In order to serve us and fine-tune the rear derailleur, we first need to understand how it works.
Below, Calvin Jones here, Parkpool Company.
The rear derailleur is a mechanism that shifts the chains at the rearrest pockets.
There are many different models, but they all share the same basic design and functionality.
Derailiers are secured to the bike with a mounting bolt to the frame's derailleur hanger.
The upper pivot of the derailleur is connected to the lower pivot by a linkage system that forms a parallelogram.
derailleurs use a cage fitted with two pulleys.
The lower pulley pulls back to give tension to the lower section of chain.
It is called the tension pulley or T pulley for short.
The upper pulley guides the chain to the sprockets and it is called the guide pulley or G pulley.
On mechanical-type derailleurs, a cable is secured at the pinch bolt.
When the cable is pulled, the cage is moved inward.
A return spring in the linkage will move the derailleur in the opposite direction when the cable is released.
With electronic shifting systems, an electrical signal triggers a motor to move the linkage.
There is no cable involved in these systems.
A derailleur cable passes through the housing to a shift lever.
Bikes commonly use an index shifter that moves the cable in small predetermined increments Pulling or releasing the cable moves the guide pulley.
Barrel adjusters can be located at the derailleur body or the shift lever.
They are used to correctly adjust the shifting between cogs.
Unthreading or turning the barrel adjuster counterclockwise effectively shortens the cable.
table, moving the G pulley to the left, or inward.
Turning the barrel adjuster clockwise effectively lengthens the cable.
This moves the G pulley to the right, or outward.
Derailies are fitted with limit screws, which are adjusted to stop the derailleur from shifting too far inward, or too far outward.
We can actually see what the limit screws are doing in this model.
The end of the limit screw strikes the linkage and this stops the pulley travel.
The L screw determines the limit of the inner most cage travel.
Tightening the L screw will restrict the limit to the left side.
The H-crew determines the limit of the outermost cage travel.
Tightening the H-limit screw will restrict the limit to the right side.
It is common to find another adjustment screw on the derailleur body called a B-screw.
This controls the size of gap between the guide pulley and the sprockets.
Those are the basics of the rear derailleur.
Now that you have an understanding of how it works, watch this video to learn how to properly adjust your derailleur.
To see all the derailleur videos in one place, check out the Park Tool Guide to derailleurs.
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