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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
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    @DanielRHicks: Why don't you expand your comment and convert it to an answer? :) – Jahaziel Nov 24 '11 at 16:04
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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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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.

Humans are peculiar. Humans produce power best at less than 100 RPM. Very similar to steam engines and very dissimilar to internal combustion engines, turbines or electric motors. Bicycle wheels rotate at 250 RPM when riding on a flat road. Thus, you need about 3.2x gearing if riding on a flat road.

250 RPM for electric motors is extremely low. Typically, optimal motors like to run at 10 000 - 20 000 RPM.

There are ways to make motors generate useful power at 250 RPM, but these make the motor heavy, expensive and perhaps inefficient.

Not only that, but a hub motor requires a reaction arm and electric cable to the hub, making changing a flat tire needlessly difficult.

The best e-bike motors are at the bottom bracket and use reduction gearing. With reduction gearing, you can reduce speed by more than 10x and improve torque by more than 10x. It won't let you use a 20 000 RPM motor, because such a motor would require 200x torque multiplication and 200x speed division. But, by creating a slower electric motor AND by using reduction gearing, it is feasible to build an e-bike that produces useful torque.

If seeing a hub motor torque somewhere, don't believe it! It is probably the torque when the wheel is not rotating. With increasing speed, the torque reduces.

Furthermore, the bottom bracket motor torque is affected by the drivetrain gearing too, so it is possible to produce both high speed (high gear) and high torque (low gear) using the same motor.

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I've had a direct drive hub motor and the latest is a geared hub motor, the geared is much smaller, lighter and faster with better low speed torque and easier on the battery.

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  • This doesn't appear to answer the question, which is comparing hub motors to external motors. You're comparing different types of hub motor. – Móż Sep 29 '15 at 21:45

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