I am putting the finishing touches on a single speed built from a grab-bag of parts. I was intending to use just a single cog from an old 9-speed ultegra cassette, but I don't think it is working to well. The chain has good tension, but whenever I push hard on the pedals, the chain pops off the cog. Do I just need to buck up and buy a single speed cog? Is it the deep teeth or the wide body that make these work? thanks.