|Front Derailleur Adjustment
The front derailleur simply shoves the chain off one front chain ring and onto another ring. The cage surrounding the chain is pulled in one direction by the inner wire. A spring in the derailleur returns the cage to the other direction when the inner wire is relaxed. A properly adjusted front derailleur should shift the chain between the front chain rings but will not throw the chain off the rings. The basic adjustments for the front derailleur are the height, rotation, limit screws and inner wire tension (index setting).
It is generally a good idea to lubricate the pivot point of the front derailleur with a light lubricant. The cable inner wires should also be lubricated. If you have loosen the inner wire pinch bolt, lubricate those threads as well. When tightening this pinch bolt, secure to approximately 30-40 inch-pounds. This is about 10 pounds of effort hold a wrench 3 inches from the bolt.
|Front Derailleur Height Adjustment
Before checking limit screw adjustments, check the derailleur positioning. If the derailleur cage is too far above the large chain ring, it is more likely to shift poorly. If the derailleur is too low, it may scrape against the chain rings or jam the chain when shifting.
Some front derailleur models do not have height or rotation adjustments. These models mount on a plate and both height and rotation settings are pre-set. Additionally, some bikes have a bracket commonly referred to as a "braze-on" for front derailleur mounting. This bracket will allow limited height and rotational settings.
|Front Derailleur Rotational Angle
The front derailleur cage should be approximately parallel to the chain. If the derailleur cage is rotated too far from this position, it will shift poorly. If the derailleur mount is a clamp type, its rotation can be changed. Generally, the outer cage of the front derailleur should be aligned to the chain. The middle image below shows adequate alignment.
NOTE: Some front derailleur models do not have height or rotation adjustments. These models mount on a plate and both height and rotation settings are pre-set. Additionally, some bikes have a bracket commonly referred to as a "braze-on" for front derailleur mounting. This bracket will allow limited height and rotational settings.
Front Derailleur Limit Screw Settings
Limit screws stop the inward and outward travel of the front derailleur cage. Limit screws are marked "L" and "H". The L-screw will stop the motion of the derailleur toward the smallest chain ring. The H-screw will stop the motion of the derailleur toward the largest chain ring.
If the limit screws lack any marking, you will need to determine which are the "H" and "L" by testing. Begin with the chain on the smallest cog. Relax the cable tension completely using the cable barrel adjuster. Place a hand on the derailleur body to feel for any lateral motion, and select one screw to turn one full turn clockwise and then counter-clockwise. If you felt motion in the derailleur, this is the "L" screw. If there was no motion, do the same test with the other limit screw. You should consider marking the "L" screw permanently. The other screw by default is the "H" screw.
Although the limit screws will stop the derailleur, it is the inner wire and derailleur spring that make the derailleur move. If the inner wire has too much tension, the derailleur will not rest on the L-screw stop. If the inner wire tension were to change, the derailleur inner limit would also change, possibly causing the chain to fall off the rings.
|Inward Travel Adjustment
The L-screw controls the inward most travel of the front derailleur. Set adequately, it allows the chain to shift to the smallest ring, but not beyond so it falls off.
|Outward Travel Adjustment
The outward travel of the front derailleur is stopped by the H-screw. When viewing the H-limit adjustment, make sure there is enough tension on the inner wire by either keeping extra pressure on the lever, or by pulling the exposed inner wire taut by hand. Use a rag to protect your hand if pulling the inner wire. After the H-screw is adjusted the inner wire should be re-tensioned to assure proper shifting.
|Index Setting - Triple Chain Ring Cranks
Some front derailleur systems have an index setting. If the shift lever has three distinctive stops or clicks, it is indexing. If the front shift lever is friction, there is no index setting. If the front shift lever has multiple clicks, such as some twist grip style shifters, it is shifted similar to friction levers. Set indexing only after completing all adjustment above.
|Index Setting - Double Chain Ring Cranks
Some front derailleur systems have an index setting. If the shift lever has two distinctive stops or clicks, it is indexing. If the front shift lever is friction, there is no index setting.
Front Derailleur PerformanceThe front derailleur should shift the chain between chain rings without throwing the chain off the extreme outer and inner rings. It is possible that the front derailleur will rub the chain slightly even on properly adjusted bikes. This is likely on some bikes when riding in the largest sprocket in front and the smallest cog in back. As the bike is pedaled with force the frame flexes and moves the chain rings side to side, which cause an intermittent rubbing noise. Loosening the H-limit would move the front cage out more, and may stop the rubbing, but it may also cause the chain to shift over the largest ring and come off. If all aspects of front derailleur adjustments are correct on this bike, the rider is simply exceeding the engineering and design capabilities of the machine.
|Information and photos are courtesy of Park Tool.
Visit their site at ParkTool.Com