Jay’s Technical Talk

I purchased a surplus 250 watt motor with a matching wheel hub and 90 tooth sprocket. The motor has an 11 tooth sprocket, and both the motor’s sprocket and the hub’s 90 tooth sprocket are designed for #25 motor chain, which is slightly smaller than standard bicycle chain.

To get the hub and 90 tooth sprocket on the wheel, I had to remove the existing hub and rebuild the wheel on the new hub. Luckily the two hubs were close enough in size that I could re-use the existing spokes. Following the instructions on Sheldon Brown’s website, and also refering to this website I was able to re-spoke the wheel.

The motor came with a bracket that had four holes threaded for M6 bolts. As my bicycle already had a wire basket mounted on the front handlebars, I used fender washers and a piece of angle iron to mount the motor to the basket directly over the front wheel.

Of course, the front basket was not sturdy enough to support the motor without bending once the motor turned on. (The motors pulls itself towards the hub along the chain.) Once the motor pulled down enough for slack to develop, the chain would pull off the sprockets.

To buttress the basket, I machined a honeycombed support strut out of T6061 aircraft aluminium using my 3 axis CNC machine. Well, not really. Instead, I used a cheaper material widely available at the local hardware store that can be machined using simple hand tools….that’s right, schedule 40 PVC pipe! Although not quite as strong as aluminium, it cost me less than $1 and has one other nice property. When somebody sees an electric bike locked up at the MARTA bike rack with a piece of PVC zip-tied to the front forks it screams out “Don’t even bother trying to steal me!”

If you look closely at the bottom of the pipe (visible more clearly on the right hand side of the top picture), you can see that I cut a wedge out of it, leaving about half the pipe. I then drilled a hole of suitable size for the axle (using a 1/4″ drill bit, and a dremel, as somebody had snapped off the 3/8″ drill bit into a tree stump). My original intention was to attach the pipe to the axle using another nut, but my local hardware store did not have M10 nuts with a 1.0mm thread spacing in stock. I found that a few zip-ties worked well enough. I also cut a small notch in the top of the pipe that supports a horizontal wire from the basket. In addition to the notch which does a good job of aligning the pipe to the wire, I used a few zip ties around the pipe, and through a few holes drilled in the top of the pipe to provide extra horizontal and vertical support where it connects to the basket. So at least the top and bottom of the pipe have matching zip-ties.

Update:
The local hardware store has re-stocked their M10 (1.0mm thread pitch) nuts, so I was able to use one to keep the PVC pipe support strut attached to the front axle (in addition to the zip ties).

Update 2: This simplistic motor mount worked fine for five months, until the chain fell off when I hit an excessive pothole. Check out my new better motor mount (Well, $7 better) by clicking here.