Friday, June 22, 2012

Motorized Bicycle - Attaching the Sprocket

motor sprocket

Here's a bit of a warning for anyone wanting to attempt this on their own.  The manual states that you may have to open up the diameter of the hole in the center of the sprocket to clear your hub.  The hole needs to be perfectly centered or your sprocket will run out of true and you will throw your chain.  The manual recommends taking it to a machine shop and having the work done on an engine lathe.  Good advice, but they should do a better job of making people aware of this before they purchase the kit.  Of course, the sprocket didn't fit over my hub.  Luckily I am equipped to deal with this, but unless you have access to machine tools, you may need to plan accordingly.

I ended up securing the sprocket on my mill with some hold-downs and using a fly cutter to open up the hub hole about 1/16".  Two heavy rubber gaskets (I think they are just pieces of old car tires) go on either sides of the spokes and then the sprocket is bolted through the spokes to some metal plates.  The rubber gaskets were warped which made lining everything up evenly very difficult.  I torqued down the bolts and left them overnight to let the gaskets settle.  The next day I loosened the sprocket and carefully centered the sprocket over the hub.  I gradually tightened all of the bolts (several times, as the sprocket kept shifting), then loosened one at a time and added blue Loctite to keep them from coming loose.  A quick spin of the wheel confirmed that everything was centered.  This was one of those steps that looked like it should take 15 minutes but ended up taking a few hours.  Several companies make sprocket adapters for these motors that make attaching the sprocket much easier.  Of course they cost more but it may be worth it just to save on time and frustration.

front view

rear view

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