Bicycle Mechanics - Triple FD with Double Crankset?

Trying to be cheaply creative, or is it creatively cheap? Anyway, here's my plan:

Using an Ultegra 6500 triple front derailleur with a Suntour Superbe Pro 52/42 double crankset (with the appropriate 109 mm BB).

In theory, as long as I have the inner limit screw set, the Ultegra triple should work just fine. In fact, I thought I read somewhere in this forum that the Ultegra triple FD is optimized for the 10 tooth drop, whereas the double is optimized for the 14 tooth drop of the now standard 53/39 setup.

Does this sound right?

The part that doesn't sound right to me is the 109mm BB spindle. I don't know what spindle length that particular crankset requires but I suspect that a crank that old will need a longer BB spindle.

The part that doesn't sound right to me is the 109mm BB spindle. I don't know what spindle length that particular crankset requires but I suspect that a crank that old will need a longer BB spindle.

Hmm. I just checked out Sheldon's BB database and it lists the length of the Superbe Pro double as a "Campagnolo 111," meaning I assume that if I can't find a Superbe Pro BB, I could use a Campy BB.

Better to measure the front chain line before spending the extra $$, though. 109.5 mm is standard on the Ultegra road double, and the Suntour double may use it too.

http://sheldonbrown.com/chainline.html

I've been doing this for a while and it will generally shift just fine with more than a 10 tooth drop but a lot of micro-indexing is necessary to avoid chain rub. I'd check into a BB that puts your inner ring just inside of where the middle ring of an Ultegra triple sits. There's less lateral travel with the triple FD sitting well inside of the chainline it was designed for. For all I know about Suntour chainring spacing though, that may well be what you want.

I've used a triple FD with a double crank. Obviously it wasn't designed for it, but it'll still work

I've been doing this for a while and it will generally shift just fine with more than a 10 tooth drop but a lot of micro-indexing is necessary to avoid chain rub. I'd check into a BB that puts your inner ring just inside of where the middle ring of an Ultegra triple sits. There's less lateral travel with the triple FD sitting well inside of the chainline it was designed for. For all I know about Suntour chainring spacing though, that may well be what you want.

This is interesting, because I was under the impression that the cage of a triple FD would be wider than that of a double.

This is interesting, because I was under the impression that the cage of a triple FD would be wider than that of a double.

I'll bet they are but if you have a 14-16 tooth drop and a 52-42-30 derailleur then the chain runs below that portion of the cage where it is supposed to ride with a 10 tooth drop. I'd have to take yet another look to be 100% sure but I think the outer portion of the cage is closer to the seatpost at that point. I've never been able to completely stop it from rubbing the outside of the chain when I'm cruising in 39-15, 39-14, ... without resorting to micro-indexing.