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Fitting a new cable to the front and rear derailleurs

Fit the new cable into your shifters first, using the instructions, then follow these steps to connect the cable to your front or rear derailleur. You’ll need to adjust the cable tension once you’ve fitted the cables.

Conventional / top-normal derailleurs (including Shadow)

For the rear derailleur, push the cable through the barrel-adjuster on the back of the derailleur. Lift the back wheel off the ground and turn the pedals so that the chain returns to its neutral position – the smallest sprocket for most derailleurs, the largest for rapid-rise types. For rapid-rise, see below. Screw the barrel-adjuster all the way into the derailleur (turning it so the top of the barrel moves towards you), then back out a couple of complete turns. Look carefully at the cable-pinch bolt; there are often several possible ways to fit the cable under it, and only one of them makes your gears work properly. Look for a groove or a slot indicating the right place. The most common place is on the far side of the pinch bolt, pointing almost straight forward. Once you have the cable in the right place on the pinch bolt, pull it gently to the right, toward the front of the bike. Use the shifters to step all the way down and up through the gears. You should feel the cable pull your hand towards the derailleur as you shift down, then relax so your hand moves away as you shift up. Keeping a gentle pressure on the cable, change all the way up so that the cable is at its most relaxed and your hand is furthest from the derailleur. Guide the cable under the cable-clamp bolt and tighten up with a 9mm spanner or 5mm Allen key. Turn the pedals and shift all the way across the gears and back again. This will shake out any slack, so undo the pinch bolt again, gently pull through any slack cable and tighten the pinch bolt firmly. Cut off excess cable and fit a cable end. Now you have to adjust the indexing.

Fitting a new cable to the front and rear derailleurs

Low-normal (rapid-rise) derailleurs

Push the cable through the barrel-adjuster on the back of the rear derailleur. Lift the back wheel off the ground and turn the pedals so that the chain returns to its neutral position – the largest sprocket. Screw the barrel-adjuster all the way into the derailleur (turning it so that the top of the barrel moves towards you), then back out a couple of complete turns. Pass the cable under the clamp bolt – a groove in the derailleur will indicate the correct place. Use the shifters to step all the way down and up through the gears. You should feel the cable pull your hand towards the derailleur as you shift up, then relax so your hand moves away as you shift down. Keeping a gentle pressure on the cable, change all the way down so that the cable is at its most relaxed and your hand is furthest from the derailleur. Guide the cable under the cable clamp bolt and tighten up with a 5mm Allen key. Turn the pedals and shift all the way across the gears and back again to the largest sprocket. This will shake out any slack in the cable, so undo the pinch bolt again, gently pull through any slack cable and tighten the pinch bolt firmly. Cut off excess cable and fit a cable end.

Front derailleur

Pull gently on the cable and shift back and forth to get familiar with the effect the shifter has on the cable. You need to start with the cable in its slackest position, which will correspond to the chain being in the smallest chainring. Before you fit the cable, check the derailleur is properly fitted on the frame. The derailleur is harder to adjust if it’s not fitted correctly. Take hold of the cage and pull it out from the frame so that the outer plate of the cage is level with the outer chainring. You may have to manually lift the chain onto the middle chainring.

The front section of the outer plate of the derailleur cage should be exactly parallel with the chainring. There should be a gap between the top of the chainring teeth and the bottom of the derailleur cage of 1–3mm (1⁄16–1⁄8 inch). If either of these needs adjusting, let the cage spring back towards the frame. Loosen off the derailleur fixing bolt (usually with a 5mm Allen key), move the derailleur, retighten the fixing bolt, and check again. You may need to repeat this several times. Feed the cable under the cable clamp bolt. A very common error is to fit the cable the wrong way around the bolt. Check the derailleur carefully – it has a distinct slot or groove where you are supposed to put the cable. Generally, if the cable comes up from below, it sits behind the clamp bolt. If it comes down from above,
it sits in front of the clamp bolt. Keep a gentle pressure on the cable and tighten the clamp bolt firmly, making sure that the entire diameter of the cable is trapped between the clap bolt and the derailleur. Turn the pedals and shift through the gears several times. Don’t worry if the chain won’t reach the largest chainring at this stage. Shift back into the smallest chainring. Undo the cable clamp bolt, pull through any slack cable by hand and retighten the cable clap bolt. Cut off any excess cable, bend it back so that it doesn’t get caught in the chain, and squash on an end cap. Now you will have to adjust the indexing.

Source : BIKE MAINTENANCE TIPS, TRICKS & TECHNIQUES
See also bike maintenance tips, tricks and techniques “Fitting drop bar shifter cables”