This is possibly the single most important adjustment that you will learn to make on your bicycle, and it’s not difficult. There are always clear signs when you need to adjust the tension of your rear derailleur cable. If the derailleur doesn’t respond to your shifter, if it shifts more than one sprocket when you click the lever or if the chain rattles and clatters as you shift, it’s time to look at your cable tension.
The important thing to remember during this procedure is always to start in the same place, with the cable tension at its slackest and the chain in the smallest sprocket. Otherwise, it’s easy to confuse yourself, matching up the third shifter click with the fourth sprocket, or whatever. You will either need a workstand, to keep the back wheel off the ground, or the assistance of a friend, to lift the bike up for you at the appropriate moment.
Before you begin changing the cable tension, familiarize yourself with the action of your shifter. Follow the casing that emerges from the right-hand shifter to where it joins the top tube or down tube, emerging as bare cable. Hook your finger under the middle of this section of bare cable and operate your shifter up and down one gear. You’ll feel the cable tension pull through, then release. When the cable tension is exactly right, the chain sits exactly below each sprocket as you change gear. This maximizes chain life and stops the chain from clattering on the sprockets as you ride. Since the sprockets and shifter clicks are evenly spaced, once you have the adjustments for the two smallest sprockets, the others should work automatically.
Once you’ve worked out which action makes the cable tighter, and which makes it looser, shift so that the cable is as loose as possible. Start by checking that the shifter is in the high gear position – turn the pedals and click the shifter so that the chain moves all the way into the smallest sprocket. Keep clicking, in case there’s slack in the system, until you run out of gears. Keep turning the pedals and click the shifter exactly one click in the down direction, so that the derailleur moves towards the larger sprockets.
Step 1: The chain should move across to the next largest sprocket and sit directly underneath it. Use the barrel-adjuster to make fine adjustments. To get the barrel to move, hold it as shown with your thumb on the top of the barrel. Turning it one way tightens the cable and moves the chain away (A) from you onto larger sprockets. Turning the other way slackens the cable and allows the spring to pull the chain towards you (B), onto smaller sprockets.
Step 2: If the derailleur doesn’t move when you click the shifter, the cable is far too slack. Undo the pinch bolt, pull through a little more cable by hand, tighten the pinch bolt. This often happens when you fit a new cable. Start again in the smallest sprocket, clicking the shifter several times to make sure it is at the slackest position. Now increase the tension by half a turn of the barrel-adjuster and repeat until the chain lifts onto the second sprocket.
Step 3: Once you can move the chain from the smallest to the second sprocket, try shifting back from the second to the smallest. You may find you have to tune the position further – try a quarter-turn at a time. Once you’ve got the right tension, shift into the second-smallest gear and look at the chain from behind. The top jockey wheel, the one with the chain around it, should sit vertically below the second sprocket. Use the barrel-adjuster to finish off the tuning.
Source : BIKE MAINTENANCE TIPS, TRICKS & TECHNIQUES
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