Juha G

With more power now in my 993, there is need for more contact patch as well. As I have wrote before (in several topics..hahha), I would like to go RSR or GT2 wide in order to fit 285/315 tires on the car. Well… I tried fitting a 10” rear tire with a 285/30/18 hoosier up front, and to my surprise, it fit perfectly!!! No rubbing whatsoever. Granted there is very very little clearance in between the fender lip and the tire side wall but it seems to work. There is also a lot of space to be gained by rolling the fenders.
I suppose the main purpose of the RSR or GT2 fender flares is to make clearance for the track to be pushed out that 30mm.
i.e. same tires will fit the car, whether It has fender flares or not but with the pushed out suspension, one can use much wider track (different offset wheels).

In the rear it seems to be a bit different case, I don’t think there is room for anything wider than a 295 without going with flares…?

Paul902

Juha,

Do you expect that there would be no contact even when the suspension is compressed, and/or when the wheels are turned sharply? The fact that they fit at all surprises me. 285 seems like a lot of tire up front. What size did the GT2 run up front?

ninjabones

I think you will find that you will have rubbing when you actually drive the car (especially at full suspension compression with the wheels turned). Perhpas you can get away with it as your ride height appears to be pretty high for a track car. Even with my flares, the 275/35/18's are an extremely tight fit... there is quite a bit of rubbing on the fender liners, especially on the driver side along the edge of the auxilliary oil cooler). I fabricated fender liners from aluminum sheets and "sucked" them in a bit to allow for better clearance. Keep a close eye on that before you shred a tire and have a blow-out. It is particularly problematic if your run wets (the nubs stick out quite a bit more than on the R6's).

Falcondrivr

At my ride height, 245 hoosier R6 rub at full lock on my02s (18x8,et50)

Martin S.

First of all, you are kidding, right? But in case you are not pulling our collective legs, you'll get better turn it...but this can have consequences

You have lots of potential problems here:
1. Front to rear rolling radius issue, the fronts and rears need to be <5%, I believe is the number. When the ABS senses a mismatch, it will shut itself off and your dash will look like a Christmas tree with all the warning lights iit up,
2. Oversteer potential...lots of grip up from, getting a big bite going around a corner, rear cannot keep up, and around it comes...never pretty.
3. Porsche does not recommend such a set up, even with their racing cars
4. More money for bigger tires

If you are running 18" wheels, 8" front, 10" rear...and you are tracking the car,
245 on front
285 on the rear

With 17" wheels, 7" and 9"
235 front, maybe even 225 if you are a stickler for details as to tire width and wheel width,
275 rear

Generally there is a 5% difference front to rear in side wall ratio to the tread width. If you have 275mm(Tread width)/40 (% side wall height to width) /17 on the rear, fronts will be a little taller, 235/45/17, as an example. I have used 275/40 and a "40" front...it did work but it is close to the ABS limits.

You were kidding, right?

tcsracing1

Under load the tires will rub....

pretty cool fitment however! I may try my 997RS 265/18 front track tires on my 993.

Juha G

Guys, I'm not joking. I am really looking into fitting wider tires on the car.
Right now I'm running Hoosier R6 245/35/18fr and 285/30/18r. I need more rubber for better grip through the corners.
A 275/35/18 + 315/30/18 combo would have about the same front to rear width ratio as I'm currently running.

I fit the 285 rears on there just to see how much I need more room. Even the 245's rub on the liners at full lock, but I don't really care. If (when) I need full lock, I've already screwed up, so little plastic liner rubbing is not a big deal.

The point of posting this was to ask if my assumption was right; The fender flares in front are there not only to fit wider tires but to make more room to bring out the hubs that magical 30mm per side...!?

Lighter car, fatter rubber,more power= better lap times,right?

Oh, and my car is at RS height...should I go lower? I know it helps but then everything gets so much more complicated (getting over bump, getting the car on the trailer etc.) is it worth it?

Bill Verburg

It rarely hurts to do what the factory did
RSR
9.5x18 ET38
11x18 ET40

245/645 x18
285/645 x18

104mm front height
93mm rear height

front
toe 0 - +5'
camber -4*
caster -4*30'

rear
toe +15'
camber -3* 40'
caster 3* max

when you fit a 10ET65 to the front most of the space used in inboard
12.4mm out and 38.4 in compared to a stock 8ET52 225/40 setup

the RSR 9.5 compared to a stock 8 ET52 is mostly on the outboard side
33.05mm out 5.05mm in

the thing you have to watch out for when moving the tires outboard is the change in scrub radius which is felt as kickback in the steering, I haven't seen it mentioned but there must have been a change in the strut top mount to move it outboard too.

bobt993

I run a 10inch front rim with an offset of 57mm and have the camber set at 3.6 degs. Ride height runs low at 88mm (close to 90mm as Bill has listed). Back wheels are 12's with a 40mm offset. I do not have steering issues, but if you don't mod the strut mounts then you need good camber plates with a big +/- range or simply flip them from side to side to gain more reduction in camber (yes reduction because when you move the suspension out 30mm the camber increases to around 10 degs neg). Add max camber on the factory adjustment (positive camber) then minimize the negative camber with the plates. This will move the inside wheel/tire away from the strut a bit more. The big rims and tires will rub on the strut if you don't do it this way. As far as rubbing, no real issue on my car. I don't crank the wheel anymore than required at turn I don't have an inside fender rubbing issue. The ABS is pretty tolerant and I have run 255s with 315s at times during race practice with no issues. The 275/315 setup is well balanced. You will get a little more grip, but the biggest advantage is tire wear and heat is reduced during longer race stints.

Bill Verburg

That's exactly the king of mod I was suggesting to minimize scrub radius change and the resulting steering issues

trophy

Interesting thoughts Juha, As long as you go with your plans (moving out the a-arm mounts to RSR/GT2 locations) i think steering issues will not be an issue. It is really amazing how big a wheel and tire will actually fit under the front of these cars.

The front to rear balance is a tough one, I would be interested to hear from Bob/Bill etc. My experience when i tried a 255 front 285 rear combo was there was actually too much grip on the front making the rear very loose. I did dial some of it out with suspension so the car was at least driveable but it wasn't pleasant. There was also a huge challenge in managing tire heat, the rears would overheat (more sliding) and the fronts were difficult to get upto heat.

I do like the 245/285 combo, for me provided the right balance of turn in crispness and balance, I have found this to be similar to the 235/265 combo I have run in the past. When I switch between these 2 I don't have to fiddle with shock/sway settings.

I plan on going down the RSR width route (with appropriate suspension changes as per Bob) and think I will end up with a 265/315 tire combo for me I think it will provide me with a good choice of tires and balance.

The only other issue I have is the big weight difference these bigger tires have. Was also wondering from Bob if he had noticed any issues or differences because of weight?

bobt993

Steve,

No real issues with the added weight on the tires. I had two sets of wheels when I first race this car only one week after completing the upgrade. 285/245 combo and the 275/315. The wider tires were definitely faster into the corners and better straight line braking. Glen set up his car up similar to mine and he migrated to a loose oversteering car. For racing this is what you want and the combination has produced some track records for both me and Glen so the car in this configuration appears very competitive even with modern aero based cars. The biggest mistake made on the track is not using the grip on entry and you need to be patient on return to full throttle. You tend to get very deep and committed on entry hanging on till past the apex. 255/315 works well also, but a bit of difference in diameter for the ABS system. My last task is converting to motorsport ABS so that will be a non-issue soon.

mcipseric

great thread!

Bill Verburg

On the RSR and Cup the factory followed it's usual practice for converting a street car to a race car
widen track
use the widest wheel for the chosen tire
lower the car as much as possible
lighten the car as much as possible

Data on the RSR is very hard to come by, but they spent all winter before the 993 Supercup debut tire testing in Spain, they finally settled on the 245/285 setup as the best compromise of grip/weight/life for the Cup cars

they obviously liked the 245/285 as that was the tire package the cars were delivered w/ just wider wheels, the delivered wheel/tire package was a wet setup

Bob if you ever get a chance I'd love to see some pics of the rear tire clearance at the inner wall, it must be very close

Kein_Ersatz