[vttramper]

My tandem recently started having chain skip (hop) problems under load (hills, out-of-the-saddle, etc.), not moving up or down a gear, but jumping on the cog. Here is what I have tried to fix the problem:
1. cleaned, lubricated and adjusted derailleurs - it shifted beautifully until under load, then same skipping problem.
2. replaced cables and housings, re-adjusted derailleurs. Shifted even more crisply... until under load. Skip.
3. replaced both chain and cassette (both are shimano 9-speed). No change.

I considered no. 3 above to be the ultimate fix for this problem so I am now at a complete loss. Any suggestions?

 

[mikerp]

Check to see if the RD hanger is bent.

 

[.je]

As above, you could have a small misalignment from the hanger, or the axle isn't seating evenly, or (maybe not you now) the cable is dragging enough in the housings, or under the BB, or somewhere, that the RD spring doesn't quite pull it enough in the low-tension (small gear) positions.

 

[wim]

As you said, your number 3 fix would make it very unlikely for the chain to skip over a tooth on a cog. I wonder: have you actually seen the chain do that? If your diagnosis is based on the pedal momentarily dropping into thin air followed by a loud pop, the problem might be with the freehub.

 

[vttramper]

As you said, your number 3 fix would make it very unlikely for the chain to skip over a tooth on a cog. I wonder: have you actually seen the chain do that? If your diagnosis is based on the pedal momentarily dropping into thin air followed by a loud pop, the problem might be with the freehub.

Yep - a friend watched the chain "jump" when pedaling hard.

Another thought: I notice that there is play in the spindle within the bottom bracket - if I push/pull laterally on the crank arm there is noticeable side-to-side movement. Maybe enough, when under sudden stress, to cause the chain to snag on a tooth and skip? Everything is tight (crank bolt as well as cartridge assy) so I'm thinking the ceramic bearings may be shot. This is a 13 yr old octalink BB...

 

[wim]

Yep - a friend watched the chain "jump" when pedaling hard..

Well, I'm stumped for now if your friend actually saw the chain jump a tooth on a cog. Usually, a chain skips (jumps a tooth) under load if the driving force is no longer divided among a reasonable number of teeth, but is concentrated on one or two teeth only because of chain / cog wear.

Can't imagine a crank or BB problem causing a rear chain skip, but I suppose anything is possible. If you have a repair stand, hand-pedal hard against some rear brake drag. You might be able to spot the problem better. Worked for me a few times.

 

[Roland44]

Check to see if the RD hanger is bent.

Good suggestion, that's pretty much 90% of the problem in this situation...

 

[vttramper]

Check to see if the RD hanger is bent.

I thought about that, but it looks good visually and hasn't been banged or anything. I don't have a hanger alignment gauge so it could be off I suppose, but I don't know why that would happen sort of out of the blue. Worth checking at the LBS tho - I sure have no other ideas.

 

[QuattroCreep]

Check your Jockey wheels. Make sure they are not worn or gunked up.

Check the B alignment screw

Get a hanger alignment tool and check front to back (horizontal) alignment of the hanger. A little harder to eyeball then vertical alignment.