Matt mentioned electric bikes in passing yesterday; as it happens, Hack-a-day had a roundup of electric recumbent projects the same day.

I can't say that I really want an electric bike — they tend to look dumb, and are heavy besides. And I just don't need one, frankly — my legs are more efficient, require less expensive upkeep and are filled with fewer toxic chemicals than EV bikes.

But I do occasionally daydream about a bike with regenerative braking. Partly this is stupid: if you're paying for a drivetrain and motor you might as well go whole-hog and build a complete electric bike. But it's at least a little bit defensible: by designing a system solely meant to capture energy and then release it as a from-a-dead-start acceleration boost, you may be able to lower the requirements for the motor. Certainly your energy-storage requirements would be diminished — no bulky batteries! An array of (admittedly expensive) ultracapacitors could likely do the trick.

Some actual EV bike manufacturers have crunched the numbers on regenerative braking and concluded that it's a bad deal. But they did so assuming a chemical battery; ultracaps don't have the same slow-recharge limitations, and are considerably more efficient besides.

It may seem like a pretty trivial thing, but I think this could actually make a big difference for cyclists' comfort and safety. When you've been biking for a while you begin to appreciate just how precious momentum is, and how irritating it is to be forced to abandon it due to a motorist's trepidation or inattention. This leads to a lot dangerous behavior — when observing a stop sign comes at the cost of sweat and burning muscles rather than a penny's worth of gasoline, it's tempting to try to make sure things are safe enough and then proceed without pausing. If I could get even half of my momentum back after being forced to stop I'd be a lot happier about pulling the brake lever. It'd let me avoid downshifting when I stop, too.

UPDATE: The Halfbakery folks considered the same idea a while ago using mechanical systems — hydraulic, pneumatic and spring-based. It sounds sort of promising! It's a bit more dangerous than an electric system, but would probably be somewhat cheaper and much more efficient. Unfortunately I know basically nothing about mechanical engineering, so there's not much more I can say about these systems besides noting that they sound cool.

Photo by Flickr user nedrichards, used under a Creative Commons license