so many variables that are unknowable beforehand, with championships routinely awarded based on margins with no statistical significance. The mental advantage of a driver preferring and trusting the tires he's on could easily be more important in the crucial moment than the physical grip level. And the opposite might be true for another driver, or that driver on a different run. If you win at a high level, you are, by definition, doing things properly regardless of what a thousand engineers with data from a thousand identical cars driven by identical robots on identical surfaces in perfect conditions in a vacuum have to say about it.

In any case, in this club's events the value of a few mm of tire width is buried so deeply within the noise of compromised course design, poor surface quality, fickle timing equipment, weather variability, and skill level that as long as you're not doing something that's obviously wrong (completely uncompetitive compound choice, worn suspension parts, seriously bad alignment), you're probably doing enough to win your class.
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Re: Thinking about tires - input appreciated

Postby Rocwandrer » Thu Nov 15, 2012 12:44 am

I don't think you understand me very well, and clearly you have no idea what an engineer does, but if it stresses you out to "listen" to me, then don't. FWIW, I actually agree with your take on psychological factors as I read it in your last post. I disagree with most of the rest of the post, but we don't have to agree. We clearly have a different take on what is fun about autocross, and how to approach it, which is fine.

I was going to say that you don't have to defend your every point for people in the discussion to realize you haven't been swayed by the points made by others, when I realized it could be easy to interpret my replies that way... That interpretation would get tiresome.

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Re: Thinking about tires - input appreciated

Postby DM » Thu Nov 15, 2012 8:06 am

We engineers find that a signifigant portion of the "fun" is doing that development work, non-engineers will tend to take what others have developed and work on the "other" factors of which their are many. The engineers have the educational background to discuss and develop their theories based on the underlying math and physics. The non-engineers don't have that depth of fundamental science & understanding. Then there is the "level" of engineering focus, with development types like Richard and Chris being very detail oriented whil Mark and I view issues from a different perspective and apply a more forest vs individual trees perspective.

Since most of the club's drivers are not at the driving skill level required to drive a vehicle that is unpredictable or goes away rapidly the more consistent setup will usually be preferred. Ultimate fast times intermixed with spins or plowed cones does not make for a good method to improve driving skills though it may result in a trophy now and again.

Ultimate grip is only achieved when the tire reaches a desired temperature with out going over it and getting greasy. With autocross runs averaging 40-50 seconds too much tire can be detrimental as it won't get up to temperature. Properly sized tires perform best due to the geometry of the tread and sidewall as well. Tread squirm and tread slip angle are very difficult to quantify but are essential to maximizing grip. Tire development does extensive testing to determine what works best. An dsome autocrossers who can't do advanced math or have even heard of physics are going to be taken as experts. Based on at best anecdotal evidence due to obviously poorly developed test proceedures. UGH

Jamming a larger than optimum tire on is not the answer, more is not always better.

Richard sets up his vehicle based on his Development Engineer training and experience and he is about as detail level an engineer as I have ever met. I'm a forest person, he analyzes the cell structure of the tree.
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Re: Thinking about tires - input appreciated

Postby Rocwandrer » Thu Nov 15, 2012 10:41 am

Down to the quote of the month, I like everything about that post :D

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Re: Thinking about tires - input appreciated

Postby Zoomy » Fri Nov 16, 2012 8:29 pm

some input from a "seat of my pants" driver: everyone has valid points. Richard's analytical mind far supersedes mine and I am often stood scratching my head when he talks to me about tires and all of the stuff that he takes into consideration to have the best setup he can for his car (or at least in accordance with his own testing and research,etc.). And for the most part I would have to say that his research and testing has worked well for him up to the point where the actual "drive it like you stole it" aspect comes into play. Here is where I agree with Josh. Having confidence in your setup or trusting your own skills or mental preparation can often surpass a perceived "useful life" of a tire. Point in case, a year or two ago when Richard wanted me to drive his car at the jetport (because he felt his tires had surpassed their useful grip...but wasn't sure) in an effort to see if it was the tires losing their grip or him being in a driving slump...long story short...I ran a time that was about 1.5 seconds faster than his...he eventually caught and surpassed the time I set later on. But the kicker was the look on his face when I told him my run time because I think that he had actually believed that the tires were past the point where he could get any more time out of them. When questioned by him what I did to get such a fast time...I believe I said, "there's plenty of grip left on these tires...just drive it". :) ...good conversation on this topic...quite interesting seeing everyone's input.
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Re: Thinking about tires - input appreciated

Postby WildPony » Sat Nov 17, 2012 7:50 am

I believe it is easy to over-think the tire issue due to many of the points stated above. Primarily the fact that our budgets are small and can't afford a new set every other event and the conditions loaded with variables- including the same venue never being exactly the same twice...I subscribe to the seat-of-your pants theory as Carl suggests.

If I could afford more tires I would put more thought into compounds and upcoming events to maximize any potential. My rs3's are currently junk and they fell off rapidly just prior to Loring when I had them flipped. I'm now pondering if I use them an event or two to start next season before replacing and potentially saving them a bit more for Loring. This, however, will not help my earn points.

Zoomy wrote:"there's plenty of grip left on these tires...just drive it".

There is a lot of merrit to the placebo affect :wink:
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