Commuting - Ever had this happen? (crankarm failure)

Riding to work this morning, I came to a short but steep hill on the MUP I regularly travel. I could see a patch of ice crossing the path, so I got off the bike to walk around it because I didn't dare try to crank up a hill on ice. I got around it and mounted the bike, but realized I had left it in a fairly high gear. So I stood up and just let my body weight push the left crank downward and then *SNAP*. I heard metal hit the ground as my foot landed on the pavement. I assumed my pedal had broken, but I looked down and saw this:

http://img.photobucket.com/albums/v214/lamplightsg/Rockadile/DSCN0104small.jpg

And then this:

http://img.photobucket.com/albums/v214/lamplightsg/Rockadile/DSCN0100small.jpg

They were 1994 Shimano STX cranks with probably about 4-5000 miles tops. And for the record I weigh between 185 and 190, probably close to 200 considering how many layers I was wearing. :p Luckily I avoided injury; I just had to walk home and get another bike. So, have you ever experienced this? It was a terrible sound!

Hercules.

Just a couple of weeks ago, my right crank arm broke about an inch below the pedal. It was a Galli that I had been using for about 25 years, so I gues it is not too shocking. When I looked hard, I found a crack in the left arm, too.

I've snapped cheap one piece cranks and there's at least one website showing pictures of, mostly older, failed cranks. Just visually check them when you're doing your weekly clean (:rolleyes: what clean?) and you'll be fine.

I broke a crank on a spin cycle at the YMCA. Actually, it might have been the bottom bracket that snapped where it joins the cranks. Tthe part probably rusted out from all the sweat in spin classes, but I like to think it was from the tremendous amount of torque that I generate.

Impressive.

No, never happened to me. Probably never will, given that I'm 120 pounds and a spinner.

After removing the arm (and replacing both!) it looks like it fractured about half-way through, and then bent what was left until it snapped completely. Looking at the cross-section of the arm, the metal on one half is much darker than the other and has a strange circular "grain" to it, where as the other half is nearly white and doesn't appear to have any "grain" at all. Of course now I'm nervous about all of my bikes. :o

...the metal on one half is much darker than the other and has a strange circular "grain" to it, where as the other half is nearly white and doesn't appear to have any "grain" at all...

darker section is the fatigue which is usually a gradual process

lighter section is your strength >> remaining metal

http://www.google.com/search?hl=en&q=metal+fatigue

Very interesting, thanks. I had some brand new Sugino cranks on the bike, but I had temporarily put them on a Miyata touring bike and replaced them with what I had lying around, which happened to be these. The Suginos are now back in place, needless to say.

Wish I were strong enough to break crank arms. Wish I could fly like Superman.

Doesn't the darker area indicate it was cracked for a while, and oxidized in the cracked section? Maybe there was a hairline crack there. I can't really tell, but it looks like there's a small nick in the crank right where the crack started.

It's lucky you didn't crash!

Cracked cranks pictures (http://materials.open.ac.uk/mem/mem_ccf4.htm) -- FAIL (http://pardo.net/pardo/bike/pic/fail/FAIL-001.html).

The closest I came to it was bending a Campy C record crank when I used to race. (It was a long time ago)

Bought a Dura Ace set after that, and did not bend that one.

Glad you are ok!

Glad you weren't hurt!

My 105 had a very big fracture. So big the LBS wouldn't let me out of the shop until it was switched. Good chance to upgrade to Ultegra! :D

I broke an Ofmega right crank about 22 years ago. It cracked from the pedal axle threads and the whole pedal broke off and I went down onto the pavement. I was pedaling up a mild incline at the time, luckily not far from home.

The cirdular "grain" is typical of a fatigue crack that spreads from the stress raiser. Then the white crystaline grain is the tensile fracture that happened under your weight.

I've never had this happen, though Sheldon sez standing up puts an unnecessary strain on the machine.

Guess I'll see, eventually. ;)

Quadzilla!

I went through multiple bottom brackets and countless chainrings on my previous bike, but I had the same crankarms for 17 years. I've never busted a crankarm, but I have seen it on other bikes than yours.

HGH......is not just for elite athletes anymore!

Broke a seat tube once but not a crank (yet).

I'm 165 pounds and a spinner and had a crank crack from the dust cap threads to the outside. That was enough to get the crank bolt to work loose and have the crank come off during a ride. But nothing like the OP!

Uh, don't wait for the crank to fail. Both the OP and myself lucked out. Usually there's some tell-tale hair-line fractures.

I've never had this happen, though Sheldon sez standing up puts an unnecessary strain on the machine.

Guess I'll see, eventually. ;)

The circular grain that you see is a textbook example of a cyclic stress fracture, which is exactly what you would expect to see in this kind of equipment.

I had a cheap Taiwanese one-piece crank fail, at the hole where the pedal was threaded.
The pedal just fell off when it rubbed against a curb [kerb].
I replaced it with a one-piece Schwinn crank that I had lying around.

I had a professor in a failure analysis class who did a bunch of consulting work. He had scores of bike cranks that were in some way involved in court cases, all of which failed like the one above. Glad you're ok!

After removing the arm (and replacing both!) it looks like it fractured about half-way through, and then bent what was left until it snapped completely. Looking at the cross-section of the arm, the metal on one half is much darker than the other and has a strange circular "grain" to it, where as the other half is nearly white and doesn't appear to have any "grain" at all. Of course now I'm nervous about all of my bikes. :o

I'll bet it creaked before it broke. The one I broke did. Creaked like crazy no matter what I did to adjust it...I was just to dumb to figure out what it was:o

Can you take a larger picture of the fracture surface? I want to see if I can identify the striations in the metal.

Thanks!

I've snapped 2 cranks:
-Cheap Sugino on 1997 mtb
-Kooka Bonnie Forged. Quite a surprise but I did have like 40k miles on those cranks....

Currently running a Race Face Atlas (DH crank on my commuter!) cause snapping cranks is scary. other choise was a Saint crankset.

I'll bet it creaked before it broke. The one I broke did. Creaked like crazy no matter what I did to adjust it...I was just to dumb to figure out what it was:o

You know, now that you mention it I do recall hearing a strange sound all last week when I was riding it. I kept thinking the chain was rubbing the front derailleur, and since I have friction shifters I kept moving it around trying to get it to stop, yet it never did. I guess I know why now. :o


Can you take a larger picture of the fracture surface? I want to see if I can identify the striations in the metal.

Thanks!

No problem. These shots aren't very good because I didn't have my tripod at home, but hopefully they'll be sufficient.

http://img.photobucket.com/albums/v214/lamplightsg/Rockadile/DSCN0108WEB.jpg

http://img.photobucket.com/albums/v214/lamplightsg/Rockadile/DSCN0106WEB.jpg

Been riding bikes since I was knee high to a grasshopper but I've never broken a crank arm.:D

Guadzilla!

I went through multiple bottom brackets and countless chainrings on my previous bike, but I had the same crankarms for 17 years. I've never busted a crankarm, but I have seen it on other bikes than yours.

Fixed that for 'ya....:D

I didn't think that kind of thing happened with normal riding. It has never happened to me. Closest I came to that is having the threaded end of the BB (Adjuster Cone side) snap in half.

You know, now that you mention it I do recall hearing a strange sound all last week when I was riding it. I kept thinking the chain was rubbing the front derailleur, and since I have friction shifters I kept moving it around trying to get it to stop, yet it never did. I guess I know why now. :o


On the bright side, at least you'll know what to listen for the next time;)

Last September I busted a new-ish (less than a year old) mid-range crank arm right in the middle, while accelerating from a stop. Unfortunately, my leg had enough momentum that my knee buried itself into the very sharp stub of the crank still attached to my bike, and I had to get get a whole bunch of stitches. I ended up complaining to the manufacturer, who, to their credit, handled it very professionally and more than compensated me for a new crankset, a missed day of work and some pain and suffering.

I hadn't noticed any creaking or cracks developing (although closer inspection probably would have shown cracks developing, as the fractured surface had dark areas).

I now run Profile Racing chro-moly cranks--pretty much indestructible and no aluminum fatigue worries.

I run big DH/BMX platforms like OP's and I can't help wonder if these pedals are a contributing factor to such failure. Specifically, the force applied to the outer edge of such pedals would get more magnified when transferred to the crank arm because of the greater width of the pedal compared to typical road pedals (and, perhaps, because they are stiff and less likely to absorb force by flexing themselves). Just a theory...

Thanks for posting more pics. Very interesting I must say. It appears that a crack formed(see the darker section) and then propagated to failure. It must have happened over a longer period of time due to the extent of the discoloration. You can see the striation lines very clearly. The lighter section shows what happens at a brittle failure in a component that has been cast. Very cool indeed.