This is my first project e-bike. I had an inexpensive and rusty Exitor Magna mountain bike, so it needed a lot of new parts. The bike was bought at a surplus auction a few years ago. Kept outside in hot, humid Florida weather for all that time.
I needed to remove the rusty derailleur and cassette on the 24in. rear wheel. I wanted the simplicity of a single freewheel.
I have a number of 18v and 20v power tool batteries and a 60v electric lawnmower used in my landscaping business, so why not use my existing supply and swap them out as I need them? I’m pretty sure they have the own BMS with the separate chargers. I haven’t decided how to monitor discharge going into bike motor yet.
I’m focusing on low speed / high torque and want to avoid creating overvoltages on the Bafang motor. Not looking for cross-country biking with this setup.
I needed to find a DC-DC buck converter to drop my batteries from 60 volts down to a manageable 48v which is the level the Bafang motor is designed to run. Fortunately, I found on Aliexpress a good fit: a 60V/30A Buck Converter Step Down DC Regulator, by SX Electronics. Also from China, I ordered the Bafang BBS02 48v e-bike kit from CNE Bikes.
I removed the Bafang plastic cover plate and added more axle grease(Slick50 One brand) into secondary reduction gear, as recommended on electricbike-blog.com. Easy 5 minute job.
Now let the grinding begin! The bike had the old American type of bottom bracket, with a one piece crank.
Although it had the required 68mm width for the Bafang kit, I still needed an American-to-Euro bottom bracket adapter to conform to the diameter of the Bafang crankshaft. Unfortunately with this adapter, I still had to cut off a few millimeters on each side of the bottom bracket since the shoulders of the Euro adapter added a few extra millimeters onto the 68mm width on each side of the bracket shell. Also I had grind down the internal bracket threads so the Bafang crankshaft could slide on easier. Tools used: 120vac electric grinder with metal cut-off discs, Dremel high-speed rotary tool, with metal grinding discs, stones and coarse-grit sanding drums.
Remember the rule, class: the front chainwheel sprocket uses the lowest numbers of teeth for low gear, while meshing with a back (rear) sprocket which will have the highest number of teeth for low gear. High torque means front low T, back high T. High speed is just the reverse: front high T, back low T.
For comparison on the original Exitor Magna gearing, its lowest (first) gear used a 24T/28T front/back sprocket combination. As stated at beginning, I wanted higher RPMs and power on the e-bike rather than top speed, and especially since I have the older cantilever brakes, which means high speed will not be a priority on this e-bike. I’m not going any faster on the road than I can reasonably stop.
Second point. Here in my neck of the woods of Central Florida, my experience has shown that it’s easy to get bogged down when biking off-road, especially on certain trails when you run into sugar sand. You’re DOA in first gear, giving it all you got in pedal power (no electric motor for PAS). You end up walking your bike out of that patch. In my case, the electric power is not for riding uphill, there aren’t any mountains here to climb.
At this time, I am keeping the original 46T Bafang chainwheel. To go to even lower gear ratios, a 34T or 36T chainring will be installed in the near future. OTOH for a back sprocket, I screwed on a 20T Shimano SF-1200 single speed freewheel to replace the old cassette. So my fixed gear ratio is now 46T/20T front/back. That’s a low gear ratio but about 2.5x higher than the lowest gear on original cassette gearing. Enough to keep the Bafang electric motor spinning fast, and giving me a reasonable compromise for street-pedaling a fixie on 24in tires.
Finally a compatible chain for single speed gears is needed: a KMC Z410 chain 1/2” x 1/8” for single-speeds or internal gear hubs meshes with the teeth on the Shimano single freewheel. The normal 1/2” x 1/8” chain didn’t mesh correctly with this freewheel.
Theoretically, I should be able to use this gear ratio, given enough PAS to run through any of those notorious patches of off-road Florida sugar sand. Or will additional tweaking of controller software and/or going for lower gears be the order of the day? It remains to be seen. Stayed tuned on this thread.