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Join Date: Aug 2009
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Double derailleurs will actually work as triple derailleurs without modification. The amount of "pull" on the cable is the same for one gear click.
That being said, there are a couple of things that will not be optimal:
1) The chain will rattle against the cage in the extremes on the inner gears. So, you'd have to trim the front more often.
2) The chain may drop more often when shifting to the granny.
3) The double derailleur may not have the range to get to all three chainrings, depending on the bottom bracket width. Sometimes the granny is too far in and the derailleur cage hits the seat tube before the correct position can be found, or if the outer chainring is too far out the cage may not be able to get to a position where the highest gear doesn't rub.
Try it and see is the only advice I have. If it doesn't work, just getting a Sora 9-speed (3403) is only about 25 bux new (the difference in front der's between product lines is minimal). Ebay the Ultegra for about 20 bux, and it's a wash. Just changing the cage would be difficult, plus you'd have to find somebody with just a cage for sale...
A bigger problem is the rear derailleur- a short-cage double won't have the capability to wrap enough chain to allow both the big-big and little-little combo's on a triple. Assuming a 23/11 cassette, 53-32 + 23-11 = 33 teeth, and the max capacity of a SS short cage derailleur is only 29 teeth (maybe 31 if the stars are aligned and you don't care what Shimano says). If you want to insist on using the SS cage derailleur, then it would be best to make the chain just long enough to accommodate the big-big combination, since a chain that's too short for the big-big can snap when shifted into a larger gear, but a chain that's too long for the small-small will just hang loose (still not great, since it can jam).