I use a biopace chainring on my fixie. I like it a lot, but learned something
VERY IMPORTANT- when you bolt on the rear wheel, setting the chain tension,
make sure the Biopace is placed in the 'loose' position. (pedals facing forward
and backward) That way, the rotation will slightly tighten the chain then return
it to your set tension. Do it the other way, and the chain may bounce off the
gears when the ride gets bumpy.