Shimano 105 front derailleur pivot broke : what am I doing wrong?

Question

Sunday, 40 km in and after a steep uphill, the pivot point on my front derailleur snapped in two parts.

I remember a whipe noise, one of the chain link opened and when I tried to place the chain back in after removing the faulty link I notice the silvery part of the derailleur was rubbing on the big gear and found about the material failure.

My question is : is that a common issue ? What did I do wrong (maintenance ? Usage?) and how can I prevent the issue in the future.

The bike had 6000 km in about a year of use and was serviced by the manufacturer one month before that.

edit : I brought it back to the manufacturer, they replaced the piece and it was covered by the warranty (including the replacement work).

Answer 1

In general, it is possible that you did nothing wrong and the derailleur broke. I bet that bike equipment in general has been getting more reliable over time, as processes and materials have improved. However, occasionally, you'll have something fail because somewhere along the line, the processes weren't followed or they got a bad batch of materials, or some other error. To some extent, you're protected by Shimano's two-year warranty (3 for Dura Ace).

In general, keep your bike clean, and pay attention to how things are working. That should let you detect any failures as early as possible. There was some discussion of periodic maintenance tasks here. For example, once in a while, it's good to check the tightness of your bolts - for example, my rear derailleur mounting bolt backed off, which I think may have contributed to it getting caught in the spokes in big-big. If you do this, you also don't want to overtighten the bolts.

Generally, structural failures like this should be rare in high-quality parts from the major manufacturers. Some readers may remember that Shimano is semi-recalling some 11s cranks because they are more likely to de-bond and break (the failure rate is at least 0.7% - the official statistics likely miss some unreported failures, and failures may be more likely in wet climates or climates where the drivetrain frequently encounters salted roads). This failure mechanism wasn't acceptable, as Shimano should have reacted much sooner, and their proposed fix is arguably inadequate. However, it does also show how rare major structural failures are. The recalled cranks were their lightest ones, and this failure was due to design rather than process or materials defects (I think).

Answer 2

I've also never seen that, but you mention "steep climb"

Is it possible that you shifted the front derailleur while pushing hard on the pedals ? Essentially shifting under load so the chain and derailleur were fighting each other, and then either the plastic failed or the chain caught somewhere and wrenched it?

Answer 3

I’ve never seen or heard of this happening. But only yesterday I was looking at my bike (while sitting in the train) and marveling at how thin and fragile those front derailleur linkages look ;)

I can’t think of anything which would cause this, except an impact or crash which might not have broken it immediately but weakened it enough (possibly also bending it slightly) for it to break later during normal use. Shifting under extreme cross loading and with lots of force on the pedals would probably lead to the biggest (sideways) loads on the derailleur, so that should be avoided. But I think usually you’d struggle to push the shift lever hard enough to cause any damage that way. I broke the pawl of my left brifter that way on a Shimano 105 5700 STI brifter but the Shimano CX70 front derailleur it was connected to keeps working 10 years later.