Originally Posted by
CCrew
May also need a rear derailleur. Most triples from Trek come with an SGS (long) rear cage and double only need a mid or short cage depending on gearing.
Triple rear derailleurs work fine with double cranks. They just don't look as traditional as a short cage and weigh about 10g more than an equivalent double which will cost a 140 pound climber .5 seconds and hour he spends off the front in the mountains (IOW, if you don't race, race but aren't competitive, or race well but weigh more than 2 pounds/inch of height don't worry about it).
Fronts are different, built to work with a different chain line, different ring diameters, etc. I couldn't get a Campagnolo triple front to shift a compact crank in all gear combinations without throwing the chain. The right front derailleur is flawless.