Shimming Rear Derailleur Hanger

Lawr

Hello. On Friday, I had cycled from Limerick to Ennis and back, and was hearing all kinds of moaning and pinging. Yesterday, I pulled off the crank and bottom bracket and checking the bearings and greasing everything and putting it all back together and tightening everything up real well. Since I cannot figure out where the pinging/popping came from, I figured it could have been the bb bearings or the bearings in the wheels. I checked the front wheel. There was play, so I tighted everything up.

When I took the back wheel off, I noticed that the line taken by the chain from the top of the crank back to the cassette was not in line with the line taken by the chain when it returned from the bottom pulley to the bottom of the crank. I looked at it from behind, and it appeared to me that the derailleur was at not parallel with the wheel, but angled in three or four degrees from the mounting point to the bottom pulley. It is difficult to measure that kind of angle, but it looked as if my hanger might be bent.

I pulled everything off. The hanger lay relatively flat on the ground, so I began to suspect the frame (aluminum with carbon seat stays). Rather than risk cracking the frame, I cut a thin steel washer into quarters and placed a quarter of the washer between the hanger and the frame. It straightened the derailleur out quite a bit. I might have even put another quarter of the washer on top of the other quarter to make it even more straight, but it seems close enough now that the angle of the chain over the top and underneath are more in line with one another. The lack of severity in the bottom line means, too, that the chain is putting less tension on the pulley as it exits the derailleur. I think one of the noises I hear is the conflict between the pulley's line and the chain's line. The chain had been coming off the derailleur at a pretty severe angle, and it was forever snapping off the pulley rather than rolling off as one would expect it to do.

Has anyone else ever shimmed a rear derailleur hanger? I know the derailleur is under a lot of tension. Is shimming dangerous?

cdaly_

Dunno but they're not that expensive...

doozerie

If it's a carbon dropout I'd be concerned about the uneven stress you may be putting on it with the shims. A metal dropout should be less fragile but I'd still be reluctant to shim it.

Replacing the hanger is relatively cheap as cdaly_ says and would certainly solve the problem, but you could also try to bend the hanger straight while it is on the bike. You can buy a tool to bend it back or you might be able to do it by hand by using the derailleur body as a lever. Make sure you have the rear wheel clamped in the frame if you try bending the hanger, as that'll provide support for the dropout. It's a judgement call as to whether to do that, some people are quite happy bending hangers back in line and I imagine it's seen as a perfectly safe and routine job by many, but personally while I'd have no qualms about doing it with a metal dropout I'd be hesitant if the dropout were carbon.