Hub motors come laced into a standard bicycle wheel, and are spaced to fit in either the front fork(100mm) or the back dropouts(135mm). While both front and back hub motors are pretty much drop in replacements, their are a few differences.
One of my front hubs drilled for ventilation and secured with a custom torque armFront hub motors
Being pulled around by a front hub can be a fun little departure from a solely rear wheel drive bike. They also let you keep your bikes rear wheel and rear gear stock. They do however have some difficulties with aluminum forks, or higher powered setups; it's hard to properly contain the torque of the motor in the front fork.Good:
Rear hubs need to use freewheels; a some what dated technology. It's become hard to source quality freewheels, especially the 11 tooth type that allow you to pedal comfortably at high speeds. A front hub motor allows you to keep your drive train stock, so you won't have to spend $20-$30 for a freewheel.
Front hub motors are pretty much a drop in install that can be done in minutes. They can also be uninstalled even quicker, so it's easy to de-electrify your bike for normal riding.Bad:
Because of the torque the fork must contain you simply cannot safely install a hub motor into a fork that is not steel. This is problematic because only the cheapest suspension forks are made from steel. Be sure to check with a magnet if you are getting a front hub motor. The magnet should stick to steel. Another option is to reinforce the fork, but this is quite a job!
A reinforced suspension fork
Even with a steel fork you will still need a torque arm to safely contain the power of the front motor. Given enough time, the dropouts holding the motor axle will fail. The best case scenario for this happening is the motor spinning and tearing up the wiring and the fork. The worst case scenario is flying over the handle bars at 30 miles per hour. A torque arm transfers the rotational force from the axle to the much stronger fork.
A universal torque arm
What happens without a torque armRear hub motors
Eventually you'll get power hungry and go with a rear motor; It's easier to safely contain the force of the motor in the stronger rear dropouts, and with enough power it's hard to keep a front hub from losing traction. Rear hubs can be a bit of a pain to install some times though. Good:
The drop outs in the rear are much stronger, so you won't need a torque arm with a steel frame. Although losing a rear wheel isn't nearly as bad as losing a front wheel, I'd still recommend a torque arm.
You'll get better traction with a rear hub. Front hubs are limited in power by their (lack of) traction.
Try this with a front hub
It's a lot easier to do a "stealth" electric bike with a rear hub!Bad:
You'll need to buy a freewheel for a rear hub. Your also limited to 6-7 gears in the rear.
Some bikes have clearance issues that require spacers and finagling. It can be a pain to get certain disc brakes to work properly as well. Especially when the hub motor is pressed right up against the brake caliper adjusters!
Clearance issues on an old "thread on disc" style 9c hub by El_steak
It's not a very hard choice to make; If you have a steel fork and you are getting a low powered kit, a front hub is easier and cheaper. If you don't have a steel fork or you think you might be interested in overvolting and other upgrades, then get a rear hub motor!