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Tension refers to the amount of pull the derailleurs and shifters apply to the cables. If the rear derailleur tends to skip a gear when shifting to an easier gear, there is too much cable tension and the barrel adjuster (usually found where the cable meets the shifter or derailleur) should be screwed clockwise. Or the cable should be loosened by the cable clamp. Always do this in the smallest gear. If the rear derailleur skips when moving to a harder gear, then there is too little tension and the barrel adjuster should be screwed counter-clockwise until the chattering stops.
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If the front derailleur cannot shift into a harder gear, then tension must be added by resetting the cable or turning the barrel adjuster counter-clockwise (vise versa for down shifting). The cable is set correctly when the chain does not rub on the front derailleur while in the middle chain ring with any gear combination from the rear derailleur. If you know the cable has proper tension, then it might be a limit problem.
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The limit screws control how far a derailleur can move. There is one for the highest and the lowest gear on the front and rear derailleur. Tightening these screws will make the derailleur move inward, loosening will make the derailleur go out. Adjust the rear derailleur so it sits under the lowest, then the highest gear so the chain does not jump or chatter. The front derailleur works the same way.
- If there is excessive chattering or a clunky feeling in the easiest gear ratio, and the limits and tension are OK, then the B-screw will need to be screwed in. It is found on the back of the rear derailleur, touching the derailleur and derailleur mount, and it controls how close the derailleur's pulley wheels sit to the gear cluster. If the chain skips in the hardest (smallest) gear and the pulley wheels are more then 2 cm from the gear cluster, then the B-screw might need to be screwed out.
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The front derailleur should be positioned so it is parallel to the chain rings. If it knocks against a chain ring while shifting, it should be raised. If it cannot shift from the big chain ring to a smaller one, and the cable tension and limits are properly set, then the derailleur might need to be lowered.
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If you are in the smallest gear on the front and rear, and the rear derailleur is completely stretched out but there is still some slack in the chain, then the chain is too long. If you cannot get the rear derailleur to extend in the smallest gears with no tension and the limit screwed out, but the chain is tight, then the chain is too small. If you have a single-speed or internally geared hub and the chain falls off, then it is too large or your gear's teeth have eroded away.
