I have been doing DE's the past few years with my stock wheels and summer/performance Michelin PS's on my '04 C4s. I'm ready for some lighter wheels and street/track tires, I've settled on 18" wheels, and gonna leave the back alone at 11" wide. But wonder what experience others have had on their C4S or TT with 8" vs 8.5" wide wheels in the front? Would a larger wheel provide any benefit with stock tire size? Would the larger wheel with a 235/40 upset the all wheel drive or be an issue because of the slightly larger diameter- leaving the rear stock? Balancing that with slightly larger 305/30 in the rear might be a tough tire to find. I wonder if others with more experience have found the sweet spot with wheel and tire size on this car.

 

In general first choose a tire you want to run. Pick the compound and see available sizes, than see if there is a set of sizes that work for you. Than find the best wheel for that tire. 8.5 will yield slightly better mechanical grip and better steering feel with a 235 tire compared to a 8" wheel with the same 235 tire. All wheel drive cares about rolling diameter, not wheel width.

 

Thanks, I see lots of cars with 8.5 wheels up front, sounds like all benefit and no downside. Probably going to use Michelin Cup tires, I think those sizes are available in "N" designation.

 

Adding a wider tire up front will also change the brake bias slightly, since you'll be increasing the patch size on two tires. Not that this is a bad thing, but it's a thing.

As far as down sides: If you were a n00b, making the car harder to understeer might be a negative, but in your situation, I don't see a fatal flaw...

 

Ah, that's the point of the question, to dial out a (little) bit of the understeer in this car without going to suspension mods- yet. So a 295/30-18 is 25" in diameter in the rear, stock 225/40-18 is 25.1" in diameter in the front, 235/40-18 is 25.4" which I've read on this site is too much (0.2" is max). But what about just changing the front rim size from 8" to 8.5" and leaving the tires at 225/40-18? Will increasing the rim width by 0.5" increase the tire patch enough to make it a bit harder to understeer on the track? And be otherwise optimal for overall diameter, PSM and AWD to function as intended.

 

8.5/ 11" sounds right for 225/ 295 cups. It won't have a tremendous effect on your over all balance. You might consider removing the drive shaft. If you like the balance after that dump the weight of the front diff and axles. If not, bolt the drive shaft back in. I think it's that easy but look in to it.

 

I can tell you a rear wheel drive carrera benefits significantly going to a 9" front wheel and a 245 front tire, over the stock 8" with a 225. There is a ton of understeer to overcome, and reducing the stagger and fixing the alignment go a long way at making the car more neutral

 

I've tried a 245 front tire on 8.5" rim and didn't like the steering response and ended up going back to the smaller size up front. You're better off just putting a rear sway bar on instead of mucking with the front IMO.

 

That's good info alpine, do you mean you changed to a smaller front tire like 225 or 235 but kept the 8.5' rim? If you used the 235's, has the diameter difference from stock 225 been an issue?

 

I changed it down to a 235 IIRC but will have to double check as I've switched out my rim/tire combo 4 times now. I'm also on a C2 RWD setup so I've never paid attention to overall diameter, but you certainly have to take that into consideration with your awd setup.