I'm curious about an explanation for why a wind-up spring KERS (Kinetic Energy Recovery System) isn't worth it for a bicycle.
The system I have in mind is instead of braking the rear wheel you engage something that winds up a spring or elastomere as the bike slows. Then some ratchet mechanism stops the elastic from recovering and spinning the wheel in reverse. To use the energy stored in the spring you have a control to release the ratchet and engage some gears that reverse the spin, from reverse to forwards.
Let's assume there are significant losses, say 80%. Given rush-hour start-stop traffic even a 20% energy recovery assist might make sense in terms of the weight/complexity budget.
So how come we don't have such a system already, what is this scenario overlooking?