Shifters will wear out over time. The levers can be vulnerable in crashes. Occasionally they have a tendency to get tangled up when you pack your bike into cars, snapping off when you pull them out. It’s surprising how crisp new ones feel – you don’t notice the old ones getting sloppy and sluggish until you replace them.
New shifters almost always come supplied with a new cable, already fitted, which saves you from having to load the cable into the shifter. It’s worth taking a bit of care when fitting the new shifter though – avoid bending the cable where it emerges from the barrel-adjuster. Kinks in the cable will make it difficult to adjust your cable tension accurately. The boxes in which new shifters are packaged are carefully constructed to protect the cable; if you take the shifters out of the box to have a look at them, make sure you repack them carefully until you’re ready to fit them to your bike. Don’t allow the cable to drag on the ground, where it will pick up dirt. Keep it coiled up until you’re ready to fit it into the shifter.
Since you’re fitting a new shifter with a new cable, it’s worth replacing the outer casing at the same time. The old casing will have picked up dirt, which will quickly transfer itself to the new cable, making it sluggish. Make sure to fit ferrules on either end of each section of outer casing. Some shifters come supplied with outer casing – you’ll need good-quality cutters to shorten these to the correct length. Use your old casing as a guide when you cut the new lengths. Once you’ve fitted the new shifter, use the instructions to attach the cables to your front or rear derailleurs. Lefthand shifters operate front derailleurs, right-hand shifters operate rear derailleurs.
Ease the shifter clamp open to avoid scratching the bars
Experiment with a comfortable angle for the shifters, then tighten the fixing bolt firmly
Fitting Trigger Shifters
Remove bar ends and grips. Even if you’re replacing the grips at the same time, don’t cut them off, because you risk scratching the surface of the handlebar, which can weaken it. The ideal tool is a chopstick – slide this between the grip and the bar, spray light lube under the grip, twist the grip and pull it off the handlebar. (Chopsticks are very useful. Keep any you get with takeout noodles in your toolbox. If you don’t ever get takeout noodles, live a little more.) Slide the grip off the end of the handlebars. Undo the Allen key that fixes the brake lever on, then undo the fixing bolt for your old shifters. Slide both off the ends of the bars. If they won’t slide off easily, use a screwdriver to lever the fixing clamps open a little bit – just enough to get the levers and pods off without scratching the bars.
Slide the new shifters on, then slide on the brake levers, easing the clamps open in the same way if neccesary. Leave both fixing bolts loose, then refit grips and bar ends. Everyone swears by different stuff for this – degreaser, hairspray (my favourite), Photomount, Renthal Grip Glue from motorbike shops and more. If you don’t have any such gear, pop the grips in a bowl of hot water for a few minutes, then slide them on, taking care not to burn yourself. Sit on your bike in your normal riding position, and roll brake and gear levers around to a comfortable position. If you’re not sure, start with the brake levers at about 45 degrees to the ground, with the gear shifters tucked up close underneath them. If you have big hands, you might want to leave a gap between the ends of the grips and the brake levers. Once you’ve got the position right, tighten both brake levers and gear shifters firmly. Thread the cables through the outer casing, lubricating sections of cable that will end up inside casing with chain oil. Check that the grips have stuck firmly – don’t ride away until you know they have!
Source : BIKE MAINTENANCE TIPS, TRICKS & TECHNIQUES
See also bike maintenance tips, tricks and techniques “New cables for STi shifters”