Stock wheel tire sizing for autocross

thothtp · 07-15-2013, 03:32 PM

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.

135iMike · 09-08-2013, 09:58 PM

Bump for answers

Jazzy · 09-08-2013, 10:13 PM

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.

Kgolf31 · 09-09-2013, 10:06 AM

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.

mimtbr · 09-09-2013, 02:44 PM

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...

02rsxpilot · 09-09-2013, 06:40 PM

Quote:

Originally Posted by Jazzy

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.

I'd probably disagree with the idea that 235s are too wide for a 7.5 inch wheel. No experience on the 135i wheels yet, but coming from the Acura RSX world, the 05-06 Type S runs a 215 on 7 inch wide wheels. It was no problem upsizing to 225 on that wheel (Hankook RS3) so why would 235 be too wide for a wheel that is another half inch wider?

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!

Kgolf31 · 09-09-2013, 08:05 PM

Quote:

Originally Posted by 02rsxpilot

I'd probably disagree with the idea that 235s are too wide for a 7.5 inch wheel. No experience on the 135i wheels yet, but coming from the Acura RSX world, the 05-06 Type S runs a 215 on 7 inch wide wheels. It was no problem upsizing to 225 on that wheel (Hankook RS3) so why would 235 be too wide for a wheel that is another half inch wider?

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!

18x8 wheels with 235 RS3s (apologize for large picture)

The tire is over the wheel, and not stretched in any bit. Half inch smaller wheel and IMO that's too much tire.

02rsxpilot · 09-09-2013, 09:45 PM

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?

07-15-2013, 03:32 PM	#1
thothtp Enlisted Member 6 Rep 39 Posts Drives: 2007 BMW 328i Join Date: Sep 2007 Location: Rochester, NY iTrader: (0)	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.
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09-08-2013, 09:58 PM	#2
135iMike Lieutenant 40 Rep 398 Posts Drives: 2012 White 135i Join Date: Apr 2012 Location: United States iTrader: (1)	Bump for answers
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09-08-2013, 10:13 PM	#3
Jazzy New Member 3 Rep 28 Posts Drives: 135i manual,Mpackage Join Date: Nov 2009 Location: Vancouver iTrader: (0)	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
Kgolf31 Brigadier General 460 Rep 4,531 Posts Drives: 2007 Z4MC, 2012 128i Join Date: Sep 2008 Location: Ohio iTrader: (4)	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
mimtbr Enlisted Member 9 Rep 45 Posts Drives: White 135 6speed M pkg Join Date: Nov 2012 Location: Detroit Area iTrader: (0)	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, 09:45 PM	#8
02rsxpilot First Lieutenant 47 Rep 327 Posts Drives: 2012 BMW 135i Join Date: Apr 2013 Location: NorCal iTrader: (2) Garage List 2012 BMW 135i [10.00] 2013 Mini Cooper S [0.00] 2002 Acura RSX [0.00]	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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