BionX puts the motor in the rear wheel. Or, rather, you do. This aftermarket kit transforms most any frame into a 350-watt crotch rocket with a top pedal-assist setting of 400 percent, meaning for about the same effort as you’d put out for a nice stroll, you’ll hit—and maintain—Tour de France speeds.

Assembly is required. But there are only three main parts: A 36-volt lithium battery, rear wheel with a built-in motorized hub, and a digital control display that affixes to the handlebars. Swap out the old bike’s rear wheel, bolt the battery onto the down tube’s water bottle mount, and run cabling between all three. The biggest challenge is making sure the cabling is taught and safely situated (hint: zip ties and patience).

The BionX system was also the most dynamic of any system we tested. There are four pedal-assist settings for adding torque, plus “negative” settings that turn the motor into a generator that lets you recharge the battery by pedaling or coasting down hill.

The whole kit weighs just 17 pounds. So if riding a 50-pound e-bike (see Kalkhoff and Giant) doesn’t seem appealing, bolt the BionX kit onto an old 25-pound bike (not too hard to find), and you’ve got a commuter you can carry up the stairs (we’d still suggest the elevator).

On the other hand, once you’ve installed it, there’s no easy way to switch back and forth from e-bike to analog. And, at $1,900, the kit costs more than the Giant, which is a complete, ready-to-go ebike.

designer: bionx
period: 2012