Dalilean

I have heard a lot of people recomending the use of 15" wheels for autocross. Why? I would have thought that larger wheels worked better as you get more traver per revolution with the larger tires.

I was also wondering, what would be a good consern in width? Wider is better or not? Wider rear, thinner front, the opposite or the same?

BruceWard

I suspect that they are using wheels in the stock size and offset so that their cars qualify for stock classes.

I run stock 16" wheels with slightly wider 225 front and 245 rear tires on my 968 for autocross.

Smaller wheels and tires are often used by drag racers. The lower rotating mass allows them to accelerate faster. It is typically said that removing a pound of rotating mass is equivelant to removing 5 - 8 pounds from the car. Additionally if the outer diameter of the tire is smaller then it makes the gearing lower.

But autocross is more about keeping speed than gaining it.

xsboost90

yeah, my car is great w/ the short slicks i run at the track. The gearing is up high enough that taking off out of a corner is much quicker. Also keeps your rpms up alittle higher.

tifosiman

If you can run 15" wheels (meaning that you haven't done some crazy brake upgrade), I would use those on your car for autocrossing. On an N/A, there is a law of diminishing returns when it comes to wheel size and width. Also, try to go for the same size all around, that will help dial out a little inherent understeer that the 944 is famous for.

While I agree with people that state that autocross is all about keeping speed, larger (and thus heavier) wheels and tires are detrimental to the N/A at an autocross, particularly on courses that are tight and have a lot of tight bus-stops and slow corners.

Z-man

Exactly. Well said, tifo!
-Z-man.

sweanders

I partly agree with tifo.

When driving with the stock 225/245 combination I prefer to adjust the understeer issue with more camber instead of wider tires up front. This since when track cornering the angle of the wheel needs to be change to maintain a good wear pattern or it will eat the tires unevenly. A shame that I don't remember the camber setting but this also gives better turn-in which I think would be great at an auto-X.

M758

My preference is for 225/50 on 15" wheels on each corner. My 83 autocross car is nicely balanced on stock suspension using the 225" tire at all corners. This also allows great tire life as you can rotate front to rear. The shorter diameter vs 16" tires also puts the car in a better gearing range for typical autocross speeds. Even on the track the shorter gearing helps since the NA does not have enough power to reach top speeds on most tracks. Better to have shorter gears to get to the motors power sooner. Plus 15" wheels and tires are lighter than 16" for the same wheel and tire type. They tires and wheels also tend to be cheaper in 15" vs 16"

TheRealLefty

IMO, 15's are GREAT for autocrossing, and not just to squeeze into stock classes. Here are some of the benefits.

1) Very light weight, especially with Hoosier stickies.

2) Great rear drive ratios, I'm using 225/45/15 on all four corners which creates an effective rear drive of something like 4.11:1 from my OE 3.89:1.

3) Easy front end tweek. 15X8 Fuchs with 10.6 mm offset (OE rear on NA's with the 7/8 Fuchs option) increases the front track by half an inch, which combined with something around 2.5 degree negative camber makes the car turn in like a bear.

4) Big Red and Big Blacks are overkill for autocrossing. Yes, they're great brakes but the OE NA single pot caliber with good pads and cross drilled rotors will stop you just fine all day long in any autocross.

5) You don't really need 245's in back with 150 HP I'm running in the Top 20 in fields of 80+ cars filled with big bore Vettes and exotic new Porsche water coolers, about 3-4 seconds off FTD and about a second behind the fastest Miatas in my class (CSP).

I recently bought a set of 16" Club Sports with decent BF Goodrich tires still attached for a future track car project and I was shocked at how much heavier they were than my mounted 15" autoX racers. I lift 'em and lug 'em all the time and I was truly amazed at the difference.

RMills944

Centripital weight sucks. The farther the weight is from the center of the wheel, the harder it is to get moving. Also, the more weight you have on the wheel, the harder it is to get moving (requires much more torque. The Porsche-designed 15" alloy wheels are very light and therefore pretty easy to move. By the definition of torque, the farther away from the center of the wheel, the more power you need to get it moving. This is why SCCA cars like my uncle's Honda Prelude use 14" wheels - they're easier to move.

Compare that to the truck in my avitar... It's got 15" steel wheels weighing quite a bit more (harder to move). Now, add in the tires - 38.5" tires weighing in at 98LBs. each - yes, you read that right. Now, the only way to move this is with a big engine or with REAL low gearing (~105:1 low range).

DAMN!! I broke out physics and killed the thread!