trackjunky

I am currently running Toyo RA-1's on my F stock 944 Turbo. I can only run 8" front and 9" rear per the rules (18" diameter). I have always run 245's on the front and 275's on the rear because that is what everyone else is running. But, I've begun to rethink things a bit.

Here's my thinking. World Challenge TC's run 235's all around and post laps approximately 2 seconds faster than PCA E & F cars. The WC cars run 2700 - 2900 lbs and have between 225 and 250 HP. Am I "over tired" with my current set up? With 217 hp at the flywheel, do I really need 275's in the rear? Would I gain cornering force by running a 245 on a 9 inch rim vs the 275?

Thoughts, ponderings and opinions?

Larry Herman

It's a good question. If your tires are too wide, there's a lot more drag down the straight, but a car with wider tires (on the proper sized rims with an optimized suspension) will have more grip than one with narrower tires. Since you are running that size on the front, why not just try it?

PedroNole

Your tires don't create force, they react to it. So, in theory, the more rubber, the more force you can apply. Now, it's true more rubber on a straight isn't necessarily a positive but I don't think you should base your tire decisions on handling on the straight either.

The interesting thing to me is that the WC cars are running narrower tires all around. As they spend a WHOLE lot more time on these issues than most of us, they probably run them for good reason...that is currently beyond my understanding.

I'm not trying to be a smart ***, I'm just from the school that I can't have enough rubber right now. So, when someone is pondering going the other way, I don't understand it. That said, again, the WC guys are probably on to something.

Veloce Raptor

Bill, FYI, um, WCT cars have a whole lot more HP than that.

An additional upside of running a 245 in the rear is that it will really stretch the tread & give you a firmer, more vertical sidewall, which (if your suspension is set accordingly) can have benefits using more throttle in the corners. the downside, however, is that your frints are still on 8" rims.

Greg Fishman

Why don't you run a 10" width? I think that is an acceptable size for an F stock 944 Turbo.

Veloce Raptor

IMO, 10" would be too wide for a 245 tire.

Greg Fishman

I meant for the rear wheels so you could fit a 275 or 285 properly.

Cory Jump

10" wheel would only be legal on a prepared 944 turbo in F not a car that started out in F . You can only go 1 inch wider in the stock classes.

trackjunky

Everyone has made some great points.

VR: I imagine that you are correct about the WC Touring Car horsepower, but I went off the specs on the SCCA site. If those cars that they (you) are running are making that much horsepower and can still put it down with a 235 x 17 tire, why should I be running a 275 x 18 tire on a car with 217 fwhp?

Pedro: I understand your points. What got me thinking about this was last years club race at TWS. One of the guys in "F" (I can't recall his name) but he could have run bigger and wider tires on his 911 and wasn't. I didn't think it odd until qualifying and the race when he qualified and finished in the top three. I asked him why he wasn't using the same tires as everyone else and he responded by saying that he felt that too much tire tended to bind the car up too much. He said that by having a smaller contact patch in a low horsepower car meant that the car could come off the corners faster. It seemed to work for him.

I remember doing something similar in Karting where we would adjust the suspension set up for a little less grip so that the additional grip wouldn't bog the motor in certain corners. It seemed counter intuitive, but it worked.

I am going to try 245x18 on all four corners this weekend. Of course my alignment is set up for the other tire size, but it should be a good starting comparison. I'll let you know how the car droves.

Greg Smith

Take it for what it's worth but a BMW club racer I know was faster on a 235/40/17 RA-1 on a 8.5" rim than a 255/40/17 RA-1 on the same rim on the same day.

Geoffrey

I think there is a balance to be had and there are a lot of factors to consider. The current thinking from the racing tire manufacturers is to have the tire installed on the widest rim possible. As mentioned above, this allows the tire to be under tension and resists floating on the rim. If you look at most of the newer race cars the rim is actually wider than the tire. The question becomes is the benefit from a wider rim vs tire better in your case than a wider tire vs rim? I have found that I prefer the wider wheel vs rim path on my own car because it gives a more reactive and solid feel.

Veloce Raptor

Ah...my misunderstanding. Sorry.

Bill, I am pretty sure most of the WCT cars that are competitive are putting out 280-300 at the crank. They have extremely finely tuned suspensions & aero packages, and yeah, I agree with you, the 235 Toyo's seem to work EXTREMELY well when you build an entire car around them!

Greg is correct--we both know the same guy who was faster with 235's than 255's. Again, that is in a BMW M3 like mine. Ironically, this same racer had me for his instructor in his first-ever DE as a green student. Despite that, he has become an excellent drover.

M758

One thing to remember is that in WC the rules limit the teams to 235 width tires. All would run wider tires if they could legally.

The talk about match tire width based on wheel width is an important one. Where classes limit wheel width alone you tend to want squeeze the widest tire you can on a narrow rim. The reason is to get the widest tire. In classes like WC they limit tire width and also rim width as well.

The other thing to consider is balance. On a 944 chassis especially one with stockish hp levels I think the ideal balance is achieve with same tire width front and rear. The issue is that with the PCA rules as they are you feel like you are "giving up" some rear tire when you do this. Nobody likes to "give up" tire. The other issue when selecting tire width is how it impacts diameter. Depending on the sizes available you may be able to go 20 mm wider, but the tire then grows a 1/2 taller. So you get more grip, but also get slightly taller final drive as a result. So... which is better? Again not a simple call. I have seen that a 225/50 16 is slower than a 225/50 15 on a 944 NA. That is due to taller gearing from the taller tire. However is a 245/45 R16 (16x8) slower than a 225/50 15 (15x7)on the same 944 NA? Hard to say. Gearing is same as the 225/50 16, but you get a wider contact patch so is it faster... Can't say.

Larry Herman

I think that you need to take suspension setup into account as well. A stiffer suspension is going to keep the tire flatter on the road surface, and will give more grip with a wider tire. A car that leans will work better on a narrower tire with taller sidewalls, as that tire will be able to maintain contact patch especially as it is losing camber.

Willard Bridgham 3

I run 245 front/275 rears on my modified 944T, but it is to control power oversteer....I would run 315's if they would fit under turbo S bodywork..My son's 944T with less modifications, but not stock, runs 245/255

Mine is s street/track car, so I'm not changing the bodywork and the track is only a hobby.

Many fast 944T track cars with stock (hah!) engines run with 245 all around, but they're also 500-600 lbs lighter than mine and much stiffer.