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Bike is a relatively new Cannondale Carbon Synapse with the Ultegra 6800 groupset (although front chainrings are FSA). Has about 1000KMs on it.

Bike has been great, but I went out on a ride today and about 40K in or so it started losing the chain off the inner front ring. Prior to this it was tuned perfectly and then suddenly I couldn't pedal more than 10 rotations without the chain coming off (not shifting at all).

I took out my tools and adjusted the L screw to tighten up the gap and it still happened. Then I thought I'd just keep out of the lowest rear gear so I could just get home (I was stuck going up a big hill). Still no luck. Must have tried 12 times and lost the chain every time within 10 feet of starting up again - making adjustments every time. By the end of it both the front and rear derailleur were adjusted to be pulling the chain away from the inside brutally - just trying to get home. Eventually I had to give up and call for a pickup.

The only other thing I noticed was that immediately before this happened the rear cogs were getting funny - it kept wanting to shift down (before the hill - was in a higher gear so in the big ring at the font - didn't lose the chain, but was going all squirrelly in the rear).

I can't see what is doing it. Chain rings don't have any bent teeth. Seems to only really happen under load. I can tighten up the L screw to the point where it is rubbing the chain and it still will manage to come off if I put any load on the drivetrain. I don't even see how that is possible but it is what happens.

I have no idea what is going on - any advice?

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Did you make any changes to the bike recently? on a bike that new, your shop should fix it for free. –  Batman Jul 27 at 2:33
    
I would wonder if the chain has come partly apart. Did you check it to make sure all the links look straight? –  Daniel R Hicks Jul 27 at 2:58
    
I did slightly adjust the rear derailleur before the ride but ever so slightly and it rode great - had brought the lower limit in a bit. Did not really check the chain though - will do. Bike store will probably fix it but it's a long drive and I'd rather learn to fix it myself! –  WillyC Jul 27 at 5:17
    
front derailleur cable loose? When it slips, what kind of tension is in the cable? What happens when you put the bike on a stand? Can you shift up and down the gears? It sounds like the front derailleur might be slipping. –  PeteH Jul 27 at 8:33
    
Not a bad suggestion - I'll give it a shot. I can shift it in a stand without reproducing the problem but I haven't checked the cable tension. I'll do that after breakfast! ...note that this problem (in the wild) wasn't happening on a shift, just under load while in the gear. Still worth a look though. –  WillyC Jul 27 at 13:48

3 Answers 3

up vote 1 down vote accepted

One other possibility is a problem on the chain itself. - Pedal backwards and watch closely to see if any any links are twisted. - Make sure that every link pivots the way that it should.

These could have happened when you had problems in the rear.

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In the end it was @DanielRHicks who provided the answer, although he didn't post as an answer so I haven't been able to give proper credit. The Shimano connector pin had slipped out of one of the plates. On casual inspection the chain didn't look bent, but when I stood the bike up and ran the chain watching it closely I could see it catching on this pin. I removed the pin and the bent plate, replaced it with a KMC quick link, and all has been well for many hundreds of kilometres since. I now bike with a spare quick link and a multitool with a chain breaker in case this happens again! –  WillyC Sep 19 at 18:26

I have had this happen due to a worn rear derailleur spring. the rear derailleur isn't able to take up the slack fast enough. also check your chain line, bent rear derailleur hanger, front derailleur cage alignment. see if you can fit a chain catcher on your bike.

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Maybe FD is broken. Check if it does shakes.
Maybe whole crank is bent - not separate teeth. Check whether the crank is closer to frame at one point.

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