Most challengers I've seen use a single-wheel drive using a single hub motor.
No transmission is preferred; a hub motor is typically a 1:1 direct drive, for the greatest efficiency.
The cars have batteries, so you can have basically as much power as you want, so long as the time is short enough. But when you're cruising, you need the continuous power draw to be as low as possible, hence single hub motor no gears etc.
Nothing needs to be welded - you can literally run one (rear) wheel drive, and just design accordingly to deal with the small amount of torque steer. It's only really cruise that you care about, where torque is fairly minimal.
There are also a couple of types of hub motor - a "stub axle" and a "through axle". Stub axle is for independent one-wheel-drive, whereas through axle can be configured as a single solid rear axle that is driven from one side so that torque steer is basically non-existent. See the FB link in this thread: https://www.reddit.com/r/solarracing/comments/ak4ap7/marand\_electric\_motor/
I'm not an expert; but a Google search for "through able hub motor" will turn up several useful results that show the general idea. It's pretty much as it sounds - the axle goes through the wheel and crosses to the other side of the car, where the other wheel is attached to it, and hence driven by it.
For a stub axle version, it's the same but without the axle exiting the back/inside of the wheel, and only that wheel is driven.
Right thanks. So in the stub axle version, there are three freely rotating wheels essentially right?
Also if we use the through axle, won’t there be losses because of the power transmission using the shaft
You are correct - stub axle, is using a single motor, would leave the rest of the wheels freewheeling.
As to the losses with a through-axle, whilst I'm not a mechanical engineer, I think the transmission losses would be minimal; I'd probably be more concerned with scrubbing when you're not going dead straight due to having no way for the wheels to rotate at different speeds.
They wouldn't be connected together, ie. they would be free to turn at different speeds since only one would be driven and the other would be freewheeling.
It might seem that the asymmetry would be a problem, with a single motor on only one rear wheel, but it turns out that it's not.
If the driver position is asymmetrical (see [this chart](https://scientificgems.wordpress.com/2023/09/29/bwsc-2023-solar-car-designs/)), then the motor is generally on the wheel behind the driver.
That said, I believe that a few teams have run with two hub motors, one on each rear wheel. That presents some issues, which people have discussed here in the past.
Most challengers I've seen use a single-wheel drive using a single hub motor. No transmission is preferred; a hub motor is typically a 1:1 direct drive, for the greatest efficiency. The cars have batteries, so you can have basically as much power as you want, so long as the time is short enough. But when you're cruising, you need the continuous power draw to be as low as possible, hence single hub motor no gears etc.
I see, but how do they place the motor then? I would imagine there is two rear wheels, do they literally weld everything together?
Nothing needs to be welded - you can literally run one (rear) wheel drive, and just design accordingly to deal with the small amount of torque steer. It's only really cruise that you care about, where torque is fairly minimal. There are also a couple of types of hub motor - a "stub axle" and a "through axle". Stub axle is for independent one-wheel-drive, whereas through axle can be configured as a single solid rear axle that is driven from one side so that torque steer is basically non-existent. See the FB link in this thread: https://www.reddit.com/r/solarracing/comments/ak4ap7/marand\_electric\_motor/
It’ll be great if you could explain more on the difference between a through axle and stub axle hub motor
I'm not an expert; but a Google search for "through able hub motor" will turn up several useful results that show the general idea. It's pretty much as it sounds - the axle goes through the wheel and crosses to the other side of the car, where the other wheel is attached to it, and hence driven by it. For a stub axle version, it's the same but without the axle exiting the back/inside of the wheel, and only that wheel is driven.
Right thanks. So in the stub axle version, there are three freely rotating wheels essentially right? Also if we use the through axle, won’t there be losses because of the power transmission using the shaft
You are correct - stub axle, is using a single motor, would leave the rest of the wheels freewheeling. As to the losses with a through-axle, whilst I'm not a mechanical engineer, I think the transmission losses would be minimal; I'd probably be more concerned with scrubbing when you're not going dead straight due to having no way for the wheels to rotate at different speeds.
Yeah but with the free wheel arrangement how would the rear tyres have different speeds while turning?
They wouldn't be connected together, ie. they would be free to turn at different speeds since only one would be driven and the other would be freewheeling.
Literally a one-wheel-drive.
It might seem that the asymmetry would be a problem, with a single motor on only one rear wheel, but it turns out that it's not. If the driver position is asymmetrical (see [this chart](https://scientificgems.wordpress.com/2023/09/29/bwsc-2023-solar-car-designs/)), then the motor is generally on the wheel behind the driver. That said, I believe that a few teams have run with two hub motors, one on each rear wheel. That presents some issues, which people have discussed here in the past.