it is not under-tired in the front at all. plenty of grip from 245's to steer the car. the understeer is primarily alignment, although spring and bar settings do play a part also.

If anything, it is under-tired in the rear. if you are doing track work, I would recommend 245 front and 295 rear. this is best done with 9" and 10" wheels. make sure that the overall tire diameters match front to rear.

 

A simple or maybe not so simple question. Is the stock tire size stagger for 18" CS optimal for balanced handeling? Since the tires are the ultimate arbiter of traction, what is the best solution for maximum cornering power? . . .

That's a pretty difficult question to answer - I think you would have to do some extensive testing (skidpad, slaloms, lap times) where the only variables are the tire sizes - in order to come up with an objective answer. That's pretty much out of the question for individual owners like us. One would think that Porsche would have done that during development - but who knows?

I have recently started running 245/275 on the track, and still tend towards oversteer; I would agree with joel-cs, I could probably use 285 or 295 on the rear. The problem is finding tires in those sizes that maintain close to the same diameters as OEM. Personally, I think the stock sizes are fine for handling on the street, where I don't expect to approach the handling limits of the car.

 

diverdog: Finding the best solution for maximum cornering power would require testing a lot of wheel and tire size combinations on a skid pad. Then we'd still wonder about transitions, turn-in, etc. I haven't tried any other wheel or tire sizes yet, so I'm just guessing that inflating the front tires a few more psi and getting a more performance oriented alignment would help reduce the mild understeer noticably. That's all I've done, and the handling seems about neutral on track and at autocross.

Since the stock CS already has mild understeer, I don't think it needs wider rubber in the rear instead of the front. I may be wrong, but the RAC 9 and 10" wheels with 245 and 275 tires should provide well balanced handling and performance for our 300-hp mid-engined cars. My guess is that much more wheel and tire than that on the stock CS would start a curve of diminishing marginal utility where the extra unsprung rotating mass would eventually reduce overall performance. I don't even try to guess about the wheel/tire limits of a highly modified (engine, suspension, brakes, etc.) CS.

 

On the street stock tire sizes are fine. I drive very conservatively and only about 150 miles a week or so.

On the track with stock tires at 38F / 39R hot the car feels pretty neutral to me. It rotates nicely. I think maybe some AX seat time where I can safely push over the edge will be helpful.

I'm planning of modifying the car for DE's as I gain more seat time. I'd like to do it in a logical progression. Safety & durability items items first, then performance.

The car has PASM and I'm not planning on lowering it as I scrape on my driveway at stock height. Suspension mods I'm considering are lower control arms and adjustable bars & links F&R . Also an LSD.

I will be getting new wheels & R series tires for the car next year & would like to chose wisely.

Thanks for the helpful observations.

 

I'd say someone who says the 245/275 combo gives enough tire in the rear should do some rough math. Weigh the car on scales (or look up someone elses weights). Go get a rough idea of how many square inches of contact patch your front and rear tires have. Then tell me how many more psi your rears see than the fronts on that combo.

It's considerably more. In fact, I'm pretty sure if you do that math again with 245/295, you'd find you don't really have quite enough rear tire. Now, that rough math is not the end-all-be-all of calculations, but it certainly gets you in the ball park.

Sure, I've got 400HP in mine, but 245/295 is still very easy to cause oversteer with.


--Donnie

 

I've got 400HP, 245/295

--Donnie

Is that the most tire you can run without fender mods?

How much work to fit 275/315? or 275/335?

 

Understeer adjusted via camber and toe

My first post. Don't own CS. Starting to think about getting one.

A few years ago when driving Autocross and lapping events in a Boxster and Honda S2000, I had maximum front wheel camber and a toe adjustment. The result was the cars went from understeer to a much sharper turn in on corners, especially tight corners around the cones.

However, the adjusted toe does not allow the steering to return to neutral as one would normally expect. So, driving down the freeway the car could be a little darty if not paying attention.

BTW, the knowledge level on this board is very impressive.

 

My first post. Don't own CS. Starting to think about getting one.

A few years ago when driving Autocross and lapping events in a Boxster and Honda S2000, I had maximum front wheel camber and a toe adjustment. The result was the cars went from understeer to a much sharper turn in on corners, especially tight corners around the cones.

However, the adjusted toe does not allow the steering to return to neutral as one would normally expect. So, driving down the freeway the car could be a little darty if not paying attention.

BTW, the knowledge level on this board is very impressive.

I'm planning on having an alignment done that will bang out maximum neg-camber, wouldn't the car still seek center if the toe is re-adjusted back to where it is now? I always thought that more neg-camber and toe in would provide over steer and may introduce bump steer, but still pull to center on highway.

When I say max neg camber I mean -1 deg btw

 

With the right suspension setup i bet you could squeeze a 305 on the back. it would be a very tight fit and you would probably need very stiff springs to make it work. i am looking into it and will let you know what happens...

 

joel-cs, we already tried with 800 pound rear springs and you'll get rub. At the ride height I really want it's pretty easy to get rub with the 295's, in fact.

Andrew, my best experience right now is with PS2's during the One Lap. 40psi hot.


--Donnie

 

I'm about to buy a set of 18" track tires myself. I was also deciding between 235 and 245 in the front. Doesn't the GT3 come with 235s? The question comes to mind, why would Porsche use 235s when 245s are available. Is there an advantage to the thinner tire?

 

Yes.

The advantage is that it understeers more, and Porsche is less less likely to have to deal with idiots that slide off the road backwards.

Brian

 

So the consenses seems to be that more tire at all corners is better. lol But a shift to more tire is the rear is really needed do to the rear weight bias of the car. Tune the handeling with camber & bar adjustments.

As far as fitment with no interference I'm thinking of 8.5" F & 10" rear wheels with BFG-1 245-40-18F & 285-30-18 rear. The rears are a little shorter than would like & I hope the rake angle will be OK. Perhaps a set of PSS9's will be necessary. Ouch in the wallet!

Anyone running these tires on the track?

 

I'm thinking of 8.5" F & 10" rear
245-40-18F & 285-30-18 rear.

The shorter rears will help with gearing.
If those are Hoosiers you're talking about I would look for 9.5" f & 11" R wheels.

If you're gonna run 285/30s you might as well go for 245/35
for the front. Front and rear will then be the same dia. 24.9".

I don't think you want the fronts 3/4" taller than the rear.

 

looking at BFG g-1's or nittos. I guess until someone makes a lower profile 245, a 275-35 is the only tire that will give similar diameter to the 245-40.