After punctures, chain repairs are the most common task you’ll carry out. Nine out of ten broken chains occur when undue pressure is applied to a chain that’s in the process of shifting between gears.
The combination of angle and stress is too much for the fragile, put-upon chain and it gives way, leaving you suffering a sudden loss of drive and balance and possibly vaulting over the bars if you were stamping hard on the pedals at the time, too. Learn how to use your gears correctly and you’ll avoid unnecessary problems; similarly, keep your drivechain in good condition, as old, worn and neglected chains are more likely to let you down under pressure. For most chain problems, you’ll need a chain tool. These can be annoying to carry as they only do one job but it’s extremely hard to botch a chain repair using anything else. Several manufacturers, like Park and Topeak, make good chain tools that also fold so invest in a version that you’ll know you’ll carry and then keep it with your toolkit at all times.
Your first step is to go back and retrieve your chain. It may have stayed with the bike but is more likely to have unravelled onto the trail. If you were moving at speed it may be some distance back up the path so retrace your steps and collect it. Then follow the steps below to repair it.
Checking the length of the repaired chain
When you’ve finished the steps below, it’s important to check that the repaired chain is still long enough to reach all the way around your drivetrain. It will be slightly shorter, since you will have removed damaged links. It is essential that there is still enough slack in the chain even in the largest sprocket, so that the derailleur is not strained or twisted. Otherwise, you risk tearing the derailleur off, damaging both the derailleur and the part of the frame to which it attaches.
Get someone to lift up the back of the bike for you, then change into the smallest sprocket at the back and the largest chainring at the front pedal with your left hand and change gears with your right. Then change gears click by click towards the largest sprocket at the back, while watching the derailleur. As you move into larger sprockets, the derailleur will get stretched forward. Check the tension of the lower section of the chain, where it passes from the bottom of the chainring to the rear derailleur. If this section becomes tight, stop shifting. If you force the chain into a larger sprocket once the chain is tight, you’ll damage the derailleur.
If the derailleur is struggling to reach the largest sprocket at the back, it’s important not to change into this gear as you ride along. Try to remember not to use this gear. We prefer to readjust the endstop screw on the rear derailleur so that you cannot accidentally change into the largest sprocket, because it’s all too easy to forget once you start riding. Shift click by click into larger gears until the chain becomes taut, then screw in the ‘low’ end-stop screw until you can feel resistance – it will touch the tab inside the derailleur that limits further movement. Once you get home, replace the chain with a new, longer one (you’ll almost certainly need a new cassette too) and readjust your end-stop screw so that the chain reaches the largest sprocket. For more about end-stop screws.
Step 1 : Once you have the chain, find the break and look at both ends. One will end in a narrow segment and one will end in a wider segment. A complete link consists of one narrow and one wide segment. You may find that the plates that form the end of the wider segment are twisted and damaged, so this complete link (both the wider damaged plates and the narrow segment they’re attached to) will have to be removed.
Step 2 : Look carefully at the chain to choose the right place to break it. When you come to rejoin it, you need to match up a narrow and wide segment. Once you’ve selected the correct rivet, lay the chain over the chain tool as shown. If your tool has more than one set of supports, choose the ones furthest from the handle to support the chain.
Step 3 : Turn the handle of the tool clockwise so that the pin approaches the chain’s rivet. When you get close, line the pin up with the centre of the rivet very carefully; if the pin is on the edge of the rivet or the plate instead, it will bend and damage the tool, possibly irreparably, as well as further damaging the chain.
Step 4 : Continue to screw. It will start to push out the rivet. You should feel some resistance but if you’re having to turn very hard the pin probably isn’t correctly lined up. If it’s a Shimano chain and you have a spare replacement rivet, or if you’re going to be rejoining the chain using a Powerlink, then push the rivet all the way through. For all other chains you need to stop before the rivet is loose of the rear plate, as you’ll be using it to rejoin the chain.
Step 5 : Once the rivet is sticking out of the back of the chain and is free of the inner plate but not the rear plate, remove the chain tool by backing off the handle and flex the two halves of the chain gently to slide them apart.
Step 6 : You’re taking out a complete link – one wide section and one narrow section – so repeat the process, two rivets along, on the other side of your twisted link. You should now have a broken link and a slightly shortened chain; one end should end in a wide segment, the other in a narrow segment. Turn it so that the rivet at the wide end faces towards you.
Step 7 : Feed the end of the chain with the narrow segment between bottom tension jockey wheel and the tab at the bottom of the derailleur (B), then between the tab and the top guide jockey wheel (A). Don’t go around the outside of the top tab; the shifting still works in a way, but things are noisy! If you have another bike, use it as a reference. Pass the chain around the front of the guide jockey, then over and back to the bottom of the cassette.
Step 8 : Continue around behind the bottom of the cassette, up and forward over the top, and then forward toward the chainset. Pass the chain through the front derailleur. It will eventually have to sit on the chainrings but, for now, pass it around the front of the chainset, then drop it into the gap between the chainset and the frame to give yourself enough slack to rejoin the chain easily.
Step 9 : If you’re refitting a standard chain, ease the two ends together, flexing the chain so you can slide the inner segment of the chain past the stub of rivet sticking through to the inside of the outer plates. Once you’ve got it, though, the stub will make it easy to locate the rivet in the hole in the inner plates, lining the two ends of chain up.
Step 10 : Lay the chain over the furthest away chain supports on the chain tool. Turn the handle clockwise until the pin on the chain tool almost touches the rivet on the chain. Wiggle the chain to precisely line up the pin with the rivet.
Step 11 : Keep turning the handle, while pushing the rivet into the chain, until there is an even amount of rivet showing on both sides of the chain. Remove the tool.
Step 12 : Rejoining the chain usually squashes the plates together and makes the link stiff. free stiff links. Finally, reach around behind the chainset and lift the chain back onto a chainring. Stand up, lift the saddle up with your left hand and push the pedal around with your right foot so that the chain can find a gear.
Source : BIKE MAINTENANCE TIPS, TRICKS & TECHNIQUES