Front derailleur cable tension pulls the cage outwards, shifting your chain onto larger chainrings, but when you release the tension, the derailleur relies on a spring to pull the cage back toward the frame and to pull your chain onto smaller sprockets. If your pivots are dirty or worn, the spring won’t be strong enough to pull back the change and shifts into lower gears will be slow.
Front derailleur: clean and oil
If your front derailleur is on strike, your first remedy is a good clean with a long soak in light oil (WD40, GT85, Superlube). Periodically work the derailleur back and forth as far as you can – gradually increasing until you work across the full range of movements. This is easiest with the chainset removed. Dirt will probably ooze from the pivot points as you go. Wipe it off.
Oil all the pivot points carefully. Use the shifter to slacken off the cable; shift it into the smallest chainring position. Take hold of the derailleur cage, pull it out from the frame as far as it will go, and push it back. Repeat a couple of times to work the oil into the pivots. Once the cage is moving smoothly, wipe off all the excess oil – don’t leave any oil on the surface of the derailleur: it’s sticky and picks up more dirt. For extra points, clean the derailleur cable.
Work the derailleur back and forth
Front derailleur: reshaping
The shape and condition of your front derailleur are crucial for reliable shifting. Old-fashioned front derailleurs consisted of two very simple flat plates, one on either side of the chain, which pushed the chain from side to side under the control of the front derailleur cable. Modern derailleurs are shaped to lift the chain quickly into place, allowing you to shift accurately under pressure. When new, they work faultlessly, but wear, crashes and brutal gear changing will take their toll.
If you’re having problems with your front shifting and adjusting the cable tension and end-stop doesn’t cure it, it’s worth inspecting the derailleur cage quite carefully. You can often identify the source of problems, and occasionally cure them, with some judicious bending. Check that the inside of the cage isn’t worn out first though – the most usual problem area is the inside surface of the inner cage plate. This is the surface that pushes the chain from the middle chainring to the smallest one, and from middle to outer. It’s tricky to get a good look, which is why wear in this area often goes unnoticed. Unless the derailleur is brand new, you’ll see the marks where the chain has scraped across the cage. With time, these marks get deeper. If the marks are so deep that you can feel ridges with your fingers, the derailleur will need to be replaced – the chain will catch on these ridges as you change gear, twisting it rather than lifting it cleanly onto the next chainring. In extreme cases, the chain will wear all the way through the derailleur cage and it will snap. Long before this, the clumsy changes will have worn and damaged your chain, so it makes good sense to replace worn derailleurs sooner rather than later.
If the derailleur isn’t worn out, check the shape. The curve of the cage should match the shape of the chainring. The front section of the outer plate should be parallel to the chainring. The top section of the cage, which links the front plate to the back, should be flat and horizontal. Clumsy shifting bends the derailleur cage, preventing it from following the contour of the chainset.
Use pliers to bend the cage gently back into shape. Ideally, you need to reshape the derailleur with one movement – using the pliers to lever the cage backwards and forwards will weaken the metal. Check the condition of the derailleur pivots. Take hold of the back of the cage and wiggle it up and down. You will be able to feel the cage flex a little, but if you can rock it up and down, or you can feel it knocking, the pivots are worn out and the derailleur must be replaced.
Source : BIKE MAINTENANCE TIPS, TRICKS & TECHNIQUES
See also bike maintenance tips, tricks and techniques “Setting the end-stop screws (Part 2)”