You could try to somehow attach the generator directly to the pedals/crank, that way you won't lose energy to the chain and the bearings on the rear wheel. It'

You could try to somehow attach the generator directly to the pedals/crank, that way you won't lose energy to the chain and the bearings on the rear wheel. It's probably not a meaningful improvement if the bike is well-kept and the rear wheel is lifted from the ground.