GaryK

I'm not a fan of big heavy cast wheels, like most of the aftermarket 17's and larger out there, especially for track use. The stock Club Sports are strong and relatively light, but the 7" front is too narrow to run a decent size tire. The very rare 8" Club Sport would be ideal, but another option is the 16x8" 52mm offset forged flat dish wheels from the 928. Or the 16x8" cast Design 90's from the 964 or S2, which are 55mm offset and only a bit heavier than the Club Sports.

I run the 8" 928 wheel up front and the original 9" Club Sport in the rear with a 1/4" spacer, all with 245/45-16 Toyo RA1's. The Toyo's are faster than the A032R's, just as streetable, and much quieter. But I agree with Fast951; if you're inexperienced, run some well worn street tires on the track initially.

With a wider front contact patch and track, more negative camber particularly in the front, and the 968 M030 bars with the adjustable rear set full stiff, your car will turn in much better than stock.

And the other item for the brakes besides track pads and adequate cooling -- the 33/5 proportioning valve from the 928S4. Your car will brake much flatter.

hally

Russ,
the 17x9 993 all round sounds like a good compromise, allowing rotation. I gather you are satisfied with this setup? Would you say rotation doubles your tire life?
cheers

ninefiveone

I'm very interested in hearing what people are running for camber at the track on r-compounds. Anyone?

Danno

I'm using -3.25 and -3.00 on the fronts and -2.75 on the rears. This worked well for the Yokohama A032Rs. The Kumho V700s wanted about 0.50 degrees more negative and I'm sure Hoosiers would want another 0.50 degrees on top of that!

hally

Danno whats your take on wheels, is it simply a case of wider is better, or is there a point of diminishing return? What about the significance of the wheel / tire weight?

Danno

Hmm, I think there has to be a balance with everything. A mistaken assumption is that wider tires give you larger contact pathc, but that's not true. It just changes the shape of the contact patch from a square to a wide rectangular one with roughly the same contact surface area. What this means though, is that any given point on the tire as it rolls thought the contact patch will be in touch with the ground for a lower amount of time on a wider tire. Without going into complicated physics and calculus, the effect of this lower contact time with wider tires is that they cause heavier and more twitchy steering.

So there's a limit somewhere, but I'm not sure how one would derive it mathematically or any way other than through testing with emperical data. The gains and limit are probably not scaled linearly as well, but as an asymptotic limit approaching some barrier, but never quite reaching it.

But along the way, there are some compromizes. Wider bodywork is one thing that needs to be addressed. Which leads to more aerodynamic drag. Brake discs stuck in the center of wider wheels probably won't get as much cooling.

There's only so much room on the inside before you start rubbing things. And the best way to widen is to split the difference between the inner & outer edges to maintain stock offset for even bearing loads and steering geometry. Eventually, you can only add more width to the outside edge. This has the effect of worsening the scrub-radius on '86 & earlier cars (more yank on the steering when you hit a single-tire bump), while improving it on the '87+ cars. More rim-width on the outside will also increase the load on the outside wheel-bearing. However, this may be offset by a better distribution of load between the bearings under cornering (inside bearing overloaded in stock configuration for cornering).

If you haven't figured it out, there's no simple easy answer. You have to balance your goals with the tradeoffs, and this may change with the type of driving and the tracks you're on.

So when I upgraded my wheels, I went up to a reasonably sized set of 17" wheels anticipating an eventual maximum tire size of 275/315mm. I got BBS racing wheels in 17x10.5/11.5" and they weigh 17/18-lbs. This is about 2-lbs heavier than the 16x7/8" Fuchs that came with my car. However, they're still 4-5 lbs lighter than Porsche wheels and 7-8 lbs lighter than aftermarket wheels. I'm planning on shaving off another 1.5-lbs apiece by going to magnesium centers.

The first set of Yokohama A032Rs in 255/40-17 and 275/40-17 sizes were tried first for fitment and clearance measurements. They weighed about 20-lbs total less than my 245/45-16 BFG street tires. The 275/40-17 I wanted to use in front won't fit without wider fenders. That's because the tire is about 1" taller than stock and combined with a 0.5" lowering, the top of the tire is actually 1" higher up into the fenders than stock. So I've only got about 8-degrees of steering on the tire before it rubs the fenders.

On the rear, I'll have to get wider fenders as well to run a 315/35-17 tire. I might be able to squeeze them in now, but the 275-40/17 tire is already rubbing on the inside AND outside. And going up to a 12" wide rim for the 315mm tire will only make the rubbing worse.

Long story short? The maximum I'd use on a street car and occasional track car is a 10/11" wheel combo in 17" or 18". This will let you use 255-265mm front and 275-295mm rear tires without having to modify the fenders. Beyond that, even more width would really only benefit a full-time racecar.