[tkwetzel]

The only thing that I can think of that would allow this is a clutch like in a car with a manual transmission. There are one-way clutches that would allow you to free-wheel, but only in one direction, and I am not exactly sure how a bike hub works to allow it to free-wheel in both directions.

One thing that might work is a centrifugal clutch. We have one on the mini-chopper that my friend and I built. It is mounted on the output shaft of the engine and lets the engine idle up to a certain speed. When the engine speed increases, there are clutch shoes that move outwards and grab the drum around them to turn it. There is a sprocket already attached to the drum. I do not know if there is a cenrifugal clutch that would work on a FIRST robot due to the speed and torque required to engage it, but it is something to look into.