Chain won't seat onto big ring gear teeth...help?
 

Hey guys, this problem is really stumping me.

When I shift to the big ring, the chain keeps jumping off my bike to the outside, and I've adjusted the derailer back and forth in hopes of getting it to stay. Now the derailer is to the point where it barely pushes the chain over far enough to the big ring - but still the chain jumps track!

Finally, I noticed it seemed like the chain wasn't catching properly and took this picture of it balancing atop the cogs. http://bikeforums.net/attachment.php...hmentid=408397

Has anyone seen this? I'm wondering if something is worn out and needs to be replaced, or is it a matter of something misaligned.

Any ideas help.

Thanks!

Davey

Oh, it's a shimano 105 and an FSA compact crank if that helps...

What would help most is knowing how long it's been a problem, and what changes, if any, were made before it started.

For example; if this is a new chain on a very old chainring (doesn't look that way) then that would point to some possibilities.

One diagnostic that you might try. Remove the front derailleur, (or slip the chain put and let the FD swing as far inboard as possible so it's out of the way), and manually shift the chain back and forth with your fingers and see if it settles nicely. If finger shifting works, then you know it's simply a FD adjustment issue.

BTW- something about the photo bugs me. Please measure a 12" (24 links) length of chain with a ruler. Tell us how far beyond the 12" mark the 25th pin is (should line up exactly like the first, but will be farther out on a worn chain.

Hey thanks for the tips. It's all the original stuff and nothing new has changed. The bike is 5 years old or so. It just started doing that seemingly out of the blue.

I tried the manual finger test to shift and I discovered that the chain will 9 out of 10 times ride high like the picture.

When measuring 12 inches of the chain, I found the links to be about a 16th/inch longer. Would that cause it?

Thanks!

Check the alignment of the derailleur with the chainrings as well as height (Park Tool Co. » ParkTool Blog » Front Derailleur Adjustments). If the derailleur was knocked and moved to a non-parallel position it could push the chain too far.

Thanks for the link, there's a lot of good stuff on there.

Yes, I've made adjustments to the derailleur and lined it up perfectly. I'm starting to think that the FD is fine and something else is the problem such as a worn chain.

I don't think 1/16" inch longitudinal wear is not enough to cause shifting problems, but a chain that developed more side flex can shift erratically. Try putting the chain in the large/small combo (high gear) and push it sideways midway on the chainstay. Compare the deflection to another bike.

Davey, I replace my chains when they reach 1/16" stretch or just slightly before 1/16". In your case a new chain maybe all that's needed to bring shifting the FD back to how it should perform.

I have one bike with a non ramped/pinned crank set, also lacking specially cut chain ring teeth, using indexed shifting and there seems less forgiveness for anything not within spec.

Brad

One thing I see is that the chain isn't on the pickup pin. If chains shift elsewhere than the designated place they aren't phased for the new sprocket to mesh, and will ride the tops until in phase before dropping in.

If you look at the photo, you'll see the chain starting out with the roller on the tip of a tooth, as you go farther along toward the rear it gets better and better, and I'd expect it to be dropping in within a few more teeth.

It's possible that the pickup pin is worn so there's no longer control in when it shifts, in which you've lost the "glide" in hyper glide, and it'll shift at any time and ride the top until it's in phase then engage (like before hyperglide was invented). Sometimes older chains act this way because they are more flexible than the design requirements, or possibly the inner ring isn't timed correctly -- is it the original and did anybody switch or remove it at any time?. Chainrings have timing marks so double check that they're aligned correctly.