carpediemracing 

This applies to all bikes.

A long cage rear derailleur, designed to take up a lot of chain slack (i.e. 35+ tooth difference, like 20T up front and 15T in the rear), has a lot of "swing" in the cage. The spring is no stronger than a short cage rear derailleur. If you remove the third ring off the front, you end up with a lot of extra chain and a derailleur with a lot of extra slack capacity. The right length chain and a short cage derailleur will tighten things up.

Almost any steel cage front derailleur can be squeezed a bit. I routinely did this on bikes over a 15 year period in a bike shop, probably thousands and thousands of bikes. It makes even the worst derailleurs shift better - faster downshifts, more solid upshifts, no more rubbing than normal. You have to have two things done first:

1. Your derailleur angled left-right correctly first. Usually the outer cage is parallel with the chain in the big ring, but you should read the derailleur's instructions. If you don't have any, and the inner cage isn't parallel to the outer one, then make the outer cage parallel to the chain in the big ring. Since outer cages aren't straight (they typically curve in at the front) you should make the longest straight part of the cage parallel.

2. You have to have the derailleur as low as possible. I put mine so low I can tell when my pivots develop a bit of play because my front derailleur will start scraping the chainring - typically <1 mm of clearance, not the 4 or 5 mm which is recommended. If you're dealing with a used derailleur you'll have to raise it a bit because the act of pulling on a slightly worn derailleur will make it flex/move in directions it's not supposed to move. With the case of virtually all current front derailleurs (four pivots), the first bit of cable pull moves the cage of a worn-pivot front derailleur *down*, not up/out.

There are exceptions:
1. You have a dent in the seat tube from an over enthusiastic wrencher and now your front derailleur only sits in one place. Or you're paranoid about denting your seat tube so you leave the derailleur alone (this is the case with my current bike - my derailleur clears the bit ring by a massive 5-6 mm).
2. You have a carbon, ti, or aluminum cage (high end Shimano have aluminum for a while). Can't bend them or at least you shouldn't.
3. If the distance between the front plates is about equal to the width of the chain then you can't squeeze it any more (i.e. you've already squeezed it a bunch of times).

hope this helps,
cdr