It is possible that derailleur’s are one of the most misunderstood, misadjusted and misused pieces of equipment on a bicycle. When I used to work in a shop I couldn’t count the number of times someone would come in with a horribly maladjusted derailleur because they had tried to fix it themselves. Usually it was a simple fix, which was made into a more complex fix from their fiddling. With puppy dog eyes they would look up at me trying to avoid eye contact and tell me they had messed it all up, or they would try and deny anything had gone wrong – JRA. When a derailleur goes out of adjustment, most of the time (80-90%) it is a simple fix; it is usually a matter of adjusting the cable tension. The rest of the time assessing the problem can be difficult, check out this document from Shimano on the 100 possible causes of bad shifting. This guide will stick to the simple stuff, mostly to avoid any finger pointing if your adjusting escapades go awry and because I want to avoid this turning into a small novel. And really, if it is anything more than a simple fix, it is probably a good idea to take it to your shop.
Is it Bent?
First things first: Check to see if your derailleur bent. There is no point spending the time fiddling around with your derailleur, trying to tune it, if it or the derailleur hanger is bent. A bent or broken derailleur will never shift properly. Have a look at your rear derailleur from behind your bicycle , it should hang down straight from your rear dropout; if it is tilted to either side; something is bent and your best bet is to take it to your local shop.
Get to know your Derailleur
Make friends with that little gizmo on the back of your bicycle, a good relationship with your derailleur can be rewarding and fun. That was a lie; they are often frustrating like a Rubik’s cube or relationships. But enough contrived similes. There are three main adjustments you can make to most rear derailleurs: B-Tension with the B-Tension adjustment screw, cable tension using the barrel adjuster at the back of the derailleur and or one on the shifter, and limits using the high and low limit screws (usually marked with an L and an H at the back or side of the derailleur). Before you adjust anything, though, start off by figuring out what is wrong.
Most of the time, almost all of the time if it was adjusted properly from the start, it is just a simple matter of changing the cable tension. Do this properly and you will find yourself with a properly adjusted derailleur. How can you tell? (Click on the pictures below to view a close up of derailleur’s and barrel adjusters
Not shifting into a higher or lower gear quickly and smoothly with each click: Cable Tension
Most of the time derailleur’s are properly adjusted at the shop and only need small adjustments to compensate for cable stretch (especially on new bicycles). Cable stretch is normal on new bicycles or anytime you install a new shifting cable, the cables are wound and stretch during the first few rides. 9 out of 10 times this is the culprit of your shifting woes.
Too little tension: most often the problem
On a normal pull rear derailleur (which if you are reading this I am guessing you have); if you go to shift into a larger ring on the back and the derailleur is disobedient then you need to add tension to the cable. You can do this by twisting a barrel adjuster counter clockwise. Don’t get too excited, and go turning the adjuster all willy-nilly, try a quarter turn first and see if that improves the shifting, if not try another quarter turn and continue until the chain eagerly jumps up to the larger cog. If you go too far you will notice that the derailleur no longer shifts down to a smaller ring proper. Don’t worry; it’s not a big deal. To remedy this just turn the barrel adjuster the opposite way in small increments. Once you’re shifting is smooth, stop playing with your derailleur and go riding. Resist the urge to do anything else.
Too much tension
If you go to shift to a small (more difficult gear) and nothing happens or your chain starts jumping around like a Mexican jumping bean then your cable probably has too much tension. Turn the barrel adjuster clockwise until the chain moves quickly and smoothly up and down and voila!
Limit Screws: Where things get interesting
The limit screws adjust the two limits of the derailleur: the high limit (littlest cog) and the low limit (biggest cog) consequently there are two screws for this task usually marked with an L and H respectively. The simple explanation is that the upper pulley wheel should align with the largest and smallest cogs and no further. Usually this is set by your friendly neighborhood dealer and is not something you should worry about or adjust yourself, as it can cause quite a mess. Usually if the chain is not shifting or is over shifting in your biggest and littlest cogs then something has been bent. However, if they are out of adjustment here is how to fix it.
Too open and the chain comes off
If the L limit is too open when you go to shift into your big ring you will quickly hear a large amount of metal on metal unpleasantness as your chain hops off your cassette into your spokes, often making quite a mess of things back there. If your H is too open then the chain can come off and wedge itself between the dropout and your cassette or may just continue to jump off the small cog.
To fix this you’ll need a small Phillips or flat head screwdriver. Shift into either the lowest or highest gear on your rear cassette and adjust the L or H screw by turning them clockwise until the upper pulley wheel aligns with that cog.
Too closed and you’ll never reach the limits
If your chain won’t shift up into the largest cog or down into the smallest cog and nothing is bent and your cable tension is correct then you will need to open the limits. Using either your small Phillips or flat head screwdriver, open the limit by turning the screw counter clock wise, work in small increments until you can get the chain into the correct gear. Once your chain is cooperating, adjust the limit so that top pulley aligns with the cog. Now you can go riding.
The B- tension screw is something I am going to leave out. In essence this adjustment screw ensures that the derailleur’s upper pulley wheel is the correct distance from the teeth on your cassette to ensure proper shifting. If you are feeling mechanically brawny I have included links to instructions from both SRAM and SHIMANO where you will find the answers you are looking for on this adjustment.
Now you are ready to make sure your derailleur is running perfectly. I’ve probably already said too much about rear derailleur adjustment, or maybe just enough to get you in trouble, use patience when you are making these adjustments, derailleur’s can be frustrating. But with a little know-how and some patience, you should be able to get yourself dialed. If for any reason you feel like you are getting in over your head, visit your local dealer and the will get things sorted for you.
Or the technical documents for SRAM Derailleurs in PDF