Update/Summary: Di2 + Praxis rings shifting problem solved by switching from a Shimano Dura-Ace CN-7901 chain to a KMC X10SL chain.

*****
I recently built up a BMC TM01 with Ultegra Di2, Quarq BB30 Elsa, and Praxis rings. Shifting has been great with one frustrating exception for which I need wrench advice:

When I'm in the small chainring, and the 5th or 6th cog, and I attempt to shift to the large chainring, the chain intermittently jumps off, inboard towards the bottom bracket. It doesn't happen every time; maybe 1 in 20 shifts. I have not had this happen in the other cogs.

I've checked the limit screws, cage height and cage alignment. They all look good, but I must be missing something. Video and photos below. Any suggestions would be greatly appreciated.

YouTube VIDEO: http://youtu.be/ALhIlgvIpBU

Photos:

At the very least, a chain catcher may be enough to fix the issue. What's really odd is that this happens when upshifting, I don't know that I've seen anything like that before. For that to happen, the derailleur would almost have to move in first to drop the chain then out to make the shift. Any chance you could get a closer take of the derailleur as it drops?

Here's another video, a little closer:

http://youtu.be/fc9cumZMlmQ

Okay, so what that looks like is the derailleur is pushing the chain onto the big ring too slowly (or something along those lines) causing the first few links to sit with their side plates on the teeth rather than meshing properly with the ring. Then when the chain fully releases from the small ring, the partly-shifted section of the chain no longer has anything holding it in place and falls off.

If anything, I think the derailleur may not be at fault here. It's possible that the derailleur's stops aren't quite right or something, but this seems more like the chain and big ring aren't playing nicely together. How's the condition of the pins and ramps on the big ring? If the pins are worn down, that might explain why the chain loses tension below as it comes off the small ring.

Thanks for the quick replies. The big ring is virtually new; pins and ramps are perfect. Now I'm wondering if Praxis rings play nice with Di2...others have reported they do. For the record, I'm running compact rings (50/34) for the hilly Wildflower triathlon course this weekend.

I have a spare set of Sram Red (Non-Yaw) chainrings, as well as a spare set of Sram Red Yaw chainrings I could try. I also have a spare KMC X10sl chain (the bike currently has a Dura-Ace CN-7901). Thoughts on whether any sequence or combination of those is worth trying?

I can't tell from your picture explicitly, but it looks like the 7901 chain is on inside out: The model designation should face outward.

If it won't be too much work, throw the KMC chain on and see how that works. I don't think the Red rings should make a difference, since they're basically identical to the Praxis rings. (In fact, I think they actually are the same manufacturer, just branded differently. I think Shimano is the only big company that makes their own rings anymore.)

The previous comment about the chain direction is a very good catch.

Thanks guys. I'm at work now, but I'll double-check the DA chain orientation and try the KMC chain when I get home.

Not true. SRAM rings are stamped steel, where the Praxis rings are forged. Totally different process requiring totally different manufacturing techniques. Praxis claims to make their own rings using their "cold forging" technology. From what I've seen, the Praxis rings have a much better finish to them.

_________________
2013 Madone 5
2012 SpeedConcept 7
2013 Superfly

Chain too short? If there's too much tension because its too short, it will pull back and the chain will pull across the ring. I'm on 7970 Di2, Quarq Elsa 10r with Praxis 52/36 rings, dura ace 7901 chain and the shifts are great. Never had that issue. As well, looking at the first pictures, your FD is too high relative to the big chainring. Its supposed to have only about a 2 - 4 mm gap. Looks like a lot more.

Thanks for the suggestion, but I'm pretty confident the chain length is good. I did the big:big+2 links method and confirmed with the equation method. I also can successfully shift big chainring + big cog without issue.

Also, I'm glad to hear you've had success with a similar setup.

Also a good suggestion. I optimized for a ~1-2mm gap near the front of the derailleur (using a penny), though that may be hard to see in the picture. That said, the back half of the cage does have a bigger gap. Is that something I should investigate?

There's nothing to worry about at the back end of the cage, that's normal. The front end of the cage is what does all the work, while the rear mostly keeps the chain in check. Use that derailleur on a compact chainset and it'll look even goofier. I noticed the same thing, but on further inspection it does look spot-on at the front.

As for the Praxis/Sram rings--different construction methods, but if you ever hold two side-by-side in your hands, the shaping is identical. Likely they use the same dies and stamps, but with different alloys, tempering methods, etc. As far as shifting goes (especially unloaded like in the video), they should perform the same. The difference is in weight, stiffness, finish, and durability.

And now that I think of it, isn't "stamping" just lay-speak for cold-forging? Stamping is another term for drop forging, and unheated drop forging is typical for thin metals. Sounds like cold-forging is just different marketing speak for the same thing. [Disclaimer: I grew up with a machine shop, so I'm not just making stuff up here.]

No, as long as the front half is ok.

Back to the chain length. I know that you've used the big/big plus 2 method, but that basically creates a length that's the absolute minimum you can have. In other words, big/big without the "plus 2" will usually be too short. Depending on the bike and the chainstay length, that still could be too much tension. I've had this happen to me on another bike, and adding 1 pair of links did the trick. Longer is ok as long as you don't get any chain sag. If its possible, try to add a pair of links. Or, as an alternative, try winding in the B screw on the rear derailleur to marginally reduce chain tension.

I always thought of stamping as just cutting away excess material and forging as pounding the metal into a certain shape. The latter changing the internal structure of the metal making it stronger. I'm not a metal smith though, that's just what I always thought.

_________________
My Cervelo R5
My Cervelo S3
My MTB