How important is tire width and sidewall size for autocross?

Hi everyone,

I’m getting closer to having my 84 VW Rabbit GTI finished and ready for autocross this spring and I’m debating what I want to do for wheels and tires. I have 2 different sets of wheels right now which are both 6" wide. I'm debating if it's really worth spending another $400-$500 min. just to have a 7" wide wheel instead.

So here’s what I have:

1) 14” x 6” Factory VW “Snowflake” wheel - 15.8 lbs. - 185/60/14 tire size (I actually have 7 of these which is an added perk)

2) 14 x 6” BBS RA Wheel – 18.7 lbs. 195/50/15 tire size

Regardless of size I pretty much want to run Direzza ZII Star Specs.

What I’m wondering is how much would added wheel and tire width play into performance for autocross and track use?

Is that extra 1” and one size wider tire section width really going to give me any advantage?

If I go wider on the tire and I’m putting a 205 instead of the 195 on a 15 x 6” wheel am I making handling worse?

Is “more sidewall” actually advantageous for handling? Say the 185/60/14 vs. the 195/50/15?

Would that induce more body roll?

Or could it make turn in easier?

I'm leaning towards the 14 x 6 OEM snowflakes and calling it a day if the 1" extra width wouldn't be a clear advantage and the extra sidewall height isn't a detriment.

You're probably going to have an easier time finding tires in 15" versus 14". The stickiness of the tire compound is going to have more impact than anything else. The size of the contact patch comes next, sidewall probably third.

Using a bigger tire (in diameter and weight) will possibly change gearing and reduce acceleration slightly. Maybe.

Beyond that, there are tons of variables.

Yeah, the disadvantage to the 14 x 6s is the ZII Star Specs are the ONLY 200 treadware tire available for it. If they decide in a couple years to drop 185/60/14s I'm kinda screwed. I guess I don't see myself going to full on racing compound anytime soon though.

Lots more rubber available for 205/50/15s then 195/50/15s even.

+1 for getting out of 14" hell. I'd think 15x7s would be easy to find and then you could run Z2SS in 195/50/15

+1 also for dumping the 14" wheels and picking up about anything on CL in a 15x7 (heck, I have an extra set for sale right now) and getting into 205/50 15 rubber.

FWIW, I tried the Dunlops on my Miata last year and found them.....disappointing. They just never felt like I could lean on them the way other autocross tires felt.

Another +1 for getting 15" wheels. I think 14" tire options are only going to get worse as time goes on.

KyAllroad wrote: FWIW, I tried the Dunlops on my Miata last year and found them.....disappointing. They just never felt like I could lean on them the way other autocross tires felt.

Really? I love my Dunlops. I've punished them for a full day on the Tail Of The Dragon and they never cried uncle once. My left leg actually hurt that night from bracing against the door panel. I can also flatfoot my Miata through a lot of our autocross courses without breaking traction. Granted, they do not like it when you start heaving the car around.

Back to the theory, assuming all else being equal...

Sidewall flex means slower transitions. Between your two choices, the 15" will be faster. Even better would be a 15x8 wheel inside a 205/50-15.

Your Rabbit uses the same bolt pattern as a Miata, so there are piles of inexpensive 15x7 wheels available that are around 13 lbs.

Z1SS worked great for me.

Konig Helium 15 x 6.5 are less than $100 from Dicount tire, and only 11 pounds... 205/50 FTW.

+1 to 15" wheels.

15x7 & 7.5" TRM wheels are dirt cheap also. 8" too, but weird offset (+20)

The extra 1" of wheel width will be very noticeable.

I've got it in my head (for better or worse) that about a 50 aspect ratio sidewall is the ideal. That's approx what they ran in F1 back in 2004, so it must be the magic number. I'd feel ok going 10 in either direction, but 35 isn't compliant enough, and 65 is too squishy.

For Autocross, I doubt the extra 10mm tread width is worth the weight penalty for the bigger size. Particularly on a low power car. That said, will you even notice from the driver's seat? Probably not. Only way to know for sure is buy both, and get out your stopwatch.

Wider wheels do help, though.

Keith Tanner wrote: Your Rabbit uses the same bolt pattern as a Miata, so there are piles of inexpensive 15x7 wheels available that are around 13 lbs.

I could conceivably have 1 or 2 sets of lightweight 15x7s available in that bolt pattern. Let me know if you're interested.

Duke wrote:

Keith Tanner wrote: Your Rabbit uses the same bolt pattern as a Miata, so there are piles of inexpensive 15x7 wheels available that are around 13 lbs.

I could conceivably have 1 or 2 sets of lightweight 15x7s available in that bolt pattern. Let me know if you're interested.

I just sent you a message Duke. Thanks.

Thanks guys. Follow up question then. If I were to stick with the 15x6s I own would it be a disadvantage to oversize the tires by going 205/50/15 instead of 195/50/15s? Is more rubber bad in that case?

I've had mediocre luck at best trying to pinch too-wide tires onto a narrow wheel. Usually it makes the sidewalls squishy. Unless there is a compelling class-driven reason, I don't recommend it.

On my ITB VW I always ran 205 tires on 6" wide rims. Another competitor ran 225s on his. We were limited to 6" by the rules, otherwise we would have been on 7s or 8s.

My rule of thumb for autox is that the car will be the fastest with the widest wheels and widest tires that will both fit without too much rubbing and be legal under the rules. The speeds are slow enough that you basically can't "overtire" it. As for pinching it -- people use to run Miatas in Stock with 275 Hoosiers on 15x6 wheels. It certainly wasn't the ideal wheel width for the tire, but it was the fastest way around the course.

If you already have tires for the 14" wheels, just run those to start with and upgrade later as you finish sorting the car. If you're buying tires anyway, and have space in the budget, then getting 15 wheels now is probably worth doing.

You should be able to stumble on used 15x7 wheels, so they should be affordable, plus, you do want tire choices in the future.

On my Rabbit GTI I've run 15x6.5 and 15x7s, both with 205/50R15s. The 7" width was noticeably quicker and is what I've stayed with. This combo is tough to fit in the wheel well. I've got about 2mm clearance between sidewall and strut body in front and it's very close at the flares, but it doesn't rub. Nowadays though, I take this setup off and put on my 13x9s with R compounds for FSP autocross fun. What class are you in now, anyhow? STS?

flogger wrote: On my Rabbit GTI I've run 15x6.5 and 15x7s, both with 205/50R15s. The 7" width was noticeably quicker and is what I've stayed with. This combo is tough to fit in the wheel well. I've got about 2mm clearance between sidewall and strut body in front and it's very close at the flares, but it doesn't rub. Nowadays though, I take this setup off and put on my 13x9s with R compounds for FSP autocross fun. What class are you in now, anyhow? STS?

I'm wrapping up the suspension build now and I haven't ran it with SCCA yet but STS should be my class.

I am a bit worried about clearance too. I've got Koni Yellows and Ground Control Coilover sleeves on it. The car is so clean I really hate to start messing around with the fenders at all or rubbing.

What offset and camber specs are you running? Any recommendations for that?

Contradiction wrote: Thanks guys. Follow up question then. If I were to stick with the 15x6s I own would it be a disadvantage to oversize the tires by going 205/50/15 instead of 195/50/15s? Is more rubber bad in that case?

Yes it is, you'll be squeezing your tires and reducing contact patch width, not to mention reducing tire life by causing more wear in the center of the tread.

GameboyRMH wrote:

Contradiction wrote: Thanks guys. Follow up question then. If I were to stick with the 15x6s I own would it be a disadvantage to oversize the tires by going 205/50/15 instead of 195/50/15s? Is more rubber bad in that case?

Yes it is, you'll be squeezing your tires and reducing contact patch width, not to mention reducing tire life by causing more wear in the center of the tread.

Good point, I'll stick with 195s if I stay with the 6" wheels then.

Giving this a bump to see if it catches Flogger's attention again so he can answer my question about offsets that he's running.

Why no love for 13's?

I've run 13x8s on my '84 GTI with great success.

Smaller diameter wheel means less rotational/ unsprung weight. Depending on tire choice, it can also mean smaller outside diameter, which can change your final drive ratio very positively on a low powered car.

You can have a big contact patch, reasonable aspect ratio, AND improve the shift points.

Can anyone tell me why everyone always jumps to the 15's and does not consider the 13's?

I agree about the 14's.

For me, 13s won't even begin to fit over my brakes.