Since my Bafang SWXH was well used to begin with (~6000km) and showed some frequent freewheel clutch problems after about 1000km of use I wasn’t very hesitant to use it as a test platform for an oil cooling experiment.
There is a long thread at the Endless Sphere forum about oil cooling hub motors where I got the general idea and the specific oil type to use. The general consent was to use ATF. The next gas station offered Shell ATF 22 so I went with that.
The construction of the Bafang SWXH makes this a pretty easy and straight forward business. Take out two opposing screws of the disc brake adapter and fill in 50-60ml ATF trough the holes with a big syringe from the next pharmacy. Use the upper hole to let the air out and the lower one to fill in the oil. Make shure all disc screws are tight afterwards. Mine came with a Nylon spacer that acts as a gasket now.
Following the advice from the Endless Sphere thread I installed a little ‘breather tube’ in one of the disk brake flange holes in the beginning but found out later the motor ‘breathes’ the warm air out through the motor cable without problem.
Don’t fill in too much oil or you will produce quite a mess in the following days when the excessive oil will get pressed out of the bearings and motor cable. Initially I filled 50ml ATF into my SWXH and it worked great. The second time I filled in 80ml and my trike marked its territory with oil for the following week…
Since the ATF will dissolve the grease in a geared hub motor its a good idea to change the oil after about 100km of use and fill in fresh ATF.
So far my little ATF cooled Bafang is holding up great on my daily 35-45km commute and ran some 900km in the past month. To make things a little harder on the motor I relaced the SWXH from a 20″ rim to a 26″ rim and changed the 17 ampere 6-FET controller to a 35 ampere KU123 12-FET controller. On top of that its running off a 14s/10Ah LiPo battery pack (52V nominal, 58V hot off the charger) instead off my 10s/11Ah LiMn battery (37V nominal, 42V hot off the charger).
The poor little ‘250W’ SWXH is now producing roughly 650W continuous at 45km/h and up to 1400-1500W peak power while accelerating my trike. The housings get quite warm to the touch and really warm after flogged for 15min up a slightly hilly road at 40km/h to keep up with traffic on my daily commute. My best guess is about 60°C since I can still touch it but will not like to keep my hand on it for a longer period of time.
While the hub motor runs now at about double the power level than before the internal freewheel clutch works flawlessly for any length of time. Lubrication with ATF instead of the stock grease really does make a difference here.
I think oil cooling geared hub motors with ATF has its merits when you intent to run them above their specified power levels for any length of time since my ‘500W’ Bafang BPM II developed a similar infrequent freewheel clutch problem after about 3000km of use in its stock configuration with a 15s LiPo battery (55V nominal, 63V hot off the charger) and 35A KU123 12-FET controller (BMS Battery is a shitty vendor btw, buy somewhere else if you can).
While an 80ml ATF filling ‘fixed’ the stuck freewheel clutch in the BPM II for a short while the clutch started slipping some hundred km later. After opening the BPM II I’ve found a lot of fine metal shavings stuck to the magnets but to my amazement the Nylon planetary gears looked like new.
I haven’t found time to rebuild my Bafang BPM II yet but found the image of an opened freewheel clutch belonging to a Bafang BPM in this thread. Along with the images in that thread comes a well written description how geared hub motors in general and the one-way clutch in particular actually work.
That image saved me the burden to open up the damaged clutch of my BPM II to satisfy my curiosity how it is built exactly. The one-way clutch assembly is riveted together and not made to be serviceable in any way. The goal would be to find the right spots for some well placed holes drilled into the side panels of the clutch for easier entering of ATF to improve lubrication of the rollers for trouble-free long-term operation.