Friday, June 22, 2012

Motorized Bicycle - Chain Tensioner

custom chain tensioner

If you are very lucky, when you mount the motor you will be able to adjust the chain length perfectly so that both the pedal chain and the motor chain are taught without any need for a chain tensioner.  Chances are, you won't be that lucky.  I needed a chain tensioner to keep from throwing the motor chain.  Unfortunately, the chain tensioner that came with the motor kit was the ugliest monstrosity I had ever seen.  If I was Grubee I would be embarrassed to include it with the motor kit.

stock chain tensioner

before finishing

First, the stock unit would not have fit the oversized chainstay anyway.  Second, I'm pretty sure the plastic wheel would have been quickly chewed to bits by the moving chain.  There are aftermarket solutions which are much more elegant, but again they cost a lot.  So I decided to make my own.  I started with a length of 3/16" x 1" aluminum bar stock cut to length.  Then I milled a 3/8" slot for an idler sprocket where the bracket met the chain.  I drilled and tapped the bottom of the bar to mount to the hub-brake mount.  To mount the top of the bracket, I drilled and tapped again and used one of the motor mount caps to secure it to the seat stay.

lower mounting

repurposed motor mount for top mounting

Surprisingly, the hardest part was finding an appropriate idler sprocket.  Most of the ones I could find were way too large.  I finally found a 10 tooth sprocket on Amazon that was a replacement part for a lawnmower.  It worked perfectly but I had to grind down the bolt head a little to clear the spokes.  When it was all finished I threw it in the blast cabinet to give it a brushed look that matched the motor.  The new tensioner is rock solid, easy to adjust, and only cost about $10.

idler sprocket


  1. Would it be possible for you to make one of these chain tension arms for me? I'm in Jim Thorpe Pa. and I travel to NYC once a month or so.

  2. Sorry, no. I don't have any more motor mount caps. And I don't have the bike to get the dimensions from. It's really simple to make though. All you need is a piece of aluminum bar (available at Home Depot), a 10 tooth sprocket, and a drill.

  3. Nice article. I think it is useful and unique article. I love this kind of article and this kind of blog.

    4 Stroke Bicycle

  4. Agree, excellent placement and idea, thanks for hashing the work out for us!