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Re-set the derailleur; tighten the cable (probably replace it). The der may be mis-aligned. If it moves correctly this won't happen, but you can usually correct a mis-shift with the der and crankset in motion.
 

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Yeah, this still happened to me with a new chain and a well set up new front der, but an old inner chainring. All 10 speed. A new inner chainring has solved the problem. I can still make it happen but only by deliberately doing awful shifts.
 

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Cycling Dolomiti Friuli
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Discussion Starter · #7 ·
Shortened the chain a couple of links which has helped with chainstay slap but no help with chain intermittently spinning freely when shifting from big ring to small ring. Thinking of removing the chainrings, seeing if prior owner/shop maybe installed some chainring spacers (I've heard of people doing that on compact cranks so they can use small ring/smallest cog without rub). If not I'll try a new inner ring (they're like $50-$60!)
 

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Hmm
I can always MAKE my 10s do this if I am in 8 on the back (10 is the smallest) and I downshift the front at very low rpm. Even with new chain, chainrings, well adjusted front der.

But it never (almost) happens in real riding.

How many miles on your chain & inner chainring? Are the inner chainring teeth still sharp pointed, I think that makes a difference. Not sure though.
 

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Dunno. I am far from an expert. It must be frustrating
Warped inner or outer chainring?
Front der properly set up? Especially inner plate parallel to chainring. Also its height, maybe try a bit higher?
Accidentally put on an 11s chain?

All real long shots. Maybe some other expert will come along.
 

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How about some pictures?
pictures of your chain in big/big and small /small. pics of the FD looking down on it. How slack is the front cable. If you take this to a shop to look at My guess is that they will be able to ID the issue cheaply and very quickly.
Chain came in Campy box marked 10 speed
This has been happening when shifting from big ring to small ring (outer plate of FD pushing chain toward frame).
 

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Happened to me today

This happened to me today, so you are not alone. I like to think of my setup as "well adjusted" (just like me ;)). First time in hundreds of miles.

I was in 10 on the back (smallest cog), big ring, going down a steep hill to a sharp rise. I shifted the back to 8, then dropped the front at low revs with no power on. It skated, I panicked because I was losing momentum fast and there was traffic. Managed to get it back on the big ring, but somehow the chain sucked and crunched into my chain stay. Fortunately I had helicopter tape so no real damage except to my ego.

I really don't know what causes it, apart from bad shifting style. Sheldon Brown says "avoid shifting in this way".

This bike has the older style EPS chainrings, my other bike with the newer MPS chainrings has never done this. The MPS inner ring tooth design has changed to be more pointy, I wonder if it can catch a skating chain better?

It has only ever happened to me in 8 on the back. No other gear does it (touch wood).

My front der is about 1.5 mm from the teeth, inner cage parallel as per Campy instructions, well adjusted for chain rub. My chain has 200 miles on it, chainrings fairly old (now swapped to newer ones).
 

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The EPS chainrings have Brevet Campagnolo - Made in Italy stamped into them along with the size. They are always bright metal.

The MPS chainrings have clear machining marks and have painted logos (at least the Centaur ones do). I don't think they have any stamps of any kind. MPS was launched in about 2010 for Centaur & Veloce. You can download from Campag the spare parts catalogue for 2012 and find the part number. Bound to be expensive, Campag spares always are!
 

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You might want to do some more diagnosis. As I said, this only ever happens to me in 8th cog on the back on a 12-25 cassette, 39 x 53 chainrings. And even then only once every few hundred miles on my EPS bike, when I am not paying attention.

I am not sure "just" an MPS chainring will fix the problem. I think MPS is better at "rescuing" a shift that has gone wrong than EPS, but those wrong shifts should not really be happening.

How often does this happen to you? Which gears & what circumstances? Compact or standard chainrings?

Are you running the "new shape" front der (launched in about 2007/8?). It is much stiffer and IMHO better than the "old shape" from the early 2000s. Is your front der set up exactly as per Campag instructions, ie 1.5 mm above big chainring, inner cage exactly parallel to the chainring, aka outer cage about 1 mm tail out? Do you have low front der cable friction?

Photos? More info?
 
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