Michael Ross wrote:
Lots to like about this effort, and the video is easy to watch, and
provocative in good ways. Bill D mentions unsprung weight. If you
aren't a performance nut you can manage clunky handling. Bet those
4 motors aren't heavier that 4 steel truck wheels.
I generally agree with Bill Dube', though perhaps not quite so
forcefully. Wheel motors sound good on paper, and can be made to work in
practice. But they have so many "feathers on the scale" against them
that they haven't proven to be practical (at least, so far).
First, the drawbacks of multiple motors vs. a single motor:
1. They cost more per horsepower:
2. They are heavier for a given horsepower.
3. They are less efficient (the bigger a motor, the better its
efficiency).
Second, the drawbacks of putting a motor in the wheel:
4. Higher unsprung weight (worse handling_.
5. Have to get high-power flexible cables to each wheel wheel.
6. Have to get cooling to each wheel motor.
7. Lower motor reliability, due to shock, vibration, water, dirt.
Third, if you eliminate the gearing:
8. Motors work best at high RPM (only limited by strength of
materials and magnetic losses). Wheel motors force
*low* RPM, so they don't deliver high power/weight.
Thus, a high-speed motor with gears works out better.
(Almost all eBikes do this.)
Fourth, the safety and reliability issues Bill mentioned:
9. Failure of one motor or its controller can lock that wheel,
or make it madly try to run at the wrong speed.
10. More motors and controllers means more points of failure,
thus lower reliability.
With all these "strikes" against them, wheel motors usually strike out.
A conventional high-speed motor and differential are a better choice.
Wheel motors have thus wound up as a niche solution that only gets used
for special cases (like small, or light, or low-speed vehicles; or
because they sound like a fun idea, or as bait to lure investors).
--
Engineers like to solve problems. If there are no problems handily
available, they will create their own problems. -- Scott Adams
--
Lee Hart, 814 8th Ave N, Sartell MN 56377, www.sunrise-ev.com
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