Photo montage showing bolts holding modular drop-out in Bicycle frame, with derailleur and cogs

Not what I was expecting

During my last ride on Kuroko, I experienced a sudden grinding noise from the drivetrain, perhaps the crankset. I brought Kuroko to the Workshop in the Sky on my return home, planning on a complete drivetrain cleaning and inspection. I expected to find perhaps a worn chain or failing bottom bracket bearings — something Kuroko has had an appetite for in the past.

Detail of bicycle rear derailleur and modular drop-out separated from frame
That’s … not supposed to happen

After setting up the new repair stand, I got busy cleaning the chain. With that out of the way, I decided to clean up the teeth on the chainring and jockey wheels, and then have a go at the sprockets on the rear wheel.

As always, I loosened and removed the thru-axle, and then tugged the rear derailleur out of the way so I could free the rear wheel from the dropouts. And that’s when the unexpected happened: the derailleur dropped from the frame and dangled in midair from the shifter cable.

At first glance I thought the derailleur hanger had broken — a not unheard-of occurrence. But a closer look showed something even more unusual: the modular rear dropout had separated from the frame. Further inspection showed only one of four bolts remaining in the frame.

These bolts have loosened and fallen out before. I had an issue with the thru-axle gradually loosening until I realized these bolts were loose, after my return from England. I’d had to buy some replacements then and still had some spares on hand, I’m glad to say.

Photo montage of small bolts and plastic container of a variety of bolts
Lone survivor

I tried the same size bolt again, but it wouldn’t hold. I ended up using some bolts that were half again as long. They stick out the opposite side. They don’t interfere with anything, the chain or the derailleur. It just looks a bit naff. But if they hold the thing in place, I’m happy.

Photo montage showing bolts holding modular drop-out in Bicycle frame, with derailleur and cogs
Screwed right

With that taken care of at the chain oiled, I ran the derailleurs through their paces. On the new workstand at least, the shifting works flawlessly and there’s no noise from the bottom bracket. Finally, I checked the chain for wear (it has nearly 3,000km on it since the last replacement), but it’s all in spec still.

Detail of bicycle chain with chain wear guide applied
No wear detected

What portents for the future?

As this is a repeat failure, I’d like to find a more permanent fix. I’m going to try to contact the maker and ask for a replacement drop-out/derailleur hanger, as well as some spare bolts. If I can get those, I’ll use a thicker Loctite than the standard Blue 242: maybe the Red 271. The bolts shouldn’t be taking a lot of force — the clamping force of the thru-axle against the hub holds everything in place and should be taking the brunt of the load. At the same time, the bolts may need to be removed if the derailleur hanger ever breaks (which can happen).

If I can’t get a response out of the maker, or I do but this doesn’t resolve the issue, then I may have to resort to more drastic measures.

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