RaceDriver7

iTrader: (9)

Pettit recommends the following alignment settings for the FD:

Front Wheel: Toe Camber Caster
Wheel Diameter: 16" 17" 18"
Street: 1/16" in -1.2 -0.9 -0.8 +6.0
Long Track Event: 1/16" in -1.5 -0.9 -0.8 +6.0
Short Track Event: 0 -1.8 -1.1 -1.0 +6.0
Autocross: 1/16" out -1.8 -1.3 -1.1 +6.0

--------------------------------------------------------------------------------
Rear Wheel: Toe Camber KEEP REAR THRUST ANGLE SET TO ZERO
Wheel Diameter: 16" 17" 18"
Street: 0 -1.1 -0.3 -0.0
Long Track Event: 0 -1.3 -0.5 -0.2
Short Track Event: 0 -1.5 -0.5 -0.2
Autocross: 0 -1.5 -0.5 -0.4

In case that's hard to read, here is the link: http://www.pettitracing.com/rx7/index_information.htm

My question is why do they recommend different camber settings for different diameter wheels? Why does this make a difference when the overall diameter of the wheel with the tires is always the same?

The only thing I can think of is that they are assuming you are running a wider wheel with the larger diameters, in which case it makes sense to run less camber to prevent rubbing. But then why not reference it by wheel width instead of diameter?

I'm running 17" wheels with 255/40's front and 275/40's rear. Does anyone have recommendations on how much camber I should dial in?

Thanks,

Michael

maxcooper

I don't really know the answer. Sidewall height may be an important aspect (a pun! ); maybe even more important than width.

-Max

RaceDriver7

iTrader: (9)

Damn, I would have expected you to know, Max. I feel less stupid now.

I can see how it could have something to do with the aspect ratios too...

I know someone that is running 1.8* camber front and 1.5* camber in the rear with 18" wheels and pretty wide tires... he is pretty happy with it and actually said he is actually going to try to increase the camber next time. So I can't decide whether I should follow Pettit's recommendation for a 17" wheel or just go ahead and try the 16" recommendations. Maybe I will just try something in between.

If anyone actually knows the answer, PLEASE post up! I'd also love to hear any feedback from guys that road race or autocross about how much camber you are running with what size tires.

maxcooper

Get a tire pyrometer and use that to decide if your camber is correct. Immediately after a session (before the tire cools down), measure the tire temp in 3 places on the tread -- inner, middle, and outer. Get someone to help so they can do it as soon as you pull off track. The inner and outer should be close to the same temp if the camber is optimal. If the inner is cooler than the outer, run more negative camber. If the inner is hotter than the outer, run less negative camber.

This will help you set the tire pressure, too. There should be a linear gradient of temps across the three readings, like 140, 145, 150. If the center is hot, like 140, 150, 150, the tire is overinflated. If the center is cold, like 140, 140, 150, the tire is underinflated.

Since the inner is always 10 degrees cooler than the outer in my examples here, a little more negative camber might be a good idea, too, but only 10 degrees difference isn't too bad.

-Max

RaceDriver7

iTrader: (9)

Very good tips, thanks Max. I'd like to get a pretty good alignment the first time, though... and I'm not quite sure where to start.

DamonB

If tire diameter stays constant then bigger diameter wheels insist on lower profile (less sidewall height) tires. For a given tire the less sidewall height the less negative camber it will require for maximum performance.