Bafang geared hub motors (I think all Bafang hub motors are geared) have a freewheel which allows the wheel to roll forwards without drag from the motor and gears. This allows you to ride an ebike without drag when pedalling but not motoring.
Sometimes the freewheel fails, usually by locking up and not freewheeling. When this happens, you can feel the drag of the motor and hear the whirring of the gears when you push or pedal the bike forwards – the same as you get when you push a normal hub motor wheel backwards. This really slows you down when descending hills. The first time this happened to me I thought I had a stick stuck in the mudguards, because of the noise and drag.
Here is what you see when you open up a Bafang hub (there are different motor types, but they’re all the same in principle:
Hopefully your hub motor isn’t blackened and burnt like this one. It’s an SWXH 250w rear motor which was cooked by long slow hill climbs, and I think it also has water damage as well.
The freewheel is the steel disc on top of the motor, on the left. It is attached to the motor shaft with a key and circlip. The nylon planet gears are attached to the freewheel, underneath. Under the nylon planet gears is a short cylinder: the magnet drum. This spins when the copper windings underneath it are powered, and it transmits its power to the nylon gears with a little steel sun gear in its middle. The nylon gears then drive the hub by gripping onto the ring gear which can be seen in the motor case on the right.
The freewheel spins freely in one direction, but grips firmly on the shaft in the other direction.
This is the freewheel once taken off the shaft:
|On this side of the freewheel you can see the stub shafts for the nylon gears (big rivet heads) and the small rivets, all holding the layers of the freewheel body together|
The freewheel can be opened by grinding off the rivet heads on the opposite side from the nylon gears, and then wedging off the thin steel faceplate with a thin chisel. [Actually, lately I've found a better way to remove the rivets by punching, similar to unriveting a chainsaw chain]
Here you can see the rivets ground down, and the faceplate coming off.
Inside, it looks like this:
You can see a boss in the middle which keys onto the motor shaft. Around the outer edge of the boss there are 3 hardened steel rollers which sit in a tapered space and jam the boss (with help from springs) when the freewheel turns the locking way, and allow the freewheel to turn freely the other way (when the rollers are pushed toward the springs). In this photo the outside of the freewheel can turn anti-clockwise freely, but locks when turned clockwise as the rollers are jammed in the tapering space.
I recently opened up 2 jammed freewheels this way. One had a cracked roller:
The other simply had a roller jammed in its tapered space, which I was able to lever out with a screwdriver and the freewheel then worked fine. I greased the rollers and reassembled it and plan to re-use it. Maybe it will just jam again, then I will check the ramp surface for damage and perhaps try to fix that.
It wasn’t hard to reassemble the freewheel. I found some galvanised flat-head nails with the right diameter, and cut them to length as rivets after setting them in place with a punch. Here is a nail after setting in place (punching it down into the hole to get the head down low):
Here is the nail being cut to length with a hacksaw, about 2mm above the surface of the face plate:
Here is the re-rivetted freewheel. I haven’t tried this out yet, so I can’t promise it won’t have any trouble.
Opening and repairing freewheels isn’t for everyone, and some freewheels won’t be repairable. Luckily you can buy replacement freewheels from Greenbikekit.com for a fair price. These come with nylon gears (make sure you get nylon gears with the right number of teeth – usually 28 or 36).