Your front derailleur cable pulls your front derailleur outwards when you operate the shifter, shunting the chain from the smallest chainring to the middle and then largest chainrings. When you release the cable tension by clicking the triggershifters or rolling the twistshifters back, the spring in the front derailleur pulls the cage back, shunting the chain back into the middle and then smallest sprocket.
Pull the derailleur outward, over the middle chainring
If your front derailleur cable breaks, the derailleur spring will simply pull the cage back over the smallest sprocket, leaving you with only your lowest range of gears. Cycling home in the smallest front sprocket is likely to be annoying. You will probably want to try to set the front derailleur so that it runs in the middle sprocket, rather than the small one, unless your whole route home is up a steep hill.
If the low ‘L’ end-stop screw (A) will reach far enough, you can use it to set the derailleur position so that the chain runs in the middle ring. Reach in behind the chainset and lift the chain from the smallest chainring so that it sits on the middle chainring. Pull the front derailleur cage outward so that it is centred over the middle chainring. Holding it there, tighten the low ‘L’ adjustment screw (turning it clockwise) as far as it will go. Release the derailleur cage with your right hand and assess where it sits.
The front derailleur is ideally positioned when there is a 1 mm (1⁄16 inch) gap between the outer plate of the front derailleur cage and the chain, with the chain in the smallest sprocket at the back. If the cage is too far out, so that the gap is bigger than this, undo the ‘L’ adjustment screw by turning it anticlockwise until the derailleur sits in the right place. You will now be able to use the rear gears normally.
You may find that even when it’s screwed in as far as it will go, the ‘L’ end-stop screw will not push the derailleur out far enough not surprising, since this is not really what the end-stop screw was designed to do. If you can persuade it to sit mostly over the middle ring, you may still be able to use the larger rear sprockets normally. Avoid shifting into the smaller sprockets as this will cause the chain to rub on the derailleur, pushing it over so that you drop back annoyingly into the smallest chainring.
Alternatively, if you can’t make any progress with the end-stop screw, you may be able to wedge something behind the front derailleur to hold it out from the frame. Nature is packed with suitable devices – sticks are perfect. Pull out the derailleur and tuck something in to hold it in place. This is one of those occasions when it’s handy to have a sharp knife in your toolkit, so that you can shape the wedge. The spring in your front derailleur may be enough to hold your wedge in place, but zipties can help to stop it rattling free. You may even be able to set the derailleur so that your chain runs on the largest chainring. If you come to a hill, stop at the bottom and pull your wedge out, climb the hill in your small chainring, and refit the wedge at the top. Again, you should still be able to use your rear derailleur, but don’t shift into gears that mean that the chain rubs on the derailleur cage – it will slow you down and wear out the cage.
Coil up the broken derailleur cable and secure it out of the way so that it doesn’t get tangled up in your wheel, brakes or suspension. The frayed broken ends are surprisingly sharp. Make sure they don’t stick out and stab your legs. Undo the cable clamp bolt and remove the stray piece of cable. Retighten the cable clamp bolt so that it doesn’t rattle loose.
Derailleurs that have been badly bent in a crash may not respond to this treatment. If the cage is too twisted to allow the chain to pass through it, you will have to remove the derailleur completely. Undo the cable clamp bolt and coil the cable out of the way. Undo and remove the front derailleur fixing bolt. Your chain will still be trapped in the derailleur cage. Avoid splitting and rejoining the chain, which can be fiddly and weakened. Instead, undo and remove the bolt at the back of the derailleur cage and slide the chain out from inside the cage. Sit the chain back onto one of the chainrings. You may not have this option on recent front derailleurs. You would expect the chain to stay in whichever chainring you’ve chosen, but without the guidance of the front derailleur, it will usually jump off. Mostly, it will head for a smaller chainring. You can minimize unexpected changes by pedalling as smoothly as possible, and by staying in the larger sprockets at the back, which helps to keep the chain tension high. Occasionally the bolt that holds the back of the derailleur cage together can rattle loose. This tends to make front shifting sluggish rather than hopeless, but you can secure the two halves together with a ziptie through the bolt hole to restore full function.
Source : BIKE MAINTENANCE TIPS, TRICKS & TECHNIQUES