Have been having a problem now for awhile which no one seems to be able to fix. Periodically when I am riding in the big ring, especially up hill under some load, and switch to the small ring, the chain fails to engage the small ring. Suddenly no power at all which isn't good. It looks from the riders perspective like the chain is lined up correctly, not too far L or R, but just doesn't engage the teeth on the small ring. The bike is a Look 585 with Campy Record gear, 50/34 compact front. 8 years old. One bike shop told me it was a derailleur problem, replaced the front derailleur, and put on a new chain, made the problem better but not completely fixed. Second shop said they called campy, think it might have something to do with the crankset not being completely lined up correctly because of being taken apart and put back together. This has only been done at most 2-3 times to replace the bottom bracket. They don't seem to think they can fix it that the crankset needs to be replaced. Really? After 8 years, probably 40-45 K miles. I am 5'7" weight about 145 so I don't think I put a lot of stress on it. Oh and chain rings themselves are not 8 years old, more like 2, have maybe 7k miles at most. Second shop took 3 links out of the chain, said it was too long but that didn't fix it. Any thoughts?

Are the crank and chain not the same "speed", i.e. is an 8-speed era crank using a 10-speed chain?

This used to be very common back in the day before gated shifting. If you look at the shape of the chainring's teeth you can imagine what's happening. If the chain hits the ring with a roller to the tooth tip and you spin the crank because there's no load, the teeth will skitter below the chain keeping it out at the edges of the teeth. It's like shifting an auto transmission with the engine RPM not matched closely enough -- gr-r-r-r-i-i-nd.

A contributing factor could be chainring separation. If the chain is a bit too narrow for the crankset, (ie 10s chain, 9s cranks) it can lean against the outer ring with the plates lined up to the chainring teeth, rather than moving over so it drops in.

Also if you have unmatched chainrings, or don't have the little diamonds lined up properly the chain is more likely to land out of phase making your problem more likely.

The FIX,

1- check chainring gap, and/or chain width. test by putting the chain against the inside fce of the outer ring and see how it lines up.

2- check phase marks and rotate the inner ring to a new position if necessary. Test by hand shifting the ring through the outer ring's gate and seeing how it phases as it comes in contact with the inner ring. If it lands roller to tooth, rotate the inner ring to a new position. There are 5 positions each 1/5th tooth apart in phasing, so one will be close.

3- there are no synchro rings on bicycles, so you have to help the chain engage by timing you shifts and easing power off for a 1/4 second so the chain can engage. This is learned technique like toeing the clutch for a fast 2nd to 3rh shift in your car. Once you get it down it'll be second nature and you'll never think about again.

What FB is tring to say is that shifting technique has a place here. How much is unknown but as a life long wrench i have test riden thousands of bikes. One of the constant aspects of test riding a bike when you're real experienced is that you can opperate a bike to make it work well, or not. As a mechanic who wants to get on to the next job, one tries to make the test ride go well. And a good rider can make that happen.

So a rider who is not paying attention or is less skilled can opperate a bike to not work so well, independent of the state of tuning. I see this all the time when the customer says something like "the shifting gets rough only after 30 miles". Meaning only after they get tired and don't pay attention to their technique as much as they should is the problem happening.

The bike does not care if you're in a race, just riding around the block or trying to keep up with your buddy. It does not care if it's in a well tuned state or worn out. It does work better, for what ever state it is in, when the rider ALWAYS cares. Andy.

Yes, I always say that the rider is a key part of the mechanical system. Good riders get good performance even from problem hardware, and poor riders make good hardware work poorly.

But I intentionally listed rider habits last. The OP should take a few minutes to verify that there isn't a clear mechanical cause, testing both the separation, and the phasing. By working that first he gives himself the best chances, then the rest is technique.

Nice thoughts but no. It's a ten speed crank and a ten speed chain. The phase marks(diamonds) are lined up I have checked that myself and also two different professional mechanics have looked at it. As for the rider experience I have 20+ years of 4-7 k per year, and the last eight years on this bike, never happened before this year, so technique is not an issue. Perhaps by applying FB's suggestion about easing off a little I can keep it from happening but the fact remains for 7 years and many thousands of years it never happened at all, so there should be a fix, perhaps a new crankset but I wish someone could explain to me what has happened to the current one

That may be a clue. The rings have something like 35k miles on them, and I assume you've been through a few chains, so it could be as simple as a wear pattern affecting the shape of the chainring teeth. A new crankset isn't indicated, but possibly a new inner ring is.

I had this problem when replacing the stock rings on a crank for some that are custom-sized. They were spaced / offset slightly differently than the originals, and so the chain could often skate between the middle and big rings without being engaged. This couldn't happen with the stock rings because there was not as much space between them. Since you said that the rings had been changed, I would first look to see if they are the problem - even if they are also Campy rings then they could have been designed for a different crank since they were bought 6 years later. So, try some different rings and see if this fixes it.

The newer rings are Campy record exactly the same as the ones they replaced and were onn the bike for well over a year before this began to happen.

First follow the good advice found in the posts above.

Beyond that, my questions are:

1. What was the very last thing done to the bike before the problem started (i.e. FD shifter and/or FD cable replacement, rear wheel/ hub/gear-set change, etc)?
You mentioned that the "a shop said it might be due to the crankset not being lined up correctly because of being taken apart and put back together. This has only been done at most 2-3 times to replace the bottom bracket." I would, as others have mentioned, start with the crankset and bottom bracket. I'd carefully check the newest bottom bracket length against the original BB (if possible). However, I think the "2nd shop" and FBinNY are correct in saying that the problem is due to "taking the crankset apart" and then re-assembling it incorrectly. This could very well be your problem.

2. Did the bike suffer any damage before the problem started (i.e. crash or moving damage, slightly bent rear triangle, etc)?

3. Did you yourself measure the chain line and, if so, is this measurement with specifications? Note: measuring the chain line "from the riders perspective" does not always mean that it's within specs. Also keep in mind that some crankset/bottom bracket and/or FD combinations can be "incompatible" in regard to proper chainline and/or shifting.

4. Is the FD shifter and cable in good condition? And is the cable of the proper size/type and correctly routed?

I too would suspect an improperly assembled crankset (if all else checks out OK). This, of course, means that you'll need to either correctly re-assemble the crankset (as mentioned by FBinNY, etc) or replace the crankset (as mentioned by the "2nd" shop.

I have the same issue with a 70's bike with Campy Gran Sport crank. Checked all the usual suspects but nothing jumped out at me. It only happens when I gear down preparing for a hill while the cranks are still at speed. If I slow down the cranks a bit to allow the chain to engage it's not an issue. I use SRAM 8 speed chains and I'm thinking there is mismatch between the chain and chainring. I have seen similar issues with SRAM chains and Shimano components that were solved with a Shimano chain.

I re-read the OP, and now see that they aren't the original rings. That convinces me that it's either an issue of separation or phasing (or both).

Check the separation carefully by placing your chain against the inner face of the outer ring and seeing if the plates touch the tops of the inner ring's teeth. They shouldn't, the chain should be forced over so the inner plate is just outboard of the tip if the inner ring's teeth. Otherwise it'll ride the teeth rather than dropping in cleanly.

It could be that this was OK for a while, but with some tooth wear the problem worsened. If both rings are Campy brand, it's possible hat you have a 9s ring on the outside rather than a 10s. Or if it is a 10s, it's of a later vintage, and lacks the relieved (I don't know if they changed since 2004) area on the inner side of the mounting area. This was intended to move the ring inboard on 9s and earlier cranks (used on OEM 10s until 2004 at least).

Either way, if you see a chainring gap issue as described, fixing it (by any means) will solve the problem.