November 2012 (with some updates)
I recently had my first breakdown of a hub motor. After about 3 years and 7000km of mountain riding, the nylon gears failed in the first ebike motor I ever bought. My brother in law pulled over to the side while climbing a steep hill to let a car pass, and when he tried to start again the wheel just gave a nasty buzzing sound and a bit of shaking when throttled.
The motor is a Bafang SWXB geared front hub model, which are very easy to open – only 6 philips-head screws holding on the side plate.
Inside we found that the 3 nylon planetary gears (which take drive from the little sun gear on the motor out to the ring gear on the hub shell) had their teeth mushed off.
After some correspondence with sales at Greenbikekit.com, I was able to order some replacement gears. There were some challenges working out what part went in what type of motor, but eventually I took delivery of a small pile of spare nylon gears for both the SWXB, and the other model motors I have in stock and have sold.
The SWXB – an older model of Bafang – takes 28 tooth gears, 38mm outside diameter. Some more recent SWXK and SWXH take 36 tooth gears. All gears came with sealed ball bearings inside them and replacement circlips. I paid US$5 each plus shipping, which came to about $25 a set.
I have no resentment about the failure of these gears after this much work by the motor. I’m sure they would have lasted much longer riding in normal terrain – up here my motors spend a lot of time hauling up hills at around 10km/h, resulting in high strains and high temperatures in the motor. Replacing the nylon gears every few years is a minor maintenance task, biggest problem being having to get home up some big hills without assistance when they fail. I’m not tempted to install steel gears to get a longer life. I expect this would result in much faster wear of the steel sun and ring gears and thus would shorten the repairable life of the motor.
Replacing the gears on the SWXB was quick and easy – only a few minutes work once the wheel was off the bike. For people doing a lot of hill climbing on e-bikes, I think it would be worth keeping spare nylon gears in stock – minimal cost and great saving of time and trouble once they fail and leave your bike broken down.
Since ordering the gears from Greenbikekit.com, I see that they now stock them on their website - previously I couldn't find them listed anywhere.
April 2013 update
Since repairing my old SWXB I've opened a few other Bafang motors. SWXH - the Bafang/8fun 250w rear motor - comes in 2 different models, as well as different speeds for different diameter wheels.
SWXH old-style (threaded side plate)The older type of SWXH opens by turning off the threaded side plate which has the threaded freewheel boss on it. Here it is:
Opening this requires a special tool which keys into the 3 holes around the freewheel boss and a lot of force. Like a freewheel, it has a normal RH thread which tightens as you pedal, so it comes off anticlockwise.
I made a tool for this job, fabricated from mild steel:
This was held tight in place using the wheel nut and a big square washer:
It was so tight that I put the tool in the vice and turned the wheel (like removing a freewheel/spin-on-cluster):
Tools can be bought from Greenbikekit.com.
This SWXH had 28t nylon gears. Another which looked the same externally had 36t nylon gears.
I have had trouble with one SWXH when I needed to replace the spin-on freewheel, but the whole side plate spun off instead. Removing the freewheel required me to remove the side plate, and use the side plate tool on the inside of the side plate with a bolt holding all together. This sort of trouble makes the bolt-on side plates look good.
SWXH new style: side plate with philips-head screwsNewer SWXH motors I've bought are much easier to open. 6 philips head screws hold on the side plate with the freewheel boss:
These new SWXH have 36t nylon gears (as far as I've seen so far).
The other side of both SWXH motors look the same: