Between an
engine and the wheels, there needs to be:
- some way to disengage so that the engine can idle, and so it can run fast enough to work while the car starts moving (a clutch or torque converter)
- a reduction of speed (and corresponding increase of torque) so that the fast-turning engine can work with the slower-turning wheels (gear reduction)
- a way to change the ratio of speed reduction to suit different road speeds (a multi-speed transmission)
- a way to move the vehicle in reverse while the engine still turns in the normal direction
- a way to split the drive power between two wheels, allowing them to turn at different speeds in a corner (a differential)
There also must be some way to mount the engine.
A manual transmission uses a clutch, traditional automatics use a torque converter. A transaxle is the transmission (for #2, #3, and #4) combined with a differential (for #5) all in one housing. Air-cooled Volkswagens and Porsches use a transaxle, with the engine mounted to the back of it.
Between an
electric motor and the wheels, the speed reduction (#2), differential action (#5), and some way to mount the motor are all still needed. With many motors (having a narrow band of speeds at which maximum power is available), a multi-speed transmission (#3) is helpful, too. With DC motors, either a reversing contactor system (and some compromise on brush timing) or a reverse in the transmission (#4) is needed.
The car's original transmission is bigger, heavier, and more complex than needed for the electric motor (because not so many speeds are needed, and because a clutch is needed only if shifting), but it's already there and designed to fit in the car, so most conversions use the original transmission and final drive (differential) or something like it to perform these needed functions. I suspect that every off-the-shelf conversion kit for an air-cooled VW uses the original manual transaxle.
So that's why the original transaxle is usually used. Now for the alternatives...
There is a recent trend to using
motors from scrapped production EVs. One way to avoid the use of the original VW transaxle entirely, if using a production EV motor, is to use it complete with the transaxle that comes as part of a used EV drive unit. That transaxle generally only has one ratio (not multiple speeds as in #3), but that's okay for these motors. There is no clutch in these, because it is not needed. The combination of the motor and transaxle (and maybe the inverter) is commonly called a "drive unit" (at least in this forum). Nissan Leaf and Tesla Model S drive units are the only common candidates for this (although most production EVs use a similar design), and the only one likely to fit in the VW is one of the small Tesla drive units, because it sits on and behind the axle line.
The other way to not use the original transaxle is to use some other aftermarket transaxle, or to build something yourself. For DIY systems, various belt or chain arrangements (for the reduction) and special differentials are typically involved.
Both complete drive units and DIY solutions are much more difficult installation than using the stock VW's manual transaxle, so I'm not suggesting it in this case - I just listed the idea of not using the original transaxle to round out the list of choices.