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Aieeee! Broken crank arm on winter bike #5343949 02/07/20 06:01 PM
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Number_35 Offline OP
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Here's yet another unforseen failure on Jr's winter bike. He's a tall strong young man, and can apply a lot of force to the cranks on his single-speed winter ride. I picked up a replacement crank arm from a LBS for $13. Not the most costly repair, and of course it was easy enough to remove the pedal! Another first for me. LOL

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Re: Aieeee! Broken crank arm on winter bike [Re: Number_35] #5343956 02/07/20 06:08 PM
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AZjeff Offline
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Any chance the pedal was loose and worked it? I know some strong guys riding SS full time and have never seen that either. Not Alivio or some other low level cranks?



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Re: Aieeee! Broken crank arm on winter bike [Re: Number_35] #5343969 02/07/20 06:16 PM
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cjcride Offline
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I'm sure he is a strong rider as you say yet I bet hard landings with feet planted hard on the pedals probably contributed

Re: Aieeee! Broken crank arm on winter bike [Re: AZjeff] #5343975 02/07/20 06:23 PM
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Number_35 Offline OP
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It's possible, but in my experience a loose pedal will tear the threads out of the crank first.

Just thinking out loud here, but the utility I worked for specified Charpy-compliant line hardware for any critical application. Charpy is a rating of the steel to resist cold-weather brittleness. A notch is made in the metal, and the sample is subjected to x number of Joules impact energy @ -20 C. Because high strength and lack of cold-weather brittleness are a trade-off, typically a very high-quality steel must be used, and the part must be forged rather than cast. This drives the cost way up, to about 3x conventional hardware, if I recall correctly. Anyway, I've joked in the past about needing Charpy-certified car front-end parts here, after seeing ball joints break in extremely cold weather, and now wonder if the cold was a factor here.

Re: Aieeee! Broken crank arm on winter bike [Re: cjcride] #5343980 02/07/20 06:24 PM
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Number_35 Offline OP
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Very possibly - and of course we have lots of potholes here. Hard to avoid with our freeze-thaw cycles.

Re: Aieeee! Broken crank arm on winter bike [Re: Number_35] #5344027 02/07/20 07:23 PM
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Imp4 Offline
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Originally Posted by Number_35
It's possible, but in my experience a loose pedal will tear the threads out of the crank first.

Just thinking out loud here, but the utility I worked for specified Charpy-compliant line hardware for any critical application. Charpy is a rating of the steel to resist cold-weather brittleness. A notch is made in the metal, and the sample is subjected to x number of Joules impact energy @ -20 C. Because high strength and lack of cold-weather brittleness are a trade-off, typically a very high-quality steel must be used, and the part must be forged rather than cast. This drives the cost way up, to about 3x conventional hardware, if I recall correctly. Anyway, I've joked in the past about needing Charpy-certified car front-end parts here, after seeing ball joints break in extremely cold weather, and now wonder if the cold was a factor here.


Um, no, not really.
Impact strength or toughness is the ability of a material to absorb energy and not fracture. Often times it is measured at low temperature where brittleness can become an issue.
One way to measure impact strength is to perform a Charpy impact test in which a sample is subjected to an impact and the amount of energy the sample absorbs is measured.

Being "Charpy rated" really doesn't mean anything because a Charpy impact test is a method.
Typically what is stated is that the material must absorb a certain minimum amount of energy when the test is performed at a prescribed temperature.
For example, "four tests performed at -50°F in which the average value of energy absorbed is 56Kj/m2 with no single sample less than 41Kj/m2" or something along those lines.
Requirements are often stated with test specimens taken from both a longitudinal and transverse orientation with respect to the grain flow of the material.
Longitudinal requirements almost always exceed transverse requirements.

I'm sure there are some good YouTube videos out there.
An interesting side note is that toughness is defined as the area under the stress strain curve from the moment of initial plastic deformation to the ultimate tensile strength.

I'm sure somewhere along the line I've erred in my description. Someone with more knowledge will come along shortly with a few corrections...


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Last edited by Imp4; 02/07/20 07:44 PM.

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Re: Aieeee! Broken crank arm on winter bike [Re: Number_35] #5344050 02/07/20 07:51 PM
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buck91 Offline
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Interesting. I never had a failure like that winter commuting in Michigan back when I was light and in shape at ~230lbs, plenty of power! I'd betcha that pedal was over torqued causing the aluminum in the cheap crank arm to stress. Was it greased? It should be, and tightened well but not crazy tight.


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Re: Aieeee! Broken crank arm on winter bike [Re: Imp4] #5344076 02/07/20 08:24 PM
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Number_35 Offline OP
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I think you're right, my wording was not good. We specified that the line hardware had to be Charpy Level 1 compliant. Level 1 was the least-stringent level, and I think specified 20 J applied @ -20 C. (Obviously it's not enough for the sample parts to be tested; they have to pass the test.)

The Wiki article seems to back up my statement that as strength increases, so does brittleness:

https://en.wikipedia.org/wiki/Charpy_impact_test

"Generally, high-strength materials have low impact energies which attest to the fact that fractures easily initiate and propagate in high-strength materials. The impact energies of high-strength materials other than steels or BCC transition metals are usually insensitive to temperature. High-strength BCC steels display a wider variation of impact energy than high-strength metal that do not have a BCC structure because steels undergo microscopic ductile-brittle transition. Regardless, the maximum impact energy of high-strength steels is still low due to their brittleness."

In any case, my point, made jokingly, was that given our severe winter temperatures here (it was -30 C earlier today), it might behoove cyclists to use (non-existent) Charpy-compliant parts.

Re: Aieeee! Broken crank arm on winter bike [Re: buck91] #5344078 02/07/20 08:27 PM
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Number_35 Offline OP
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Originally Posted by buck91
Interesting. I never had a failure like that winter commuting in Michigan back when I was light and in shape at ~230lbs, plenty of power! I'd betcha that pedal was over torqued causing the aluminum in the cheap crank arm to stress. Was it greased? It should be, and tightened well but not crazy tight.
Good point. Well, I was the original installer, so if it was too tight it was my fault. Yes, I've learned the hard way to always apply anti-seize compound to pedals and bottom brackets! Doubly-so on winter bike parts!

Re: Aieeee! Broken crank arm on winter bike [Re: Number_35] #5344247 02/08/20 02:28 AM
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Think it's 'flexure'. That aluminum arm hit it's terminal end life cycle.
No pun intended. there's a wiki page about aluminum that mentions parking old high time aircraft because of it.

Re: Aieeee! Broken crank arm on winter bike [Re: Number_35] #5345125 02/09/20 08:08 AM
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NYEngineer Offline
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Did he hurt himself on the top tube when that failed? I hate it when stuff like that happens.

A while back I was back riding after a broken ankle. I was in the woods in a state park with my then ten year old son. My right pedal spindle snapped off. We were just far enough away from the car that walking would have been a bummer. I picked up my pedal, clipped in my left foot to the remaining good pedal and pedaled out with one foot. My son was behind me laughing the entire time.
The best part was when I got home and showed the broken off pedal to my wife. It was the right side and it was my left ankle that had been broken. She had no idea and yelled What, you have a bionic ankle now? I let her think that. Crank Bros Waranteed the pedals.

Chuck, my money is on there was already a small crack. We are having a relatively warm winter down here. I was cringing reading about riding in icy potholes.

Re: Aieeee! Broken crank arm on winter bike [Re: Number_35] #5345321 02/09/20 01:09 PM
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Lubener Offline
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Originally Posted by Number_35
Here's yet another unforseen failure on Jr's winter bike. He's a tall strong young man, and can apply a lot of force to the cranks on his single-speed winter ride. I picked up a replacement crank arm from a LBS for $13. Not the most costly repair, and of course it was easy enough to remove the pedal! Another first for me. LOL


Strange you posted this, last week I noticed the left pedal didn't quite feel right while on my way home, two more revolutions and the pedal popped out of the left side crank arm. Came close to raising my voice a couple octaves.Totally stripped out, no threads left in the pedal hole on the crank arm and I did nothing but some hard biking. Still trying to find a new crank arm (Sugino (VT), old and somewhat rare and hard to find..

Last edited by Lubener; 02/09/20 01:13 PM.

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Re: Aieeee! Broken crank arm on winter bike [Re: Lubener] #5345556 02/09/20 07:09 PM
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Number_35 Offline OP
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Originally Posted by NYEngineer
Did he hurt himself on the top tube when that failed? I hate it when stuff like that happens.

A while back I was back riding after a broken ankle. I was in the woods in a state park with my then ten year old son. My right pedal spindle snapped off. We were just far enough away from the car that walking would have been a bummer. I picked up my pedal, clipped in my left foot to the remaining good pedal and pedaled out with one foot. My son was behind me laughing the entire time.
The best part was when I got home and showed the broken off pedal to my wife. It was the right side and it was my left ankle that had been broken. She had no idea and yelled What, you have a bionic ankle now? I let her think that. Crank Bros Waranteed the pedals.

Chuck, my money is on there was already a small crack. We are having a relatively warm winter down here. I was cringing reading about riding in icy potholes.
NYE, good story! That's a great argument for toe clips (which I abandoned about 15 years ago after a couple of falls). I broke my R leg skiing two years ago, so am partly bionic with a titanium rod.

Jr fell off the bike when the crank arm broke, but was not injured. Fortunately he was on a side street when it happened. Vw7674 suggests metal fatigue, which seems likely.

Originally Posted by Lubener
Strange you posted this, last week I noticed the left pedal didn't quite feel right while on my way home, two more revolutions and the pedal popped out of the left side crank arm. Came close to raising my voice a couple octaves.Totally stripped out, no threads left in the pedal hole on the crank arm and I did nothing but some hard biking. Still trying to find a new crank arm (Sugino (VT), old and somewhat rare and hard to find..
I've done the same, and for some reason, including your experience, that's 3 out of 3 on the L side. Too small a sample to draw conclusions I suppose. Good luck finding a replacement! Hope you don't have to buy the drive side as well.

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