Morning folks,
Suddenly my race bike has started dropping chain when I’m in the last two biggest cogs, while on the small ring: it doesn’t happen if I’m on the big ring and the last two cogs, or if I’m on the small ring and below those cogs. If I am, in a matter of 8-9 pedal revolution it will come off. If it helps, it’s a 2x10 campy Record (2010 or so I think). Any tip? Unfortunately I’m on vacation, and I can’t resort to my usual bike shop of trust, so I either fix it myself or find a good LBS.
It sounds like you need to check the low-limit screw on the front derailleur. 
I believe it’s this one on the 10s Record FDs. You’ll want to shift into the small chainring and biggest cog before driving it in slowly until it’s just barely not touching the chain (2-3 mm).
The good news is if this is the problem you’ll only need a Phillips screwdriver to fix it. 
Good luck!
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Use a JIS screwdriver instead of Phillips to avoid rounding off the screwheads
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I don’t have either :-/ I guess I’ll either ask the local shop below or see if a supermarket has it. This is the time where I regret not buying a multitool to keep with me at all times…
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Although if one is careful, a regular screwdriver will work. Just be careful to not put too much force on the screw head and allow it to slip. For a quick turn or two, a standard #2 should suffice.
But Park Tool has the screwdrivers for you! The ‘Phillips (JIS)’, and a ‘flat blade’ too. ($34 each!)
Maybe try borrowing one? It shouldn’t take much time to do it. (and is this a new symptom, or one that has been happening for a while?)
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Brand new: two rides ago, nothing at all. One ride ago, it dropped a couple of times, one of which truly surprised me as I was standing (the second one I was shifting down from the big ring, I thought it was operator mistake). Today it started and I couldn’t pedal for more than 9-10 revolutions in those cogs.
I’m surprised if it’s the derailleur that needs adjustment: should the chain stay on regardless of the derailleur position?
Well, if this is a new symptom, there has to be something else wrong. Did you change the chain recently? Is it possible that it’s too long? Have you checked that everything is attached tight? I had a loose front mech screwed-up by the shop I bought it from 
, and never thought to check that when I was having issues. (The assembler was fired a week before I came in with the problem, but who doesn’t check a bike before it’s shipped?! I stopped spending money there for sure)
It just seems too pat to be a limit screw problem, out of the blue like that. Think back to what has changed recently, and start there…
‘Should the chain stay on regardless’? YES YES YES!! Especially if nothing else has changed. To start all of a sudden is so odd. Yeah, it’s possible to undershift and toss the chain off the bottom, but that’s a set physical limit, it’s not made of rubber, so to have this happen like this is really odd (to flog a dead horse)… Something changed
That’s the problem: nothing changed. I have not changed chain or anything. The only thing I did in fact was to wash the chain and lube it again, but even if that was the issue, I had a ride after that and everything seemed fine. The only thing in between was that I packed it in the car, but it’s not the first time that I do it, and I don’t recall doing/happening anything strange.
When you packed it in the car, did you have the wheels on, or off? If off, was there any possibility that the weight was resting on the rear mech, or that the frame was wedged between items that could have shifted? It’s a long shot thought, but something changed
(I packed my bike in the car for a return trip from a ride, and unbeknownst to me, the rear rotor contacted a suitcase and warped in the first leg of the return. I stayed at the same hotel that had a wicked steep climb out the backdoor, and wiped my bike out, put the front where on, and DENIED!! I was so gutted. HAH!! Poop happens!)
I took wheels off; bike was on top of luggage, nothing on top of it. I agree something must/could have happened, but I have no clue what it could have been.
I just turned the bike upside down and spinned it (I don’t have a tripod, being on vacation), but I wasn’t able to replicate the issue on those cogs. To my eye, everything seems fine and aligned; to my eye the front derailleur seems to be right as is. If anything, when back spinning it in the last cogs looks like there’s a bit of drag that pulls the back derailleur, but I think that’s because of the diagonal chain position between the small ring and the last cog.
I guess either way, adjusting the limit screw should allow you to ride, but the reason for the sudden change is disconcerting. That doesn’t ‘just happen’.
Is the crank loose, the chain ring loose? 
The chain is likely to drop if your limit screw isn’t where it is supposed to be. It’s there for a reason after all! 
Dropping your chain when shifting from the big ring to the little ring (especially when at least halfway up the cassette) is a sign that your FD isn’t where it’s supposed to be. Even if there is another issue somewhere else, your FD should stop your chain from dropping.
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I also thought about the crank, but I just checked and it’s not loose; same about the rings. The chain should not be worn out, I tested it two months and it was fine.
Also: why is this happening also when I’m not on the bike? I tried to pedal it while keeping the wheel off the ground (and upside down) and the chain doesn’t drop.
things perform differently when they have the load of a rider on them.
we should probably clarify, is the chain dropping off the chainring or off the cassette?
by the way, you probably could have found a shop at this point that could have looked at your bike in person. this type of issue is something that a good shop should be able to diagnose and likely fix in a matter of minutes while you wait. It’s worth a shot!
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Yes, so I’d start looking at all the things that aren’t likely to happen like tweaked frame, cracked chain stays, or other parts. (A local shop had a bike in that confounded them, and it turned out to be a small hidden crack in the chain stay connection to the BB shell. Under force, it could flex enough to cause issues. The frame was eventually replaced, and they were able to keep the frame and had someone cut it apart to show the crack, and the amount of resin and fiber mats involved in manufacture. It was a mystery why the stay cracked, but it was replaced, so good on them)
Off the chainring; sorry if it wasn’t clear.
Re: local bike shop, I wish: I’m in a three thousand people village, and the only shop is an eBike rental, which I didn’t trust to begin with, and even less after I talked to them.
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Maybe you should buy a multi-tool from the local bike shop.
Also, is the FD mounted (to the seat tube) with a clamp? if so, make sure the derailleur (with the clamp) didn’t just get bumped and all rotate about the seat tube, thus changing the limit screw setting (this was always the biggest aggravation with setting FD in the '80s and '90s)
This might seem obvious but closely inspect your chain and chainring. Maybe even get a marker and mark what link/ tooth it’s dropping from to see if there’s any consistency. I’ve had similar issues appear suddenly and in one case found a bent plate on the chain, and in another found a bent chainring tooth. Neither were obvious with a quick glance but both were very clearly the cause of the problem once I did notice them.
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Not obvious at all - I’ll try and do that. Unfortunately it would be very hard to find the specific moment it drops, since it only happens when I’m on the bike, and rather suddenly without any hints that is about to happen. Meanwhile I found a shop to take the bike to tomorrow, let’s see what they say.
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