SilverStretch

I have less than 500mi on a brand-new bike with Ultegra FD, RD, chain, and cassette, and an FSA triple crank. Problem: During climbs, when shifting from the middle ring to the smallest ring, I occasionally encounter a condition where the chain "skates" on top of the smallest ring, not engaging it at all, leaving the crank to basically freewheel while the chain remains stationary. This immediate and complete loss of resistance in the drivetrain is a Bad Thing. It is scary, disorienting, hard on my knees and the disruption to the ride has about gotten me killed a couple of times.

This thread has a picture of my exact problem (I have one as well, taken just today, if it would help)

This thread seems to indicate that the smallest ring might be installed...backwards? The same thing is suggested here as well for a Champy setup. Seems strange to me that a ring would be asymmetrical and installable either way, but I'm awfully new at this.

This thread suggests a worn chain as the culprit, but the whole drivetrain is so new on my bike that this seems unlikely...?

I am going to take the bike back to the LBS that assembled it and ask them what they think (they include a tune-up once the cables stretch a bit), but frankly I'm just as interested if any of you have insights!

Thanks in advance for any ideas...this has got to get fixed before it causes me a serious problem.

Al1943

Several considerations.

1) Cross-shaining. Where on the cassette was the chain when the skating occurred on the small chainring? If the chain was running on the right side of the cassette this can cause contribute to the problem of the chain not dropping over the small chainring teeth.

2) Too much tension on the front derailleur cable.

3) It may be possible that the small chainring is reversed, I'm not very familiar with FSA cranksets.

4) Thickness of the chainring teeth. My wife's TA chainrings were so thick that the chain would skate on the middle chainring when shifting from the big to middle if the chain was running on the right side of the cassette. I filed a bevel in the tips of the teeth.

5) Chainline. Ideally a Shimano triple cranksets should have a 45 mm chainline. You could try measuring from the center of the downtube to the center of the middle chainring. But 45 mm is not necessarily best for every triple. A bottom bracket spacer can be an easy way to move the crankset out 2 mm. This would help the chain drop over the teeth.

Since it is a new bike the best thing to do is keep after the dealer until he gets it right. But you should also continue to learn the cause of the problem and the cure.

SilverStretch

Thanks very much for your quick and thoughtful reply Al1943.

It has not occurred during cross-chaining; it happens on the 3rd or 4th largest cog when going from the middle to smallest ring.

As to the other possible causes you mention, I will take them up with the LBS.

I have been thinking about replacing the FSA triple crank with an Ultegra triple since I chose the bike; having one component of the drivetrain not match the others never quite sat right with me. If the problem is the thickness of the teeth or a some kind of spacing mismatch, that might be all the prompting I need!

FBinNY 

When the chain drops to a smaller ring, there's no assurance that it'll drop phased for the rollers instantly drop between the teeth and engage. If the rollers land on the teeth and you're pedaling hard the sprocket may skate exactly as you describe. It's like driving with a non-synchro auto transmission, you have to help it engage by matching revs a bit. On a bicycle that mainly means easing up pedal pressure until the chain settles and engages, which takes a fraction of a second.

Also, chainring separation can also be a factor. If the rings are slightly wider apart it's possible for the chain to ride the tops of the inner sprocket on the plates while resting against the face of the larger ring. Narrowing the gap forces the chain over so the rollers and spaces between them are centered over the teeth.

One other thing you can try if you don't have matched and phased chainrings (these are made in matched pairs with small timing marks that are usually lined up with the crank arm). Rotate the inner trial and error (provided the number of teeth doesn'r divide by the number of mounting bolts) trying the various mounting positions until you find the one of five that gives you the smoothest shifts.