Tandem Drivetrain Problem
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?
When you say "The cranks rotate noticeably more in a single frame when the crack occurs" I assume you mean the stoker's cranks? If so then there is probably no issue with the timing chain and associated chain rings.
Normally this issue is caused by either a slipping freehub or a mismatch in wear between chain and either cassette or chainrings (or both). Have you double checked that you have actually changed the chainrings in question and not unintentionally put and old ring back on? Also try pulling the chain away from the front of the chain ring (https://cdn-ctstaging.pressidium.com/wp-content/uploads/2020/12/how-to-check-for-bicycle-chain-wear-26.jpg if it looks like that photo there is an issue)
Can you see from the video whether the chain is jumping forward with the cranks or not? Also, video of the rear wheel and cassette would be worth checking to see whether the freehub is slipping if you haven't already. Perhaps mark the chain cassette with a dark mark on one link/tooth to make this easier?
Does your GoPro have a slow motion mode with a very high frame rate? That would really help you diagnose this. Also shooting video in direct sunlight will give you the highest shutter speed so the motion blurring in each frame will be reduced.