Sorry if this is a stupid question. EVs use single speed transmission because the AC motor has instant torque. But they still use a transmission, the motor turns gears that turn the wheels. My question is, why not just directly spin the wheels with the motor?
In: Engineering
You can do it without a transmission. Build the motor into the wheel hub. But there are some problems – the biggest one is ‘unsprung weight’. The motor in the hub is going to be heavy, and when the wheel hits a bump, it is thrown upwards, and it takes time for the springs to push it back down again, meaning the wheel spends some time in the air. So almost all designs mount the motor to the chassis, and use universal joints or CV joints to drive the wheels.
And once you have that, you gear the drive down so you can use an efficient, light, high speed motor.
Because is also hard to make a motor that drives slow enough. Typical highway speed of a car wheel is well under under 1,000 RPM, and that is really slow for an electric motor. Especially if you want a light, high power one. They spin at 10,000 RPM or higher. In order to get decent torque at that less than 1,000 RPM you need high current, which means thick, heavy copper windings. And a hub mounted motor cannot be heavy.
That’s a possibility, but there is some math to wheel speeds during a turn that could become bothersome.
In a right hand turn, for example. The passenger side drive tire is rotating slower than the driver’s side drive tire by necessity. This is usually handled by the rear differential. On a direct drive EV you’d have to have a way of controlling the speed of each motor as a function of the vehicles steering mechanism.
I’m no expert, but it seems to me that as an engineer, you’d be safer and more efficient if you used systems that already exist when designing a commercial vehicle.
Electric motors spin crazy fast. It’s easier and usually more effective to use gears to get a slower rotation and a “stronger” rotation.
Electric cars with actual gearboxes will also have more strength in the lower gears for hillclimbing and starting but still be able to spin the wheels really fast at higher gears.
They can spin very fast. The motor has a much higher max RPM than it would ever be able to use due to power limitations and the problem of wind resistance. Torque, however, is something that is very useful. With gear reduction you gain torque to the wheels at the expense of max speed.
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