EBike Motors Hub
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An ebike hubmotor is a motor within the front or rear wheel of the bicycle. They come in two basic types: direct drive and geared.
Stength of the bike's dropouts and potential clashes between the motor and disk brakes are key factors to consider when using a hub motor.
Direct Drive Hubmotors
Direct drive hubmotors are the simplest type of motor, since they do not have gears or a clutch. The electric motor is built right into the hub, with the windings attached to the axle, and the magnets arranged around the winding on the outer rim of the hub. Compared with geared motors, they are simpler and have fewer moving parts. They are also called outrunner motors, since the moving part is on the outside. This page describes potential failure modes for direct drive motors.
- Harder to pedal or loses speed when the motor is not being used, due to the motor being directly connected to the wheel, therefore unable to freewheel.
- Usually larger and heavier than geared motors. 
Common Direct Drive Hubmotors
Aotema hub motors used to be marketed primarily by Wilderness Energy (now out of business) and sold as the BL-36. It is a brushless motor with no sensors. Drawing ~750w on 36v expect a top speed of around 23 mph with 700c tires.
Geared hubmotors have a more complicated design. The motor inside turns a set of planetary gears (typically using a 5:1 reduction ratio). The motor spins faster than for direct drive motors, due to this gear ratio. Most planetary gearmotors also have a freewheeling clutch built into them.
- Usually smaller and lighter than direct drive motors
- More efficient for take-off and mild hills,  with less heat generated in these situations
- Motors with a freewheeling clutch coast better than direct drive motors. 
- Noisier than direct drive, due higher motor speeds and spinning gears.
- gears and freewheel are extra parts which can wear over time. However many modern geared motors are claimed to be maintenance free.
- the freewheel function suspends regeneration  Regeneration can typically extend range by 5-15%, depending on the hills and the amount of stops.  
Find a comprehensive list with characteristics of brushless geared hubmotors on the ES forum by voicecoils 2008.
This graph was created by the ebikes.ca drive simulator, to compare the efficiency, torque and power of a common direct vs a geared drives with equal top speed.
Common geared Hubmotors
- Bafang 250W
There are various 180-250W outrunner motors for both front and rear wheel (SWXB, SWXH, SWXK, SWXU....), max. diameter: ~120mm or 143.5mm, gear reduction ~4:1 or 4.43:1 (depending on version), different winds are available for 24-36V and 20"-28" wheels.
- Bafang BPM and QBPM
- Bafang CST
similar to BPM, but with a freewheel compatible to shimano hyperglide cassettes, instead the common BSA threads
- Bafang CST Mini
Similar to SWXH but with a freewheel compatible to shimano hyperglide cassettes, instead the common BSA threads
Bafang official homepage: 
Article with lots of pics from BPM and MAC 
These three motors are very similar. Their very high pole count and a 5:1 gear reduction give them increased torque. They are available in different winds for 20-28" wheels and 24-48V battery voltage. The BMC has thinner laminations and freewheel threads made of steel (rather than alloy).
- Rated power: 500-1500W,
- weight: about 4.5kg (10 lbs)
This thread discusses the differences. It is common to swap the nylon planet gears for stronger composite gears or steel gears, these are available at various e-bike shops on the net.
similar to Bafang SWXH
Main Article: TongXin_/_Weifeng
Friction drive inrunner hubmotor. Very lightweight (<2kg) and small, upto 180Watts continous output. Almost silent and a very high reduction, but slightly reduced efficiency. There are two versions, the original and the newer and even more light weight Keyde.
All cute motors are inrunner motors with a power rating of about 250W. The Q128 is said to be the weakest one, even if rated at 500W. The Q75 that came to market in 2013, is the smallest (92mm/110mm spokes flange diameter) and most lightweight (1.2kg) Hubmotor.
helical geared inrunner hub motor with steel gears 
These have a much better climbing ability than common single speed motors of the same size. The second gear is introduced by spinning the motor backwards and engage a different gear by an additional freewheel. There is no need for mechanical actuation. This is called retro-direct 2-speed drive and is well known since decades. However, the drive can normally not be spun backwards since the two gears would fight each other. But this issue was solved with the 2-speed motors. 
Lifespan and Maintenance
Motors with brushes need the brushes replaced from time to time.
However, most motors are brushless. The only wear items on brushless motors are the bearings, so these motors can last 10,000km or more. Hub motors are usually sealed and do not require any maintenance.
Front vs Rear wheel
This thread discusses the pro's and cons of front vs rear hub motors: http://endless-sphere.com/forums/viewtopic.php?f=3&t=26625
Installation on the front wheel is often simpler, however the rear wheel is safer- especially for more powerful motors.