Sorry if this is a lame question but in a case like in a small pick up truck for example , why cant you just mount the electric motor directly to the rear differential and not use the transmission or driveshaft ?
You can, but, its much more effecient and better to use an AC motor because they turn at higher RPM. You would have to calculate the needed rmp range based on the rear end gearing to determine if a DC motor could be used. DC motors run the most efficient near 3600 RMP (at least ADC Motors).
Because you really need to have some sort of gear reduction…
Ives Meadors of Synkromotive, converted his old truck and had a huge DC motor going right to the rear driveshaft, and a custom controller… and it accelerated like crap… they’re using a transmission now.
The right gearing in the differential would make this feasible, with enough h.p. in the motor. The White Zombie uses a direct driveline, no clutch, no trans, with a 3.58 gear ratio, 355 volt lack and 2400 amps of available discharge. He turns 10 second 1/4 miles! See this car here: Welcome to Plasma Boy Racing, home of White Zombie, the world’s quickest street legal electric door slammer in the 1/4 mile drag.
Big problem is weight. Motor mounted on a live axle rear end would be a nightmare on the road. Industrial equipment uses direct coupled motor/differential but travel at low speeds.