By Jim Langley

As promised in last week’s episode, this Tech Talk covers a few additional chain-drop, chainline and front derailleur issues raised by readers in our Comments section. As a reminder, a few weeks back, in Issue 732 of RBR Newsletter, I wrote about helping a friend diagnose and solve his chain-drop issue. We’ve received some excellent comments on these topics in the weeks since. So, I’ll share a few of the most insightful ones and add a few more tips.

Front Derailleurs are Just Plain Difficult to Adjust

First, an important point to appreciate is that front derailleurs may be one of the hardest things on a bicycle to adjust perfectly. I say this because as a bicycle shop service manager, I found that it was always among the most challenging things for new mechanics to learn and master. Some could never get it right and would have me do the final fine-tuning for them.

What makes it so difficult is how front derailleurs mount to frames. It’s nothing like the rear derailleur that simply bolts on and is automatically in the optimum position (assuming the dropout and derailleur hanger are aligned).

Instead, the front derailleur can be positioned in an almost infinite number of positions – even when mounted to a braze-on on the frame. Even if you understand the basics of front derailleur adjustment, it’s easy to make it too high, too low or angle it incorrectly. And, if you do any of these things, it won’t shift properly, if at all.

One of the challenges is being able to see what’s going on and to determine what change is needed to solve whatever adjustment issue you’re experiencing. That’s because, judging the gaps between the chain and the front derailleur cage (both sides), and also the gaps between the cage and the chainrings can be difficult. But if you can’t see it or recognize that something’s misaligned, you can’t correct the problem.

FrontDerailleurAlignment.WEB

To help, here’s an excellent tutorial from Park Tool.  Their photo above shows how slight changes in the derailleur angle can mess up the shifting.

And Now For Your Comments

Operator error

Chuck Davis made a great point about chain drop: that it can be caused by operator error, too. He said, “failing to trim when you’re on the big chainring and on the largest cogs lets the front derailleur take a full swing at an enhanced chain angle. And that can cause the chain to come off.”

That’s absolutely correct. As you shift up onto the largest cassette cogs, the chain angle increases to the extreme. If you decide to shift to the smaller chainring when your chain is angled like this, it’s almost asking for the chain to come off. Ideally, you would realize you’re in a gear that’s creating a risky chain angle and shift to a smaller cog to improve the chain angle. Then you can safely shift to the smaller chainring.

Chain rub or chain drop, those are his options

JeffDH explained, “My problem with chain drop is that if I adjust the limit screw so that the chain NEVER drops, then in the outside cassette gears the chain rubs on the derailleur cage. This happens both with the large cog/small chainring and small cog/large chainring. But, if I adjust the limit screws 1/4 turn in/out so that the chain doesn’t rub on the derailleur cage, I will occasionally drop the chain, especially if I shift the front when the rear is already in the last couple cogs on that side of the cassette.”

These symptoms, where you either have chain rubbing or chain drop usually indicate that the derailleur cage angle isn’t aligned correctly with the chainrings. If it’s angled even slightly wrong, it will rub and/or can throw the chain. So, I would check closely by sighting from above. If you systematically shift into all of your gears one by one and look at the alignment of the cage, you should be able to see which way it’s off and how to change its angle to fix the problem. (The above photo clearly shows how the angle can be off either way, in A and B, and correctly aligned, in C.)

A worn chainring can throw the chain, too

Larry K. pointed out that “worn chainrings can cause the chain to drop when backpedaling, too. I had one that did that even though the teeth weren't hooked enough to cause skipping. The teeth were worn to a point, however, and that seemed to be the cause of the unshipping – especially when the chain wasn't in the middle range of the cogs. Chain drop might occur when pedaling as well but I have an inside chain retainer on my bike so it never did so for me.”

With modern chainrings it can be hard to visually spot a worn ring since so many have different shaped teeth to optimize shifting. Larry mentions hooked teeth, referring to how teeth that are symmetrically peaked when new, take on a wave-like hook with enough miles.

This is kind of hard to judge, so what you could do is snap a close-up photo of your chainrings with your phone and then use it for reference when you think your rings are wearing out and shifting poorly. Or, if you keep some new replacement rings on hand, a surefire way to gauge wear is to hold the new rings right next to your old ones to compare the tooth profiles.

Steve’s chainline tip

Finally, Steve Andruski mentioned, “Park Tools made the CLG-1 and CLG-2 chainline gauges. They are both now discontinued, but there's still an extensive article about chainline on the Park website."

Thanks a lot, Steve and everyone else who commented. Here’s to smooth shifting and chains that stay on! If you've got more tips related to these topics, let's hear them in the Comments below the Newsletter version of this article.


Jim Langley is RBR's Technical Editor. He has been a pro mechanic and cycling writer for more than 40 years. He's the author of Your Home Bicycle Workshop in the RBR eBookstore. Check out his "cycling aficionado" website at http://www.jimlangley.net, his Q&A blog and updates at Twitter. Jim's streak of consecutive cycling days has reached more than 8,000. Click to read Jim's full bio.

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