Which Front derailleur???
 

Our 99 C Dale RT1000 tandem front derailleur broke today. 9 speed crank, 53/42/30 chain rings. 10 Speed in the rear, 10 speed chain.
I know that most Shimano triples are set up for 50/39/30 and I need the correct 34.9 clamp, not sure which one to look for..I have also read that there is a difference when having a 42T vs a 39 middle chain ring. Pls advise which FD I need to look for. After wading thru Shimano tech docs, it looks like a 6603, 7803, and 7803 are rated for 52T, most other triples are rated for 50/39/30, not sure about the 42T middle ring...
cross posted in Tandems

My Guess; Road, triple [53/42\39/30] oversize band , or adapter band and a braze on FD.
I doubt the 3 more teeth on the middle ring matters mostly the arc of the outer cage is different,

more when its a 44t MTB compact triple outer..

Why not replace it with what was on your tandem. Many parts are available on E-bay as New Old Stock (NOS). Used stuff also has a lot of life left as well. THe gearing on you bike, if it truely is a 99, appears to have been modified as 10 speeds were not available in 1999, it was 9 speed era so you or someone already has done some modifications if you have 10 speed rear/chain.

As far as I know, you could use a 9 speed triple FD such as a Dura Ace FD-7703, rated for 53/39/30, or Ultegra FD-6503, rated for 52/42/30 but should work for a 53T large chainring. You could even try a 10 speed triple Ultegra FD-6603, rated for 52/39/30.

I've never considered a triple front so please educate me if wrong. However I don't see where the size of the middle matters to the front derailleur. As long as the DR cage can accommodate the chain drop from the large to small, I'd think there'd be no issue.

STI front derailleurs are designed for a particular size difference between the middle and big ring. If the middle ring is much bigger than the derailleur was designed for, the inner plate of the derailleur may contact the middle chainring. Moving the derailleur up can buy you some clearance on the middle chainring, but then the derailleur is positioned a bit high for the big ring, not to mention it reduces the capacity for a small ring.

The lower edge of the inner cage plate needs to clear the tops of the middle ring. So a der with too "deep/tall" an inner cage plate shape could rub an a 42T ring but clear a 39t ring. Also the cage's contours might not match where the chain is running past them if the ring was the wrong size, affecting the chain's lift during shifting. Andy