I use a jack-shaft arrangement with a nexus inter3 . I spin that bastard up to 2000 RPM on my way to my 40mph cruise speed. No danger of breaking them if you spin them up faster.
HP = TORQUE x RPM ÷ 5252
If you notice the equation above, with the same HP value of your engine, solve for torque at any given RPM.
You will find that increasing the RPMS transmitted by the jack-shaft, the load it deals with lowers.
I recommend the nexus over a sturmey-archer due to my experience with the reliability of the nexus. Dont listen to the nay-sayers.
I recommend this hub as a jackshaft.
BEST VALUE FOR MOTORIZED BIKE SHIFTER APPLICATIONS PERIOD!
I have thousands of miles to back me up on this
We're working on a 4.2 HP engine mount for my ride that will couple to the tranny.
We are betting on hitting 55-60mph. Right now we get 43 mph with a 1.5 HP engine.