JXB

Does anybody else have this problem? Standing going up hill and the chain jumps a tooth or two on cassette (loud clunck).
My stoker (wife) screams everytime it happens. Cassette and chain are not worn, derailleur is adjusted. It only happens when we are standing. Cranks are in sync. My thinking is that there is some jerk motion caused by our padaling forces not being completely in sync.

Butch

I had a similar problem on my Shogun. The rear derailleur was not free. The large pin that allows it to rotate had lost it's lube and wasn't allowing it to return freely. This occurred in the smallest (least teeth) sprocket the most. WD-40 to loosen and light oil to lube and all is well.

TandemGeek

What year/model of tandem, what brand/model hub, how many miles are on the tandem, and how many miles have you logged since having the rear hub's internal mechanism cleaned and lubed?

My knee jerk reaction is to suggest what you are experiencing is pawl slip. Basically, one of the internal pawls is failing to fully engage the ratchet ring which causes the cassette carrier to "slip" and then "catch" under very hard pedal loads. If my assessment is correct, and if left unaddressed, you'll eventually experience more skipping and cause more internal damage. Depending on the hub, it can often times be remedied with a simple cleaning and re-lubrication of the internal hub mechanism if it's caught early enough. If you find that a pawl has cracked or is excessively worn, they can often times be replaced to remedy the problem. The higher-end hub manufacturers will often times do the repairs for a nominal fee, including shipping and parts.

JXB

I have a Santana Sovereign. It is about 10 years old.
I built new wheels a few years ago with Hadley tandem hubs.
They don't have that many miles on them.
They are very high quality. I don't think it is the hubs but it may be possible.

Brian

Frame flex? We get it on our tandem, but worse case is a thrown timing chain.

TandemGeek

Mike Hadley's hubs are fairly easy to service so it would be worth while to give them a quick cleaning, inspection, and relubrication with a light oil or perhaps a 50/50 blend of Phil Wood waterproof grease and tennacious oil to see if that solves the problem. Even a few pair of Santana's brand new Sweet 16's were recently discovered to have been over-lubricated with a heavy grease instead of the appropriate type of oil. The thick grease kept the pawls from fully engaging the ratchet ring and caused the same type of "slippage" under heavy climbing conditions that you experienced. In fact, as I now recall, at least one set of our Phil Wood hubs had the same ailment many years ago right out of the box: it was caused by a slightly offset ratchet ring.

Again, if all the other bits and pieces of the drive train are in good order -- no bound-up chain links, no bent cogs, no excess cog or chain wear, and the rear derailleur is dialed-in and not rubbing against the cassette's largest cogs when you're in the 34t cog -- it's the only thing that comes immediately to mind per your description of the problem.

JXB

I will give that a try and report back later.
Thanks for the advise.

merlinextraligh

Sounds like a stiff link. Run the chain around slowly backwards, and see if there is a link that doesn't want to bend around the cogs. If there is, you can get it lose by bending the chain sideways a bit until it's freed up (textbook will tell you not to do this, but it works) also cleaning and lubing can help with a stiff link.