Demount Or Release The Cable
Some people prefer to set a limit screw before the cable is connected, or with the cable demounted so it can’t pull. That means even if the cable breaks in use there is no risk of the chain shifting off the cogs, although this is an optional step.
Pedal the bike gently, push against the rear derailleur to shift into a lower gear. When you release the derailleur the chain should drop back promptly on to the highest gear. Turn the pedals faster and let the chain drop suddenly to be certain it won’t move too far right and jump over the smallest cog and fall off. Turn the pedals backwards and the chain should run smoothly through the derailleur when the derailleur is in it’s default – highest gear – position.
If you need to make further adjustments move the limit screw a quarter turn at a time and check again after each adjustment. That way if you go too far you know how much to go back.
Low Limit – Diagnose
Pedal the bike gently and push the derailleur towards the lowest gear position.
If the chain won’t go into first gear – the biggest cog – you need to turn the ‘L’ limit screw anti-clockwise to allow it to move further. If the arm of the derailleur starts to rattle on the spokes or the chain goes over the biggest cog and falls off you need to turn the ‘L’ limit screw clockwise to limit the derailleur's range of movement. As you get closer to the target make small defined adjustments so if you go too far you know how much to go back.
Some bikes are equipped with a plastic disc between the first gear cog and the spokes, this is a ‘spoke protector’ sometimes called a ‘dustbin lid’ or a ‘pie plate’. If the ‘L’ limit screw is kept in the right place you don’t need it. It can be removed by taking off the cassette or freewheel and unclipping it from the spokes.
Some derailleur work in the opposite direction. The spring pushes towards the low gears and the cable pulls toward the high gears. If you have one of these the process is roughly the same. Start by positioning the ‘L’ limit screw then connect the cabl(link) to pull the rear derailleur right to check – and if necessary adjust – the ‘H’ limit screw.