1 • hearable signs .. each chain link makes a more single rattling sound at the chain wheel tooth ..
2 • feelable signs .. the chain can be moved around and lifted more from the chain wheel at 9 o'clock with two fingers ..
3 • visible signs .. small & bigger gaps at 6 o'clock between the chain and the teeth of the chain wheel .. the teeth look like shark fins ..

// dropped my chain two times today uphill out of the saddle .. phase 3 showed up end of winter .. just waited to replace the drive train for the new saison .. oSo >>

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Like my chain might be too long...

I'm running a 53-39 and 11-32.

Would that cause it to jump off under load, when not in the 39-11 position? I'm virtually certain I was no further cross-chained than 39-13 and would guess I was in 39-14.

I'm having a hard time visualizing how a too-long chain causes chain pop-off under load. Not enough chain contact around the front/bottom of the chainring?

In potentially related news: I just discovered that no matter what I do with my B screw on my rear mech, there is no adjustment of how close or far the rear mech sits from the cassette cogs. When I screw it all the way in or all the way out, it makes no difference. The rear mech seems to just fall past the position (closer to cogs) with no effective limit.

Is something broken in my rear derailleur? It has taken some hits in a couple crashes. Is there something I can do to fix it?? It's it's a Dura Ace di2 9070, I think.

Oddly, there is no weird shifting, and only weird noise when the cassette jockey wheel appears to be contacting the 32t cog. This issue WAS present before the chain started coming off... so potentially related.

Any ideas from anyone are appreciated. Have already learned a bunch. Thanks to all.

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Dr. Alex Harrison | Founder & CEO | Sport Physiology & Performance PhD
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Noted. I've at least gained the confidence that if I learn how to keep my bike components properly maintained/replaced as needed, I won't need to replace my frame. Thank you very much to all.

Now just to address the issue of if my rear mech is broken. Would love input on that! (my previous comment, posted a couple minutes ago)

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Dr. Alex Harrison | Founder & CEO | Sport Physiology & Performance PhD
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• small - small .. there should be space for a finger between the chain and the guide jokey wheel ..
• big - big .. the rd should not be overstressed and the guide jokey wheel not touch the cassette ..
• the wrapping angle and distance to the cassette of the rd is adjusted with a screw facing the drop out ..

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I think I have achieved the first two.

But the screw you mention here, which I called the B screw (not sure if correct), does nothing. No change in derailleur position for any amount of turning. It's as if something internal to the rear derailleur just allows it to move right beyond the typical catch point.

I can confirm the screw is contacting the hanger.

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Dr. Alex Harrison | Founder & CEO | Sport Physiology & Performance PhD
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.. does the tension spring in the rd work .. proper ??

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https://si.shimano.com/...DM-DA0001-10-ENG.pdf .. page 30 [& 64 ..]

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Yep, that chainring is cactus!
BUT, also check your bottom bracket bearings
Several years ago I had an issue where I would drop the chain and found it was the non-drive side bearing had worn enough that whole rocked rocked back and forth and it was enough that in the right circumstances it would drop the chain

Feels like yes, there still is some spring action but maybe not enough?? Seems to swing pretty freely beneath the cassette.

I've got snap ring pliers arriving tomorrow from amazon to investigate further. I'll report back. Thank you!

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Dr. Alex Harrison | Founder & CEO | Sport Physiology & Performance PhD
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Duly noted! I'll investigate.

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Dr. Alex Harrison | Founder & CEO | Sport Physiology & Performance PhD
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I wouldn't be so quick to blame the chainring, especially when you say it happens on both the small and big ring.

Those 'shark fin' shaped teeth on there are normal, see attached pic, thats how they look new, at least to my eyes....

Yeah, I was going to say that ring is worn, but not crazy worn. Might as well replace, but not convinced it's 100% of the issue.

That chainring is done. It's not so much the pointy teeth it's the wear between the teeth that concerns me. You can see how deep they are getting.

I'm flying out of Tucson tomorrow. I'll try to hook up with you next year.

.. even on mine .. there is a kind of fin shape [reminds me nicely on the old da stuff that could be mounted in the wrong direction] .. so it's the distance between the contact flange .. and a more & more shark fin shape .. oSo >>

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Those are my thoughts. The chainring is worn and possibly getting towards/at replacement (difficult to be sure from that pic), but it's unlikely that both big and small have worn equally such that they both start throwing the chain at the same point in time. It definitely won't be the frame, but I'd be doing more investigation to see if there is any movement coming from anywhere (loose chainring bolts, worn bottom bracket bearings etc) and doing more investigation into that rear derailleur - if it isn't providing enough chain tension that is a very likely candidate. Try and look at a friends known good derailleur and see if yours provides similar tension. Also check the chain for a bent link, or something similar.

Oh, it won't be the frame!

I agree.

I've also purchased a new Ultegra crankset, so now moot point re: chainrings.

But, if it was a chainring issue, I'd love to have your insight on what it might be before I go "test." Thoughts?

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Open to other suggestions. Crankset is being replaced with new Ultegra.
I'm investigating the derailleur spring as I write this.

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Well, if you want to cover all bases....
- get new cranks installed
- replace chain
- maybe replace cassette if you think might be worn? Just throw a cheap 105 cassette on, they work fine.

Then maybe try some hard sprints on an indoor trainer so you don't face plant if it happens again

Since you are replacing the crankset, you should replace the bottom bracket too. They are cheap.

It really sounds, and now looks, like worn chainrings. There's a decent chance your chain might have 'stretched' at a quicker rate to compensate for the worn chainring too and not be very new anymore. A stranger on the internet can't confirm that for you, but this is why people will replace the chain any time they introduce new chainrings or a cassette to a drivetrain.

The third post about a quick link not being installed correctly tends to manifest more on the cassette than the chainrings. It happens with some frequency on 12 speed MTB drivetrains with curved quick links.

1) Chainring in the pic looks toast to me (I know you said it's all been replaced anyway, just furthering the discussion about drivetrain maintenance, etc). It's not so much the sharkfin profile, some of which is by design for shifting performance as others have mentioned, but rather how rounded off and short they are relative to how big the 'valleys' in between have gotten. Basically, the gaps get sloppy relative to the spacing of the link rollers in the chain so they don't mesh snugly enough anymore; over time, the chain elongates too (commonly referred to as 'stretching' but it's just the cumulative play in the rollers adding up as they wear, rather than the plates actually stretching) so it will still work w/ the old chainring as they wear together, but then when you replace the chain the new links are shorter again like they're supposed to be so they don't mate w/ the worn ring like the old chain did.

2) I realize the natural tendency is to assume the chain is jumping off both rings for the same reason, but it could be that they are showing 2 different issues that just happen to present similar symptoms. I'm thinking the chain jumping off the big ring is just classic wear as above, but jumping off the small ring may or may not be so simple. It's certainly possible that wear is also an issue since they're presumably of the same age (although smaller rings will often wear faster since they don't spread the load over as broad a contact surface per revolution, but they also don't tend to get used as much depending on how hilly your typical ride is and of course they aren't usually subject to sprinting loads; before someone says a climbing load is the same while standing, the downforce might be similar but the RPMs are way different so it's easier to throw a chain on a road bump or some such during a sprint than during the slower rhythm of a standing climb). However, the chainline most likely being sharper is a contributing negative factor, as well as the possible issue of chain tension if the new chain is too long and/or the RD tensioning spring isn't working like it's supposed to.

3) easiest way to test the B-screw function to tension the chain rearward is to just slip the chain off the front rings so it's slack ~ does the RD automatically swing all the way back? You can simply pivot it forward/back on the hanger w/ your hand and feel the spring tension (or at least you should if it's working properly). That won't be an issue so much in the big ring since there's more system tension anyway having to wrap around a larger circle, but it could show up when in a small/small(ish) combo.

Thank you! It might be the rear derailleur. It doesn't behave like my wife's bike. More in a moment in reply to another user.

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Dr. Alex Harrison | Founder & CEO | Sport Physiology & Performance PhD
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I recently had to replace a frame where the bottom bracket area went all noodly. The first symptom was the chain scraping the sides of the front derailleur during high torque situations. I also dropped chain a couple times, like you're seeing.

I verified the cause by locking the rear triangle firmly in a fixed turbo trainer (the kind that doesn't let you rock). Then I placed my phone under the BB, put it in a big gear, cranked some high torque, and recorded some video. I could see the chain ring moving laterally nearly a full centimeter (more than enough to crash the chain into the FD). It was pretty shocking to see the video. I wondered how long I'd been riding like that.

My next step was to verify it wasn't the bearing or bearing assembly, so I swapped with a new BB assembly. No difference.

So I tossed out the frame.

The B tension spring appears to be allowing travel in both directions, pretty easily. Do these wear out? Are they sold separately somewhere? Any info helpuful!

Is this compatible? Or must it be dura-ace?

Results of my investigation:

I can install as pictured on page 30, no prob.

When I adjust B screw as pictured on page 64, nothing happens, because....

As pictured and discussed on page 93, I DON'T even have to use a 3mm hexagon wrench to get the claw on the stopper plate to cross the claw on the bracket body. I can do it with my hands/fingers. It's not tension-free, and it is challenging with fingers, but definitely no 3mm hex wrench needed. This makes me think the spring is worn out or loosened. I've uninstalled and reinstalled twice to ensure that I'm doing it in the correct orientation.

Visual inspection of spring looks fine. Only thing to note is that the springs ends (y) and (z) (Page 93) appear exactly opposite each other when the spring is unloaded, which is not the case in the Diagram 1 on page 93.

Global question for my understanding. At what point should that spring allow disengagement of the claw on the stopper plate from the claw on the bracket body, during normal bike operation?

That claw interface is what appears to communicate the B screw "limit" at all times, on my wife's bike. As in, the derailleur can't go past (closer to cogs) than the limit screw at any time. Hard stop.

On my bike, there is no hard stop at all. It floats right past what should be the stop, though there is a tactile "stop" there, but the aforementioned "claws" don't stay engaged.

FYI: A year ago, probably when the derailleur started functioning as it does now (apparently improperly but good enough to ride for a year without much concern), I had crashed by sliding out on the right side of the bike so the derailleur took an upward-directed hit from the ground, which seems to have eliminated that "hard stop" and allows it to swing past the B limit, closer to the cogs.

Biggest question: is the RD issue likely to cause chain pop-off? I guess maybe yes... especially if paired with worn rings. And probably-worn bearings... I'll check those out when I install the new Ultegra crankset.

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Dr. Alex Harrison | Founder & CEO | Sport Physiology & Performance PhD
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.. huuh .. my technical english competence makes it hard to answer .. maybe some one can jump in to sort the movings of the rd out and give an advice for correct geometrical function ..

.. the chain can jump of the small chainring easier with not enough "pre-tension" ..

Sorry & Cheers

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