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07-15-2013, 03:32 PM | #1 |
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Stock wheel tire sizing for autocross
I am due for a new set of tires, and I am trying to figure out the ideal sizes. I am running a 135i in SCCA D stock Road Tire, which means no camber (other then alignment pins) and stock size wheels (7.5x18 front, 8.5x18 rear.) Those numbers can't change. I do have front sway and shocks.
What is my best sizing for autocross performance without rubbing on daily driving? I'm thinking Dunlop ZIIs in 235/40 front 255/35 back, is that my best bet? Or would I be better off with narrower treads? Or would I be better off stuffing even more on (can you fit more?) Or could I fit 245/40-18 BFG Rivals front and back (only size available now) What is the best possible size/brand combo if you can't change anything on the car? Thanks for the help, I've searched but can't find any specific sizing advice on a stock setup for autocross. |
09-08-2013, 10:13 PM | #3 |
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I'm running 225/255 ZII's in d stock. 235 front can probably fit but I have read that some think this might be too wide for a 7.5" wheel and slow you down due to too much slop in the tire.
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09-09-2013, 10:06 AM | #4 |
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Don't overstuff the wheel.
If you say, fit a 255 tire on a 8" wheel, you'll end up loosing feel and overall grip. This is because the wheel is just too big on the wheel itself, so when you go into a turn, the tire will shimmy back and forth on the wheel before deflecting and reaching max grip. Since the tire is overstuffed, it can't deflect as much as a properly sized tire, and cannot achieve max grip. General rule is +10mm on the tires. +20mm can get you into trouble. I have 18x8 and 18x9 on my Z4M and run 235 front, 265 rear. OEM is 225/255 Keep in mind, Dunlop and Hankook tires run wider than spec'd, so a 235 tire may actually end up being a "245 contact patch" with the square shoulder profile. I'd personally go to 225 and 245, respectively. |
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09-09-2013, 02:44 PM | #5 |
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There are a few posts on this subject:
http://www.1addicts.com/forums/showthread.php?t=495105 http://www.1addicts.com/forums/showthread.php?t=219124 http://www.1addicts.com/forums/showthread.php?t=214652 http://www.1addicts.com/forums/showthread.php?t=446923 These are just the ones I booked. I have 245/35-18 / 275/35-18 Hoosiers like most DS cars, on stock wheels. My car has the M-sport suspension. I suck at autox, but with this setup, it's reasonably competitive. I bought this R6 compound setup used for $250! On craigslist from a Porsche boxster owner. The R6 does'nt get to temp all that well with a single driver, but if you have two drivers it does ok. Anyway it was cheap. At last weekend's autocross the fronts corded. So I got one track day, plus three autocross out of the fronts, the rears should last another trackday. Just bought new fronts on tirerack special for $143ea, Hankook Ventus Z214 autocross compound, so we will see how those last... The 245 racetire rubs the strut slightly but it's not causing any durability issue: The 275 rears rub the fender. It's light and only on big bumps. Again, not enough to create a tire issue. This might be remedied with rear camber adj. Anyway, my car is all stock including alignment. Racetires make this heavy car change direction very quick... |
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09-09-2013, 06:40 PM | #6 | |
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Drives: 2012 BMW 135i
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Quote:
Other than that, I agree in principle that you are better off with a proper size tire on the wheel than going for maximum tread. Open to hearing from anyone who's tried 235s on the stock wheels though! |
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09-09-2013, 08:05 PM | #7 | |
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The tire is over the wheel, and not stretched in any bit. Half inch smaller wheel and IMO that's too much tire. |
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09-09-2013, 09:45 PM | #8 |
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Drives: 2012 BMW 135i
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Interesting. Thanks for the pic. I see Tirerack agrees, citing the recommended wheel width for any 235/40 extreme performance tire as 8-9.5".
The aforementioned 225s I had on the 17x7" RSX wheel were 225/45, which according to tire rack are suitable for a 7-8.5" wheel. Assuming the difference is due to the 8 mm of sidewall? |
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