wheel size advice for 951 track car

[Matt.S_951]

Looking for some advice on wheel sizing for an 87 951.

I want to use the car for autocross and DE's, with the focus on DE's. I will be using street tires designed for autocross, such as bfg rivals, because I don't want to swap wheels at the track

The car is currently stock. Long term plans include coilovers and about 350 HP from a Vitesse stage 3 kit. I want to make sure that whatever setup I pick will still work well at that power level and at the slightly lower ride height of the coilovers.

First question, should I go with 17 or 18 inch wheels? My inclination is 17", due to the Porsche service bulletin recommending against 18".

Next question is wheel width. I would prefer not to have to roll my fender lips. There are a couple of options that I see here:

9 f, 10 r. This seems to be a popular setup. It also seems to be about the most you can fit without rolling the fender lips.

9 f and r. Square setup, which is nice for extending tire life. Not sure if it's enough rear tire to handle my future power.

8.5" fronts also seem popular, is there a reason for this? Pairing them with 9 or 8.5 inch rears would work, but either of these options seems inferior to 9" all around.

Thanks for the help everyone.

[smudo]

For track (and not only) I would use in front 7.5'' or 8'' wheels with 225/45 tires with 9'' wheels in the back with 255/40 tires. Those are popular sizes also for many semi slick tires and correspond to original tire sizes for turbo S with cup wheels.

If you will need more grip in the rear due to big power, you could put 265/40 or even 285/35 tires on 9'' wheels. Wider and bigger diameter wheels will be slightly heavier which is not good for the track.

Another thing to look at is the offset. It should be around ET 50 in front and slightly below ET 50 in the rear to not have problems with fitting them.

As a second set of wheels I have Keskin Kt 2 8.5'' and 9.5'' 18'' wheels (both ET52) with 225/40 and 285/35 in the rear with no fitting/rubbing issues. For all above sises you do not need spacers and can use standard wheel bolts.

[DannoXYZ]

I'd go with 17x10" 65mm offset all around for maximum traction. That's due to the sidewalls being vertical or even pre-stretched. No waiting for the sidewall to flop sideways until it can be in tension.

Use 65mm offset to fit under stock fenders. This is what Porsche did in going from Turbo to TurboS. Measure the clearances inside& outside yourself and see that there's only room on the outside for 1" extra before the tyre rubs. But there's easily 2" on the inside. Increasing the offset puts more ofthe extra width on the inside where there's more room.

Lots of great DOT-R tyres in 255/40-17 size. And you can rotate the tyres for more even wear. The fatter increase in front also reduces the stock understeer and gives you faster lap times.

For maximum traction with 18" I've used 255mm front tyres on 10.5" rims and 295mm rear on 11.5" rims.

[geno951]

I feel like my 16's are perfect just because I don't trust rubber with less than 35 profile. Just raises my blood pressure thinking about when they'll blow the bead.

~on the phone~

[geno951]

And 225/265 front back OR 255 on all 4 so you can reeeeeeally rotate.
My 2c.
Literally anyone else will have more experience than I so take what I say lightly.

~on the phone~

[DannoXYZ]

It's just difficult to find the widest 16" tyres in the 245/45-16 size now. At least most of the DOT-R track tyres are still made in that size.

If you read over the rule books, POC, NASA, SCCA, etc., they only govern rim-widths, not tyre width. That's because the rim-width makes a bigger contribution to traction than tyre width. Squeezing a wide tyre onto a narrow rim will cause it to distort the contact patch. Especially under cornering as the outside sidewall needs to be bent inwards inboard of the rim-edge before the tyre can pull on the rim (try pushing something with a rope). So it's best to get rim-widths that are about the same as the tyre widths, and vice-versa.

Way back when, I bought some used wheels from a racer at Laguna Seca. Went out on lunch-break to swap my tyres over from the stock 16x7/8" wheels onto the new 16x10.5" ones. On the first lap, I flew off the inside of turn 2! Even with the same tyres, the wider rim gave so much more traction, that it didn't need as much steering-input. Ended up being about 2-sec/lap faster in the afternoon sessions than morning on stock rims. All my clean laps in morning was within 0.4-sec from each other, as well as the afternoon ones, so I can attribute the difference to the only change made, the wider rims.

[Pete R]

I would use 17's. Tire sizes are plentiful and cheaper than 18's. If you run a square set up and asymmetrical tires you will be able to rotate them all over and get maximum wear out of the set.

[Matt.S_951]

Danno,

Do 10 inch fronts fit without rolling the fender lips? Also, do they fit with the stock suspension?

Thanks

[DannoXYZ]

Originally Posted by Matt.S_951

Danno,

Do 10 inch fronts fit without rolling the fender lips? Also, do they fit with the stock suspension?

Thanks

No fender-rolling needed front or rear with increased TurboS 65mm offset. In front you get 1" extra on the outside and 2" on the inside. In rear, extra 0.5" on the outside and 1.5" on inside.

On stock front suspension, you have about 2.0" clearance between the rim-edge to the spring-perch with 16" wheels. Since the struts are angled inwards, 17" rims should give you an additional 0.25" clearance to 2.25" That's cutting it pretty close.

I've always went straight to 2.5" coilovers and swaybars as the very first upgrade since it makes a bigger difference than wheels. So I never had a chance to fit 17x10" wheels on stock suspension. I recommend getting the wheels and doing a test-fit to see. If it rubs, you can get the 2.5" coilovers for an additional $250.

[Matt.S_951]

What offset is needed for 9" wide front wheels?

Thanks.

[Matt.S_951]

Also, what wheel / tire sizes are people running on SP3 prepped club racing cars?
They've probably got a good idea of what works best.

[DannoXYZ]

Originally Posted by Matt.S_951

What offset is needed for 9" wide front wheels?

Thanks.

You can go with stock 52.3mm offset in 9" front wheels. That will limit you to a 225mm tyre though.

Using TurboS offset of 65mm will let you run a 255mm tyre without rubbing fenders (no rolling needed).