If you are designing an electric car from the ground up, you may find that a fixed gear or two gear transmission is the most efficient solution. If you are converting an existing gas vehicle to electric, re-using the existing transmission is a very attractive option. It allows you to reuse the existing transmission mounts and the mechanical drive system downstream of the transmission, like axles and drive shafts.
The gearing on a gasoline engine will do a decent job of covering the gears we want for the electric motor. The electric motor will have a lot more torque available at zero RMP, so 1st gear will pretty much be useless. An electric car will be quicker off the line in 2nd gear than the gas motor starting in first. Electric motors are happy turning higher RPMs than gas motors, so 3rd gear is usually good for highway speeds.
We need to come up with an adapter to join the electric motor to the transmission. This will be two adapters, actually. The first is a chunk of aluminum to physically join the body of the motor to the bell housing of the transmission. The second is a hub that mounts to the output shaft of the electric motor, and bolts up to the flywheel. This hub is a two piece system. By drawing the hub together, the taper squeezes the motor shaft with quite a bit of force.
The VW Doka is a rear engine, rear wheel drive that uses a trans-axle, where the differential function is included in the gear box. The Sprite is a traditional front engine, rear wheel drive configuration. Both will use a similar arrangement to connect the electric motor to the transmission.
The goals of a good adapter: keep the electric and transmission shafts rigidly aligned, and locate the flywheel into the transmission bell housing the same depth as it was with the gas motor, so that the clutch fork will correctly engage the pressure plate to release the clutch disk.
That's the theory. Now to put it into practice...
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