VK3CKC's eLecTricks Trike Design and Build.

Feb 20, 2013
Axedale, Victoria, Australia
Could I make a better mid-drive from a bottom bracket? The basic requirement, assuming that there will be some sort of freewheel on the rear axles, is to accept drive from the front chain and drive output with a drive gear for each of the rear axles, as well as a conventional multi-gear cassette on the right side.

The significant difference between a wheel hub and a bottom bracket: The wheel hub rotates around the fixed axle on bearings whereas a bottom bracket has a rotating spindle held captive within its bearings. How difficult would it be to attach a freewheel and an output gear to the right hand side of a bottom bracket and an output gear to the left hand side? The output gears could be turned by the rotating spindle providing the rear axle gears were freewheel. I owed it to the project to find out.

The two output drive gears on each side of a re-purposed bottom bracket could simply be the 28t sections of two existing chainring sets with their fixed pedal crank arms cut off. Attaching a freewheel to the one on the right hand end might be a bit more challenging.

I managed to find a modified bottom bracket/chainring set from a build from years ago to try. I won't use this one beyond this test as the pedals are held on by a nut, I would prefer a bolt for a purpose I have in mind, and the left hand pedal looks like its rusty old thread will not allow me to remove it anyway.

I removed the centre of the chainring to which the pedal arm was attached and put its 28t chainring aside for later. I cut the discarded end of one of the two hubs used in the Mk1 mid-drive version (earlier post) flush with what was the inside of its spoke flange.

I had to make what was left of the cut off pedal arm a little smaller so that the hub end would fit over it. Not only was it too large, it was also out-of-round, having been only roughly finished with an angle grinder. Due to a not very well equipped workshop, my options were limited and I could not access anything more professional due to Covid-19 and I didn’t really want to spend anything anyway.

Once again, a makeshift lathe came into play. I fitted the chainring centre to the old bottom bracket, clamped it in a vise, clamped the vise to the workbench and brought the bench grinder up close enough for the side of the grinding wheel to make contact with the area that required reducing. Turning the left pedal rotated the part against the rotating grinding wheel and, in far less time than I expected, it was small enough for it to fit inside the hub. The discarded hub end fitted nicely, so the hub part used in the Mk1 mid-drive was cut flush with the inside of its spoke flange and it too fitted nicely in place. It was the same size so no real surprise there.