The DiskDrive, by British startup Skarper, is a 6.6-pound object that snaps onto your bike, then powers it electrically by driving the rear brake rotor. Pop it off to recharge it (or prevent it from being stolen).
Using the system does require you to swap out your existing rotor, with this one of Skarper's design:
"Skarper have engineered a full gearbox into the space of a rear disk brake," they write. "When not electrically driven, the disk behaves exactly as a normal disk without any compromise on performance or function. Your bike is ready for electric drive at any moment. Precision features on the disk allow the direct connection of a Skarper motor drive unit whenever desired."
I'm sure it may vary bike to bike, but it's incredible how quickly and easily the unit snaps on and off in the demo video:
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It takes 2.5 hours to charge the device from empty to full. The 250W motor has a maximum range of 60km (37 miles) and can propel you to a top speed of 32kph (20mph).
The DiskDrive is intended to ship next year, and Bike Radar says the target price is £1,000 (USD $1,239).