Description of the Problem: The derailleur shifts fine when the chain is on the smallest or largest cogs, but the shifting in the middle of the cassette isn’t satisfactory.
Possible Sources Of The Problem
1. Bent Derailleur Hanger
The derailleur hanger is the connection point between the frame and the derailleur. On modern bikes, the hanger is made of soft aluminum. This is a strategic choice meant to protect the frame.
If the bike falls, the soft hanger bends first and does not transmit stress to the chainstay.
When the derailleur hanger is bent even slightly, the rear derailleur gets out of position and cannot shift accurately.
There are two solutions to this problem:
a. Re-align the derailleur hanger with the help of a derailleur hanger alignment tool.
b. Replace the derailleur hanger with a new one.
The repair option is more expensive at first because an alignment tool isn’t one of the cheapest instruments.
However, in the long run, it can save you money because you won’t have to buy a new derailleur hanger when damage occurs. If you’re often working on different bikes, purchasing the tool makes even more sense.
Note: Older steel frames come with a derailleur hanger that’s part of the frame. In that case, replacing the derailleur hanger is not really an option. Technically, it’s possible to cut it and re-weld a new one, but that would be an illogical action when one can simply re-bend/re-align it. Steel is elastic and allows for similar manipulations.
2. Bent Derailleur Cage
The symptoms of a bent derailleur cage are similar to that of a misaligned derailleur hanger.
Usually, the cage can be re-straighten by eye until the derailleur shifts with sufficient accuracy. In extreme cases, the derailleur or at least its cage will have to be replaced.
3. Worn Cables and Housing
The shifting cables connect the shifter to the derailleur. They move in housing which prevents rubbing against the frame and ensures smooth motion even when the cables bend.
If the cable is old and/or the housing is contaminated, the shifter will be prevented from moving the derailleur sufficiently. The result will be incomplete shifting (the chain not quite getting on the targeted cog).
The most straightforward solution is to replace both the shift cables and the housing.
If the cables and the housing are relatively new, however, they’re unlikely to be the culprit.
4. Improperly Adjusted B-Screw
The b-tension screw is used to adjust the distance between the top pulley of the rear derailleur and the cassette or freewheel.
When the b-screw is screwed in, it pushes against a lip on the derailleur hanger. In consequence, the derailleur “decompresses” and “descends”.
If the B-screw isn’t adjusted adequately, it may create shifting issues.
In most cases, the B-screw gap should be between 5-6mm. If it’s less than that, the derailleur may start hitting the cassette. If it’s more, the derailleur could have a hard time shifting accurately.
5. Broken/Malfunctioning Shifter
Another source of the problem could be the shifter itself. If the shifter is broken, or the shifting cable hasn’t been properly routed through it, the rear derailleur may be moving inaccurately.
6. Cassette Wear
The middle cogs of a cassette are used the most and wear down the fastest. With time, the cogs become thinner and pointer. As a result, the chain begins to slip. This problem becomes even more apparent when the chain is new.
The only solution, in this case, would be to replace the cassette. However, if the cassette is worn, the chain and even the chainrings could also need replacement.
7. Insufficient Cable Tension
If the shifting cable isn’t properly tensioned, it will fail to move the chain to the needed location. As a result, the shifting will be subpar.
To fix this issue, it will be necessary to re-tension the cable and adjust the barrel adjuster. The barrel adjust is a knob found near the rear derailleur or the shifter which finetunes the tension of the shift cable.
8. Stiff Chain Links
If the chain has been shortened, the connection point could be a little stiff. To remedy this issue it will be necessary to examine the chain to see if the links are properly installed. If everything is fine, flexing the stiff area laterally should recover its flexibility.
9. Wrong Chain For The Cassette
If the chain is too wide for the cassette, it will have trouble shifting properly.
With each gear increase, the space between the cassette cogs gets smaller. The cogs get ever so slightly thinner too. The goal of this engineering is to ensure that one hub can accept cassettes of different speeds. This eliminates the need to replace the hub when switching to a cassette with more gears.
The reduced spacing demands a thinner chain. Otherwise, the chain rubs against the adjacent cogs. For example, if the user relies on a 10-speed chain while running an 11-speed cassette, the chain will underperform.
10. Broken or Improperly Assembled Rear Hub
If the rear hub is broken, or it has been improperly re-assembled after a service, it could have serious play resulting in movement of the wheel and the cassette. The unstable cassette will prevent the chain from moving to the needed cog accurately.
11. Rear Wheel Moving In The Dropouts
If the rear wheel hasn’t been securely installed, it may be moving in the dropouts when the rider exerts a lot of force. As a result, the cassette will also move out of place, and the derailleur will be prevented from shifting accurately. This scenario could be the case only when the bike has forward-facing dropouts. If the dropouts are vertical, the wheel has nowhere to move.