combo encoder and chain tensioner

Hi folks,

The last few years we’ve always seemed to mount our quadrature shaft encoders somewhere difficult to reach. I was thinking that this year we’d try to mount one with an idler sprocket on a chain tensioner. Any thoughts?

  • Steve

Avoid using an encoder as the tensioner shaft. The bearings in it probably aren’t capable of supporting the radial load of a tensioner.

Instead, install it on a tensioner that supports its shaft independently. (See here for some examples of shaft-mount encoders that you might be able to use.)

Use caution. If the encoders don’t like side loading, you’ll need to support the other end.

However, I have another suggestion that’s worked before. You’ll need, in addition to the encoder, sprocket, and encoder mounts:
–an axle of appropriate size for your sprocket
–A pair of mounts that allow the axle to be moved side to side as needed to tension the chain. (Could also be up and down.)
–a piece of surgical tubing and a pair of zipties.

The best picture I can find of this method is at If you can zoom in right in front of the close tranny, you’ll see a small metallic boxy thing with a sprocket on it. The other end of the sprocket’s shaft has been machined down to the size of an encoder shaft, and the encoder is on the other side of the holding metal. The encoder shaft is flush against the axle’s end, and surgical tubing is used as a sleeve to keep them together (after being stretched a little and ziptied down).

Agreed. The plan is to support the idler axle on both sides of the sprocket, extend axle on one side, then fix coder to that side.

Eric - thanks, but that photo’s a little grainy in that area, I can’t quite make out what’s behind the sprocket (or is it hidden by the sprocket anyway?). Also, I planned on using surgical tubing or equivalent to give a bit of compliance. We’ve had problems other years with mounting plates that didn’t quite line up the encoder axle with the axle it was encoding.

Thanks very much.

  • Steve

Here’s better pictures of what we did in 2007 and 2008.


Thanks, those pix make it very clear. I had in mind spring-loading it on an arm, kind of like a bike derailleur, rather than mounted right on the frame. I think we’d be okay if there’s not a lot of play to begin with, otherwise the slack being taken up during acceleration might give inaccurate readings.

  • Steve


I’d be worried that when the slack is taken up during acceleration, the chain gets loose enough to skip, defeating the purpose of the tensioner.


Bike chain derailleur’s work as they do for the sole reason that the chain is only ever driven (intentionally) in one direction.

When you put a spring loaded tensioner on one side of a drive chain that goes both directions, in one direction you shouldn’t have torque startup issues, but in the other the initial start may be enough to pull the tensioner and loosen the chain adding quite a bit of slack.
And more slack = higher chance of popping off

Also the main reason bike derailleur’s are spring loaded is the gear changing system, fixed drive bikes simply slide the back wheel forward and back to tension the chain and no spring is involved.

Thanks, all. It looks like a single sprocket that’s actually taking up tension is probably going to give us problems. I’ll shelve it for now and lean on the mechanical guys to give us a more conventional means while keeping my sketchpad handy.