recumpence wrote:Unsprung rotating mass is still an issue.
A single power transfer stage like a belt or something likely will make sense for at least a few years still until the right topology exists to put the motor into the rim while adding negligible additional weight.
Right now, using a very today-feasible technology like Mile's ultra-light hubmotor magnetic design, a very high power motor could be added to a wheel for less than the mass of a motorcycle tire. Once the structure of the rim starts serving the second function of also being the motors rotor (maybe induction?), things will start getting very light weight.
Essentially, the greater the surface you can get your motor to occupy, the higher the power per unit of weight the vehicle will be capable of once optimized. If you take the magnetics for your electricity to torque conversion machine and wad them all into a little compact ball, this is kinda the worst case for continuous power (which is always a thermal limitation). If you take the same mass of magnetics as the small motor has, but spread them out on a working radius the size of the rim, you improve the continuous performance by perhaps an order of magnitude.
Think of taking the same ~2lbs of iron and copper in a compact RC motor, and distribute that same mass of iron and copper around the diameter of a rim. The slot for each tooth is maybe 2-3mm deep or whatever, it's going to have maybe 500 teeth or whatever, the magnets (or go short-flux-path switched reluctance) are maybe 0.2mm thick, perhaps in a halbach array or whatever. You end up not weighing any more, but making the torque through applying a force on a long moment arm this way, and the parts that get hot have a whole order of magnitude improved surface for thermal dissipation.
I realize that there is nothing like it for sale today. This is because EV drivetrains are still very much evolving in a mindset tainted by previous limitations of ICE engines that needed to be tiny and high RPM to have decent power density. Electric doesn't suffer any such limitation to be light and powerful, one may simply push the limits of the geometry and manufacturing processes until you arrive at the electricity to torque converter one finds best suited towards turning a wheel.
It is the inevitable path forward, because all other paths are performance/efficiency compromises. May take 5-10 years or something before we see anything like it available off-the-shelf though.
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