Skipping gears, chain not moving to correct gears and the chain jumping off are good examples of the derailleurs requiring attention. This guide will hopefully help you to understand how the derailleurs operate and how to set them up so they work as they should.
To make cycling more energy efficient a bicycles chain can be moved (derailed) between a number of gears/sprockets. The size of the sprockets determines how quick you must pedal and at the same time how much energy is required for each rotation of the crank arms. By selecting a lower gear, the gear shifter cable adds tension to the derailleur forcing it to change up to a larger sprocket. This reduces the amount of force required by the cyclist, however the distance travelled is also reduced - this is perfect for climbing hills. The opposite effect is given by selecting a high gear, when the shifter cable tension is released allowing the derailleur to drop the chain to a smaller sprocket using its internal springs. This is great for downhills and straights.
Adjusting the Derailleurs
The rear derailleur must be functioning correctly in order to adjust the front derailleur successfully.
You're going to need:
Adjusting Rear Derailleurs
There are five variables on the rear derailleur including a b-tension adjuster (B screw), a high gear limit stop (H screw), a low gear limit stop (L screw) and two cable barrel adjuster - one on the rear derailleur (C screw) and the other on the handlebar shifter.
B-Tension Adjuster: This controls how close the derailleur sits in relation to the selected gear sprocket. The body of the derailleur should be close to the sprocket but not close enough to touch it. If it is touching the sprocket, you need to increase the tension by rotating the B screw clockwise until it just misses (using quarter-turn increments). If the body is quite far away from the sprocket you need to turn the B screw counter-clockwise to release some tension. In this case turn the B screw until the body touches the sprocket and turn clockwise so that it just misses.
High Gear Limit Stop: This prevents the chain from jumping off the smallest sprocket (the highest gear). If you need to, adjust the cable barrel adjuster (C screw) to release any tension from the gear shift cable. If you position yourself behind the rear wheel and look at the chain moving on the smallest sprocket, the chain should be running central to the lowest sprocket. If it's trying to jump into the axle, rotate the H screw clockwise (using quarter-turn increments). If it's trying to jump on to the larger sprocket then rotate the H screw counter-clockwise (using quarter-turn increments). You can now adjust the C screw to add tension to the gear shift cable so that the next gear down is engaged when shifted.
Low Gear Limit Stop: This prevents the chain from jumping off the largest sprocket (the lowest gear). Shift down to the lowest gear and look to see if the chain is set central to the sprocket. If the chain looks like its jumping into the spokes of the wheel then turn the L screw clockwise (using quarter-turn increments) until central to the sprocket. If it's trying to jump down to the next sprocket, turn the L screw counter-clockwise (using quarter-turn increments) until central.
Cable Barrel Adjuster: This changes the tension of the gear shifter cable which determines how much the derailleur moves when changing gear. Take a look at the gears from the rear of the bike after working through the gears first going up two gears, down one, and then down two gears, up one. Make sure the chain is sitting central to each sprocket at each step. If the chain looks as if it wants to jump to the larger sprocket, the gear shifter has too much tension. To reduce the tension, turn the C barrel adjuster clockwise. If the chain wants to fall to the smaller sprocket you need to add tension so turn C counter-clockwise. You can also use sound to adjust the tension. If you can hear a metallic sound then there's probably too much tension - turn clockwise, and if you hear clicking then there's probably too little tension - turn counter-clockwise. On most gear shifters there is another barrel adjuster so that you can make minor adjustments whilst riding.
Once the rear derailleur is working as you'd like it remember to lubricate all of its moving parts - this will ensure smooth gear shifting and prevent corrosion and wear.
Adjusting Front Derailleurs
Ensuring the rear derailleur is setup correctly first, you can now adjust the front derailleur. There are five adjustment variables including height, angle, low gear limit stop, high gear limit stop, and the cable barrel/bolt adjustments.
Height Adjuster: Firstly the cage height and angle must be set. For this you will need to loosen the clamp that attaches the derailleur to the frame. This adjusts both height and angle of the derailleur. Manufacturers usually instruct you to leave about 2mm of space between the underside of the derailleur cage and the top of the teeth of the largest sprocket, but the closer the better as long as it's not touching. Make sure none of the cage is touching the sprocket and partially tighten so that the clamp holds the derailleur's weight.
Angle Adjuster: This can require a bit of patience as the cage isn't always straight. If you look at the derailleur from above and imagine a centreline running through the middle of the derailleur cage - this needs to be lined up parallel to the centreline of the frame. When you've got it, tighten the clamp fully.
Low Gear Limit Stop: You should be able to see two screws sat side-by-side on top of the derailleur body. There should be one or two markings (L and/or H) on the plastic next to the screws. The position of the L and H screws has changed on more recent derailleurs - we need to adjust the L side first. This adjuster prevents the chain from jumping off the smallest sprocket into the bottom bracket. So change the gears so that the chain sits on the largest rear sprocket and the smallest front sprocket. The inside of the cage of the derailleur should just miss the chain on the side closest to the frame. If this is too tight for the derailleur, just adjust the L screw in quarter increments until the chain sits nicely in the centre of the sprocket.
High Gear Limit Stop: This prevents the chain from jumping off the largest sprocket onto the pedals. Change to the bikes highest gears, so select the smallest sprocket on the rear and the largest sprocket on the front. Now using the H screw adjust the derailleur cage so that the inside of the cage (nearest the pedal) just misses the chain - the chain should sit centred on the sprocket.
Cable Barrel/Bolt Adjusters: These adjust the tension of the gear shift cable so that the chain has enough slack to change gear but not too much so that it jumps of the sprockets. Change the gears back to the largest sprocket on the rear and the smallest on the front. Now loosen the bolt that clamps the cable to the front derailleur whilst holding the cable, pull the cable to take out any slack and tighten again. Change to the middle sprocket on the front and move through each gear on the rear ensuring the chain doesn't keep hitting the front derailleur cage. Use the barrel adjuster on the front gear shift lever to fine tune if the chain does hit.
You're gears should now be working properly - if they're not it's nothing to do with us! If you've tried everything we've suggested above and you're still having problems then I'd probably take it in to your local bike shop to get checked over.