Bicycles/Maintenance and Repair/Derailleurs/Rear Derailleur Adjustment

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Bicycles

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Figure 1: Shimano Ultegra rear derailleur

Introduction

A rear-derailleur is the gear-changing assembly on the rear wheel of a bicycle; it moves the bicycle chain from one sprocket to another.

It is cable-operated using the handlebar shift control. The derailleur allows the rider to choose the ease of pedalling, for example, for hill climbing or a downhill run. For those who take things a bit more seriously, gearing allows the rider to maintain the most economical pedalling rate, regardless of minor changes in terrain.

Some bicycles have both front and rear derailleurs. This page concentrates on the rear-derailleur .

Because the 'de-railing' of the chain involves deflecting it, the chain has to be moving at the time. That is to say, a derailleur can work only while the rider is pedalling.


[edit] Terminology

To better understand the material that follows, consider the following points:

Figure 2: Typical locations of adjustment points
  • The cogs in the rear wheel of a bike are often referred to as sprockets, and the ones on the pedal area are called chain-rings.
  • When there is more than one cog they are arranged in a stack called a cluster or a cogset; these come in two forms: cassettes, which do not include a ratcheting mechanism, and freewheels which include a ratcheting mechanism. When installed these are hard to tell apart, but once off the hub the difference is clear.
  • The highest sprocket in a cluster is the small one closest to the outside of the bike, and the lowest sprocket is the largest one nearest to the inside of the bike. These points have meaning for adjustments.
  • The top pulley of a rear-derailleur is called the guide-pulley.
  • The lower pulley is called the take-up pulley or tension pulley.
  • The dropout is the slotted part of the bike frame that the wheel fits into.
  • The hanger is the thin mounting plate for the derailleur. It is fixed to the dropout.
     


The overall gearing of a bike is set by both the choice of front chain-ring and the rear sprocket. Simple bikes have only a single chain-ring on the pedal assembly so all of their gear shifting is done on the rear wheel. This is the most common case, so this page concentrates on that situation.

The manufacturer's documentation for a derailleur is by far the best source of information, but when it cannot be found either in the bike handbook or on the internet, refer to the adjacent drawing (Figure 1 ) for the usual adjustment points, and the procedures that follow.

[edit] Preliminaries

Make sure before carrying out adjustments that the derailleur hanger is not bent. The hanger is the part that the derailleur threads into; on steel bicycles it is part of the frame; on aluminium bikes it is a separate piece. If the hanger is bent it can be seen by viewing the bike from the rear; the derailleur should hang vertically, perpendicular to the wheel hub and not bent inward. The smooth running of the drive depends on the hanger being straight, and even a small bend can upset adjustments and give an unsettled feeling to the drive.

Repairing the hanger: On steel bicycles the only option is to bend the hanger straight, though aluminium hangers can also be straightened if they are not too severely damaged. It is important to keep in mind that aluminium fatigues much more easily than steel. As a rule of thumb if the hanger doesn't break during the repair (a real possibility) it is good to be ridden on. Any professional bicycle shop will have the proper tool for this (the Derailleur Alignment Gauge from Park is recommended), but it is unlikely that the home mechanic will want to make such an investment. Hanger repairs are unlikely to be adequate without the right tool for the job and indexing might not work properly.

Replacing the hanger: On aluminium bicycles the hanger is a cheap sacrificial piece, being designed to bend on impact instead of the expensive derailleur. Unfortunately bicycle frame manufacturers have make numerous hangers over the years, and no bike shop has more than a small fraction of them in stock. However, if the part is available, the replacement is easily done at a bike shop, or even at home, and is comparable in tool use and difficulty to the installing of the derailleur itself.

Sometimes a hanger is not bent enough to be visibly damaged, but can still adversly affect the indexing. If after following the instructions on this page the indexing is not working properly, and all parts are compatible, then checking the hanger alignment with the proper tool is the first problem solving step.

It may be helpful to note that the adjustment procedures on this page are devised for the most common shift-cable behaviour. That is to say, the assumption is that shortening the cable results in moving the rear-derailleur inward, toward the largest sprocket. Other arrangements exist, and for these it will be necessary to refer to the manufacturer's instructions.

[edit] Adjustment Summary

There are three main adjustments for a typical rear-derailleur: These are;

  • The limit stops, high and low, are adjusted independently of the indexing and cable, to prevent any movement beyond the high and low sprockets in the cluster.
  • The 'B' adjustment, the body-angle screw, is made to move the upper pulley and chain to a position close to the underside of the largest sprocket.
  • The indexing barrel-adjuster is set to make the indexing align properly with the individual gears.
     


[edit] Set the Chain Limits

Figure 3: Another screw layout

As the jockey wheel moves back and forward across the rear cassette to change gears, it could be pushed beyond the intended limits. To avoid this, the limits are set unconditionally, that is to say, independently of anything being done by the cable or the indexed gear-shifter on the handlebars.

On every derailleur will be found a pair of screws, closely-spaced and typically marked as 'H' and 'L', for "high" and "low" respectively. These screws are the limit-stop screws, and serve to establish the physical limits of travel. Their exact locations are best found in the bicycle handbook or in documentation for the derailleur, though the manufacturer's internet site is often useful too. Figure 2 shows a typical layout for these adjustment screws for commonly installed models.

The setting of limits is best done before the cable and chain are added, although most adjustments need to be made long after installation as well. Both methods are included.

[edit] Prior to Cabling

The limits can be roughly set without a cable or chain installed, and this method is the one to use when a bike is first being assembled. It is done by simply pushing the derailleur by hand to the required position. If this method is appropriate, then roughly set the limits as follows:

  • The high limit screw is adjusted so that the top pulley's center lines up with the outer edge of the smallest sprocket. This is assumed to be the default position for the derailleur when a shifter cable does not impede it.
  • The low limit screw is similarly adjusted, but so that the pulley's center lines up with the center of the largest sprocket in the cluster. If there is a chain installed, the pedals need to be cranked to attain this position.


The above adjustment is best done with the bike in a repair stand, but at a push could be done with the use of an extra pair of hands.

[edit] After Cabling

If the derailleur is to be adjusted with the shifter cable connected, first screw the barrel-adjuster as far in as it will go clockwise, (whether the barrel is on the handlebars or on the derailleur itself), so that there is the minimum of cable tension. For each of the two limit adjustments the object is to set the tightest screw setting that can be tolerated without misalignment noises or faulty shifting. Once again, these adjustments need a repair stand, a helping hand, or some other improvisation to work with the rear wheel off the ground. The adjustment sequence is as follows:

Adjust the high limit :

  • If there is a choice, select the largest front chain-ring.
  • Shift the chain onto the smallest rear sprocket using the shifter on the handlebar.
  • Then, further shift the chain onto the second-smallest sprocket by pulling the cable while cranking the pedals.
  • Release the cable and note whether or not the chain moves cleanly onto the smallest sprocket without misalignment noises.
  • If all is well tighten the high limit screw by one-quarter turn, and repeat the shifting exercise. Continue tightening and checking, tightening the screw by one-quarter turn each time, until eventually a point is reached where the adjustment will be too tight, as evidenced by a slight misalignment noise or faulty shifting.
  • When this fault point is reached, unscrew the high limit adjustment by one-quarter turn, to obtain the correct limit setting.
     


Adjust the low limit :

  • If there is a choice, select the middle front chain-ring.
  • Shift the chain onto the second-largest rear sprocket using the handlebar shifter.
  • Then, shift the chain onto the largest sprocket by pulling the cable while cranking the pedals.
  • Note whether or not the chain moves cleanly onto the largest sprocket without misalignment noises.
  • If all is well tighten the low limit screw by one-quarter turn, and repeat the shifting excercise. Continue tightening and checking, tightening the screw by one-quarter turn each time, until eventually a point is reached where the adjustment will be too tight, as evidenced by a slight misalignment noise or faulty shifting.
  • When this fault point is reached, unscrew the low limit adjustment by one-quarter turn, to obtain the correct limit setting.
     


At times these adjustments have a fairly wide adjustment tolerance, and it is difficult to identify an exact point to call the right adjustment. At these times as long as the shifting for the top and bottom sprockets is clean and noise-free the greater part of the matter will have been solved.

At this point the intermediate gears will make a noise, since the barrel-adjuster was completely tightened prior to making the limit adjustments. This matter is corrected in the section on indexing.

[edit] Set the Body-angle

The 'B' screw is otherwise known as the body-angle adjustment. It is used to move the entire derailleur to a position beneath the sprocket-cluster and close to it. The closer the pulley is to the sprockets the more teeth are engaged with the chain. This reduces wear and tear on both the sprockets and the chain, and in addition tends to make gear shifting more decisive.

Typical rear-derailleur specifications, for example, for the SRAM SX-4 and its relatives, specify 6mm as the gap to aim for, though these shifters still work well when the setting is a bit wider.

This is the 'B' adjustment procedure:

  • Make sure that the low and high limits are already set.
  • Move the rear shifter onto the largest sprocket; that is, the smallest shifter number.
  • Locate the 'B' adjustment screw using the bicycle's handbook. If the handbook is not available then note that the body-angle screw is one of only three adjustment screws found on a rear-derailleur. The other two are the limit screws mentioned previously. The screw is most often located near the hanger. See Figures 2 and 3.
  • Observe the gap between the derailleur guide pulley and the bottom of the largest sprocket.
  • Adjust the gap in accordance with the manufacturer's specifications for the derailleur. If the documentation is unavailable then it may be possible to access it on the internet. Failing this, set the gap to about 6mm, or as close as is reasonable without obstruction.
  • Move the shifter through all of the gears to make sure that the sprockets are clear of the guide-pulley in every position.
     


Rear derailleurs are designed to handle a range of cogsets with different numbers of teeth on their largest cogs. They cannot take any number of teeth, but bikes using the same derailleurs are often fitted with quite different cogsets. The mechanic uses the B-tension bolt to adjust the derailleur for different sizes of large cog. If the size of the cog is outside the acceptable range, then perhaps adjusting the B-tension bolt, even to its extremes, will not get the derailleur working optimally. As a rule, Shimano derailleurs can handle a wider range of sizes than SRAM, though both of these may work reasonably well outside the preferred range.

Sometimes despite best efforts, and even after b-angle adjustment the bike design makes for one gear that is harder to shift into than others. It has been pointed out by some riders that a slight back-peddaling to remove the tension in the chain before moving forward in the troubled gear can be useful in such cases.

When the body-angle adjustment is complete, move on to the indexing adjustment.

[edit] Set the Cable Tension

[edit] Barrel-adjusters

The barrel-adjuster is usually found on the right handlebar or on the derailleur itself, at the point where the shift cable enters. See Figure 1 for an example of the latter. In the usual case it does not matter where the adjustment-barrel is located, screwing it in (clockwise) slackens the cable and unscrewing it tightens the cable.

The handlebar gear-shifter moves in discrete steps, and each step is matched to the exact distance between the respective sprockets on the wheel-cassette. Thus, moving the shifter on the handlebars, regardless of the starting point, will lengthen or shorten the cable by an exact amount. The only bug in this system is the residual slackness or over-tightness in the cable itself. To see why the barrel-adjuster is needed, consider these two examples.

  • If the cable has too much residual slackness when the handlebar shifter is operated, say from the smallest sprocket to the next-smallest, then a part of the resultant cable shortening will be used in first taking up the cable slack, then in moving the derailleur. As a result, the shortening is not all available to move the derailleur, and it is unlikely to reach far enough.
  • Similarly, when the cable is too tight, shifting between say, the largest sprocket and the second largest will result in the shifter lengthening the cable by one unit. Because of the residual over-tension in the cable that tends to move the derailleur inward, not all of the intended lengthening will be available to the derailleur. It will not quite reach far enough.
  • In both of the examples above the chain will likely make a noise as it shifts, but if the tension in the cable is not excessive, the chain might still pull itself onto the right sprocket; if the tension in the cable is too high then the chain might rattle somewhere in between the sprockets.

The entire function of the handlebar barrel-adjuster is to remove any residual slackness or over-tension in the cable so that the exact lengthening and shortening of the cable by the handlebar shifter has no error.

[edit] The Shift Cable

When the barrel-adjuster cannot align the gears within its adjustment range, then the excessive slackness in the cable needs taken up. Because the barrel-adjuster has quite a wide range, this stage is rarely needed.

A commonly observed test as to whether or not a cable needs tightened finds the process unnecessary provided that the barrel-adjuster can be set to allow changes up and down between the smallest and the next smallest sprockets. This shift is the most likely to fail if the cable is too slack so it provides a good rule of thumb. The process is as follows:

  • Screw in the barrel-adjuster(s) fully.
  • Then, unscrew the adjuster by one full turn.
  • Undo the pinch bolt on the derailleur to release the cable. It usually needs just a hex wrench, but there are some nuts that use special keys.
  • Pull the cable tight and refasten the pinch bolt.
  • Check that the existing derailleur adjustments remain valid.
     


[edit] Adjust the Barrel

Provided that both of the limits and the 'b'-screw adjustment are adequate, the cable tension can be adjusted with the barrel-adjuster. Because cables stretch with use, this adjustment might be needed from time to time, other than during a full line-up. The process is as follows:

  • Place the chain onto the smallest rear-sprocket, and if there is a choice, the largest front chain-ring.
  • While pedalling, use the handlebar shifter to change gear to the second-smallest sprocket.
  • If the chain falls short and does not reach the sprocket then the cable has too much slack; unscrew the barrel-adjuster slightly. If the chain moves too far or moves by two positions then the cable is too tight; screw in the barrel-adjuster slightly.
  • Repeat this shifting and adjusting until the derailleur moves smoothly backward and forward.
  • Shift the gears over the whole range and note whether or not shifting is adequate. The noise from chain-rattle should be reduced to a minimum. Fine-adjust the barrel if necessary to obtain smooth shifting in both directions.
     


A troubleshooting table has been included in the next section to consolidate the points made elsewhere in the text, and it contains the main rear-derailleur faults. Such a table is necessarily brief, and is intended to be used in conjuction with the descriptive materials in the main text, and not as an alternative to it.
 

[edit] Troubleshooting

The first thing to check is that all parts are compatible, especially if you are not familiar with the history of the bicycle. Indexing shifters require that the number of gears on the shifter is the same as the number of gears on the wheel. Different brands of shifters may need to be used with matching shifters: i.e., Campagnolo must go with Campagnolo. Others makers such as SRAM and Suntour have made shifters that only work with their own rear derailleurs, but also have made other shifters that work with Shimano. Shimano shifters are designed to work only with their own products.

Rear derailures have a specified maximum capacity and maximum range of teeth, and if these are exceeded, the derialleur may work poorly or not at all. The capacity describes the extent to which the unit can take-up slack when gears are changed. Also, if the hanger is not the one intended for that unit, then the derailleur may be positioned too distant for a correct body-angle adjustment.

The most usual rear derailleur adjustment faults are listed in the table below with suggestions as to their cause and correction, though users are advised to aquaint themselves with their manufacturers' websites for the official line, and with bike blogs for problems that makers tend not to discuss.

Rear Derailleur Troubleshooting Guide
Fault Condition Possible Cause Corrective Action
Chain jumps outward from small sprocket High gear limit not adjusted Tighten high limit screw to bring the top pulley slightly inward
Chain will not move to small sprocket High gear limit not adjusted Slacken high limit screw to bring the top pulley slightly outward
Chain jumps inward past the largest sprocket toward the spokes a. Low gear limit not adjusted

b. Derailleur or hanger are bent

 a. Tighten low limit screw to bring the top pulley slightly outward

b. Straighten or replace items
Shifting is slow Top pulley is too far from the sprockets Withdraw the 'B'-screw to narrow the gap
Shifting sounds rough Top pulley is too close to the sprockets Tighten the 'B'-screw to widen the gap
Slow shifting onto smallest sprocket a. Cable is too tight

b. Cable is binding

 a. Turn barrel adjuster clockwise

b. Lubricate and check cable for damage
Slow shifting onto largest sprocket Cable is too slack Turn barrel adjuster anticlockwise
Chain jumps two gears from smallest sprocket Cable is too slack Turn barrel adjuster anticlockwise

[edit] See Also

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