Anyone with a 17t cog does. 17 is a prime number, so it's always going to be relatively prime to a given chainring (except those that are even multiples of 17: 17, 34, 51, 68, etc).

this whole prime number deal is probably one of the most important facts for those thinking of riding brakeless. however, i certainly did not think about this very much when i learned how to skid. those that shred their tires every couple of months need to know this stuff.

this knowledge needs to be disseminated!

I guess I should have specified. Regular pedals refers to pedals with clips and straps. I was also refering to skipping, and not skidding. I used to ride my bike with platforms, and skidding is quite possible, but skipping requires that you unweight the back wheel and lift it off the ground, which is something I can only do with the use of clipless pedals. I was hoping that maybe I wasn't skipping properly, but only lifting the back wheel up and putting it back to the ground when I rode with clipless, and that there is a bit more technique I'm missing that would allow it to be done with clips and straps.

I was thinking about this analysis. It seems to indicate that if you have a rear cog that has a prime number of teeth (13,17,19) there will be no lower common denominator. You will always get the same number of skid patches as the number of teeth on the cog no matter what chainring you select.

Oops, I posted the last message before reading the rest of the thread. Someone already figured out what I just said.

unless the number of teeth on the chainring is a multiple of the number of teeth on your cog (e.g., 51x17), as bostontrevor has stated.

math makes my head hurt

So I found this thread recently and decided to hack together a script that will calculate this sort of thing. I can't publish it though because my comcast account does no support PHP files. Does anyone have PHP support? Wanna publish it?

Found a free host.
https://xjeffx.freeownhost.com/

well done

bravo