4.Angle adjustment (“B-tension”)
Modern derailers have two spring-loaded pivots. The lower pivot, sometimes called the “a pivot” winds the cage up to take up slack as you go to smaller sprockets. The upper “b pivot” adds additional slack take-up ability by pushing the derailer’s parallelogram backwards.
The tension of the two springs needs to be balanced for best shifting.
Most derailers have an angle adjustment screw (Shimano calls it “B-tension adjustment”). This adjusts the tension of the upper (“b”) spring of the parallelogram, and thus the height of the jockey pulley. The looser this screw is, the closer the jockey pulley will be to the cluster.
The angle adjustment will need to be set according to the size of the largest rear sprocket. If you change to a cluster with a larger or smaller low-gear sprocket, you will need to re-adjust this setting. You will also need to adjust this if you change the length of your chain.
If the angle adjuster is set too loose, the jockey pulley will bump into the largest sprocket when the bicycle is in the lowest gear (large rear, small front). This is the gear you should check the adjustment in. A larger low-gear sprocket may require a different rear derailer, for enough angle adjustment to clear the sprocket. In extreme cases, such as with a Shimano 36-tooth sprocket, a longer angle-adjustment screw may be needed — some people even install the screw backwards.
Since a derailer shift is caused by forcing the chain to run at an angle, the greater the angle, the sooner it will shift. The closer the jockey pulley is to the cluster, the sharper the angle will be for a given amount of sideways motion of the derailer. Thus, the looser the angle adjuster screw is, the better the shifting will be.
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