I'm looking to replace my Suntour derailleur on my tandem and need one that will cover 28-48-54 chainrings. Does anyone have a suggestion?

Mike..

Front or rear derailleur? The rear derailleur shouldn't be a problem; any wide range rear ought to suffice. I had a Simplex SX410 that worked fine for many years until one of the springs broke, I replaced it with a Shimano Altus that also worked fine, but seemed incongruous with the rest of the components, and now run a Campy Rally (thanks, PastorBob!) which is working very nicely. I couldn't find a front derailleur that would accommodate that wide range on the crank, though. Setting the derailleur high enough to clear the 54T ring meant the chain would scrape on the cage pin of the FD when in the 28T ring. I tried several FDs: Simplex Super LJ Triple, SunTour Cyclone, and Shimano Deore XT (aka "deer head") without success. I finally replaced the 28T ring with a 32T ring and all is well now.