Alternating clunk

[ElyDave]

I'm getting what I can only describe as an alternating clunk through the pedals on the S40 REcumbent.  Seems to not be every pedal rev, but swaps from being felt through one foot to the other, which makes me think it's drive train related. 

I've had the chain off for cleaning recently and I'm fairly sure I've put it back on the right way round.  Of course, back on the workstand I can't repeat it. 

Sticking jockey wheel?
Stiff link on the chain?
Damaged tooth on a chainring?
anything else?

[Lightning Phil]

Defo not the bottom bracket or loose crank arm?

[BFC]

How long is the chain (no of links) and what size chainring?

Could be a single link on the chain playing up, with the clunk every time it comes back to a certain point (mech, cassette or chainring).
Check the chain for missing/damaged plate, missing roller and check the quicklink has seated properly.

Watching the chains behaviour on the workstand can give a clue, but rarely shows up the same symptoms as on the road under load. Can't imagine how you clamp one of them in a workstand though.

[Kim]

Last time I had something like this, a small (~6mm) rare-earth magnet had been knocked off its usual position beefing up the spoke magnet for the cycle computer, and become attached to the chain.  It would occasionally migrate between the side of the chain, the bolts securing the chainring guard and the rear mech cage, with peculiar intermittent clunks interspersed with long periods of normal operation, before I eventually spotted it.  Infuriating!

Stiff chain link or a damaged tooth seems like a more likely suspect.  I had a damaged tooth on the granny ring of my Dawes that felt uncannily like the chain slipping at the cassette end.  Naturally, the problem would only occur under heavy load.

[ElyDave]

Defo not the bottom bracket or loose crank arm?

Nothing to suggest so, but can check before I go out today

How long is the chain (no of links) and what size chainring?

Could be a single link on the chain playing up, with the clunk every time it comes back to a certain point (mech, cassette or chainring).
Check the chain for missing/damaged plate, missing roller and check the quicklink has seated properly.

Watching the chains behaviour on the workstand can give a clue, but rarely shows up the same symptoms as on the road under load. Can't imagine how you clamp one of them in a workstand though.

Standard length chain on this one, FWD. Clamped in the worstand by the boom.

Chainrings are ultegra 52-39-30. Will give it another look.

[McWheels]

I had this on my previous CB; though not alternating like yours. It turned out to the BB bearings were starting to disintegrate. Maybe they're still rotating inside your BB but the low or weak spot is becoming more noticeable.