Derailleur Limit Screws (H-screw and L-screw) - Ballyhoura Mountain Bike Trails

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Derailleur Limit Screws (H-screw and L-screw)

Tuesday, 17 July 2007

Rear derailleurs "derail" or shove the chain off of one sprocket and move it to another. The upper derailleur pulley (called the "guide pulley") pushes the chain to the next sprocket. A proper gear adjustment aligns the guide pulley under the sprocket. Changes to the inner wire tension causes movement in the derailleur. The derailleur body is fitted with a spring that is pulled tight, or relaxed, by the inner wire. Pulling the inner wire moves the derailleur cage and guide pulley in one direction and tightens the spring. Relaxing inner wire tension allows the spring to move the body and pulley in the opposite direction.

It is normal for a chain to make some noise during a shift. The shift may appear subjectively "noisy," "loud," or "rough". Factors like the type of chain or sprocket, the wear on each, and the amount and type of lubrication will affect the noise a chain makes during shifting. The limit screws typically can do nothing to affect the noise during a shift between cogs. Noise from the chain as it rides on the sprocket is, however, a useful symptom. There is for any given bike a "base level" of noise from the chain as it passes over the sprocket teeth. When the derailleur jockey wheel is out of alignment, the chain may make excessive noise. To demonstrate the "base level" noise, shift the bike to the second sprocket by pulling the inner wire. Continue to pedal and move the inner wire slightly to hear changes in the level of noise. The quietest level of noise may be considered the base level for that bike.

Derailleur pulleys are limited in both inward and outward motions by using the derailleur limit screws. Properly set, the derailleur will shift to both the extreme outward sprocket (the smallest in size) and the extreme innermost sprocket (the largest in size). The limit screws do not control the derailleur on the sprockets between the two extremes. These are set using the barrel adjuster and tension on the inner wire. The limit screws are usually marked "H" and "L". The "H" controls the outer most limit of the derailleur, and the "L" controls the inner most limit.

Using the shift lever to adjust limit screws can cause confusion and problems because it tends to focus attention on the inner wire tension (indexing) rather than limit screw settings. Instead of using the shift lever, pull the inner wire with one hand to simulate shift lever action. This will help eliminate confusion between indexing problems and limit screw problems. With the bike in a stand, practice shifting with this method before adjusting the limit screws.

Turning the limit screws adjusts the limit of travel of the pulleys. Tightening restricts the travel, while loosening allows more travel. The purpose of the following procedure is to find the tightest H-limit screw setting that will allow a good shift to the outermost cog, and the tightest L-screw setting that will allow a good shift to the innermost cog. The location of limit screws on the derailleur body may vary between manufacturers. Always look for the "H" and "L" marked adjacent to the screws.

H-LIMIT SCREW

A. Shift chain to outermost (largest) chainring. Shift chain to outermost rear sprocket (smallest sprocket).

B. Check tension on rear inner wire. If inner wire appears to have any tension, it may interfere with the H-screw setting. Turn adjusting-barrel clockwise to eliminate inner wire tension. Proper cable tension (indexing) will be adjusted later.
C. Pedal bike at a quick cadence, approximately 60 rpm or more.
D. Pull inner wire to shift derailleur one sprocket inward. Adjust pull on inner wire until chain rides quietly on second sprocket. Release inner wire quickly to shift back to outermost sprocket and note shift. When adjusting the H-screw, be concerned with two situations:
1. The outward shift from the second sprocket to the outermost sprocket.
2. How the chain rides on the outermost sprocket.
Do not be concerned with how the chain rides when it is held on the second sprocket.
E. If the shift outward seems acceptable, tighten H-screw 1/4 turn clockwise and repeat shift. Even if shift appears acceptable, continue tightening H-screw by 1/4 turn increments and checking shift until shifting is slow or hesitant. Another symptom of a too tight H-limit screw is when the chain is on the smallest cog but makes a rattle from rubbing the second sprocket inward. View this last symptom by looking under the rear sprockets where the chain meets the sprockets. The inner plate of the chain will rub against the next sprocket inward making the noise.

F. When symptoms of a too tight H-screw appear, loosen H-screw 1/4 turn and check shift again. Repeat process of shifting and correcting by 1/4 turn increments. When too tight symptoms disappear, H-screw is at tightest acceptable setting, and limit screw setting is done.

NOTE: "Rapid Rise" or "Low-Normal" derailleurs use a reverse spring application. When the inner wire tension is completely relaxed, the derailleur sits on the inner most sprocket, which is reversed from other common derailleurs. When adjusting the H-limit screw, it is necessary to pull the inner wire until the chain is in the second to outermost sprocket then pull hard to shift to the outermost sprocket. The same concept is used to adjust the Shimano® "Rapid-Rise" or "Low-Normal" derailleurs. You want the tightest limit screw setting that allows good shifting to the extreme outer and inner cogs.

L-LIMIT SCREW

The L-limit stops the derailleur from moving inward (toward the spokes). The limit screw does not make the derailleur move, pulling the inner wire makes the derailleur move. The L-screw allows the pulley wheels to shift the chain to the innermost sprocket and yet not shift off the sprocket into the spokes.

A. Shift bike to middle chainring (or smaller ring of double chainring bikes) and to second-to-innermost rear sprocket.

B. Pedal bike at a quick cadence, approximately 60 rpm or more.

C. Pull inner wire by hand to shift derailleur sprocket inward. When adjusting the L-screw, be concerned with two situations:

1. The inward shift from the second-to-innermost sprocket to the innermost sprocket.
2. How the chain rides on the innermost sprocket.

D. If shifting seems adequate, tighten L-screw 1/4 turn, and repeat shift. Continue to tighten L-screw until symptoms of too tight appear. These symptoms are that the chain will not complete shift even with pressure on inner wire, the chain hesitates before shifting inward even with constant pressure on inner wire, or the chain rattles excessively when riding on innermost sprocket

E. When symptoms of a too tight L-screw appear, loosen L-screw 1/4 turn and check shift again. Repeat process of shifting and correcting by 1/4 turns. When too tight symptoms disappear, L-screw is at tightest acceptable setting, and limit screw setting is done.

B-Screw Adjustment

After setting the L-screw, check the "B-screw" for an adequate setting. The B-screw controls the derailleur body angle, hence the name, B-screw. Shift to the innermost rear cog, which is the largest cog. View the upper pulley relative to the largest cog. If the pulley is rubbing against this cog, tighten the B-screw to increase upper pivot spring tension, which pulls the pulley back and away from the cog. If there is a large gap between the upper pulley and cog, loosen the screw.

Rear Derailleur Changes Skips from Gear to Gear

There are several things that can cause the chain to skip.

1- Rear derailleur hanger is bent. The rear derailleur secures into a tab on the frame called the derailleur hanger. It can become bent from crashing, falling over, and just knocking it about. It should not bend from "normal" riding. See bent hanger.

2- Rear derailleur cage is bent. It is possible in some cases to align these cages, take it to a good shop. Some models of derailleurs have replaceable cages, while other require the entire unit be replaced with a new derailleur.

3- Gear cables are sticky from dirt. Clean and lubricate cables and housing.

4- Gear cable housing is cracked, dirty, or bent. Replace cables and housing.

5- Chain brand or model is not compatible with your gear. Switch to a different brand.

6- Chain is worn out. Check with Park Tool CC-2. Replace as necessary.

7- Rear cassette sprockets are worn out and need replacement.

8- Shift levers are not compatible with your rear derailleur.

9- The gear cable is not correctly mounted to cable pinch bolt.

10- Tight links in chain. See Fixing Tight Links

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