Ask a Mechanic | Avoiding Dropped Chains On a Mountain 3×9 Setup

Welcome to our Ask a Mechanic column where our expert mechanic Daniel Slusser answers your bike maintenance questions. If you have a question for Daniel, please post it on our Facebook Wall or e-mail Daniel directly at Today’s column aims to solve the age old problem of dropped chains on a mountain bike.

Dropped ChainQuestion: I ride a Santa Cruz Blur dual suspension with all the components and wheels being XTR. Please, I can’t stress enough that my knowledge of bikes and its components is novice at best. I have a hell of a time riding, plus it’s great exercise. The bike itself is 2003, as are the components (I think). I’m really happy with performance, weight, and especially the brakes/shifter which are integrated. My deal is that on downhills or bumpy, rough terrain, I’m constantly loosing my chain. From: Mark

Answer: Dropped chains can be caused by a lot of different factors. Derailleur setup and chain length are the biggest contributors to the problem, but some equipment changes can help as well.

XTR M971 9-Speed Rear Derailleur

If your 9-speed derailleur is ten years old, the springs are probably shot. Here is a good replacement.

On your rear derailleur, tightening the b-tension screw can help (the phillip’s head screw where the derailleur attaches to the frame). There is also a hidden screw inside the rear derailleur parallelogram on the older XTR derailleurs that can adjust the spring tension on the parallelogram via a 2-position cam. Use the tighter of the two positions. Switching to a mid-cage (GS) rear derailleur can help also, but you will need to be careful to avoid cross chaining (going big-big or small-small on your gear selection). It is also possible that the springs on your rear derailleur are worn out and can’t keep the chain tight anymore. The derailleur is ten years old. Here is a link to a closeout XTR M971 mid-cage 9-speed rear derailleur that would be a good replacement for you. Just keep in mind that this one may work backwards to what you are used to. Pushing the right lever up will make this derailleur go to a higher (harder to pedal) gear in the back.

For the front derailleur, adjust the high and low limit screws so that when you are in the big and small chainrings the derailleur cage is as close to the chain as possible without touching the chain. This will prevent the chain from dropping off the big and small chainrings. When you are shifted in the middle chainring and large rear cog, back out the barrel adjuster on the front shifter until the derailleur gets as close to the chain as possible without touching it. This will help to prevent the chain from dropping off of the middle ring.

Adjust your chain to the proper length using the method shown in this video:

Although this video is for SRAM chains, setting the length and shortening the chain is exactly the same for a Shimano chain. The difference comes when you rejoin the chain. Whether you do the work on the chain yourself or not, using this video as a guide can let you know if the work even needs to be done or not. May your chain retain it’s position on your drivetrain, Mark. Happy Trails!

Daniel Slusser is a professional bicycle mechanic with over ten years of experience. He holds a bachelor’s degree in business administration from HSU and a master’s degree in history from Cal Poly. When he is not riding, wrenching, or writing he enjoys spending time with his wife and two children.