That's the operative question and the reason for my initial reply: just what rear derailleur are they recommending this thing be used with ON TANDEMS?

While I've heard folks say that they were able to make 34t cassettes "work" on tandems with earlier Campy Racing-T rear derailleurs with their medium/longish cages I found the upper limit to be 28t and a stretch at 30t on tandems running 28t - 32t granny gears with 54t big rings on tandems with standard length rear stays.

Since '02 we've been using up to 32t cassettes with Campy long cage Chorus & Record rear derailleurs on our Erickson tandems and it's a stretch with 30t and 32t granny gears coupled with 54t big rings. What I've found is that while you can shift into the 32t, the amount of chain required to wrap the 32t/54t "big/big" ring combinations causes the teeth of the rear derailleur's guide or jockey pulley (that's the upper one) to "kiss" the teeth on the 32t sprocket even with the B-screw set to it's max. Again, the combination "works" but you can hear a distinct mechanical noise coming from the jockey pulley and cassette in this combination. And, IMHO, you MUST always set up a tandem so that the chain is long enough to support the "big-big" combination, lest you bind up your drivetrain and/or damage your rear derailleur.

Now, I will note our Calfee's chainstays are about 1" shorter than the Ericksons (similar to a single bike) and only uses a 53t "big ring". The shortened stays & reduced tooth count on the big ring make for a drive chain that's 2" shorter than on our Erickson which has eliminated most of the interference between the derailleur's jockey pulley and 32t sprocket with the 30t granny ring. However, I'd be surprised if there wasn't a lot of interference with a 34t sprocket. I've never even bothered to check as I don't even own a 34t cassette even for our off-road tandems: 32t has always been "good enough" for road or off-road.

Now, at least for single bikes, I wouldn't see any interference issues with a 34t on a bike running a compact drive with a 50t or 48t "big ring", and I suspect that may be the target market for this cassette.

Anyway, take it for what it's worth... As I said, I'll be interested to see what they recommend since the defacto solution for anyone who has wanted to run a 34t cassette has been to spec a Shimano XT or XTR rear derailleur that is designed with the needed capacity.

Shimano uses the same cog sizing for it's 11x32t 9 speed cassettes.