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Bafang/8Fun freewheel failure

This week I had a new SWXK motor problem: a failed freewheel.
Coasting down from Mt Glorious early this week I heard a whirring sound, apparently from the front wheel. It sounded like a stick rubbing on the tyre, so I stopped to check. Having a closer look I found that nothing was happening on the outside of the wheel, but that on rotating the wheel forward, the motor hub’s gears could be heard giving the same whirr that they give when the wheel is turned backwards. Clearly the motor freewheel was stuck on.
I needed to continue on my 11km trip to Mt Nebo, so for most of the coasting time the motor remained engaged, making the sound and giving noticeable drag. Not good. Occasionally the freewheel would go free, with a slight clunk, and the bike would coast normally, but this didn’t happen much.
Once home I quickly removed and opened the motor hub. This is quick and easy if you do it right: remove the axle nuts from the opposite side (the brake disk mount side), remove the tiny grub screws, and wind off the side plate with the Bafang opening tool.
Once open, it was easy to remove the circlip which holds the freewheel body onto the axle, and pull the freewheel off.
The freewheel was clearly sticky. Putting a finger into the keyed hole, the freewheel would not freewheel. Using a tool in the hole and pushing harder, the resistance to turning could be overcome, but it was clearly not good. Although the freewheel was clean and slightly greasy, it felt like it could be lack of lubrication causing the trouble. I ran some WD40 and then chainsaw bar oil into the joint in the freewheel, gave it plenty of spinning on my finger, and replaced it into the hub feeling like it might be better now.
One more 11km trip found that I had been unsuccessful: the freewheel was locked on again, occasionally letting go but mostly locked and slowing coasting down signficantly.
Today I opened the hub again, and replaced the freewheel with a working one from a burnt out SWXK. Again quick and easy. The troublesome freewheel was truly stuck this time, unable to be spun at all even with a tool in the keyed hole.
The freewheel unit is riveted closed, apparently with 6 rivets including the studs for the nylon planet gears. I hope to have a go at opening it up later, to get an understanding of what is failing in them.
Replacement freewheels are now available from I was curious when they had been put up for sale, not expecting that they would fail frequently enough to make them worth selling as a spare part. Bikes have freewheels in geared hubs, cassette freehubs and spin-on freewheel clusters, and these fail very rarely. Now I understand they do fail, but I’m still surprised. Interestingly, the SWXH hub freewheel on my Xtracycle sometimes sticks, but it always lets go within a short time, and it’s done over 2500km like that. This was a new motor with only 900km on the clock. So far each of my SWXKs from that batch have had a serious problem. Bad luck, or a pattern of poor quality control?

How Bafang Freewheels work

Hub motor freewheels don't click like a cassette or cluster freewheel - they are silent. Cassettes and gear freewheels use pawls catching on a ratchet ring, which make clicks as the pawls slip over the ratchet teeth. Hub motor freewheels use rollers in tapered spaces, that rub lightly when freewheeling, but that wedge between a smooth cylinder and their tapered space when gripping. 
The freewheel is part of the planetary gear freewheel unit, which is easily found and removed from the hub. 

Here's a Bafang SWXK freewheel opened for repair - note the nylon planet gears attached to the other side:
In the centre is a hub which attaches to the motor axle with a key. The large flat cylinder (the main body of the thing) rotates freely around this keyed hub anticlockwise. When turned clockwise the 3 rollers are pushed (with assistance from little springs) into their tapered spaces and jam tight, gripping the keyed hub.
Freewheel failure seems to be mostly due to these rollers, which can get jammed, or even break. The broken roller in the photo below was the cause of my problems in the story above.

I'm not sure if these rollers can be got as a spare part, but I've replaced them, using parts from other broken motors.
Repair of a freewheel is usually done by buying a replacement freewheel unit. These are cheap and pretty easy to install. Replacement freewheels generally include a set of 3 planet gears already mounted, which are a useful spare part to keep if your hub motors do a lot of work.

Opening a freewheel

Here's how I made a repair to my freewheel. First I ground off the heads of the rivets which hold the freewheel unit together:

A thin chisel helped pry the faceplate off. This is what it looked like inside with the rollers and springs removed:

After repair I riveted the freewheel back together again. I was lucky to find galvanised flat-head nails which were the right thickness to do the job:

I cut the nails to a suitable length:

And peened the heads to hold it all together:

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