Re: Skid patches...
A fixed-gear drivetrain will have a limited number of skid patches, or areas on the tire that are in contact with the road during a skid. This assumes that the rider always locks the wheel with the same leg position. The number of skid patches is influenced by the same factors that determine gain (excepting crank arm length), although in a different way. The number of skid patches is important for tire wear, low numbers of skid patches will mean that just a few spots on the tire will get all the wear from skidding. Large numbers mean that the wear will be more evenly spread out.

When I first read about it, I didn't think it was that big of a deal, until I could see the wear on my own tires after my first summer on a fixed. Now I love my 17t cog. I just keep it away from its arch-nemesis, the 51t chainring.

Noisebeam made a chart
http://www.bikeforums.net/showthread...ght=skid+patch

I remember seeing a formula around somewhere as well, but I don't have it bookmarked.