There are two different types of gear changer. Those fitted to the right hand side of the handlebar control the rear gear. This moves the chain across the six, seven, eight or nine sprockets on the back wheel. To help you keep track of which gear you are in, they are usually fitted with an indicator. When the rear changer is working correctly, gear changes are almost silent and go through very quickly. However, it’s always best to change gear well before you start struggling to keep the speed up. It also helps to take
a little pressure off the pedals and change a maximum of three gears at a time.
If a changer is fitted to the left hand side of the handle bar, this controls the front chainwheel gears.
Don’t try to change gear when the bike is standing still or coasting downhill. In addition, don’t try to take a gear changer apart, just give them a quick squirt of multi purpose lube over the exterior of the moving parts and then wipe off the surplus. 
1. On some bikes, the lever for changing gear upwards has a large thumb grip. To change up, reduce the pressure on the pedals but keep them turning. Then push the lever once or more, depending on how many gears you want to shift.
2. To change downwards, hook your forefinger round the bottom lever, pull it upwards until it clicks and then release it. If you want to change more than one gear, pull the lever two or three times but again, keep pedalling. while you do so.
3. If you find it awkward to use this type of changer, or cannot see the gear indicator, try adjusting its position on the handlebars. Just loosen the bolt on the handlebar clamp two turns and twist the whole changer assembly.
When your gears are running properly, you should be able to select any gear first time. There should be almost no noise, either from the chain running over the sprockets or when you change gears. If there’s a clicking or a clacking noise in any gear, try tightening the cable slightly.
Try lubricating the gear cable as well.
On the other hand, if the chain starts to drop off the chainwheel or it feels as if it’s trying to jump off the largest or the smallest sprocket, clean the chain and both gear mechanisms with a degreaser. Then check all the adjustments, as shown. here.
This is also a good time to check the jockey wheels for wear. On Shimano, the top jockey wheel should have a little bit of sideways movement but the bottom one should not.
If you can’t get the gears to change smoothly and precisely, get your mechanic to check if the cables need replacing and that the gear mechanisms and frame are not damaged in any way.
REMEMBER, AT THE REAR WHEEL: The smaller the sprocket, the higher the gear. So adjust the screw marked H for High. The larger the sprocket, the lower the gear. So adjust the screw marked L for Low.
BUT AT THE CHAINWHEEL: The larger the chainring, the higher the gear. So adjust the screw marked H for High. The smaller the chainring, the lower the gear. So adjust the screw marked L for Low.
To set up Shimano gears correctly, let the chain down onto the smallest sprocket. Try turning the H adjuster either way until the chain runs almost silently when you turn the pedals. Then undo the H screw another half turn anti-clockwise.
Test the change from the smallest sprocket to the next. It should click up and down without delay. If it doesn’t, give the cable adjuster half a turn anti-clockwise. Then test the top to bottom change and adjust the L screw if necessary.
Next, turn the pedals slowly and use your thumb to push the gear inwards against the spring, lifting the chain up onto the big sprocket. Adjust the L screw until the chain runs almost silently, then let it jump back down onto the smallest sprocket.
Keep on increasing the cable tension half a turn at a time until the top to second change works really well going both ways. Finally, flick up and down the whole range of sprockets several times, as fast as you can, just to check.
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