Clearance to spring perch
 

Does anyone know how to determine a minimum safe clearance between the (front) tire and spring perch?

The credit card test has always served me well. If you can easily slide a credit card between the tire and spring perch, then you should be good.

I'd personally want a bit more, especially if you plan on cornering hard. As you load up the outside wheel the tire will move towards the perch a tiny bit.

I'd say ~3mm (1/8") is plenty, but you can probably get away with less.

Oh yeah, more clearance is always better, but I have used the credit card method on 3 different cars that I autocrossed extensively and never had rubbing issues. With that said, I always used r-compound tires or high-performance street tires, so there is a lot more sidewall stiffness with those tires.

If you're using an all-season tire or anything else with a softer sidewall, then you're right, more clearance (2 credit cards!) might be safer.

Paint stick should be safe for most sidewalls.

I have just short of 1/4" to the Direzza ZII's, so it sounds like I should be fine for track days. I wouldn't have been as nervous if the clearance was to a smooth surface like the strut, but the perch had me concerned.
Thanks, guys, for the feedback

A 1/4" is more than ample clearance. I ran my Direzza ZII's with less than 2mm of clearance and they didn't rub. I put a piece of masking tape on the perch lip to see if it would rub and ran a day of autocross runs. The perch sits pretty low on the tire sidewall so there isn't much tire flex at that point plus the ZII's have pretty stiff sidewalls. You're good!
http://i1299.photobucket.com/albums/...ps34a1fdb8.jpg

Thanks fstlane, that totally answered my concerns.