Race car wheel size/offset selection

[JohnJL]

I need to upgrade my wheels and brakes. With north of 325hp at the wheels the 9x15 fuchs brake traction way too easy. I need some help spec'ing what the correct wheel size and offset should be. This is a race-only 1983SC with stock (revalved and raised front spindle) uprights/hubs. Very stiff torsion bars.

First I need to select diameter. I believe I want the smallest wheel that fits over my brakes, which is at least 17". Is there anything to be gained by going to 18's, besides weight?

Is there a decent selection of 17" slicks and R-tires for 9 or 10" fronts and 11 or 12" rears?

Next, width. I am taking a wild guess at 9 or 10" fronts and 11 or 12" rears. Is that about right?

I am planning on cutting the fenders and installing extensions so no worries about width. I would like them offset about 5mm outboard more than the stock 9" fuchs in the rear now. The 15" RA1s rub already.

I have about 2 months until I have the car back after the move and want to have all the bits ready to go...

Thanks!

[JeremyD]

I read through your build the other night - nice! Looks like you have regular flares - so will probably be limited to 8 and 9.5 widths. With a little fender rolling you can stick some meat under them - I fit 235/275 with a little effort. Jeff is also a distributor for Kodiak and Fikse (and developed the forged fuchs that Zuffenhaus is now selling)

Call Jeff Alton - Pacific West Performance Inc. Jeff Alton 604 313-3494

or Steve Weiner
Rennsport Systems
Portland Oregon
(503) 244-0990
porsche@rennsportsystems.com
Welcome to Rennsport Systems, Porsche Performance Products for the 21st Century

[Bill Verburg]

Originally Posted by JohnJL

I need to upgrade my wheels and brakes. With north of 325hp at the wheels the 9x15 fuchs brake traction way too easy. I need some help spec'ing what the correct wheel size and offset should be. This is a race-only 1983SC with stock (revalved and raised front spindle) uprights/hubs. Very stiff torsion bars.

First I need to select diameter. I believe I want the smallest wheel that fits over my brakes, which is at least 17". Is there anything to be gained by going to 18's, besides weight?

Is there a decent selection of 17" slicks and R-tires for 9 or 10" fronts and 11 or 12" rears?

Next, width. I am taking a wild guess at 9 or 10" fronts and 11 or 12" rears. Is that about right?

I am planning on cutting the fenders and installing extensions so no worries about width. I would like them offset about 5mm outboard more than the stock 9" fuchs in the rear now. The 15" RA1s rub already.

I have about 2 months until I have the car back after the move and want to have all the bits ready to go...

Thanks!

yes, 18s will have superior potential and there does not have to be a weight penalty. Each individual setup should be evaluated on its own merits but the potential for 18s wrt 17s is better

what can be fit will depend on the fender flares you use, w/ SC/Carrera fenders 245/275 is about the limit, w/ 930 fenders you can go much wider but w/ diminishing returns. early RSRs ran 9 & 11 230/600 & 260/300

i'd probably go no more than 255/305 and no less than 235/265 w/ appropriately sized wheels, probably 9 & 11

[Jeff Alton]

Tried and true on a turbo flaired car is 9/11 with a 245 or 255 front and a 275 or 315 rear(17 inch diameter), not sure if it really is optimal or not. When stepping up to 18 more width choices come into play. One can also consider running 18's on the rear and 17's on the front.

You have lots of power but the sort of driving you plan to do, the suspension set up, gearing will all play a role in how you want to configure your wheel tire program.

I got your email and we can disucss more in detail about your needs.


Cheers

[RarlyL8]

The wheel diameter chosen will depend on the tire height and width needed (asect ratio) and the brakes you must cover. I would first calculate the exact height that you need as this will define and limit your choices of tire and wheel very quickly. The wheel needs to accommodate the correct tire, not the there way around.

[Bill Verburg]

What happens when you go too wide is the tires are tougher to heat and drag goes up very quickly, both tend to negate any cornering grip gains

[JohnJL]

Thanks guys.

The flares are currently slightly rolled SC. Maybe I am too pessimistic but I had already resigned myself to changing them out or more likely just cutting them and grafting on a set of the GT Racing arches.

Can 9" and 11" in 17" diameter fit under SC flares with 255/305? Toyo R888s have a big gap between 275 straight to 315 in 17". MPSCs are not available in any 17" on that range (that I can find.) The closest rear Hoosiers I could find were both A6/R6 in 295/35 in 17". Events usually require R rated tires but I'd like a set of slicks for fun. I run RA1s now and like them.

Hoosier's closest front might be 255/45ZR17. R888s are 255/40R17. Fit under rolled SC flares?

If not and I need flares anyway I might as well go to 18s for the tire choice and brake clearance?

[skinnerd]

I had an RS fendered car that basically is identical to yours as far as fender width's go.
My Fikses were optimized for my fenders.

Up front I had 8.5x17 with 245/45x17.
In the rear were 10x17 with 275/40x17.

These were the absolute biggest wheel/tire I could fit.
I did a lot of fender rolling, moving oil lines, removing shock covers, etc to get them to fit.
Even so, they were tight and would rub on the biggest of bumps slightly.

Can't imagine you can get anything bigger with your current fenders.

As said above....go to turbo flares or RSR front and rear, and now you can fit 9.5x17 with up to 275's up front and 11-11.5x17 with 315 - 335's in the rear.
The wheels will need the correct back spacing to work correctly.
This is the direction I am going with my car this winter.

Good luck.....

[Bill Verburg]

Originally Posted by JohnJL

Thanks guys.

The flares are currently slightly rolled SC. Maybe I am too pessimistic but I had already resigned myself to changing them out or more likely just cutting them and grafting on a set of the GT Racing arches.

Can 9" and 11" in 17" diameter fit under SC flares with 255/305? Toyo R888s have a big gap between 275 straight to 315 in 17". MPSCs are not available in any 17" on that range (that I can find.) The closest rear Hoosiers I could find were both A6/R6 in 295/35 in 17". Events usually require R rated tires but I'd like a set of slicks for fun. I run RA1s now and like them.

Hoosier's closest front might be 255/45ZR17. R888s are 255/40R17. Fit under rolled SC flares?

If not and I need flares anyway I might as well go to 18s for the tire choice and brake clearance?

no, biggest under SC/Carrera body will be 245 on 8Et28+/-3, 275 on 9.5ET19+/-0(maybe 10) and that will require perfect o/s and lots of additional work

there's going to be a big hole in the supply of DoT R rubber due to the last round of green Euro regs wrt tire compounds

if you want to run 255/305 or so just add 930 fenders and then you can run them, Jeff has good o/s for 9 & 11 on SC suspension/930 flare cars

[Jeff Alton]

Although some find the look not the best, one will find a wider range of tire sizes now in 18..... Especially if you have a wide body.....

Did I mention I have a great deal on a set of FM10's in 18?

Cheers