Cyclist0108

This did not go well. About a week old Box derailleur.

I'm not sure what happened, but I suspect it must have slipped off and then bent.

I'm thinking I should try to cut the bolt flush to the derailleur after the bend, and then back what remains out.

Would putting a new one in backward help prevent a reoccurrence?

DrIsotope

Cut off the head, take it out from the thread side. Replace with something much harder. You can get Metric Class 12.9 in most threadings. The factory bolt was likely Class 8.8.

Cyclist0108

Thanks -- that was going to be my next question (i.e., what is the hardest option to prevent this from happening again?)

Darth Lefty 

Curious: why threaded so far? To reach 46t?

Cyclist0108

50T

It's my kid's bike, and I'm trying to help without getting too involved or judgmental. I think it was probably out further than needed (the derailleur is rated for 50T), and I am not sure when this happened, but my guess is it wasn't ever engaged on the hanger properly.

Cyclist0108

The plot thickens. I cut the screw and got it out (had to cut from the bent side). Fortunately no obvious damage to the derailleur, although the hanger is gouged.

[deleted some incorrect stuff here]

dsaul

I've found that a lot of hangers don't have a long enough b-screw shelf. Modern wide range cassettes make that problem worse. As the b-screw is extended, it rotates the derailleur around the mounting bolt and the b-screw angle changes in relation to the hanger shelf until it is barely resting on the edge of the shelf.

If it were mine, I would set the new b-screw where it needs to be and then loctite a nut on the end of it. The nut will provide a larger surface area to rest on the hanger shelf.

DrIsotope

The above suggestion is pretty good-- give the tip of the screw some more surface area to sit on the hanger shelf, should help resist bending.

It's weird, because on my 1X bike with a 10-42 cassette, I ditched the Rival 1 RD stock B-screw because I'm only using about 2 threads of it. Replaced it with a screw maybe 12mm long. I have far more threads on the B-screw showing on a bike with an 11-32 cassette.

Cyclist0108

I bought some with flat heads so I could put them in backward. I think it will work ok like this. I also have some nuts in case that approach doesn't work (or if I want to further reinforce the bolt). Then hanger is quite chewed up, so it is probably time to get some new ones (my steel bikes don't have that luxury). It sure seems like a weakness in the design (I know they are supposed to be weaker than the frame, so they break first, but the hanger design could be better).

Thanks again for all the advice!!

(I'm sort of going in slow motion because the kid should be in on this and it is finals week.)

redlude97

I've used the backwards b screw solution as well, you can get hex heads are home depot that are longer. You just pull up on the RD, adjust, set back down