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Discussion Starter · #1 ·
I have a Shimano Ultegra di2 set up, 53/39 in the front with a 28-11 in the rear. Chain is tight and not worn, and derailleurs are adjusted correctly.

I am having an issue with my chain coming off to the outside of the large ring when I am lower in the rear cog, anything in the 13,12,11. It has happened twice on a steep and fast downhill with a couple bumps under moderate power, and once when I was sprinting in the 53/12 under high power(I was sprinting so as much as I could put out anyway). Before this happened everything worked fine, and shifted normally. I have adjusted the front derailleur so that it is almost rubbing on the chain on the outside of the big ring to act as a chain catcher or sorts, but is there another reason that I should look towards to fix this?
Thanks for the help!
 

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Things can flex around quite a bit when you are humping down on the peddles. I would check for a loose bearing or crank arm. If they chainring is slightly bent is may be very bent under load.

Ride Safe,

Joe
 

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Discussion Starter · #5 · (Edited)
Things can flex around quite a bit when you are humping down on the peddles. I would check for a loose bearing or crank arm. If they chainring is slightly bent is may be very bent under load.

Ride Safe,

Joe
It just had the crank off about 500km ago, so I would hope everything is still nice and tight in there. I will check that out and see if the chain ring is noticeably bent or off
 

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Chain was installed in the correct direction, right? Newer Shimano chains are unidirectional, though I'm not sure if/why improper orientation would do this.

Otherwise I'd think the chain is still a bit long, which can be a bit sensitive since you're running an 11-28.
 

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Discussion Starter · #7 ·
Chain was installed in the correct direction, right? Newer Shimano chains are unidirectional, though I'm not sure if/why improper orientation would do this.

Otherwise I'd think the chain is still a bit long, which can be a bit sensitive since you're running an 11-28.
Yes right direction. I thought it might be a bit long but when it is in the 53/28 it is very tight, so tight I don't think I can remove a link without it seizing up.

What was weird to me is that it just started happening. I race a crit on Saturday with quite a few out of the saddle sprints and I was there at the end sprinting for the line with no issues. Come Tuesday of this week and it falls off.
 

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Did you check for any kind of frame damage? FD mount cracked, cracked BB? Sometimes these kinds of mysterious issues crop up because some component got compromised, somehow. Try to grab your FD cage, and see if it somehow behaves unusually. Same with your cranks, and chainrings. Any lose bolts anywhere? Cracked chainstay or seatstay? What you are describing is extremely unusual, so something is fishy here...
 

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Discussion Starter · #9 ·
I did check for something being loose/bent. I grabbed both crank arms and flexed it back and forth as hard as I could and it wasn't loose or really flexible at all. I am having a Quarq installed next week so at that point I will make sure that everything gets a solid once over but there isn't any damage from wheat I can tell
 

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Discussion Starter · #11 ·
Just wondering if you have managed to resolve this issue ... curious as to what is happening ...
I did...by putting on a Sram Red Quarq.

I think the issue was a slightly bent tooth on the outer ring. It was causing the chain to roll over towards the outside, and with enough force it would pop off. Rotor cranks are garbage in my opinion, I certainly won't be running one after this.
 

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Discussion Starter · #16 ·
You have Ultegra Di2 and you opted for Rotor chainrings? :confused:
That's how the bike came, I didn't opt for that. Presumably it came that way to save money on the manufacturers end.

The OP problem is characteristic of a worn large chainring (aka sharkteeth). So, yeah, changing the crankset will fix that too.
Under 1000km on the set, so not overly worn. They may have been bent from the front derailleur pushing the chain over under load or something weird like that. Either way I'm glad to not be using them any longer
 

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Can you post pictures? From the side and front profile of the bent tooth?

If the tooth is bent then possibly from hitting something on the ground? Would not think that the derailleur pushing the chain over under load will bend the tooth on the chainrings ...
 

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Discussion Starter · #18 ·
You have Ultegra Di2 and you opted for Rotor chainrings? :confused:
Can you post pictures? From the side and front profile of the bent tooth?

If the tooth is bent then possibly from hitting something on the ground? Would not think that the derailleur pushing the chain over under load will bend the tooth on the chainrings ...
I already attempted a straightening job which worked and got me through my race last Saturday so there's not much to show at this point. It was very evident what was happening. By turning the crank you could see the point that the chain would try to roll off of the chain ring
 
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