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Old 01-22-2019, 07:21 PM   #26
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Join Date: Dec 2018
Location: Kelso, WA
Posts: 40
Originally Posted by phook View Post
It would be interesting to see which would improve the lap times. Slightly lighter wheels vs a wider contact patch.
I have done quite a bit of testing on this on my CAM Cobra and there is a high margin of diminishing returns on wheel weight, and a bell curve on contact patch. Let me expand on that. If you run an exaggeratedly heavy wheel, say a 35 pound steel wheel, and then a regular alloy of say 25 pounds, you will not only see a difference in the lap times, but actually physically feel the momentum difference. Dropping again to say a 20 pound wheel will net more improvement, but maybe only 35-40% of the previous as opposed to the expected 50%. Every pound more you take off (at increasingly higher expense) takes off less and less repeatable time. When it comes to diameter, there isn't really a strong correlation in just 1-2" steps. In other words, if you go from a 17" wheel and 24" tire to a 19" wheel and a 24" tire (thus no overall size difference, but a 1" shorter sidewall) you will gain about as much from the sidewall height reduction as you lost from the increased weight, but it might not even make a measurable difference (in autocross!).

Way way back in the day we experimented with 13", 14", 15" and 17" rims all with nearly identical height tires on my FB RX7 autocross car and saw a bell curve of grip. The 13's were slowest, the 14's went faster, the 15's went fastest, and the 17's went about like the 14's. That's when I started figuring out that tire width to rim width was an issue. All of the tires were 205 mm but the 13's were 6" wide, the 14's 6.5", the 15's were 7", and the 17's were 6.5". You want the closest to square wheel width to tire width cross section that you can get, erring on the side of wider wheels/narrower tires, assuming good suspension travel/camber gain. (With camber issues like my 086 S has, it's better to run a much wider wheel and narrower tire to try and counteract that force trying to push the sidewall over on the outside edge).

All of that is completely useless unless you have the grippiest compound possible though. I did a lot of testing on tire compounds and sizes last season. A 305 Nitto NT05 is slower than a 275 Falken Rt615K+ is slower than a 255 Bridgestone RE71R, all on the same wheel on the rear (so no camber issues) and they are all 200TW. Compound is everything in autocross, that's why the schizoid guys are buying pallets of tires, shaving, heat-cycling, and keeping temperatures in check with warmers and/or sprayers. Getting the grippiest compound to it's absolute traction sweetspot is the majority of the game and you have to be willing to do that if you want to e on the trophies end of the stick (but at least it's not Hoosier's anymore!).
2000 Boxster S - SCCA CS / PCA P03 autocross build
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