Staying single speed in your rookie season is not a bad idea. Banebots has redesigned their transmissions for greater durability, and a planetary transmission takes less space, is easy to mount, and you can change output ratios simply with sprockets. You MUST support the outboard end of the output shaft, and they have a bearing block available to make this easy. The kit transmission will also work well, but you should consider lightening it up. We have used DeWalt XRP transmissions with the CIM motors for the past two years, and they have been bulletproof (with modifications). I would not recommend you doing this outright, but you can do a Search on CD for Joe Johnson's paper "Nothing But DeWalts". You can get up to three speed output, but two speeds are most useable.
