Bent derailleur hanger

Ron338 said:

Does anyone here have any stories to relate on how long a previously bent derailleur hanger lasted til it broke?

There's no way of telling, but one bend (and back) is rarely enough to cause failure, if it's a smooth bend.

But for peace of mind on a steel frame it shouldn't be too hard to find someone who could cut the old hanger off and fit a new one.

Steel strain hardens, meaning it gets stronger when you bend it back and forth. This continues until you reach some critical stain, where it fractures. Assuming the hanger didn't crack when the mechanic straightened it, you should be alright. However, if you bend it again you could be in trouble.

I bent the derailer hanger on my steel bike in the middle of a tour - I was ~10 miles from the nearest town. Luckily I had brought a large adjustable wrench which I was able to use to straighten it back. That was 13 years ago and the bike still works great.

artmichalek said:

Steel strain hardens, meaning it gets stronger when you bend it back and forth. This continues until you reach some critical stain, where it fractures.

That's just BS.

boudreaux said:

That's just BS.

Or it could be "hooey". Seems line bending something over and over would cause failure. As far as the topic at hand, one moderate bent shouldn't affect it's strength.

capwater said:

As far as the topic at hand, one moderate bent shouldn't affect it's strength.

That's the beauty of steel.

True. I have done it TWICE on one frame and both times the hanger was fine after using the Park tool made specifically for the job.

Everyone...Just wanted to say thanks for the replies....they were a lot of help, and they eased my mind. This is a great forum, isnt it?

Ron338 said:

Everyone...Just wanted to say thanks for the replies....they were a lot of help, and they eased my mind. This is a great forum, isnt it?

think about how many times you have to bend a paper(alum) clip before it snaps.... now compare that to the heavy steel derailuer hanger

I dont think its possible to bend the hanger that many times to cause it to snap(within reason)

boudreaux said:

That's just BS.

I could refer you to the technical literature, but you don't seem to be the reading type. It's a commonly held fact that all metals undergo an increase in yield strength when subjected to plastic deformation at room temperature. That's not an opinion. As far as how many bends you can get away with, that depends on the elongation of the material. High strength aluminum alloys, for example, have a low elongation (around 0.11 for 7075-T6) and will fracture on the first bend. For steel, the elongation is dependent on the alloy composition, manufacturing process, and heat treating.

artmichalek said:

I could refer you to the technical literature, but you don't seem to be the reading type. It's a commonly held fact that all metals undergo an increase in yield strength when subjected to plastic deformation at room temperature. That's not an opinion. As far as how many bends you can get away with, that depends on the elongation of the material. High strength aluminum alloys, for example, have a low elongation (around 0.11 for 7075-T6) and will fracture on the first bend. For steel, the elongation is dependent on the alloy composition, manufacturing process, and heat treating.

Art..it appears that you know something about metallurgy, so maybe you can give me more info. The frame doesnt say what specific steel it's made of, other than "Trek True Temper Ox chromoly steel". If i had to guess, I'd say that the hanger was bent roughly 45 degrees out of alignment. Any thoughts on this?

Ron338 said:

Art..it appears that you know something about metallurgy, so maybe you can give me more info. The frame doesnt say what specific steel it's made of, other than "Trek True Temper Ox chromoly steel". If i had to guess, I'd say that the hanger was bent roughly 45 degrees out of alignment. Any thoughts on this?

I can't tell you the plastic strain you inflicted without knowing things like thickness and bend radius, but 45 degrees is a big bend. True Temper Ox is a very high strength alloy with a low elongation. I think it's somewhere in the .1 neighborhood, maybe lower. Like I said before, if it didn't crack when it was straightened out, then it's going to be alright. It isn't really weakened, and it won't break under normal riding conditions. I wouldn't worry about taking a long trip with it. Just make sure that derailleur is adjusted properly this time.🙂

.............

jedblack said:

think about how many times you have to bend a paper(alum) clip before it snaps.... now compare that to the heavy steel derailuer hanger

I dont think its possible to bend the hanger that many times to cause it to snap(within reason)

Paper clips are not aluminium. They are steel. IF you bend aluminium it will break sooner.

tafi said:

Paper clips are not aluminium. They are steel. IF you bend aluminium it will break sooner.

ooppsss... sorry, meant to say steel

since we are "talking" about "steel" here....

boudreaux said:

Sometime when you have some time on your hands, chuck some steel stuff
in a vice and bend it. Depending on the alloy, and other factors some
will snap immediately,some will take a considerable bend.Any of them
will be done for if bent beyond their limit just once. Steel can be
stressed indefinitely if it's limit is not exceeded.Your response as
originally stated was bs.

Perhaps you can direct me to the ASTM standard for the "chuck some steel stuff in a vice and bend it" test. I'm a little bit busy finishing up my second college degree in solid mechanics, and I don't have time to look it up.
For everyone else I've attached an approximate equation for estimating plastic strain in a bent derailleur hanger. It's based on a sheet forming equation, so it doesn't account for edge effects, but it will provide a conservative estimate. Note that you don't want to bend anything past half of the %elongation, since you have to be able to bend it back.

artmichalek said:

Perhaps you can direct me to the ASTM standard for the "chuck some steel stuff in a vice and bend it" test. I'm a little bit busy finishing up my second college degree in solid mechanics, and I don't have time to look it up.
For everyone else I've attached an approximate equation for estimating plastic strain in a bent derailleur hanger. It's based on a sheet forming equation, so it doesn't account for edge effects, but it will provide a conservative estimate. Note that you don't want to bend anything past half of the %elongation, since you have to be able to bend it back.

I was under the impression that the temperature at which the bend took place made a difference. Or is if if you are above NDT it doesn't matter?

jitteringjr said:

I was under the impression that the temperature at which the bend took place made a difference. Or is if if you are above NDT it doesn't matter?

It matters a little bit. If you're above the annealing temperature, then the material won't strain harden. There's also a brittle transition temperature, which is typically very low. Relative to what it takes to affect the crystal structure of most metals, the temperature range in which people are able to ride bikes is very small.