The Ultimate Guide To Bike Rear Mech : Part 3 – Removal and refitting

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You will have to remove and refit an old rear mech to do an overhaul. Or fit a new one from scratch when the old one is worn out. There are three different ways in which the rear mech is fitted to a frame. On good-quality bikes, the mounting bolt is screwed straight into a threaded hanger that hangs down from the rear drop-out.

Be careful to keep the bolt straight as you screw it in. If it goes in at an angle and you force it, that could strip the thread in the hanger. If you do this on a steel frame, you will have to get it tapped out by a professional bike mechanic. It is also worth getting the threads tapped out after a respray, in case the threads are choked with paint. On the other hand, if it is an alloy frame, you just fit a new gear hanger.

Here a small extension plate is bolted onto the gear hanger and the mech itself is fitted to the rear end of the extension. This places the rear mech further back than usual, which allows much larger sprockets to be fitted. To remove the rear mech, undo the socket-headed bolt as if it was the mounting bolt.

The method to fix rear mech to the frame is used on budget bikes, which do not have a built-in gear hanger. Instead, a separate steel gear hanger is bolted to the rear end, with an oval nut on the inside, shaped so that it fits into the drop-out. On budget hybrids, the bolt-on bracket is extended like the extension plate described above.

To remove a rear mech fitted using a bolt-on bracket, first remove the back wheel. Then remove the bottom jockey wheel or break the chain. Finally, you loosen the fixing bolt at the rear of the bracket and pull the mech forward, away from the frame. When re-fitting, make sure the oval nut fits into the slot for the wheel in the drop-out. As you tighten the bolt, stop the oval nut turning by holding it with a spanner on the flats. There is no need to overtighten because the bracket is held in place by the wheel nuts as well as the fixing bolt.

Whatever the method of fixing, lightly coat the thread of the mounting bolt with anti-seize grease. And if you are fitting a new rear meth, bear in mind it is always best to fit a new chain as well. Set the correct chain length. Finally, set the B screw adjustment as explained below. This ensures that the jockey wheels and sprockets do not touch when you go down into bottom gear.

THE B SCREW ADJUSTER

As well as the two limit screws, nearly all rear mechs have a third adjuster, usually known as the B screw, also referred to as the chain adjuster. You do not normally need to touch it. But if you fit a large bottom sprocket, 28 teeth or more, the top jockey wheel sometimes touches the sprocket and interferes with the gear change. If this happens, select the smallest chainwheel and sprocket, then adjust the B screw to give the smallest possible gap between the jockey wheel and sprocket. Increase the gap slightly if bottom gear does not run smoothly. For SUNTOUR MTB mechs, you set the gap at 6-8mm but on their road mechs, you adjust the B screw so that the main plates are parallel with the chainstay.

A. When You Need To Do This Job

  • The old rear mech is worn out.
  • A rear mech has been stripped down completely

B. Time

  • 30 minutes to fit a new rear mech, plus another 30 minutes to adjust and test gear change

C. Difficulty: Medium

  • Care is needed when screwing the pivot into the gear hanger. Otherwise it is easier than overhauling a rear mech.

REMOVAL

This shows the way to I remove a rear meth by breaking the chain. Hold the meth with one hand while you undo the pivot bolt with a tong hexagon key as it may be tight.

REFITTING

If you intend to re-fit the mech, take the opportunity to do a quick snip down and clean. Always clean the jockey wheels, then tube with waterproof grease.

FINE-TUNNING INDEX GEARS


STEP 1

If the indexing is having an off-day, increase or decrease the cable tension by turning the cable adjuster a (planer-turn. Experiment to find out which way is best.

STEP 2

Racing bikes with STI and 4 Ergopower combined gear changers usually have a thumb adjuster on the down tube so you can alter cable tension when riding along.

STEP 3

When fitting a rear meth to the frame, it is easier to screw the top pivot bolt into the gear hanger if you steady it by tucking your forefinger behind the gear hanger.

STEP 4

Now fit the chain. Then adjust the throw by turning the H limit screw until the jockey wheels align with the outer edge of the top sprocket.

STEP 5

Fit the gear cable now, tensioning the inner before you do up the cable clamp. Snip any spare inner cable off close to the clamp and lit a cable cap to stop it fraying.

STEP 4

Turn the pedals slowly and slowly change gems one by one down to bottom gear. Adjust the L limit screw if the chain does not jump onto the largest sprocket or run there silently.

STEP 7

Check that the outer cable is arranged in smooth, large radius curves. Then go through the final indexing adjustment and give the bike a road test.

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