CanadianBiker32

Last week while riding my road bike, My Rear Derailleur Hanger broke off. What type of conditions would cause this.

Happened at top of a hill. I was just shifting to harder gear, gently so i can descend. Then whole part with my derailleur came off for no reason. Never crashed ever on this bike, never hit anything with it
bike is 6 years old. Argon 18
Suggestions to why that would break as it did? thanks

cny-bikeman

There's no point in throwing out a bunch of guesses. Metal parts whose integrity has not been compromised do not break, so either the part had an original defect or was somehow stressed since that time, whether you were aware of it or not. But your title and first paragraph refer to breaking, then you say it "came off," and finally you end asking why it would break. Did it break, with a portion still attached to the bike, or did the bolt/fastener holding it on fail or come out?

CanadianBiker32

The actual part that connects to the derailleur broke off. So my rear derailleur was floating free.

Andrew R Stewart 

The first possible reason that comes to my mind is "chain suck". This is when the chain jams between the crank's rings while shifting under power. The chain's wedging on both rings causes the chain to ride up the "back side" of the rings and the rear der cage is rotated out, feeding the chain out. In time (brief moments) the chain either gets wedged/stuck on the underside of the chain stay or the der cage's rotation isn't enough and the chain tension continues to build. The weak link is often the mounting bolt of the rear der. With worn rings or heavy pedaling pressures chain the likehood increases. Andy.

cny-bikeman

Again, that is not clear, as the hanger is the part that connects to the derailleur. Is part of the hanger still attached to the frame and part to the derailleur, is the entire piece with the derailleur floating free, or did the derailleur come off the hanger?

bradtx

Canadianbiker32, I think, cny-bikeman, would like to determine whether your bike has a replaceable RD hanger or not. My questions are which derailleur did you shift in preparation for the descent and which cassette cog were you using during the climb?

Brad

cny-bikeman

Actually that is a good point. I was assuming a replaceable hanger, as I've very seldom seen an integrated hanger break, never without the derailleur going into the spokes or being fouled by a branch or similar.

Wilfred Laurier

the two most common reasons for breaking derailleur hangers are

shifting into the spokes
trying to shift into big chinring big cog combo with too short of a chain

without a more thorough inspection of the bke
this is all the info i can offer

MAMIL1976

cny-bikeman

Acceleration puts absolutely no stress on the hanger, and neither does any kind of normal shift. Something thrown up from the street can cause catastrophic failure, otherwise it's as others have said above. Andy is right, there's little point in guessing.

Crankycrank

A photo would really be helpful to this thread.

JohnDThompson 

While integral hangers on steel dropout seldom completely break, aluminum dropouts did have a problem with this; that's what prompted the move to sacrificial hangers on non-steel frames.

davidad

One of these ? https://wheelsmfg.com/derailleur-han.../argon-18.html They are aluminum and can break.