Tom W

I'm considering moving to a class where 10.5" front and 13" rear tires are expected/required. Since wheels are not cheap I want to ensure I get the right size.

I currently run Cup II 7" (ET 55) in front (with a 4mm spcaer to fit 225 MPSC tires) and Cup II 9" (ET55) in the back with a 6 mm spacer.

So, my view is that I effectively have a 7" wheel with a 51 mm offset which means 5.46" fits between the hub and the tub. My calculations say that for a 9.5" wheel in front, I would want a .96" (24 mm) offset. To fit an 11.5" wheel in the back, I'd need an offset of 17 mm. How's my math?

The likely tires are 265/35x18 MPSC or 245/40x18 RA-1 in front and 315/30 MPSC or either 305/35 or 335/30 RA-1 in the rear.

These will obviously not fit under a C2. RSR body panels would be fit to the car to allow such big tires. Anyone else been crazy enough to do this to a C2? Any thoughts to share before I do it?

garrett376

Geez, who are you racing with? What class? Does a 964 engine make enough power to turn rubber that wide?!

Jim Michaels

My response was the same as above. You'll be adding many pounds of unsprung rotating mass, and you'll need more than precise offsets. You'll need big fenders and flares. How do the other 964s in the class do it?

Tom W

It is a new class with PRC (which runs with NASA). The rules are not finalized yet but the current proposal is to require slicks (Goodyear R600 compound) of 10.5 x 23.5 x 16 front and 13 x 25 x 16 rear or a Toyo RA-1 in equivalent sizes on an 18" wheel (instead of the 16").

One intent of the proposed class is to limit hp by requiring a stock fuel pump and air intake on the 3.6L engines. This should top out cars at about 300 hp. My car makes 293 at the flywheel according to the engine dyno. The other is to limit cornering by specifying the tire. The R600 is a long wearing slick that should last at least 3-4 days. It just isn't as "grippy" as others. So if hp and max grip are specified, driver skill in the turns should have a big impact. Limiting hp means that you can't make it up on the straights. We are discussing the idea of forcing a 16" rim (which means some of us have to change calipers) or allowing a DOT-R alternative like RA-1s in equivalent sizes.

As I said, it will require a new body (RSR panels should allow them to fit).

Almost any car can use tires that wide, it just a question of what the weight does to acceleration. The much wider tires will certainly allow a hell of a lot more corner speed.

The class was not invented with a stock 964 in mind. It's for 914s and older 911 tubs that have 3.6L engine put in them. I'm at a disadvantage as my car currently weighs 3000 lbs with me in it. The class allows 2300 lbs. That's a lot of weigh to lose, but if I can shed it, it will also do wonders for the car.

Geoffrey

The 23.5" diameter front is way too small for your C2 and will affect the ABS when combined with the 25" rear tire.

Jim Michaels

Thanks for explaining. That's too far beyond my experience for me to be of much help, but wouldn't wider wheels and tires require greater, rather than less, offsets than what you have now? I don't know anything about figuring offsets when wide spacers come into the equation. With sufficiently wide fenders and flares the rubbing concern would be with the inboard side (e.g., the oil line in the right rear fender) rather than at the fender lips, so I guess you'd be using spacers. As you suggest, your overall weight will place you at a distinct disadvantage compared to the earlier light weight 911s, also with 3.6l engines. It will be difficult (but maybe not impossible) to approach the 2300# limit. If this is supposed to be a kind of "spec" class, it's unusual because spec classes usually try to make it cheaper and easier for people to conform to specs and race rather than more expensive and harder. I don't understand the attraction to this class as an alternative to running nearly stock in PCA club racing, or with POC.

Tom W

Geoffery: I know. It's a big reason I'm lobbying heavily to allow the 18" DOT-R tires so I can keep ABS. If forced to go to the 16" rims, I'd have to get rid of my big reds and disable the ABS (and learn to drive without it).

Jim: The offset on Cup II wheels is positive. That means that more of the wheel is between the inside and the hub than if there was no offset. Since the distance between the hub and the inside of the car does not change with different fenders, that's the distance that must be maintained. As such, putting a bigger wheel on means that the offset will be less as more of the wheel will stick out beyond the hub. Look at it this way: My current wheel in front is 7" and I want to put a 9.5 inch wheel up there. That means that the wheel must stick out 2.5 inches more beyond the hub (because it can't go in any further). So, I need a less positive offset to maintain the interior spacing and move the extra wheel size to the outside.

My car doesn't fit into a stock class due to other prior modifications and there are very few people who race stock or improved classes in Nor Cal. 2 PCA does 2 races a year in CA from what I can tell, this year they were in So Cal and Nevada. Worrying about PCA for 2 races doesn't appeal much to me. I though about POC too, but it would take a year to get certified with them and they are primarily a So Cal group. I'm not fond of long tows to Willow Springs, Buttonwillow or Fontana. I'd rather go an hour to Sears Point, 2 hours to Laguna Seca or the 3 hours to Thunderhill. That means racing with ARC or PRC (or both). ARC is small and a great group of guys with cars other than Porsche. PRC is bigger and does more races but almost no one is in a stock class and there are no stack 964s or 993s that I'm aware of that race with PRC. The nicest class in PRC for me is the existing Toyo spec class. However, they don't allow 3.6 L engines. The new GTL class would be a spec class for 3.6L engined cars. It will simply take a number of significant modifications to get my car to fit into it.

I have no expectation to be able to easily get to 2300 lb. I'd be happy to reach 2500 to start.

Red rooster

TomW,
Your thinking on offset to make wheels fit is OK .The offset is also there so that the center of the wheel is right in the center of the wheel bearings .If you just move the outside rim out 2 inches the center of the wheel goes out 1 inch.
You should check that the wheel bearings will be happy with this misalignment and slicks. Its your neck but it doesnt sound good.

Geoff

Jim Michaels

Tom: Thanks for your patience with me. It was your question, but I'm learning more than you are. I understand positive offsets (didn't know that's what you had though), but have always widened with negative offsets, which also produce the wheel bearing stress problem that Geoff mentioned above. I hope you can work something out to race successfully with them, although it appears that it won't be so simple. Now I also know where Half Moon Bay is (geography lesson). I'd never been to any CA tracks, until I attended the Monterey Historics at Leguna Seca this year. That trip will go down on my list of great car events, along with my trip to Rennsport Reunion II at Daytona.

Tom W

Geoff & Jim: The bearing issues are a concern, especially as I go though a set (all 4 corners) every year. I've had more damn wheel bearing problems than anything else on the car. I finally concluded it was due to how I tied the car to the trailer (through the wheels) and since the switch to the autometrics tow hooks I have had no issues. It is my understanding that going from a positive offset to a negative offset will significantly increase the stress on the bearings (in addition to the obvious increase in stress due to the bigger wheels/tires for both weight and contact patch). All this get offset by the significantly lighter car. That also makes a big difference. What works for a 2400 lb car does not necessarily work on a 3400 lb car.

If I do this, I will also look at switching to RS, GT2 or EVO uprights and tie rods so the car can be lowered even more without bump steer issues and looking at options for wheel bearings.

Red rooster

Just taken another look at the tire sizes you may have. The 255 on the front would fit a 8.5 inch wheel and the 305 rear need an 11 inch wheel.
Maybe with a 45mm offset you will get there ! but spacers / panels sound likely especially at the rear.
Good luck.
Geoff