Originally Posted by
tandem rider
Since you did not have a problem with ghost shifting with the Ultegra 6700, I don't understand why frame flex would be the problem with the ETAP. I once had the problem that you are describing going up steep hills on the Hilly Hundred. It is unnerving and hard on the rear wheel. At the first sag stop the mechanic replaced the cassette and my problem was solved. You mentioned that a Shimano cassette was a little better than the Sram, but I wonder if something is keeping the chain from fully engaging on the cog. As WheelsNT mentioned, it could be something with your hub.
Tandems East is probably close to you. They might have an answer to your ghost shifting.
Good luck fixing your problem and let us know if you find a solution.
I'd be surprised if it were a freehub body skipping problem. I've been through three freehub bodies, pawls, etc. on my current tandem hub. And that's after destroying six hubs catastrophically prior to this one. But the recent skipping I've experienced is not very disruptive to our riding. It's obvious it's internal the way it feels and is nothing like a chain skip or miss shift. It's barely a bother. But after about a year of only occasional skipping, I disassembled it and found sufficient wear on the pawls and engagement ring. Internals were just replaced. But again, this skipping is barely a bother when riding challenging terrain off road.
Alternatively, the catastrophic freehub failures are obvious and end the ride right there. If Alcanbrad had such significant problems with the freehub internals, I'm assuming he'd notice other symptoms. E
But who knows, it would be worth investigating this as well.