SRAM make three different types of shifter. Attack shifters are designed for a 1:2 actuation ratio, and so are compatible with Shimano rear derailleurs. Shifters whose name consists of an x and a number, like x3 and x9, have a 1:1 actuation ratio (so pulling 1mm of cable through the shifter results in the chain moving 1mm to the side) and are only compatible with SRAM rear derailleurs with the same naming scheme. SRAM XX shifters are only compatible with SRAM XX rear derailleurs and SRAM road rear derailleurs like the Rival, Red and Force models.
Fresh twistshifters aren’t difficult to fit, especially as new ones come complete with a cable already installed. Keep this coiled up until you’ve installed the shifter on the handlebar and are ready to slide the cable into its outer casing. This stops it from dragging on the ground, picking up dirt.
As you’re installing the shifters, avoid kinking the cable as it enters the barrel-adjuster. It’s a vulnerable point until you’ve supported the cable by sliding it through the outer casing. Remove any bar ends and plugs. Then remove the short section of static grip – the section that stays still when you change gear. If you are replacing it, simply cut it off. Slide in a chopstick or similar instrument gently between the grip and the bar, without scratching the bar. Spray light lube under the grip and slide it off.
Find the clamp bolt for the twistshifters. These have come in a couple of different sizes over the years – usually small! Undo it enough to loosen the shifter – you don’t need to remove the bolt completely. Then gently slide the shifter off. Slide the new shifter onto the bar and set its position so that the barrel-adjuster and gear cable run under the brake lever. Try to set the position so that the barrel-adjuster is as close as possible to the brake lever, but you have to leave enough room to turn the barrel-adjuster. Tighten the shifter clamp bolt.
Slide the plastic washer onto the bars so that it sits against the end of the shifter. Next, fit the static grip back on. These are shorter than standard grips, so you need to take extra care that they are firmly stuck. For extra grip, tighten a ziptie around the outboard end of the grip. If this doesn’t do the job, your grips are worn out and should be replaced with new, tighter ones. Special short twistshift-compatible versions are available, but they’re no cheaper than cutting a standard pair to length with a sharp knife or a pair of scissors.
Set the position of the shifter easily, so that you can still turn the barrel-adjuster
Refit any bar ends and bar-end plugs
Next, the cable must be threaded through the outer casing. It’s worth replacing this when you’ve got this far. It’s only a little extra effort, and saves you transferring old dirt onto fresh cable. New shifters usually, although not always, come supplied with lengths of fresh outer casing. If they don’t, pick some up from your bike shop, along with enough ferrules to fit on each end of each section of casing. Cut the new sections of casing to length by comparing them with the old sections, fit the ferrules and slot the casing into the cable stops on your bike. Check the length of each section of casing – they should make graceful but not excessive curves. Extra length just adds friction, but if the sections of casing are too short, the casings will kink, squashing the cable inside. Check particularly sections between handlebars and frame, and sections where the casing runs from the main frame of the bicycle to rear suspension section – the cable should be long enough to allow these parts to move freely. Slide the cable carefully through each section, with a little oil on parts that will end up inside casing. Exposed parts of the cable don’t need lubrication, excess oil will only serve to pick up dirt. Once you’ve routed the cable back to the derailleur, use the instructions to fit the cable to the derailleur and adjust the tension.
Source : BIKE MAINTENANCE TIPS, TRICKS & TECHNIQUES
See also bike maintenance tips, tricks and techniques “Fitting Trigger Shifters”