With this Quick Question series we will present fast fixes and collect comments from seasoned riders around specific D.I.Y. mountain bike repairs. While much of this trailside triage is covered in our repair articles and videos, this is a space for longtime riders and readers in the Singletracks community to share their knowledge. Please type your related experiences and advice in the comments below. Do you have a quick question? 🤔 Email email@example.com.
Front derailleurs are still dead, and the dust is thickly settled on that subject. All hail the narrow-wide chainrings and derailleur clutches that saved us from the menacing 2X and 3X drivetrain overlords. We now have a wide gear range without all the faffing, and the massive cassettes are slowly losing weight year by year.
What about preventing the chain from hopping off those child’s-drawing-of-a-mountain-shaped teeth? That’s the one thing a front derailleur was relatively good at. Why does my chain come off, what’s the best way to get it back on, and how can we prevent it from ejecting again? These are the questions we bring to you, seasoned mountain bikers, to share with the newer riders in our community. We will take the first crack at answering questions like this to ignite the discussion.
Starting with the assumption that your bike has a contemporary chainring, with alternating tooth widths designed to better retain the chain, there are multiple reasons your drivetrain might chuck its serpent links. A bent or stiff link can cause a derailment, as can a chain that’s too long or one that’s not designed to pair with your particular drivetrain configuration. As mud from the rear tire builds up around the bottom bracket it eventually ends up on the chainring, which will lead to a dropped chain. A worn-out derailleur clutch will allow the chain to flop and bounce until it eventually loses the teeth. I’ve also seen a chain that was so over-lubed that it regularly came off the ring because the oil was picking up loads of trail debris that would get lodged between the ring and chain. A chain line that’s too far from its intended measurement won’t help things either.
Once your chain ejects, you will want to stop immediately. If you continue riding, especially on a rough descent, the chain can become tangled, making it more difficult to re-mount. In the worst-case scenario, it can catch on the spokes, wrap around the rear wheel, and rip the derailleur off. Even if you’re racing a gravity event, it’s probably worth stopping to get the chain back on.
If you’re lucky, the chain will be sitting on top of the bottom bracket cup and you can push the derailleur cage forward with your left hand while seating the chain over the ring and be on your way. If the chain fell off the outside of the ring it may be lodged between the crank arm and the ring. Like removing a chain from between the cassette and spokes, this extraction requires care to keep from bending links. Make sure to pull straight up, not flexing the chain side to side. It may be stuck by the force of your legs or weight, so you’ll need to pull up hard.
If your bike has a chain guide, this trailside triage might take a few extra minutes. I would highly recommend taking the plastic upper guide off to get the chain back on. That way you don’t have to fight the guide to remount it, and there is less likelihood of damaging the guide or chain. Most guides are held on by a single 4mm hex bolt and can be removed and replaced in a minute or two. If the chain has turned into a pretzel it might be better to open the quick-link, untangle it, and reinstall it appropriately.
How can we prevent the chain from dropping in the first place? For starters, make sure that the chain and chainring are designed to play together. For example, some Shimano chains don’t fit properly with certain chainring teeth, and some chainrings don’t accept 12-speed chains.
With a proper chain line and length, a fully functioning derailleur clutch, matching drivetrain components, precise adjustment, and no bent links or busted teeth, the next step is a chain guide like the one pictured above. There are several guide styles and mounting configurations, but they all do a similar job. With a well-aligned chain guide, most riders never have to consider their chain falling off. However, if it does you’ll thank yourself for packing a multi-tool.
While we are right to praise the invention of narrow-wide chainrings for their retention qualities, clutch derailleurs deserve just as many accolades. They keep our drivetrain quiet and intact, and without them that fancy chainring would fail more frequently. If your chain is dropping no matter what you adjust or replace, the clutch might be ready for a service or replacement. I have managed to finish several enduro races without a chain guide thanks to that little clutch, provided I take good care of it.
Do you have any tips for keeping the chain on your bike, or stories about epic ejections? Please share them with us in the comments below.