I would like to convert a Mazda Miata into an electric car, I plan to remove the engine and the gearbox, and have the car be a direct drive. The motor I intend to use has twice the torque of a standard Miata. If I were to use this motor, would the high torque damage the vehicle? Are there other issue I should be aware of, before going through with any conversion? Is there some way to avoid such issues?
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If you were to simply replace the existing petrol engine with an electric motor, then yes, it would ruin the gearbox very quickly, as it won't be able to handle that amount of torque.
However, even if the transmission would cope, that wouldn't be the best way of doing it - an electric motor has a very different torque curve to an internal combustion engine, so you would need to replace the gearbox anyway. The fundamental point of a car gearbox is to convert the very narrow torque curve of an IC engine into something usable over the full range of road speeds. In theory, a suitable electric motor wouldn't need a gearbox at all (those used in trains, for example, generally drive the axles either directly or through a fixed geartrain), as it;'ll have a much wider torque curve.
If I were converting an existing car to electric, I'd remove the existing engine and gearbox and attach a suitable motor to the differential input shaft, keeping the diff for the final stage so that only a single motor would be required, and to avoid adding to the unsprung mass. The motor should fit in the space previously occupied by the 'box, leaving the entire engine bay free for batteries...
I can't see you having many issues with power handling once you have removed the transmission. The driveshaft, diff and axles should be able to handle just about any torque you throw at them unless you start really getting above 400-500 ftlbs or something like that.
I bet the biggest issues you would have with this project would be ECM related and getting the electronics to work without freaking out that there no longer is any oil/trans pressure, temperature, rpm, etc.
Also are you going to convert the vehicle to a mechanical steering rack or somehow run a power steering pump separate? Again same with heating/ac you would lose those or have to replace them with an electric equivalent. Basically anything belt driven has to be removed or replaced.
If the accelerator is cable operated you will have to convert that somehow to an electrical signal to go to your speed controller.
I bet trying to get regenerative braking to work would be very difficult and advanced.
Have lots of money in the bank before you start this project.