Dancing Chain (riding up on chainring)
 

I am having an issue with a chain riding up on the chainrings. Everything works great on the stand. However, when I get it on the road, the chain starts "dancing" across the lower stretch of chain between the lower pulley and the chainring. At first, I though it was a bent RD, but after I installed a new RD and the problem persisted I did some additional trouble-shooting. While on the stand, I started adding some resistance by manually pressing on the brake caliper and noticed that I could duplicate the problem. I noticed the chain starts riding up the chainring at the bottom of the rotation. Can some explain what is happening and suggest a remedy.

I'm wondering if I have a too-think, early generation chainring. However, the large ring is a pinned, later generation chainring which came with the crankset. Although the problem is not as pronounced in the big ring as the small ring, the chain does "dance" or ride up the back of the chainring on both the big or small ring.

I made a couple videos, which I hope I am successful in posting. The first is on the road and the second is the bike on the work stand (after a few seconds, I press the caliper and you can see the chain ride up the bottom of the chainring).

Lastly, the entire drivetrain (Campagnolo 9-speed chain, Chorus 9-speed cranks, Chorus 10-speed RD, Campagnolo 9-speed cassette) has fairly low miles, though it's been pieced together from various sources.

Thanks in advance.

http://i850.photobucket.com/albums/a...ps6f9gp8ce.mp4

http://vid850.photobucket.com/albums...ps6f9gp8ce.mp4

http://i850.photobucket.com/albums/a...psb2iw9kev.mp4

http://vid850.photobucket.com/albums...psb2iw9kev.mp4

How old are those chainrings? Are the teeth worn to sharp points instead of coming up close to parallel then squaring off. New chain, old chainrings? It looks like the teeth are picking the chain up (actually not releasing it). A new chain will have a much tighter pin to pin distance than the distance between worn teeth.

Ben

All drivetrain parts are used, but not overly worn in my opinion. Here's a close-up of the chainrings, which seem to me to still have plenty of life left.

That's a good way of putting it: the chainrings aren't releasing the chain. Again, only an issue on the road or when resistance is added.

http://i850.photobucket.com/albums/a...psyz9z1sj1.jpg

are all of the spacer in place between the two chainrings ? if one or more are missing it could cost your problem . it the spacing between the chainrings that the chain getting caught by .

That doesn't make a big difference because the teeth stay in the same place. Although, often with wear, the pulling end of the tooth is angled upward so the chain is forced to ride high, which then makes the chain seem short.

Teeth can wear unevenly (I now periodically rotate my chainrings).

I use an old Campy Nuovo Record (SR?) era chainring with 9spd without problems, and now even using a Campy 9s chain.

The first thing I'd do is to physically measure the chain. How old is it? New? Used?

A tight link can create a periodic hop, but not necessarily continuous hopping. Anyway, worth looking at.

Those teeth don't look bad. Maybe a little elongation of the slots, but not too bad.

As a test, you could try flipping the inner ring around backwards and see how that affects it.

The first hint about cause is that the problem shows up when there is pressure on the drive train, either from being on the road or applying the rear brakes. As the tension on the lower section of chain between chainring and pulley is not affected by pressure on the drive train the problem must be occurring before that point. That means the interaction between chain and chainwheel, such that the chain is unable to disengage when it has been solidly seated in the chainwheel teeth.

What you are experiencing is chain suck, and the most likely cause is that your chain is new or not worn and the small chainwheel is sufficiently worn or damaged - or a combination. Although not a "shark's fin" the small chainring is definitely more worn than the large. I'm also wondering if that chainring was often ridden in more of a cross-chain combo, which could have produced some burrs. You could certainly try reversing the chainring, as well as making sure the sides of the teeth are smooth. A new chainring would certainly solve the problem. Here's quite a treatise on chain suck: http://www.fagan.co.za/Bikes/Csuck/

This needs to be corrected as soon as possible, as it can get suddenly worse, especially on a climb, resulting in the chain wrapping around, causing the pedals to seize and possibly a fall or damaged parts.

Agreed. Second vid shows the chain not disengaging the small ring. Andy

Thanks everyone for the feedback.

That link is really helpful. Thanks for your input and suggestions. It does seem I am experiencing chain suck. I think I'll experiment with another chain and perhaps careful filing of the sides of the chainring teeth, plus some extra lube, then go cautiously from there.

You mention adding lube -- is the chain clean and lubed now, or might it be a little gunked-up at present?

Chain is very clean with normal (perhaps on the light side) application of lube. Not gummed at all. However, one other potentionally important note: the chain was involved in a catastrophic incident in which another derailleur went into the spokes and damaged the dropout (since repaired). Although the chain appears straight, I do wonder if it got torqued/twisted out of shape and is contributing to the chain suck.

If the chain suck is not periodic with the crank rotational position but seems to be every 2 or 3 crank revolutions then, yes, the chain being damaged could be the cause. Chain link issues come around at an odd period compared to wheels and cranks. Andy

For posterity, the chain suck issue has been resolved. The chain rings were the issue. Upon closer inspection the small ring was more worn than I realized and had small mushroomed edges one side of the teeth which may have been holding up the release of the chain:

http://i850.photobucket.com/albums/a...psawdmxawg.jpg

I sourced a new small chainring and the chain suck went away on the small ring. However, I was still getting a bit amount of chain suck on the big ring, too. I then went about hand filing the teeth judiciously to remove the slight beginning of a shark fin shape. A new big ring will be needed sooner or later, but for now all is working smoothly.