BBSHD DIY Build Report: Motobecane

Here’s what the stock bottom bracket looks like with the RH crank removed:

And, the bottom bracket, empty:

Here’s how it lined up before trimming the front derailer mounting bosses. As mentioned above, with this frame, you need to trim the bosses. I cut the long boss off flush with the short one, and then chamfered the short one, in order to get the required clearance so I could set the gap between the motor and frame to ‘just touching’:

Once the frame was trimmed, I needed a small (0.1″) spacer on the RH (drive) side, so that the motor wouldn’t be side loaded into the frame.

I found a piece of aluminum scrap the right size. If you need to come up with your own, take the Bafang crank nut with you as a gauge. You don’t need ‘exactly’ the right size:

On to the Left side…a longer spacer is needed here. 0.8″. Same diameter as RH side.

Here’s the shape of the bottom bracket and frame interface, from below, for reference:

The downtube on the Sturgis is flattened on the bottom, and the back of the bolt boss on the BBSHD hits the downtube, leading to a rather low mounting position, with the BBSHD rotated further down than I liked. So I ground away some of that boss to gain more ground clearance. I did not hit threads or motor interior – this was extra metal, not apparently needed. I went very slowly, in case I broke through, knowing a dab of metal epoxy could repair a small hole well enough, but not a big one.

The square space you see below is not a through hole, just an exterior feature

I put a piece of innertube between this boss and the frame, then allowed the whole motor to rotate while I tightened the bottom bracket nuts. So, it is pretty tightly held to the frame. I used adhesive to attach the rubber pad to the motor, so that if the motor ever comes loose, it won’t damage the frame when it swings back up under torque.

Final motor position. It looks so tiny on this big bike:

The mighty mini 30T. There is room for a bigger chainwheel to clear the frame, without making the chainline worse. I know there are a couple of 42T chainrings out there, like the Luna Eclipse. But they move the chain inboard. Perhaps too much, as this lines up pretty well in #5 & #6 gears (10 speeds). I think I’d like to try to put a 36T on there for slightly higher gearing. But I can’t find an adapter to fit the BBSHD for standard chainrings

First gear

Pretty straight chain line in mid gear.

Speed Sensor Mounting, pic #1

Speed Sensor Mounting, pic #2

Speed Sensor Mounting pic, #3

This Luna Cycles triangle bag is a little big for the frame, but squashed to fit nicely. I was going to add foam around the perimeter, but realized I wanted to carry a spare tube, too. Spare tube=frame padding! Once I get a rear rack and bag, I’ll put the pump there, and add some foam between the pack and the inner tube, as well. This is a well made bag. Tough, good shape, straight seams, and a water seal on the zipper. Be sure to remove the bolts from the water bottle bosses on the seat tube and down tube, or they will create concentrated impact points against the inner tube and pack. Or use thicker, firm padding. I would NOT recommend just using the battery bag without extra padding, while also leaving the bolts in the frame. I think the pack could be damaged. The bag does have some padding, but why make it’s job harder than you need to?

Bike pump will fit above, snugging everything up nicely. Firm fit, not crushing. Luna Wattmeter fits alongside with no problem.

This is an XT60 splitter I made for connecting the Luna 15W headlight and taillight to the charge port of the pack. I added a 1A fuse, in case of wire damage. I used a 1-1/4″ (30mm) ACG fuse and holder. These are only rated for 32V, but I test blew one and it popped nicely, with no arcing. So I think that will work fine.

Front wiring entry to pack works well with the provided wire port. There is even a spare frame boss for a zip tie in exactly the right place

Front view of wiring

Controls

Taillight