Bicycles/Maintenance and Repair/Derailleurs/Rear Derailleur Adjustment

Terminology To better understand the material that follows, consider the following meanings:

The overall gearing of a bike is set by both the choice of front chain-ring and the rear sprocket. Some bikes have only a single chain-ring on the pedal assembly so all of their gear shifting is done on the rear wheel. This page concentrates on that situation. Front derailleur adjustment is discussed here.

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 figures 3 and 4 for typical adjustment points, and the procedures that follow.

Preliminaries
Make sure before carrying out adjustments that the derailleur hanger is not bent and that the derailleur's cage is not twisted. Each of these cases can be checked by viewing the bike from the rear where both of the derailleur's pulleys and each selected cog should be in vertical alignment. In particular the hanger should not appear to be bent inwards toward the wheel. Both shifting and the smooth running of the chain depend on this alignment, and even a small deviation can upset the normal feeling of the bike.

Repairing the hanger: On steel bicycles the only option is to straighten the hanger section of the frame. Aluminum hangers can be straightened if they are not too severely damaged though it is important to keep in mind that aluminum fatigues much more easily than steel. Most bicycle shops will have the proper tool for this (the Derailleur Alignment Gauge from Park is recommended), though it is unlikely that the home mechanic will need one often enough to own their own. The notion of hanger straightness in this process is perhaps a misnomer, in that some slight bending of the hanger may be necessary to satisfy the overall alignment.

Replacing the hanger: On aluminium bicycles the hanger is a sacrificial piece, designed to bend on impact instead of the expensive derailleur, or even worse, the frame. Unfortunately bicycle frame manufacturers have made numerous designs of hangers over the years, and no bike shop has more than a small fraction of them in stock. However, if the part is available, (numerous internet suppliers offer stocks of hangers), the replacement is easily done at a bike shop, or even at home, and is no more difficult than the installing of the derailleur itself.

It may be helpful to note that the adjustment procedures on this page are devised for the most common shift-cable behavior. 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 you should refer to the manufacturer's instructions.

Problems following a new shifter installation, might be the result of a faulty choice of shifter. This point should be understood before going any further. A derailleur has an actuation ratio of its own; a given change in cable length gives rise to a given distance traversed across the sprockets, and the reciprocal of this ratio is called the shift ratio. Similarly, shifters are made for different families of derailleur. The Shimano rear-derailleurs belong entirely to the 2:1 family whereas many SRAM derailleurs belong to the 1:1 set. Note that these family-ratios are not the actual shift ratios, but are just labels, and serve mainly to distinguish SRAM's native set of shifters and derailleurs (1:1) from their Shimano-compatible set of shifters (2:1). The SRAM shifters that are Shimano-compatible; for example, the series Attack, Centera, MRX, MRX Pro, Rocket, and TRX, are openly advertised for use with Shimano's 1.7 shift-ratio derailleurs; (the 2:1 family). Obviously, if an MRX (2:1) version of a GripShift were installed instead of the 1:1 version, the shifting would not work. In actuality, Shimano derailleurs have 1.7 shift ratios while SRAMs' native units have 1.1 ratios. See CTC on Rear Shifting for a detailed discussion, and the local Wikibooks page Gear-changing Dimensions for other tabulated shifting dimensions.

Chain length adjustment:  Some shifting problems are not so much a derailleur problem as one of a chain that is too long. Most manufacturers of derailleurs assume that the chain's length is set to optimum before making any other adjustments:  The procedure is repeated here: Starting with a broken chain, and ignoring the derailleur completely, wind the chain tightly around the largest front chainring and the largest rear sprocket. While holding the two ends tightly so that they overlap, note the length required to make them just meet. To this length add two half-inch links. Feed the chain through the derailleur and connect it for the ideal fit. Obviously, the length of any master link must be taken into account in the total, and when the bike has a rear suspension, it must first be adjusted to its furthest backward reach. It should be noted that a very small number of derailleurs need the addition of four half-inch links instead of two, so be sure to check the manufacturer's data sheets for your unit.

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

Set the Chain Limits
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 gear-shifter.

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. If not found they can be located in the documentation of the derailleur, or on manufacturer's internet site. Figures 3 and 4 show typical layouts for these adjustment screws.

Prior to Chain and 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 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.

After Chain and Cabling
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. At this point the limits are set fairly tightly. Once again, these adjustments need a repair stand, a helping hand, or some other improvisation to work with the rear wheel off the ground. To carry out this adjustment when a chain and shift cable are installed, it is best to do it after basic indexing of the gears; this makes sure that it is possible to shift onto the required cogs without misadventure. First slacken off the low and high limit screws by turning them nearly all the way out of their housings, so that initially they have no limiting action. The adjustment sequence is as follows:

Adjust the high limit :

Adjust the low limit :

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. This matter is corrected in the section on indexing.

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:

The B-tension bolt is used to adjust the derailleur to clear the cogs as it transits them during gear shifts. Usually, if the largest cog is clear of the cage, the others will be too. For this reason the adjustment is made in this cage position.

The angle of the lip on the derailleur's hanger, (the angle between the vertical and the face of the lip), sets the maximum height that can be achieved with the 'B' adjustment screw; in this case when the adjustment screw is fully withdrawn, then the cage is as high as it can go. If it happens that the cage cannot be set high enough to meet that specification, it could be a sign that the wrong hanger has been chosen.

Although the cage height affects the ease of shifting, sometimes there is one gear that is harder to shift into than others. It has been pointed out by some riders that a more conscious effort at removing the tension in the chain before moving forward in the troubled gear can be useful, though a bent hanger should also be suspected. At other times however, inconsistent shifting is a sign of a non-optimal chain length or a frayed housing end. These points should be checked before suspecting the cog itself.

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

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 the derailleur. See Figure 2 for an example of the latter. In all cases it does not matter where the adjustment-barrel is located, screwing it in (clockwise) slackens the cable and unscrewing it tightens the cable.

The gear-shifter moves in discrete steps, and each step is translated by the derailleur into the exact distance (the inter-cog spacing or pitch) across the sprockets of the cluster. Thus, moving the shifter one step, 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 move far enough to move the chain properly and make a successful gear change.


 * 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 (indexed shifting) of the cable by the handlebar shifter has no error.

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 tightening depends on whether the barrel-adjuster can be set to allow changes up and down between the smallest and the next smallest sprockets. This shift is the one most likely to fail if the cable is too slack so it provides a good rule of thumb. With the shifter and chain on the smallest sprocket, the process is as follows:

At times shifting can also be affected by frayed housing-ends or a poor choice of parts. The cable ferrule on the end of a housing must run freely in the barrel adjuster's end, but some ferrules, especially on universal cables have been noted to be too big and because they jam, cause the housing to twist when the barrel is adjusted. At other times exposed housing wires cause inconsistent behavior in shifting. The changing of a shift cable is explained in Cables and Housings and at http://www.sheldonbrown.com/derailer-adjustment.html Sheldon Brown's Derailleur website].

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 service. The process is as follows:

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 together with the descriptive materials in the main text, and not as an alternative to it.

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 etc. Shimano shifters are designed to work only with their own products.

Rear derailleurs have a specified maximum capacity and maximum range of teeth, and if these are exceeded, the derailleur 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 wrongly 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 acquaint themselves with their manufacturers' websites for the official line, and with cycling forums for problems that makers tend not to discuss.

Chain Slipping
There are many possible causes of chain-slipping (at other than a shift), and strictly speaking most are to do with the drive-train in general, rather that just the rear-derailleur. Various factors combine at any one time towards slipping, and correcting any one of them might reduce the collateral effect enough to cure the problem. This collateral effect also serves to confuse as to the most likely causes. The common causes are listed here in order of the most practical measures to attempt:
 * Chain length: If the chain length is not optimum then other factors might have more effect.   Adjust the chain length using Big-to-Big  length method outlined in the preliminaries section above.
 * 'B' Height adjustment:  Ajust the rear derailleur's cage height in accordance with the manufacturer's instructions.   If the height setting cannot be met, it could be a sign that the wrong  hanger was used.   The angle of the hanger-lip affects this range.
 * Master Link:  Make sure that any master link in the chain is right for the task, and use the one recommended by the manufacturer.
 * Chain width:  Make sure that the correct chain width is used, and that there are no protruding pins.   Again, use the chain recommended by the manufacturer.
 * Damaged shift cable housing:  If the end of a cable housing is badly frayed, with its support wires showing, then the shift cable tension might change intermittently.
 * Stiff chain links:  Observe the chain at the derailleur for any jumping links and manipulate them by hand until they are free. Oil the chain; some cheap chains when new are a bit sticky and do not have good lubrication.
 * Worn chain:  Replace a badly worn chain.
 * Worn sprockets:  Inspect the wear on sprockets especially if the problem happens only in the most used position.