How-To: Fix Common Rear Derailleur Shifting Problems

The rear derailleur is the heart of most mountain bike drivetrains. As the 1x drivetrain becomes standard issue on capable cross-country, trail and enduro bikes, we're left with one shifter, cable and derailleur to adjust for top-to-bottom gear range performance. GMBN has put together a handy guide for solving the most common shifting problems when it comes to the rear derailleur.

Bike Slang - A derailleur can also be called a "mech," especially in Europe.

Derailleur Basics

The rear derailleur is made of up pulley wheels, springs and leverage mechanisms that move when pulled by a cable. The derailleur moves inward or outward from the cassette, causing the chain to jump up or down the gear cluster as the wheels and springs keep tension on the chain while maintaining its position. The most common derailleurs come from SRAM and Shimano but BOX is also making an impact in the market as well. Each brand of derailleur has their own unique features, but the derailleurs all use a similar, traditional designs and means of adjustment.

The biggest gear on the cassette (the gear cluster on your back wheel) is the lowest gear. The smallest gear on the cassette is the highest gear. Most shifting adjustment should start with the chain in the highest gear. Before any cable adjustment is done, derailleur alignment and limit screw position must be checked.
Derailleur Alignment

Trouble-shooting shifting problems starts by making sure your derailleur is properly secured to your bike. There is one point of contact between your derailleur and your bike that is via the derailleur hanger. Some bikes have removable or replaceable hangers, while other bikes (generally older model frames) have permanent hangers that are part of the frame's structure. The derailleur hanger should not be bent and must be perpendicular to the rear wheel and cassette to get accurate shifting. If your derailleur hanger is bent, ideally, you should replace your hanger if applicable or take your bike to a local shop for professional repair. Depending on the material, bending the hanger back could weaken the structure.

Limit Screw Adjustment

Limit screws set the limit of travel on the derailleur toward or away from the spokes / cassette. The low limit screw adjusts how far up the cassette the chain will go, toward the lowest / easiest gear. If the limit screw is too open, the chain could shift over the top of the lowest gear and drop down in between the cassette and the spokes. This could cause spoke damage or a broken chain. If the low limit screw is too restricting, the chain will not reach the lowest/ easiest gear. The same applies for the high limit screw, but it applies to the smallest gears on the cassette.

The B-tension screw sets the height of the top pulley wheel in relation to the cassette. Accurate setting of this screw is important when shifting into the lowest gear. If the guide wheel is too close (high), the friction and movement of the chain becomes stressed as the cassette and derailleur contact each other. If the guide wheel is too low, accuracy is compromised because the chain tension is dangerously increased in the lowest gear . Each derailleur brand will have a specific gap measurement to achieve, but you should be able to eye up the setting with a few twists of the screw.

Cable Friction

Shifting problems may be caused by a gunked-up system. Checking how easily the shifter cable (the steel wire) slides through the shifter housing (the black hose) is fairly easy to do. Shift the system down to the highest (smallest) gear. Loosen the cable retention screw on the derailleur. The cable is now free and you can manually slide the cable back and forth, feeling for friction. Friction will be found at the ends of the housing. You may have multiple pieces of housing to check or just one, long piece that covers the entire cable. Make sure cable, housing ends and ferules (the little metal caps that cover housing ends) are clean. Once clean, a thin coating of grease will help keep things smooth. There are various lubricants specific to this issue.

If a new cable is required because it is kinked or damaged, see your shifter brand's instructional manuals for how to replace the cable. Whether SRAM, Shimano or BOX, changing a cable is generally very easy to do. If you replace a cable, it's never a bad idea to replace the housing at the same time, ensuring the entire system is clean and new. Verify that you purchase shifter housing. Shifter cable housing differs from brake cable housing, though they may look similar at first glance.
The total cost of a new rear derailleur cable and a few feet of shifter housing should be between $10 and $20. - http://www.jensonusa.com/Shifter-Cables-Housing?by=Category

Verify that the cable is accurately traveling through the system as intended. Some derailleurs have easy-to-miss cable routing. If the cable misses a guide or port, the system's performance will be compromised.

Cable Tension Adjustment

Cable tension adjustment is incredibly easy to do as long as you follow some simple steps.

Make sure your chain is in the highest (smallest) gear on the cassette and the barrel adjuster on your shifter is position that provides no extra tension (threaded all the way in).

Once your cable is guided from your shifter to your rear derailleur properly, pull the cable snug, but not overly tight, and secure the cable tension screw on your derailleur.

Pedal and shift one gear. If the chain doesn't move to the next cog, you need to add tension to the cable. If the chain moves up two or more cogs with one click of the shifter, your cable is too tight. Add cable tension by rotating the barrel adjuster the appropriate direction on the shifter. This takes out any slack in the cable, helping activate the derailleur. If your cable is too tight and your barrel adjuster is all the way in, you'll need to loosen the cable clamp bolt on your rear derailleur and adjust the cable slightly slacker. From there, repeat the shifting steps and adjust your barrel adjuster accordingly if necessary.

Once your shifting is dialed, go shred and don't forget to keep your drivetrain clean and lubed to maximize the life of its components and shifting accuracy.

Learn more about general MTB drivetrain maintenance in Vital's From the Workbench series