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Tandem Drivetrain Problem

Neil Plant2021-05-29 11:06:52


Any advice on the following would be welcome:

I've just renewed the drivetrain on our tandem (3x9), new chain, cassette and chanrings.  Timing chain is relatively new (5 years), timing chainrings, rear hub and bottom brackets 12 years old.  Drive and shifting are fine until some load is applied when, at some points, it sounds and feels like the chain is skipping - there's a loud crack and the cranks 'jump' a short distance.  To see if the chain was skipping I mounted a GoPro to film the cassette and chainrings and went through the video frame by frame.  The chain does not appear to be skipping (either riding up the teeth or changing cogs) and the cassette appears to remain 'in sync' with the wheel but there is a noticeable loss of tension with the chain going slack at the point at which the loud crack occurs - often when the cranks are horizontal (presumably max load).  Sometimes the chain derails from the chainring, sometimes not.  The cranks rotate noticeably more in a single frame when the crack occurs.  With amount of deflection on the video I'm surprised it doesn't always derail!

As mentioned, the drivetrain is new.  The freehub appears to be working ok and I can't detect any play in the bottom brackets.  The problem did occur when I just changed the chain and chainrings so I then got the new cassette but that didn't solve it.

Attached are stills of the cassette and cranks of the point where the crack occurs.

Any suggestions of other things to check?



Neil Plant2021-05-29 11:40:46

Cassette picture:

John Ernest Tompkins2021-05-29 18:05:37

Just a suggestion. Have you checked each individual link on the drive chain. Had the same problem on our Thorn Discovery the chain looked fine when stationary but one side of a link was snapped and the problems occurred as soon as we put load on. You have to check each link very carefully to find this particular problem.

Neil Plant2021-05-30 15:09:41

Thanks for the suggestion John.  The chain was ok but I found the problem and have detailed it in the other thread.  I can safely say that it is probably as unique and unusual as the one you had with your Thorn!


John Ernest Tompkins2021-05-30 16:24:22

Well done in persevering to find the problem, it’s really helpful to know about these anomalies. Enjoy the bike. Cheers John