I currently work as “The Bicycle Repairman,” and since I got the job, I see a lot of simple things that people could fix themselves. Most of these problems deal with rear derailleurs.
There are many types and styles of rear derailleurs out there. All of them need adjusted and repaired. It is one of the most common reasons people bring their bike in for repair; they can’t shift!
Learning how to adjust the rear derailleur will be to your benefit, possibly saving you a trip to the store.
First, you need to be able to operate the shifter and turn the rear wheel at the same time. For those of you who do not have a bike stand, the easy solution is to turn the bike upside down. Top Tip: Place a couple pieces of cardboard or carpet on the ground to avoid scuffing the seat and handlebars.
Next, turn the crank (where the pedals are attached) and shift the chain to the biggest front cog in the front and the smallest cog in the back.
Look at your rear derailleur and you will see where the cable attaches. It takes a 9mm wrench to loosen this nut. Loosen the nut so the cable comes off, but do not remove the nut. You will also see a thumbscrew where the coated part of the cable stops. Tighten this screw all the way down. Not too tight as you have to be able to move it by finger pressure later. Lastly, loosen the two screws on the side of the derailleur until there is no pressure on them. Take care to not remove the screws, just loosen them almost all the way out.
Make sure your shifter is in the position indicating your highest gear. (I.e. 5, 6, or 7 depending on your markings) Pull all the slack out of the cable. If you do not have a cable tool, a pair of needle nose pliers work almost as well. Pull as much of the wire out from the casing as you can. While keeping the tension on the cable, slide it under the nut and tighten it down with your wrench. This usually solves most problems, but if it still will not shift correctly then we need to adjust it.
Looking at your derailleur, you have two screws. One is closer to the gears than the other. This one is known as the top screw. If your bike was right side up then it would be, but since it is up side down it isn’t. Just remember the top screw is the one closest to the cogs. Looking at the side of your bike tighten the top screw until the first cog of the derailleur (known as the guide) is almost touching the small cog of the rear wheel. It should be about a fingers thickness away.Unless you have really thick fingers, estimate about half an inch or one centimeter.
Shift the chain all the way back to the smallest cog again. Turn the crank and shift the gears one at a time from smallest to biggest. As you are shifting pay attention to which gears rattle and which ones do not match the number on your shifter.
Then shift it back to the smallest again one at a time. While your chain is on the smallest gear going to the next size up, if it rattles adjust the top screw in or out until it no longer rattles and shifts easily.
Here comes the trickiest adjustment: when going from the second smallest to the next size up you use the thumbscrew. Put the shifter on the next number up. Assuming you have a 21-speed bike this will be numbered 5. (Six would be the smallest) Now while turning the crank, shift to 4. If it does not shift or it rattles, adjust the thumbscrew out (this is tightening the cable but do not get confused) until it does not rattle and it does shift correctly. Shift back to 5 then back to 4 to make sure you got it. Usually after this one adjustment the derailleur works well for the rest of the gears. If not then adjust for rattle with the thumbscrew through all the gears.
Small quarter turns of the thumbscrew are all it takes. Do not make large adjustments as you will end up starting over. The older style derailleurs will take the least repetitions while newer ones may take three or more adjustment repetitions until it all shifts correctly. Remember to adjust only when going from a smaller to a larger cog. Adjusting on the up shift (bigger to smaller) will undo your adjustments to where you will have to start over.
Lastly, make sure the derailleur is lubricated. Actually I should have put this first, as sometimes a shot of lube spray is the cure. However, since you are working on your bike you may as well make sure it is lubricated so that the derailleur works well for a long time. Lightweight spray lube works the best.
Let’s wrap this up by stating that if you still can’t fix the gears to shift, then by all means take your bike to the shop and let a bicycle repair person fix it for you.