I was thinking about the 2CV the other day with respect to drone motors and rollerblades.<p>A 75%-efficient 1000-watt 20000-rpm quadcopter motor is one horsepower, costs about US$150, is about 50 mm in each dimension, and weighs 250 g. If you couple it to an 8-mm-diameter output shaft, the shaft's surface speed is 8.4 m/s or 30 km/hour. Hook up a 10 cfm cooling fan to remove the 250 watts of heat (the other 25%), mount the shaft between sprung two ball bearings that press it up against a rollerblade wheel, and you have a one-horsepower roller skate.<p>An average human can wear two such skates at a time, thus providing <i>deux chevaux</i> of power, 1500 W, which is 180 N (the weight of 18 kg) at 30 km/hour or 1100 N at 5 km/hour, plenty of power to burn rubber. Accelerating a 100-kg human to 30 km/hour requires 3.5 kJ, 2.3 seconds at two horsepower, limited in practice by traction. Most of the time you would be using much less power than that, and of course you need active traction control so you don't burn rubber every time you pick up your foot and put it back down, but this seems like a tractable engineering problem.<p>If your average net power usage (losses to air resistance, road roughness, rolling resistance, and round-trip losses from regenerative braking) is 100 watts, a two-hour trip is 0.7 megajoules. This is considerably lower than the average power usage of a Citroen 2CV because a human has a lot less air resistance than a car, even with a helmet and kneepads.<p>You could provide this energy with 1.9 kg of lithium-ion batteries in a backpack, costing about US$300, but plausibly you could make it cheaper by using high-discharge-rate lithium cells for half or a quarter of that (US$75 or US$150) and US$40 of deep-cycle lead-acid batteries (4 kg) for the rest.<p>The "2CV" in the car name was kind of a tax fraud, though. An actual Citroen 2CV offered 9 horsepower even at the beginning and 29 horsepower by the end, which is a lot more than you can strap onto your feet even with modern drone motors.<p>On the other hand, if you used a larger shaft, you could get the skates to go a lot faster than a 2CV could. But I'm not sure that would be a good idea.