Q&A for How to Adjust a Rear Bicycle Derailleur

Use the tension adjuster at the gear shifter at the handlebars located under the shift system at the base of the cable housing.

Adjust your barrel adjuster until it drops into the correct gear, which may require adjusting the cable at its anchor point first.

If the cog is spinning, it's most likely that the chain itself is out of line. Guide the chain back onto the a gear and hand-pedal it forward gently until it clicks in.

If your bike used to be able to shift up to the highest cog but does not now, it is more likely the cable. Derailleurs generally don't go out of adjustment once properly set (unless the derailleur hanger is bent). Tighten the cable by adjusting the barrel adjuster out (counter clockwise). If that does not work, you may need to change the cable.

Your bottom bracket could be over-tightened, causing the crank to shred and cut off metal from around the housing.

It is a metal piece distinguishable by its shiny metal. It is there to protect the bike; if the derailleur is damaged, your bike doesn't shift well.

First, remove the chain and then remove the tension cable. Next, unscrew the rear derailleur from the frame. Then, thread the chain through your new derailleur and screw the new derailleur onto the frame. Finally, re-attach the tension cable and adjust it using the steps shown above.

Keep it lubricated so it shifts quickly.

That could be due to an old chain or old gears. An old chain will have either worn out or stretched links that no longer fit the teeth. Old gears can have worn out spaces between their teeth so that the chain doesn't match up with the gearing and can't hold like it should. This is more likely to happen when the system is under a lot of pressure and on gears that are used more than others. This is assuming by slipping you mean the chain doesn't move gears but just slips a few teeth of the gear. You can measure a chain to see if its insides are worn out and if it is the correct length. If that all adds up, it will mean you need to replace the freewheel or the small front chain rings.

Shifters (lever or grip) generally don't need service. They may need lubrication once in a while.

If you’re talking about a single speed/fixie with horizontal dropouts, then yes, pulling the wheel back is one way to adjust the chain tension. On my single speed, I use 2 metal parts called chain tensioners that keep the wheel in the correct position once adjusted correctly. But there are also other ways, such as installing a chain tensioner (different - this is a small cog on an arm) or an eccentric bottom bracket.

Your rear derailleur has two screws one that state L for Level (Out) and the other being H for Height, when you change gears for the rear side. These should be adjusted with much care, for it will take you hours to get the right adjustment.

It is likely that the chain is too long. You'll need to shorten the chain enough that the spring-loaded derailleur’s jockey wheels take up enough slack so that the derailleur doesn't ride on the cassette when the bike is in the highest gear.