How to repair a rear derailleur?
Before we can repair a rear derailleur we need to understand what one is and how it works. This understanding helps us to retain the information and allows us to complete the repairs more easily and with less help next time.
What is a rear derailleur?
Although there are lots of different rear derailleurs and lots of different brands, the fundamentals behind how they all work is the same. The rear derailleur is a mechanism which transfers the chain from sprocket to sprocket up and down a cassette. The rear derailleur attaches to the frame of a bike via a rear hanger or rear derailleur hanger.
HOW DOES A REAR DERAILLEUR WORK?
There are two important factors to think about when learning how a rear derailleur works. These are cable tension and limit positions.
Cable tension
On a ‘mechanical’ type rear derailleur, the rear derailleur is attached to a cable by a pinch bolt, this cable is connected to a gear lever at the other end to the derailleur. When the cable is pulled or tensioned by the gear lever the rear derailleur is pulled up the cassette. When the gear lever is pulled its moves by the way of indexing, so it pulls a set amount of cable which moves the rear derailleur a set amount. This set amount will be the distance between the sprockets on the matching cassette. This is why you can’t normally mix and make different brands, for example Shimano is not compatible with Campagnolo.
Every mechanical type rear derailleur contains a return spring within the linkages of the derailleur. This spring is always trying to force the derailleur back down the cassette to the smallest sprocket. This is how the derailleur changes from an easier/bigger gear to a harder/smaller gear. When the gear lever is indexed into an easier gear the tension on the cable is released by a set amount again. Due to the force of the return spring the derailleur indexes down the cassette a set amount, this set amount will be the distance between the sprockets on a matching cassette. This a how rear derailleur changes gear.
Limit screws
Limit screws, like the name suggests, limit how far the rear derailleur can travel inwards and outwards. There are two limit screws and they are called the L-screw and the H-screw.
The L-screw is used to set the lower limit of the derailleur, making sure that the upper jockey wheel of the derailleur is directly under the smallest sprocket on the cassette when there is no tension on the cable pulling the derailleur up.
The H-screw is used to set the upper limit of the derailleur, making sure that the upper jockey wheel of the derailleur is directly under the biggest sprocket on the cassette when there is maximum tension on the cable pulling the derailleur up the cassette.
How to repair a rear derailleur?
Now that you know the fundamentals of a rear derailleur, here’s Chuck putting those points into action and showing how to repair and tune a rear derailleur.
This video is from the Bicycle Maintenance Guide app. The app is made up of 60 videos containing over 7.5 hours of footage, 180+ images, over 130 sections and 50,000 words. All things road bikes, MTB’s, gravel bikes and E-bikes are covered. The app is available in both iOS and Android.
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