As opposed to electric regenerative brakes on eBikes or flywheel KERS (flywheel = lots of weight)
Why is a mechanical solution not feasible?
- Pull brake lever
- Clutch on rear wheel engages a wire that winds some spring or elastomer
- Release lever. Ratchet prevents unwinding, conserving the energy gathered.
- Pull some other lever to release the energy
- Wire transmits unwinding energy into gears that reverse rotation, so the wheel can move forward not backward
Would be useful for start-stop traffic during city commuting. Many cyclists are weary of losing the momentum they have put effort into building, which can result in more reckless riding behavior. If you can get a mechanical assist boost when you pick back up you've more incentive to stay behind a stopping vehicle instead of dangerously overtaking it.
To qualify, the mechanical solution would have to come in lighter than an eBike conversion with regenerative braking.