The Ultimate Guide To Bike Rear Mech : Part 2- How To Adjust

    Most rear mechs are indexed, so gear changes should be fast and reliable. If you are having problems, a few easy adjustments or a new cable should put things right. The first step when you are adjusting any rear meth is to make suit that the inner cable is free of friction as it moves in the outer cable.

    You then adjust the upper and lower limit screws, which control the movement of the chain cage from side to side. If you ever find that the chain has jumped off the sprockets and got jammed in the spokes, the limit screws probably need adjusting. The same applies if the chain jams between the top sprocket and the frame.

    On mountain bikes and hybrids, you select the gears with either a rotary changer built into the handlebar grip, or a trigger-type changer. But nearly all modern sports bikes and racers have combined brake and gear levers . Older road bikes have indexed levers on the down tube, in the same place as friction gear changers.

    For an indexed rear meth to work, the gear changer has to move the cable exactly the same amount for each gear change. But this set amount can only be transmitted accurately to the rear meth if the cable is under a lot of tension. If the cable stretches or the outer cable compresses, reducing the tension, the indexing stops working sweetly.

    You will probably notice the noise of the chain running slightly out of line when this happens. To increase the cable tension again, give the cable adjuster one half-turn anti-clockwise. If that does not work, go through Step 5 on this page to re-adjust the indexing.

A. When You Need To Do This Job

  • Rear mech is noisy
  • Gears will not change smoothly and accurately.
  • Chain jumps off into spokes or jams between sprocket and frame.

B. Time

  • 30 minutes from fitting new mech to completing adjustment of indexed rear mech.
  • 5 minutes to tine-tune the indexing, including test ride.

C. Difficuty

  • Basic adjustment is quite straightforward, but getting the indexing working perfectly can take a bit of patience.

Adjusting the indexing

Step 1


Check that the inner cable is not weakened or frayed anywhere. And make sure that the outer cable is not kinked or damaged, especially the short cable down near the rear mech. Then spray some aerosol lube down each section of outer cable. Do not forget to check the cable guide under the bottom bracket. Now lift the back wheel and select top gear. turning the pedals slowly so that the chain jumps down on to the smallest sprocket

Step 2


Undo the cable clamp to free the gear cable, if necessary. Then pull the rear mech backwards to make it easier for you to see the position of the jockey wheels. On index gears, a vertical line through the middle of both jockey wheels should line up with the outside edge of the smallest sprocket. To move the jockey wheels to the right, turn the H for High limit screw anti-clockwise. To move the jockey wheels to the left, turn it clockwise, the opposite way.

Step 3


Make sure that the gear lever is still in top gear position. Then screw the cable adjuster in most of the way and lit the gear cable, pulling it as tight as possible with pliers before you tighten the cable clamp. Using the gear lever, change down slowly until the rear meth is in bottom gear. The jockey wheels should line up with the MIDDLE of the bottom sprocket. If they are to the right of that point, or the rear meek will not change down to bottom, turn the L for Low limit screw anticlockwise. Turn the limit screw clockwise if the chain cage is to the left of the largest sprocket

Step 4


If you have trouble finding the H (top) and L (bottom) limit screws, they are usually on the top or back of the rear mech. But on budget Shimano and most Campag gears, they are on the side, near the cable clamp. If the limit screws are not labelled H and L, you will have to use trial and error to sort out which one is which.

Step 5


To sec how the indexing is working, give the bike a road test. First check that the chain runs silently in top gear on the biggest chainring. If it makes a rattling noise, turn the cable adjuster (large arrow) — not the H and L limit screws — half a turn anti-clockwise if the chain is trying to jump off the top sprocket. But if it is trying to climb onto the second sprocket, give the cable adjuster half a turn clockwise. Keep on road testing, changing up and clown through the gears and adjusting the cable tension, until the bike runs silently in top.

Next, change down to second gear with the gear lever and turn the cable adjuster one quarter-turn anti-clockwise at a time, until you hear a metallic clattering sound as the chain tries to climb onto the third sprocket. Then turn the adjuster clockwise until the clattering noise stops, but absolutely no further than that. Road test again to make sure that the gear changes are quick and accurate. But when you are checking out the gear change up onto the biggest sprockets, change down to a smaller chainring.

Be very careful when changing down into bottom gear the first few times as you might sovershife the chain off the biggest sprocket and into the spokes. If you cannot get the gears to shift properly, lubricate the outer cable again. Better still, fit a complete new inner and outer cable.

NON-INDEXED GEARS WITH FRICTION LEVERS

Friction gears are controlled by levers on the down tube. When setting them up, check first that the right hand lever moves smoothly. If not, loosen the centre screw slightly. But if the lever action feels loose, tighten the centre screw very slightly. Set the limit screws next. Use the same method as outlined in Steps 2 and 3 above, EXCEPT that the H limit screw must be adjusted so that the chain cage is directly below the centre line of the top sprocket. Fit the cable and then road test the bike. When changing gear, move the lever a little until you feel and hear the chain jump to the next sprocket. Adjust the position of the lever slightly if the gear makes any noise as you pedal. In top gear, the chain should run almost silently. If there is a slight but regular metallic clacking or coughing noise, try adjusting the H limit screw one eighth-turn anti-clockwise. If that makes the noise worse, turn the H limit screw clockwise a little until the noise goes away. Now change down carefully, one gear at a time, until you get to bottom gear. Turn the L limit screw clockwise if there is a clacking or coughing noise. If that makes the noise worse or the chain jumps down onto the next sprocket, adjust the L limit screw anti-clockwide one quarter-turn at a time until the chain runs silently on bottom gear. If in doubt about any of this, check the chain cage from behind to see if it has moved too far to the left, or too far to the right, and then adjust the limit screws as required. If the gears run quietly at first but get noisier, or they keep jumping to a higher gear, tighten the centre screw of the lever very slightly.

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