In my neck of the woods there are some mountain descents that are pretty substantial: say 600 meters vertically averaging 7% grade, with lots of curves and other obstacles like potholes. Typically the surface is reasonable-quality asphalt; I have a touring bike.
I’m curious if there are techniques or gadgets to moderate my speed, in order to lessen the need for braking (e.g., in a car one can downshift).
As a back-of-the-envelope, computation, consider a 110kg rider/bicycle system coasting down a 7% grade at 50km/hour, or about 13 meters/second. Due to the grade, for every horizontal 13m, s/he descends 0.91m vertically; hence, every second, the potential energy of a 110kg mass dropping 0.91m is added to the bike & rider system and must be removed in order to not accelerate. That potential energy (U=mgh) is roughly 980 joules; i.e., the cyclist must dissipate about 1kW in order to not accelerate.
This surprises me; it seems like a lot of energy to be dumping into the environment. But it's on par with what can be produced by racers to reach similar speeds on the flat.
What I don't know how to compute is how much energy is consumed by air drag, mechanical friction in the bicycle, rolling resistance, etc., to know really what extra a slowing device would need to consume.
One thing I thought of was a dynamo hub; but those seem to consume less than 5 watts, which would make no real difference.
(I should add that I know I shouldn't brake continuously all the way down. My question is about whether there are additional things I can do or add to the bike to bleed off speed.)