Elephant Bart

I received few questions in the line of “how can the Cup car fit an 18” wheel while the GT3 can’t?” Since I’ve put together the Cup car vs GT3 suspension comparison (link here if you missed it), I’ve got few pictures to illustrate the problem.



First and foremost, the cup cars rear lower control arm is mounted considerably higher on the upright and the subframe. Although the two cars seem to share same or similar rear upright, the lower bracket on the cup car is machined down. Notice the gap above the control arm bushings on both the upright and the subframe.

Also notice that the Cup cars lower bracket is V shaped, while the GT3 follows a straight line. This would indicate that the mounting is also offset, increasing rear track and/or static negative camber. It’s not immediately apparent if other arms are offset as well, but we should presume that they are.

The lower bracket and the nut is the biggest clearance issue on the GT3. In another post (link here), user Spartan was able to fit a 19” wheel only after replacing the nut with a low profile one and shaving it down.



Another issue is the front caliper clearance. Both are six piston calipers on 380mm rotors. The cup car uses a much more compact PFC brake system, while GT3 uses bulkier Brembo setup.

The PFC calipers allow for an 18” wheel without interference, while Brembo would not. There seems to be just little over half an inch clearance on Brembo kit when the 20” wheel is mounted.



The rears rotors are also 380mm in diameter on both cars; however the rear calipers are more or less similar in size, which should accommodate an 18” wheel if the lower control arm wasn’t an issue.



The tires on this particular car are Continentals. Fronts are 285/645R18 and the rears are 325/710R18

The wheels are Forgeline GT1. I’m not sure about the size and offset.

http://www.forgeline.com/products/on...block/gt1.html

The website indicates that these wheels are available in 18x9.0 +47 Shallow, 18X9.0 +50 Shallow, 18x12.0 +47 Deep, 18x12.0 +59 Deep.

In conclusion, it doesn’t seem possible to put 18” wheels on the GT3 without replacing the front brakes and the rear upright (or major modification to the upright).
What do you guys think?

Dr.Bill

Makes sense. One is a street car. One is a race car.

9972RS

Hope this is something that the RS will come with.

Zucc

Just curious what the fascination is with running smaller wheels on the new GT3? The engineers obviously optimized the car for 20" wheels. Why does everyone assume it will handle better with smaller wheels?

NateOZ

The major appeal is more tire choices and less unsprung weight.

reidry

Bart,

Thanks for the detailed answer. Very enlightening.

Ryan

Mike in CA

Thanks, Bart, You continue to post really informative stuff.

Zucc

Thanks NateOZ, I guess more tire choices is a good thing but seems like a lot of trouble to save a few ounces of unsprung weight.

SamFromTX

+1

ShakeNBake

It's mainly tire choice. they don't make slicks or hoosiers in 20" wheel sizes and the sport cups/dunlops/trofeos are not a top tier sticky DOT track tire like the hoosier R6 (R7 soon), BFG R1, etc. Without those you're probably going to have a hard time passings the 997s

tgavem

Looking at those calipers, they look similar to the Cayenne and the 997.2 Turbo.
Can anybody confirm?
If it is, the demand for the this tool will increase; assembly pin VW tool T10439
Hopefully Porsche has a shorter tool for you guys or liner must be pulled back to fit the tool. I guess you can also place the caliper on a on a jack while replacing the pads.

seapar

Clear as day. Even for me. Thanks Bart.

Elephant Bart

Thanks Everyone! Glad you're finding this info interesting.

Like others said, 18" wheels open up a lot of choices in tires. Not only that, 18" tires tend to be much cheaper than 20s, for instance Nitto nt01s are about $250-$300 a piece. They're great track tires, but available only (up to) 18".

Zulu Alpha

Bart, question: do you have any idea why cup car uses PFC while RSR uses Brembo? Or anyone of that matter? I would think they would use the same breaks, no...

dntlvet

18" tires can be purchased off of race cars for $100 compared to ~$500 for many of the street tires. For those of that are light trackers, the cost of the weekend in disposables (brakes/tires/fluids) makes a real difference.