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Some bikes, e.g. Busettii Vortex use an external motor, while others (by the same manufacturer) use hub motors. Busettii claims that external motors have higher efficiency (http://www.biyadii.com/), yet their top-of-the-line Big 50 model uses a hub motor. Other Web sites say that hub motors are more efficient.

I'm confused. Do external motors really produce more torque using less electricity? If so, how? If the issue is sub-optimal RPM in hub motors, then why would moving the motor outside the hub help with that? I am looking for some no-nonsense technical explanation of the pros and cons of each design.

Thanks!

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The external motors I've seen use a chain or belt to convey power to the rear wheel. This offers the opportunity to use different sized pulleys/sprockets to allow the motor to run at a higher RPM than the wheel, not so much improving efficiency as lowering cost, since the motor doesn't need to be designed to run at such a low speed, and hence doesn't need as many "poles". – Daniel R Hicks Nov 23 '11 at 1:30
@DanielRHicks: Why don't you expand your comment and convert it to an answer? :) – Jahaziel Nov 24 '11 at 16:04

1 Answer

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I think Daniel's trying to say that in an effort to reach the largest audience motor makers opt for a hub design because it is the cheapest. They're certainly the cheapest after-market upgrades. Motors that interact with the drivetrain can be more efficient but require solving engineering problems and so the cost goes up. Using the bike's transmission allows the motor to stay within an RPM range that can maximize power output for the energy used.

All motors have a sweet spot of efficiency vs power output and RPMs. Check out this article by Todd Fahrner of Clever Cycles describing motor performance. His choice to make Stokemonkey an external motor that drives the cranks is to allow you to shift normally and keep the motor RPM in the sweet spot of the efficiency curve of the motor.

Here's a great simulator that'll let you play around with different setups.

The Endless Sphere forums are a good place to check claims from manufacturers. Here's a thread where a large grain of salt is being applied to the claims from Busettii

Good luck!

Full Disclosure: I used to work at at Clever Cycles.

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"motor makers opt for a hub design because it is the cheapest" Actually, something of the opposite. You can use off-the-shelf external motors, since you can arrange the gearing to achieve the right speed/torque. But with a hub motor you must add poles, which adds cost/weight. The main advantages of the hub motor would be compactness, fewer moving parts, quieter, easier fit to existing frames. And probably they give a better showroom impression. – Daniel R Hicks Nov 30 '11 at 1:37

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