im wondering what gearing i should use for dh? I want to use a road cassette but im not sure which one. all i know is i want it to be from the sram pg-970 road series. the gearing options are, 11-21T, 11-23T, 12-23T, and 12-26T. im not shure which one i want. Also what size ring would i run in the front with one of these cassets.