[Bearleener]

Have you considered storing the energy as compressed air? An air motor is very compact. Have it pump air into an (LP) gas tank (in the spare tire well?) while braking, then use that compressed air to drive the motor during acceleration. If I calculated correctly, you can store about 100 Joules of energy per litre of tank and atmosphere of pressure. So a 5-gallon tank pumped up to 100 psi has enough energy (13 kJ= 3.6 Wh, ignoring losses) to accelerate a 2400-lb car to about 11 mph. Not too bad, relatively cheap and needs only a very primitive controller. This is sort of a surge regenerative energy concept like they're designing for Formula 1 cars.

I've been looking for places on my car to couple the energy in. The only place I found was the inner CV joint housings on either side of the differential. These are sturdy, 100-mm-diameter cylinders that you could theoretically attach a drive belt or rubber roller to.