Archive for July 14th, 2013

Tour Easy: Yet Another Shifter Pulley

Somehow, I think I’m never going to get around to doing a CNC version of this thing, but at least now I have more pictures…

The overall problem comes from the fact that the Tour Easy frame geometry doesn’t match the expectations of the front shifter: the cable bends over a small finger that, on a diamond frame bike, should simply hold it in position. Here’s the finger, with a very early version of the pulley that just holds the cable slightly higher than the normal position, complete with one snapped wire showing that the pulley wasn’t getting the job done:

Front derailleur cable - broken strand

Front derailleur cable with broken strand

The obvious solution involves running the cable over a nice, rounded surface that prevents abrupt bending. The most recent version looks like this:

Shifter pulley installed - left view

Shifter pulley installed – left view

Yes, the end of the cable sticks out over the chain; I haven’t tucked it in yet.

A bit of lathe work produces a 0.42 inch diameter thin brass disk with a 50 mil half-circle trench around it; in retrospect, the diameter of the trench bottom should be 0.42 inch and the OD should be about 0.45 inch. If you have really good parting-off-fu, you can produce a disk with a finished backside right on the lathe, but I had to drill an off-center hole anyway, so I thinned it on the Sherline:

Shifter pulley - thinning

Shifter pulley – thinning

It looks like this after all the thinning:

Shifter pulley - thinned

Shifter pulley – thinned

One flange is wider than the other: the thin flange faces front and gets a bunch of cutouts, the wide flange faces rearward and must support the bitter end of the cable.

I lined it up in the shifter, filed a notch to fit around the shifter finger, scribed the hole location, clamped it down, and drilled the hole:

Shifter pulley - center drilling

Shifter pulley – center drilling

I think the hole could be on-center with the larger disk; now that I’m keeping better notes, I’ll try that next time. If so, then I can drill it on the lathe, part it off to the correct width, and hand-file the backside flat. The general idea is to have the cable pass over the finger, which almost happens with the smaller diameter.

Some tedious hand-filing produces notches that index over the finger and clear some protuberances on the shifter arm. This is the front face of the pulley that sits against the shifter arm, with a 5 mm socket head cap screw for scale:

Shifter pulley with bolt - front face

Shifter pulley with bolt – front face

The rear face has one side of the trench filed away to get the cable out of the trench and around the bolt:

Shifter pulley with bolt - rear face

Shifter pulley with bolt – rear face

Then it looks like this from the right side of the bike:

Shifter pulley installed - right view

Shifter pulley installed – right view

A pleasant morning with some Quality Shop Time…