I am all for innovation and new ideas but I'm not really seeing this as being a great solution. It might be usable and a step in the right direction but it isn't there yet.

Ron and Anon 4:27 make a great point about the practicality of regenerative braking for a car vs. a bike. What may make sense for a car, especially an F1 car stopping rapidly from 200mph, does not necessarily apply to a slow moving vehicle with a total weight around 200 lbs.

The weight distribution point made by Daner and others is a very good one too.

Still, I am glad to see more engineering resources these days going toward all types of e-bike development. Time will tell if this develops into anything, but for now it is a great catalyst for more discussion. Keep the comments coming!

Regenerative braking is probably not a practical solution on vehicles as light and efficient as a bicycle, especially in the rear-hub, since only a small proportion of braking pressure is exerted on the rear of a bike.

Ebikes make sense for a lot of people who just want to get where they need to go with a minimum of fuss, so I have no problem with the concept, just the execution (Daner does a good job of outlining the problems with this one).

As far as Pete's comment, your argument really goes beyond the scope of this blog, or of bicycling, period. The fact of the matter is, the amount of influence that an individual person has on the global issue of climate change is pretty trivial. Obviously, the solutions need to be macro-industrial, political, and social ones. Personal 'greening' is pretty much just to satisfy our own egos and make us feel like we are actually significant in some way (and I say this as a car-less vegetarian for sustainability reasons, so I'm being a bit cynical beyond my hypocrisy).

Then comes the question of braking energy recovery. How many stops would one have to make to recover atleast 10% of the total battery capacity back and how much will a cyclist's range be extended?

I'll give a perspective. A 180 lb man with a 20 lb bike traveling at 15 mph needs 2026 joules or 0.56 Wh of K.E to come to a stop. With what efficiency will the recovery system be able to capture some of that? 70%? 80%? Let's assume 75%. I can get back 0.42 Wh or 1516 joules back, but I can only put this energy back into the battery at a certain charge rate which really depends on the battery's specs.

For illustration, if battery's max charge rate is 100 joules/sec, 1516 joules takes 15 seconds or more to put back in.

Now realistically, no one takes that long to stop, even though the CPSC recommends that brakes should stop a cyclist in 15 seconds from 15 mph to 0mph.

So realistically, people take about 3-4 seconds every time to stop. Using the 100 J/s charge rate, cyclist can only put in 400 Joules or 0.11 Wh back every stop. So the question again becomes, how many times does a cyclist have to stop to put back 10% of kinetic energy of braking, at 75% efficiency to get the extra amount of miles, given the limitations of the battery?

If its a large amount, it may make sense of a crowded urban area, otherwise not really considering buying costs and lifecycle costs. Also considering Copenhagen is on the route to "biking highways" in future (2012+), I expect some of the stopping needed to go away so then the question is, will a technology like this even be necessary then? Perhaps NYC can really make use of this. You can provide such a technology to those poor pedal cabbies and save some of their energy. I see a lot of practicality in something like that.