[Cloud9...68]

Maybe this belongs more in racing, but I found this to be very interesting. In the latest issue of Grassroots Motorsports, they ran a comparison of four extreme performance tires. Overall, the BF Goodrich G-Force Rival was the clear winner. But that's not the interesting part. Most of the test was run on 15 x 8" wheels (on a race-prepped Honda CRX) on 205/45/15 tires. But at one point in the test, they mounted a set of 225/45/15 Rivals on the same 8" wheels, and here was their reaction:



"The two tire sizes felt completely different, with the 205 feeling more responsive and diving into the apex more easily." The lap times for the two tires were virtually identical.



Then, they switched to 9" wheels up front (8" was the widest that could fit in the rear), and mounted the same 225/45 tires. Here's what they said:



"On the track, this combo was magic. It was simply better than all the previous set-ups in every phase of the corner: Braking was deeper, mid-corner grip was better, and power application off the corner came earlier. In the end, a half-second advantage separated it from the same tire on an 8-inch rim. More importantly, we turned our fastest-ever single lap at Harris Hill - by a fair bit." This despite the fact the the 9" rims were almost surely noticeably heavier than the 8-inchers, and the fact that they were constrained to the same 8" rims in back that they earlier found didn't work all that well with a 245-section tire.



What I find so interesting about this is the wheel/tire width combinations they're talking about. In comparison, I'm squeezing a 255 section tire on an 8.5" wheel up front (285 on 10" rims in back), and I know I'm far from the only one running similar wheel/tire relationships. But looking at GRM's results, and it's really hard to argue with the type of both quantitative data and subjective impressions they give, it seems that this may be a mistake. Unless GRM's test car is in some way unique, their results indicate that optimum cornering is obtained by having a wheel width that's very close to the tire's section width. Maybe this is common knowledge to everyone but me, but it definitely caught me by surprise.



Having wide tires sure looks cool, but on top of strongly leaning toward changing from my current staggered 18" set-up to a square, 17 x 9 approach (primarily to allow tire rotation, and to save a bunch of money on tires), I'm wondering if it wouldn't make sense to go with a much narrower tire than I had been considering. Anybody have any real-world experience, or some other form of data, that disputes GRM's findings? Thanks.

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[flash]

hmmm - sounds a lot like something somebody else has been saying all along.

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[Paul W]

I don't have any experience with this.....but having a 255 on a 8.5" rim seems much too wide for the rim. The tire would not sit correctly on the rim.



It seems like a 255 width would not be optimal for a front tire on a 968, as they came from the factory with 205's or 225's stock depending on the wheel.



Why did you choose a 255?



I also think if you went to a 17" setup, you will find that your unsprung weight goes down and the car will handle better. No matter what the car I have, the small the wheel, the better it always handles. Larger rims almost always equates to a heavier wheel/tire package. And usually a higher cost.

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[Cloud9...68]

Flash,



Yes, GRM's test results are in line with what you've been recommending, but they seem to be going a step further. As I recall, for a track-oriented 968, you recommend a square 17 x 9 wheel set-up, with 245/35-17 tires, but the GRM article recommends a wheel with that comes as close to matching the tire section width as possible, thus the 225 tires on 9" rims giving by far the best performance in their test. Unfortunately, I haven't been able to find a highly-rated 225 tire in anything smaller than a 45 aspect ratio, and I'm interested in reducing my overall tire diameter as much as possible to increase my effective final drive ratio, as we discussed over the phone recently. The Nitto NT01 does come in a 235/40-17, though, which may be a decent compromise. The Toyo Proxes R1R that comes in a 245/35-17 is directional, so they theoretically can't be rotated side-to-side (although Tire Rack ran a test on this, and didn't find a significant difference in tire performance in either wet or dry as a result of mounting the tires "backwards"), which would be a deal-breaker for me.



Paul,



My current wheel/tire selection came from talking to several experienced and highly successful 944/968 race car builders and drivers, and my uneducated desire to cram the widest possible tires onto my car. Many people have been very successful with the set-up I have, but if nothing else, it's brutal from a running cost standpoint. I wish I had done more research before before I pulled the trigger, as I've since found that a staggered 18" set-up isn't quite as "universal" as I had initially thought. Oh, well, live and learn.

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[Paul W]

Cloud.....was in Austin last weekend for a wedding....be happy you spent the money on wheels/tires vs. drinking or eating it away downtown! Wow, that's easy to do there.

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[flash]

cloud and i have had this conversation, as well as i have said it repeatedly here. the common "wisdom" is that bigger is better. that thinking is wrong when it comes to this car.



a 17" rim will have less weight than almost any 18. that is better.



a 17" tire will have much less weight than any similar sized 18, and more importantly, less weight at the outer diameter.



now, to the tire width. while having more tire seems like a great idea, it really depends largely on how that tire behaves, and how much load you place on it. increasing tire width can easily INCREASE tire temps at the outside tire. this is because the flex of the rubber under compression increases the friction and therefore temperatures. so, while you think you are going to get better tire wear and traction, you can actually be worse off. at lower speeds you will see improved traction, but once you get enough speed going to generate heat, you can see less traction.



it's a really complicated set of variables, that require a lot of math to work out the answer. i don't know of too many people who bother, and most of them just keep repeating the mistakes of others, and expecting a different result.



as for choosing the "right" width, you have to look at the tire, how the sectional width is relative to the wheel (and they vary from brand and model to brand and model) and also look at the sidewall construction. for example, you can up a size in width, if the sidewall is really stiff, but if you choose one that is soft, you need to stretch it more on the rim.



at this time, i think the optimal compromise tire would be a 245/35-17 R1R, on a 9" rim, because i would choose the smaller outside diameter over the narrower width of the 235/40. on a normally aspirated car, i think the added punch would outweigh the cornering difference. i think i could offset the front outside tire temp change by merely softening the rear springs a bit.

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[Cloud9...68]

Flash,



Agreed, it is a complex problem, although, as we discussed, it's far from rocket science. You're spot on about the weight issue - my current rear wheel/tire combination weighs a hernia-inducing 51 pounds!



I also agree about the 245/35-17 tire's advantage from its smaller diameter - I estimate it would give me 200 extra rpm at the same speed as my current set-up, which would translate into a huge improvement in acceleration out of corners. My only gripe with the Toyo (the only tire available in that size) is that it isn't (at least theoretically) rotatable side-to-side, which defeats much of the financial purpose of going to a square set-up.



Also, it fared pretty poorly in Car & Driver's most recent tire comparison, which was overseen by Tire Rack, at their facility. In addition to not doing well in any of the tests, the testers commented that is was wearing faster than any of the other tires in the comparison. However, they are a very stiff-walled tire, so they may work well on a 9" rim, despite being a little wider for that size rim than GRM's results predict. The fact that they're only 23 pounds (yowza!) is definitely enticing as well. Also, the Nitto's are an R-Compound tire, which would probably wear even faster than the Toyo's, so their advantage of being rotatable may not amount to much. Back to what you said, yes, definitely a lot of factors to consider, but a fun and interesting mental exercise. I appreciate your helpful inputs.

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[flash]

tire tests are a tough thing to gauge. tire wear is even tougher. one car might wear out a tire, while on the other it will wear like nails. this is all due to corner loading and weight transfer. what happens on an M3 is almost certainly not the same as what happens on a 968. been there done that.



admittedly, i don't know diddly about the R1R relative to other tires in that size. i would have to see how it fared, compared to others, in the same size, on a similarly weighted and set up car. that being said, when you get into that upper stratosphere of tires, the differences are not all that big. choosing the right sidewall and size for the particular car is far more important than any particular model's individual test on some other car.



regarding rotation and such, i would go make friends with your local tire guy. after a rotation front to rear, when they wear down enough, just cross mount those puppies, and you're good to go for a while longer. you should be able to get that all done for $100. that's a whole lot cheaper than buying even 1 tire.

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[Cloud9...68]

Yes, I've thought about cross-mounting directional tires to increase their life. May be worth doing. But as I alluded to above, I ran across an article on tirerack.com in which they tested a set of directional tires, then intentionally mounted them on the "wrong" side of the car, and compared results. There was virtually no difference in dry lap times, and actually identical wet lap times (which is interesting, because the main purpose of directional tires is to optimize the channels for water expulsion), and no subjective difference in steering feel, stability at the limit, or any other characteristics. So the issue of directionality may be moot, especially if the tires are rotated after each track session.

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[flash]

i've only run them backwards once. it made more noise, and had a slight vibration to it. this could easily be explained by wear patterns though.



i know they all say not to do it, and that it will cause premature wear, loss of traction, and all that. i just haven't done it long enough to find out, nor have i seen any real data to bear anything out one way or the other.

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[Cloud9...68]

[quote name='flash' timestamp='1383767711' post='151782']



i know they all say not to do it, and that it will cause premature wear, loss of traction, and all that.

[/quote]

lol - Just like "They all say" to use 18" wheels on a 968 track car! Sorry, couldn't resist... Removing tongue from cheek now.

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[flash]

roflmao - fair enough



seriously though, i do know some people who have done it and had weird directional stability issues (pulls, wandering, etc). i don't have any details on which tires they used though, so no help there.

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[MB968]

As general rule, I would say that you shouldn't run a directional tire backwards. Tire Rack's test was only for adhesion. It's not too surprising that they didn't see a lot of difference. If they looked at hydroplaning they probably would have seen a difference. But until you get into a lot of water, you probably won't see the directional nature impact the results a lot.



If the tread is mostly worn, as I think Flash suggested, it becomes more of a moot point if they are mounted backwards (from an adhesion standpoint).



Just some extra food for thought. Having worked with a lot of materials science guys they play with a lot of things to get the properties that they desire. I expect tire engineers also play with a lot of parameters to get everything they can out of a tire. So, what I don't know is if the construction of the cords/plies might be such that it isn't good to run backwards. Also most tires these days have silica wiskers in the tread for adhesion. I don't think these are oriented (but this is common to do in polymers to get the properties you want), but if they are, that could also be another reason to only run forwards. And, I don't know this for a fact, but I can imagine that running them backwards if the wiskers are oriented could lead to undesirable wear characteristics if it caused the wiskers to come away from the tread before they were worn to the point that they should.

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[Hasbro]

The thread below is tied into the GRM article and can really help people to understand traction, etc. Good read.



http://grassrootsmot...re/73662/page1/



I'm running 9.4" of tread, with a wheel/tire weight of 40 lbs.. Custom wheels would get it down to about 38 lbs..



Tires and suspension are (is) my favorite interest with cars. Science and art combined.

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[flash]

yup - making cars go faster through the corners has been my focus for 30 years. that's the primary thing my shop did. i approached it from an engineering standpoint, and cut a lot of the trial and error process out.

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[Cloud9...68]

MB,



Hmmm... Being able to rotate tires side to side is a primary criterion of mine, since the fastest turns at the track I go to are left-handers, so my right side tires wear out a lot faster than my left. Yes, I could cross-mount my tires to get a little more life out of them, but I would much prefer to be able to rotate them frequently, like after every session or two, which would get cost prohibitive to do via cross-mounting, not to mention very time-consuming. When I did a search on the consequences of running directional tires the "wrong" direction, I got the usual assortment of opinions not based on any direct experience, which is why I found the Tire Rack test so compelling - their conclusion was basically that it made no difference in any measurable or subjective way, including wet performance (which I assumed covered hydroplaning). However, they did caution that their test wasn't necessarily the be-all/end-all, and that YMMV. If it really is a very bad idea to swap directions on a directional tire, this pretty much kills the Toyo for me. I think I'll give Toyo a call and get their take on this with this particular tire.



Hasbro,



You may have posted this in another thread, but what size wheels are your currently running front and back, and what brand, model, and size(s) of tire? Thanks. As far as wheel weights, my current fronts weight 19.5 lb, and the rears are 21 lb (Enkei NT-03's). So far, the lightest 17" wheel I've been able to find are Enkei RPF1's, at about 16 lb in a 17 x 9" size. And I agree that this whole topic of tires and suspensions is a fascinating one.

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[MB968]

It would be interesting to hear what Toyo says about it. Hopefully you can get to someone who really understands what's going on with the tire in all ways, including microscopically. That's where a lot of the new science is in tires in the last many years.

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[Cloud9...68]

There's definitely a flurry of activity in the high performance tire space, probably because of the increasing popularity of track days, as not many people are too thrilled about changing tires at the track on anything but a dedicated track car. I sent Toyo an email a little while ago, and will insist on talking to someone who really understands tire technology at the molecular level to get a straight answer if the email response is too vague. I'm hoping the effect is situational - crossing my fingers that it doesn't matter much for absolute dry cornering, which is 95% of what I'm concerned about.

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[MB968]

Though I had a very memorable day autox in a heavy downpour, I don't think I'd go out for a track day in the rain. It might be educational, but just doesn't seem like it'd be all that much fun. And a bit more risky than I'd prefer.

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[Cloud9...68]

Funny - there's a company based in Texas that hosts DEs, and the owner half-jokingly told us at the drivers' meeting that he feels like he should charge extra on days when it rains, because of the learning opportunity it provides. It had rained heavily that Friday, and the track was still very wet in places on Saturday, and the cars were flying off the track like frisbees. No damage to any of them, though, as far as I could tell. Definitely a unique experience.

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