Loose B-Screw
The causes the rear derailleur to rattle and suggests that the "trim" is off. It has happened several times on my 105 R7100 group set.
A rear derailleur B-screw loosening on its own is typically caused by vibrations overcoming the light spring tension, a damaged or worn screw thread, or the screw not resting securely against the hanger tab. Apply a small amount of low-strength Loctite 222 (purple) to the threads to secure it while still allowing future adjustments.
Common Reasons and Fixes
If you are adjusting your derailleur B screw, it is recommended to adjust the B screw while holding the derailleur body back to reduce tension on the screw.
The B-screw acts as a solid limit screw, designed to be adjusted against the derailleur hanger tab to set the gap between the upper pulley wheel and the cassette.
The causes the rear derailleur to rattle and suggests that the "trim" is off. It has happened several times on my 105 R7100 group set.
A rear derailleur B-screw loosening on its own is typically caused by vibrations overcoming the light spring tension, a damaged or worn screw thread, or the screw not resting securely against the hanger tab. Apply a small amount of low-strength Loctite 222 (purple) to the threads to secure it while still allowing future adjustments.
Common Reasons and Fixes
- Vibration/Lack of Thread Friction: Use Loctite 222 (purple), nail polish, or light super glue on the threads.
- Worn Threads/Hanger Tab: If the hole is stripped or the derailleur hanger tab is damaged, the screw won't hold. A new derailleur hanger may be required.
- Wrong Position: Ensure the screw is resting properly on the tab of the derailleur hanger.
- Chain Length: If the chain is too long, the derailleur cage can sit in a position that reduces tension on the B-screw.
If you are adjusting your derailleur B screw, it is recommended to adjust the B screw while holding the derailleur body back to reduce tension on the screw.
The B-screw acts as a solid limit screw, designed to be adjusted against the derailleur hanger tab to set the gap between the upper pulley wheel and the cassette.
- Function: It adjusts the B-gap/tension, allowing the derailleur to clear the largest cog.
- Design: On the R7100, it is a simple, direct-contact screw rather than a spring-loaded tensioning screw, as modern Shimano derailleurs utilize a different design to handle wider gear ranges.
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