TimothyH

The bike has about 3500 miles on it and ran an Ultegra 6870 front derailleur with 50/34 crankset. Its a gravel bike and I'm not easy on it.

The chainrings were just replaced with Absolute Black 46/30 oval rings and the derailleur was adjusted such that I thought shifting was OK enough to ride. I paid attention to AB's installation instructions when it was set up but it felt like the chain sometimes struggled to climb onto the big ring. It broke about 40 miles after the rings were changed.

It dropped the chain once on the inside and it rubbed a little on the small front/big rear combination but toward the end of a 30 mile ride Wednesday night I heard a snap as it was shifted into the big ring...



The motor works fine. It happily moves the broken piece up and down and it even tries to trim. The motor just isn't connected to the cage any longer so nothing happens.

Not sure it if was ready to let go and changing the rings was a coincidence or if the stress of shifting oval rings was too much. Actually I'm not that hopeful that the replacement is going to hold up but particular attention will be paid to setup this time. This is getting expensive.

So, is anyone running Absolute Black 46/30 rings with a Di2 front derailleur? Anyone ever break one like this or hear of it?

Two year warranty. Maybe I should try to get Shimano to replace it.


-Tim-

metalheart44

I had the same thing happen, exact same break, but it took several years after installation for the break to happen and I just use stock chain-rings.

redlude97

Were you using the clutched RD? I'm wondering if the clutch slowed down the chain jumping up to the big chainring just enough to jam the cage against the backside of the chainring, with a mechanical FD you'd feel that and just wouldn't be able to shift momentarily but with DI2 it would continue to force the cage into the changinring until it broke, possibly in combination with the fatigue mentioned above.

Ghazmh

ChinookTx

Time to convert to 1x !!

Seriously, that's not cool. Do you think maybe the 30T ring was too small and causing too much strain on the derailleur cage (lever effect is amplified the smaller the chainring is)?

PickleRick

Testify!.....can I get an amen brother!!

79pmooney

Or, skip the Di2 in front and use a left downtube friction shifter.

Marcus_Ti

You said it happened some time after a dropped chain (after the oval ring retrofit)...I wonder if that had anything to do with it? Did you try to use the FD to undrop the chain? Just spit-balling.

With an oval ring and off-paved-roading....I'd recommend a chain catcher. K-Edge even makes blingy ones.

fietsbob

Perhaps you hit the shift button too often, while still having to force the shift, against too much chain tension..

and accumulated metal fatigue caused that piece to fail ??






.....

rollagain

Could the condition of that chainring in the photo have anything to do with the FD failure? I see pieces broken off two adjacent teeth, and what looks like shark-fin profile developing on some others. And that's at only 40 miles?!

Cyclist0108

Sorry for your loss! Mine's been shifting round 46/30T rings without issue for a couple of years now, so if it has anything to do with the change, I would suspect it was the oval chainrings. I think mine is the next older version, FWIW.

Marcus_Ti

It is an ovular outer ring. You're seeing ramps for shifting. The valleys between teeth are still symmetrical and not rounded out. Anodized rings losing finish are more misleading, unless you have a brand new ring to side-by-side compare to.

Yea, I have a 9070Di2 FD on my 46/30 and still going fine....got the D/A part instead of the FD part because of random sale pricing.

TimothyH

It was set up with an R8050 and run for 47 gloriously difficult miles in the North Georgia mountains today. I made sure it was shifting perfectly before putting it into service. I feel like the derailleur jamming against the chain as it hesitated to get onto the big ring might have been the problem. Time will tell but it was shifting very nicely today.

The oval rings felt odd at first but are really nice to ride. I'm not sure how people can say they can't feel a difference between oval and round. I could feel it immediately. It wasn't earth shattering but clearly different and I love them.





I saw that in the photo but the teeth are not broken. That's the way they come from the factory. They have <40 miles on them when the photo was taken.


-Tim-

dedhed

What metal is the broken piece made out of?
is it something a competent welder could repair.
just trying to figure out possible fabrication repair strategies

ChinookTx

I looked at mine yesterday, it looks like cast Alu or something like that. Fairly small too so I doubt it can reliably be welded. Worth a shot if it can be done for cheap I guess.

dedhed

Certainly a "I know a guy" situation, but if it was my $150 FD I'd be talking to those fab type guys I know. I'd even be looking at how it comes off and possibly maching a new part out of Al stock. Part of that is also the challenge. There even might be enough of these out there to approach a problem solvers or wheels mfg to market a repair kit.

Dan Burkhart 

The last shop I worked at had a lot of Di2 equipped bikes go through, and I only remember seeing one broken one. That one broke the link shown below.



This happened because of the owner's shifting style. He was an extremely argumentative engineer type who insisted that front shifting should be executed while turning the cranks very slowly. Of course, the derailleur would jam every time, even when correctly adjusted, and resulted in the break. The boss even went so far as to put his own bike in the stand and demonstrate that his bike would jam the front derailleur if it was shifted in the same way. The customer's adamant response was that none of us in the shop had any idea what we were doing, whereupon he was invited to take it elsewhere as we had done all we could for him.
Never did find out how that played out.

woodcraft

A while back I posted about helping a guy road-side with his Di2 front der.

& noticing that the chain side plates looked really beat up, & wondered if the forceful shifting

caused that (ability to shift under load, etc.).

You replied that there was not forceful shifting, but this suggests maybe there is.

ChinookTx

Even if it wasn't intentional, the oval rings add a force to the mix I think. As the chain moves up on the "long" side of the oval during a shift, it pushes against the derailleur. The derailleur doesn't just have to push the chain sideways now, it also fights agains the chain moving "up" against it. At least that's what I remember from college physics and resultant force theory. It's been a looooong time though, so don't quote me on that!

TimothyH

This is interesting.

I was being cautious and slow RPM shifting might have been a factor.


-Tim-

TimothyH

I think I still have the broken part and would be happy to put it on a slow boat to Milwaukee @ no charge.

PM me your address if you have a use for it and think you can fix it.


-Tim-

Marcus_Ti

There's another possibility....given that others have had this happen. With an FD, I'm not sure how it could happen, I'm theorizing here...

[story time]
I had 2 separate 6870RDs up and commit suicide under bizarrely similar circumstances. The second one I'll swear, in court, was setup to the Shimano DM, limit screws and all....which is good because post suicide it is the only one I could get meaningful data out of**. Shortest version possible--riding along the RD "re-calibrated" itself. I was riding along in gear 7/11 in the back, suddenly my RD was overshifted with the chain between the cassette and the spokes. Nursing it to a stop....Di2 itself (MT800 junction box had a Di2-direct gears readout) said the system was still in gear 7/11... I was able to get the chain out and then downshifted to highest-gear....except Di2 thought gear #1 was in fact 7/11, where I'd been riding.

Now you might say, well duh, the hanger and cage are shot and are bent AF....well....

Seing Di2's brains seemingly scrambled, I unplugged the battery from Junction A. Waited a bit. Plugged it back in. WHIRRRRR. Spun the pedals. And Di2 put the chain back on gear1, and Di2 read me as being in 1/11. So while it ran poorly (bent RD hanger and cage), Di2 was clearly brain scrambled before as to its actual gear position.


**first RD that died in similar circumstances, the RD itself sheared off at the RD-hanger bolt, as well as completely trashing the hanger . Nothing to really diagnose/debug.
[/end story time]

How this would apply in your circumstance to an FD--I don't know. I'm throwing it out there, as someone who has had not one but probably 2x 6870 RDs up and commit suicide in strange circumstances.

TimothyH

Odd Marcus, but interesting data points nonetheless.

I have to believe Shimano knows all about these failure modes, especially those that occur in software as yours appeared to have done.


-Tim-

freegeek

I have a build almost finished with rotor (non oval) 46/30 chain rings. I would not think that there is a big difference between oval and round when it comes to stress on the fd

all_seasons

Thanks so much for posting. And sorry to hear about your experience. My Di2 front derailleur snapped exactly as shown in the picture by the OP. The incident happened to me this week on my 50/34 6870 setup on my road bike towards the end of a ride. The drive train is regularly maintained and is used 100% on the road. LBS will check with Shimano on next steps towards resolving this incident.