I got an E-Bike back in ~2021, and I wrote about the first 1000 miles with my E-Bike. As I mentioned there, it is pretty good for level riding and small hills but really isn't very good for steep or long hills as it doesn't have the torque/power for steep hills and overheats at the slow speeds going up long hills. So I wanted a mid-drive bike where the electric motor power is sent through the drive train and thus you can downshift to get more low speed power. I wanted to build my own, but was always a little hesitant to drop $ into the stuff since I wasn't sure what bikes I could get the motor to fit onto. I do have a posible donor bike (Kona Dawg), but I am still not sure that it would fit a battery where I want to put it. Online I see someone built out a size large, but I think I have a medium. In any case I was still without a mid-drive when I saw one for sale at a consignment shop in Reno. It wasn't ideal, but the price seemed reasonable (cheaper than a new kit would cost). So I bought it.
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This is a Bafang BBS02b 750 mid drive motor on a Giant Trance Maestro full suspension bike (I think from 2006). It is a little too small for me (14.5" frame) but a longer seatpost covers my short legs. That does mean the battery is mounted on the bottom of the down tube and when the front shock is almost completely compressed the front wheel hits the battery. I don't know how many AH the battery is, but it is 48v and the cells say "Samsung" on them. So far I have ridden it ~20 miles and the screen says it goes from 80-100% down to 60-80%. I suspect this is not actually linear, but who knows. I should see what the actual volts are for more accuracy.
I took it on a test ride on the local Chipmunk Canyon trails. There I discovered that the settings were off (PAS 1 (Pedal Assist Setting) was way more speed and power than I wanted on the trail). It also had a tendency to derail the chainring in the lowest gear and most troubling sometimes the motor would keep running even when I stopped pedaling. It was also a lot of fun and as mentioned earlier hitting the front shock too hard hit the wheel into the battery and the motor was low enough to knock onto rocks. I did a bunch of google searching on why it might not stop the motor when I stopped pedaling and the most likely culprit required me to take it apart a bit. I had some of the tools needed, but needed a few more - so I ordered them as well as a USB programming cable to change the PAS power settings. It seemed like riding it in this state could cause permanent damage, so I stayed off it until the tools came and I could open it up. I discovered there was a rubber seal on the left side of the bottom bracket axle that could be removed with a pin with the last few mm bent almost 90 degrees. I worked it in carefully next to the axle and then turned it sideways and pulled the seal out. Then I needed a sort of a castle tool to remove some locknuts. I didn't have this and made do with some needle nosed pliers but it was sub-optimal. The next time I had to open this up I made a tool out of some copper pipe - it worked better but is too weak. I'll order one next time I have to deal with Amazon again. I found out that in fact the screws holding some wheel onto a big gear that was part of the PAS were all disconnected. I stuck them back on with loctite and re-greased the gears while I had it open.
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It was definitely better in my next test ride, but there was still some issue where the motor would try to go at least a little bit even when I wasn't pedaling or using the throttle. Over another few rides I realized that it seemed to happen when I was coasting at least 5 or so mph and hit a bump (hard to avoid on washboarded roads). I even found it happened when I turned the bike off. The motor would make a sort of grinding humming noise and sometimes turn slowly - sort of like when an electric fan or other motor fails to start. More googling didn't come up with any obvious solution. I hung the bicycle up and was able to duplicate the effect by spinning the back tire and then whacking the bottom of the motor with my hand.
I opened it up again and realized that the PAS magnet disk is supposed to have a tooth where it slots into the pedal axle so it only turns when the pedals turn. Mine had been broken off, probably when the bolts next to it were not connected. I re-created the tooth with a bit of 1" PEX tubing (I think that part worked quite well). Sadly I was still able to re-create the problem by whacking the bottom of the motor when the rear wheel was spinning. So I took the motor apart more hoping to find some loose wire or connector. I discovered that most of the controller parts had been filled in with caulking or something which meant I'd have to cut it all apart to get it more open or see what was going on. I carefully put it all back together and at least in a brief test ride was unable to recreate the problem. Hopefully fixing the PAS magnet wheel will go a long way towards fixing this. Otherwise whenever it happens pedaling a bit or getting the speed under about 5mph seems to stop it.
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I also experimented with changing the programming. It seems the settings are mostly related to the cadence, then to go faster you switch to a higher gear but with the same cadence. I set PAS 1, 2, and 3 to be pretty low for trail riding, 4, 5, and 6 to be for normal riding, and 7, 8, and 9 for higher power. There is a lot of discussion online about what the various settings do, but at least some of it conflicts, and it seems that changing one setting might change the behaviour of other settings. Still, even at a lower power setting it has way more torque than my other e-bike and I can go up much steeper hills. I used my IR thermometer to check the temps and I measured 120 F or under (on the outside) even going up a pretty steep hill. Unlike my other e-bike I am pretty sure I can go up hills with this one that I couldn't get up with my legs only mountain bike. In the rides I've done so far I have only gained about 2000 feet. Hopefully I can get the 4-6000 feet of gain on one charge to make it to trailheads and other destinations. Sadly unlike the hub motor e-bike it doesn't recharge the battery when I am going downhill. The ideal setup would have some sort of e-brake that recharged the battery.
2026-5-10 Update - there is still something odd going on, I suspect in the controller which is sadly buried in caulk (or something like that). It happens when I hit a bump going over ~5mph. It seems to happen more at higher speeds, but maybe the bump is harder at higher speeds. The motor makes some sort of groaning noise and sometimes turns the gears slowly. It goes away when I use the throttle or pedal, so I guess I can just do that for general riding. For long steep downhills the solution might be to take the battery off and carry it in a backpack. Advantage - I can hammer the front shock with impunity, disadvantage - I'd have to take it off and carry it in a backpack.