Rear derailleurs can appear to be complicated to adjust and something that only a professional mechanic should be able to play with.
In the video below I'll show you how to set up a Shimano Rear Derailleur in 5 min, getting perfect gear shifting performance. For rear derailleur troubleshooting, read below.
Troubleshooting
Bent hanger - this is often the reason for your gears to be acting weird. The derailleur hanger is a small piece of aluminium that connects the derailleur to the frame. Its purpose is to bend in case of an impact to the derailleur, protecting more expensive parts from getting damaged, like the derailleur or the frame. To check if this is the cause to your gear problems, stand behind the bike looking at the derailleur and checking that the hanger is perfectly straight. If this looks bend, take your bike to a local bike shop and get a new hanger (usually this is cheap) and your gears should work perfectly again.
Kinked Gear Cable - a gear cable can get kinked if poorly installed, caught on some obstacles trail side or even if it gets caught on the bike rack in your car. If a cable is kinked, gears will be way off when you are using higher gears but when selecting lower gears if feels okay. To confirm the cable is kinked, select the highest gear (smallest cog) and follow the cable from the shifter to the derailleur. If you find the kink, the only way to fix it is by replacing the gear cable.
Frayed Outer Cable - If you have old outer cables on your bike and you can start seeing the inside of the cable starting to fray, your gears won’t work perfectly. A frayed outer cable won’t keep the gear cable tension consistent throughout the entire gear range, causing the cable tension to fluctuate and create inconsistent shifting. You will need new outer cables.
Steep Angles on Gear Cable - this one is hard to confirm but essentially if you have the gear cable doing steep angles on route to the rear derailleur, the cable inside won’t slide as freely as it should, causing extra resistance on the shifter pads and delays in shifting. You’ll need to either shorten or extend the outer cable.
Dirty, Muddy or rusty Gear Cables - After a very wet or very muddy ride, it’s quite easy for water or grit to accumulate around the cable and make its way to the outer cables. This will add considerable friction to the gear cable and might even stop the cable from moving. Try to clean the cables and the outer cables. Failing that, you might need to change the gear and, possibly, the outer cable.
Damaged Shifter - If you feel inconsistent shifting performance and you recently had a crash, it’s possible that the shifter suffered some internal damage. Usually the shifting pads get either very stiff to move, or extremely loose when you try to operate them. You can try to repair the shifter but most likely you will have to replace it.
Old Rear Derailleur - If your rear derailleur is getting old, the spring inside might start losing recovery energy and getting fatigued. You can confirm this if your gears are moving nicely up the cassette but coming down slow.
Stiff Chain Link - Sometimes it feels like the derailleur wants to jump to a higher or lower gear when you’re pedalling. If cable tension seems to be correct, check the chain for a stiff link, a connection between two links that is not moving freely. Locate the stiff link by either looking at the jockey wheels when turning the pedals slowly, or by checking every link on your chain individually. Fix this by lubing that stiff link and/or holding the chain and flexing it laterally, to allow the link to centre itself. Failing this, ask a bike mechanic to help you out.
Chain jumping on gear - If all gears are working perfectly on the work stand but when you jump on the bike and go for a ride you feel the chain jumping, especially on higher gears, it’s because your chain and/or cassette is getting old and the teeth are not holding the chain anymore. You'll need to get a new chain and cassette.
Cable Stretch on a New Bike - If you just bought a new bike and after a few rides start feeling the gears getting out of shape, this is just the gear cable stretching, something perfectly normal. As the gear cable stretches, gear cable tension is reduced. Add tension by turning the barrel adjuster on the derailleur anti-clockwise half a turn. Continue watching the video to understand how cable tension works.
If you have any questions, leave them in the comment section below and I’ll reply.
Happy Riding!