The rear-derailleur is the gear-changing assembly on the rear wheel of some bicycles; it moves the bicycle chain from one sprocket to another.
It is cable-operated using a shift control on the handlebar or the front of the frame near the handlebar. 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.
Figure 3: Typical SRAM derailleur adjustment points.
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.
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.
Check Before you Adjust
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.
There are three main adjustments for a typical rear-derailleur: These are:
- The limit stops, high and low, are adjusted, to prevent the chain moving beyond the high and low sprockets in the cluster. This can be done either before or after a chain is installed.
- 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 barrel-adjuster is set to remove any cable slack, so that the cable-pull set by the shifter is the only factor in the operation.
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:
Figure 4: Typical Shimano derailleur adjustment points.
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 :
- 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. Noises are caused by the chain moving beyond the small cog.
- 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. Noises are caused by the chain moving beyond the large cog.
- If all is well tighten the low 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 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. 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:
- 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 always located in contact with the lip of the hanger. See Figures 3 and 4.
- 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. The tracking across the cogs in this way will be reasonable provided that the adjacent cog-size steps do not exceed those in the manufacturer's data sheet.
The B-tension bolt is used to adjust the derailleur to clear the cogs as it transits them during gear shifts. Usually, if the 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.
Set the Cable Tension
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:
- Screw in the barrel-adjuster(s) fully.
- Then, unscrew the adjuster by about one full turn.
- Undo the pinch bolt on the derailleur to release the cable. It usually needs just a hex wrench. Note where on the pinch bolt the cable was clamped since it is important to reproduce the exact route when it is replaced.
- Pull the cable tight and refasten the pinch bolt. The tightness need not be excessive; just remove all of the slack.
- Adjust the barrel-adjuster for correct indexing as in the above section.
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.
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:
- Place the chain onto the smallest rear-sprocket, the one that has the slackest cable, and if there is a choice, the largest front chain-ring. (Shimano Rapid Rise derailleurs work with the cable at its slackest for the largest sprocket, so for these units use the largest and next largest for what follows.)
- While pedalling, use the gear 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 changes gear 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 together with the descriptive materials in the main text, and not as an alternative to it.
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. Thecapacity 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.
|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||
|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||
|Slow shifting onto largest sprocket||Cable is too slack||Turn barrel adjuster counterclockwise|
|Chain jumps two gears from smallest sprocket||Cable is too slack||Turn barrel adjuster counterclockwise|
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.
- 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.