Reasons why a RD hanger might break

mugensi Posts: 559
edited September 2019 in Workshop
I was out for a cycle yesterday on my Felt VR30. After about 30km I could feel and hear a click while in the upper/bigger sprockets on the cassette and it got worse the lower the gear so when in the biggest pn the rear it was at its worst. I thought it was the BB as I could physically feel a slight pull each time it clicked. Anyway I stayed out of the bigger sprockets as much as possible and continued on home. My house is at the top of a steep ramp hill of about 300m long and 14% gradient and I didn't want to grind up it in a higher gear so I put it in the big/big combo (30/32) and spun up as easy as I could all the time listening to the creaking and feeling it in the cranks. Anyway about 100m up, there was a crack and I instantly clipped out and looked at the RD which was bent upwards and stopped short of catching in the spokes.

I walked the last few hundred meters home and inspected the bike. The RD hanger had broken at the threaded section where the RD screws into leaving a semi-circle piece of metal still attached to the RD. I could see that the rest of the hanger was bent outwards. The chain was also kinked and one of the link had popped off the pin. Thankfully I was going so slow/easy that it didn't catch and shred any spokes.

So I removed the RD which is a 5800 long cage and it appears that the cage section/arm is bent. I have three other bikes and theyre all shimano groupsets and I was inspecting the RD on each of them and the arm appears to be straight with no bend on any of them but theyre all short cage RDs and so i'm unsure if the arm should be slightly bent or not. If its supposed to be straight then I can only assume the hanger breaking and the RD being twisted upside down caused the arm to bend.

Anyway I've ordered a new rear hanger (x 2, bought a spare just to have) a new chain as I binned the old one and a new R7000 RD but I cant figure out what happened to cause the hanger to break in the first place. Is there any thing I should be looking at as a possible reason as to why the hanger might have broken?


  • keef66
    keef66 Posts: 13,123
    Well done for stopping / unclipping so quickly. I suspect the hanger was doing it's job as a sacrificial component and saved your frame.

    Very difficult after the event to work out what was the cause vs what's been damaged as a result.

    The fact you could feel a pull with each click suggests to me that the chain might have been parting company and catching each time it went through the RD. Lower gears = more sideways deflection of the chain and so more of a click. When you selected big-big the chain opened up a bit more, finally snagging the RD cage completely and pulling it upwards / bending it and breaking the hanger?
  • mugensi
    mugensi Posts: 559
    Yeah I was veering towards the chain being the culprit too. It was a reasonably new chain though but that's not to say it didn't develop some kink in one of the links. The big/big combo would, like you said, have put more strain/deflection on it causing the link to pop off the pin and catch in the RD pulling it off and breaking the hanger in turn.

    I'm just thankful my new Fulcrum Racing 5 DB wheels didn't suffer any damage.
  • RD hangers break when Jesus is cross with you.

    Time to pray
  • balthazar
    balthazar Posts: 1,565
    If you can, post a close-up picture of the break! I’d guess that most of the surface is dulled and dirty with a small bright section. This is evidence of a fatigue failure, which is how most bike components fail outside of collisions.

    It means that the crack has been developing for some time, and when the last tiny bridge holding it together gave way, the derailleur fell off and you noticed just in time. The cause is likely in either the component’s design or manufacture, not in something you’ve done.
  • StillGoing
    StillGoing Posts: 5,211
    Big/Big combo something is going to give. Be thankful the hanger did its job and it wasn't the frame.

    Whilst a bike should be capable of doing big/big, it puts added stresses on the chain, derailleur and hanger. If you've been doing it often and for long periods, your luck will last only so long.
    I ride a bike. Doesn't make me green or a tree hugger. I drive a car too.
  • keef66
    keef66 Posts: 13,123
    I disagree. You should be able to use big-big without risking ripping the RD off. Not saying it's good for the drivetrain or that it will be whisper quiet to ride like that all the time but it should be mechanically possible.

    Every time I fit a new chain I check to see that it runs satisfactorily in big-big and small-small extremes. On the triple with an 11-32 cassette the chain length is critical to allow both