Hoping someone might have an answer to this. I am using Dura Ace Di2 on my 2012 Supersix EVO with the Si crank and Canonndale chainrings.
When using the 11 or 12 tooth rear with 53 chainring combination the chain skips wildly when any pressure is applied to the pedals and before too long the chain flies off the front chainring on to the crank. I have checked the rear derailleur alignment numerous times and it seems to be perfectly aligned both with the chain on and off. The front derailleur also seems ok.
What I do notice is an obvious flex in the 53 chainring regardless of the trimming of the front derailleur, so much so the chain rubs every pedal revolution. I am suspecting the issue is the soft as butter canonndale chainrings causing most of the problems.
I am considering a set of Praxis works chainrings as I've read in so many reviews theyre shift performance is excellent and very stiff also.
Any advice would be appreciated, thanks all.

bent ring ??

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Sounds like the chain might be too short...

Check cage height and high limit.
Also, make sure there is absolutely zero play in your crank/bb system.

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Si crank and chainrings are plenty stiff. Check installation of crank. It sounds like entire crank might be loose. Chain length could not be the issue on big/small.

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For certain parts stiffer is more important than lighter.

My best friend is running Ultegra Di2 with Cannondale SISL cranks, and with the Cannondale rings he was having nothing short of major shifting problems. Finally, he swapped out the chainrings for some FSA, installed a Dura 7901 chain, and since then it's all been resolved. (He had considered the Praxis, but obviously they are much pricier than the FSA.)

Hope that helps.

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"Deserve's got nothing to do with it." William Munny

The shifting issues were not the stiffness, but the ramps and pins.

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For certain parts stiffer is more important than lighter.

Not claiming to know the how's or why's, merely that he made the above changes and all his problems disappeared. In trying to sort out his issues, he spoke to several people who were experiencing similar problems. I thought the stiffness of the Cannondale rings should be fine (some pretty strong professionals use them), but nonetheless, he's much happier now.

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"Deserve's got nothing to do with it." William Munny

Are you using a Conex link? That will do it! I switched to a KCMC link. Check that the high limit screw is set properly on the rear mech. It needs to over shift then come back. Shimano recommends one full turn out after it has hit the stop.

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R2.5 / R3 / S-Works 29er (20 lbs) / S-Works SL4 Di2 (14.7 lbs)

Connex link only has that problem if you install it incorrectly. Has to be installed with the heart shape pointing towards the centerline of chain.

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For certain parts stiffer is more important than lighter.

It was installed properly. I have done several and still have them on all my bikes except the Di2 bike. Only had a problem in the 11. Switched it and problem solved.

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R2.5 / R3 / S-Works 29er (20 lbs) / S-Works SL4 Di2 (14.7 lbs)

Thanks to everyone for all the suggestions. The Canonndale chainrings are pretty flimsy compared to the Dura Ace ones on my other bike that's for sure. The problem is not in the BB, it's tight and smooth. I'm using a KMC Ti N 10XL chain with KMC link, might get ride of the link and use a pin, never liked em' anyway. Certainly think the rear derailleur could do with over-turning the high limit screw though.