Medic Zero

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I know I posted this in the mechanics forum, but I don't know how many of you go over there, and I strongly suspect my being a Clyde had a big part in this mechanical (structural?!) failure, so I'm sharing it here with y'all for instructional purposes.



This happened to me as I went to take off from the left turn lane at a busy stop light, early in the second day of an overnight, two day bike ride. Had to catch a bus into the next town, as the town it happened about two miles outside of didn't have a bike shop, at all.

I know my weight is a factor here (280 lbs), but I suspect the pedal extenders also probably put some additional stress on the crank in a way I guess it wasn't really designed for. Nonetheless, I never even considered that such a failure was a possibility prior to this happening to me. I guess I just assumed that a cast piece of aluminum was basically unbreakable in this context.

To be fair, in addition to my weight, I ride for nearly two hours a day, seven days a week over rough roads, and occasionally end up hopping off a curb during my commute, so I suppose that poor crank has been subjected to continuous vibration and occasional severe stress.


PS: Ride report on the trip follows when I finish composing it!

bigfred 

Well done. The force of clyde is strong in you. I've done my fair share of damage over the years (BB spindle, pedal spindles, etc.) but no crank arms that I remember. Impressive.

I suspect you're correct with attributing some portion of blame to the pedal extenders.

What make and model crank?

2 wheeler

Wow. I've never seen that before.

jmp72

wow that's bad... I was thinking that I was the only one... hope you are not injured... I was scratched a lot!

last year I was sprinting at 30km/h, full power on the pedal, third pedal stroke ---> me, my bike over me, scratching on the road haha

i'm now using ultegra 6700 with no problem i'm also 280lbs

sstorkel

Wikipedia says: "One important structural limitation of aluminium alloys is their fatigue strength. Unlike steels, aluminium alloys have no well-defined fatigue limit, meaning that fatigue failure eventually occurs, under even very small cyclic loadings. This implies that engineers must assess these loads and design for a fixed life rather than an infinite life."

Didn't Mr. Beanz break an aluminum crank in the same fashion? I think a couple of guys over in the Road Forum have posted similar pictures.

Medic Zero

Well, this is the first time I've done it, and that crank has only been on the bike a few months as I had one that evidently wasn't torqued down enough work it's way loose and strip itself in the process. That said, if I remember correctly, the crank that failed came out of a spares (used) bin at my LBS, so who knows how much fatigue it had already suffered under before it had the bad luck to meet me!

I don't check out the road bike forums, I don't enjoy bikes with drop bars, am the opposite of weight conscious about my bicycle or its equipment and generally don't like road bikes. It's quite possible folks over there posted similar failures, I would have missed it. I didn't catch Mr. Beanz similar crank failure either, but if it happened before I joined about four years ago I would have missed it, because I didn't spend any time lurking before joining and discovered the Clyde sub forum right after.

It's funny, as because of what you've quoted above from wikipedia, I've steered clear of aluminum bikes for me for touring and riding around the city, because of the miles and hours I put my bikes through, but somehow I never even considered that the very same limitation would apply to cranks. Makes the utmost sense once realized, but like all assumptions was invisible up to that point!

Now the quest for some not terribly expensive off-road crankset that hopefully will stand up to the kind of abuse I dish out for some time! Welcoming suggestions!

CommuteCommando

I suspect a manufacturing flaw. In spite of the fact that that aluminum has no fatige limit, to stress a crank with no defects such as a cold seam in the casting (functions like a crack) would require something on the order of a 400 pound clyde doing a million miles on the thing. Most medium to high quality crank sets, like the ultegra are forged, which among other things, eliminates such defects.

I'm not sure your crank was just die cast, as I'm pretty sure, but not certain, that almost all aluminum cranksets are forged. There was possibly a small nick or crack in the crank arm that went undetected. I sharp inside corner raises the local stresses considerably, and small cracks can grow to big ones fairly quickly. This is why airplane windows have rounded corners, except the pilots windows which are heavily reinforced.

skidder

I wouldn't put too much faith in that Wikipedia quote shown above. Whether steel, aluminum, titanium, or some other metal, the quality depends on the metal alloy and how it the particular part was manufactured. There are both cheap alloys and cheap methods to manufacture metal parts, it just depends on the price point the manufacturer is trying to hit with the parts.

What 'level' of bike was the crank on? In general, the lower priced/lower quality bikes will ahve lower quality cranks. I had a similar aluminum crank arm snap (I felt it breaking before it actually sheard off) on a lower-end bike, and just replaced it with a better quality aluminum crank I found on line. No problems since that time. And the bike has an aluminum frame that I'm not the least bit worried about.

thirdgenbird

I've seen several pairs or dura ace cranksets that were broken by people under 200lbs. It is usually at the pedal axle, but not always.

Homeyba

I've seen that happen a number of times over the years, even to little guys. It's a manufacturer flaw. If you want to attribute it to your awesome power though who's to say different...

rm -rf

Typically, a crack starts at a defect or a nick in the aluminum. It often grows slowly, then breaks all at once.

See this example. The dark area was where the crack was there long enough to darken the metal.

It's probably a good idea to check for cracks on the crank, near the pedal hole, and also cracks at the spoke nipples on the rims.

BigJeff

Habit of not shifting down at stop lights would cause some heavy stress over long periods of time.

Medic Zero

Although sometimes it can be a manufacturer flaw, it is much more common that it is due to fatigue of the aluminum. Check out the thread I started in the "bicycle mechanics" forum here for more info.

My weight may have actually been one of the smaller factors, but it contributed. It also doesn't help that I am in hilly terrain, so I'm really putting some torque through the whole system, especially given my weight. The pedal extenders probably contributed far more, due to applying the stress/force in ways the crank wasn't really designed for, a few hard pedal strikes pedaling through turns contributed too, as well as occasionally jumping off a curb, but the constant vibration of going over awful roads two+ hours a day, virtually every day was probably the largest contributing factor.

Be aware though that these failures can even happen to little folk. I'm starting to feel like cranks are ticking time bombs!

Medic Zero

I'm actually really good about downshifting before stop lights. It's very, very, rare that I don't do so.

Definitely something for other folks to be aware of though as a contributing factor!

Homeyba

Without actually looking at the part it's hard to know for sure but I'm inclined to believe that the forging had an inclusion or cold shot that weakened the part. Just based on my 15+ years as QA for a materials fabricator. Foundry issues happen more often than you'd believe and most often it's difficult to tell you have an issue until the part breaks catastrophically. If it was solely a stress/force issue they be braking all the time. You don't really see that happen. I ride 10k miles a year and have for years, including the 2.5 miles of dirt road from my house to the paved road and never broken a crank, except for crashing. I'm not a whole lot lighter than you and I also race.

Medic Zero

It happens more often than you think. Seriously, check out this:

https://www.bikeforums.net/showthread...failure-before

and this:

https://pardo.net/bike/pic/fail-001/000.html#Crank

Reading some of the text that accompanies all of those pictures of crank failures illustrates how frequently this happens and that it usually isn't manufacturing defect, as does the shared wisdom in the thread in the mechanics forum on this.

Homeyba

I know it happens, I've seen a couple. All of the one's I've actually looked at had a defect in the casting. Stress may be the straw that breaks the camels back but it's the defect that is the initial problem. Defects in the casting, be it a cold shot/shut, inclusions, shrinkage, a bleeder etc., all can degrade the structural integrity of the part and it eventually fails. I bet, if you looked at your broken crank you will find some kind of defect in the original casting. You can usually tell by the structure of the metal around the failure. Casting/foundry processes are not precise.
There are lots of web pages showing all sorts of part failures but if you look at how many bicycles are sold annually compared to the number of real failures the number of failures is relatively small. It's pretty easy to become a chicken-little by reading all that stuff.

BigJeff

I totally understand... I've been stuck in a big gear before too...

But if I look in my tool box and pull out socket arms and spanners there are very few that I would stand on with heavy torque.... and every time I did I would expect catastrophic results.

I break lots of stuff.... It is only time before I bust mine.

Medic Zero

I like to be efficient, something about having to take off extra slow due to my not having had the foresight to downshift as I roll up to a stoplight sticks in my craw, so I'm almost always mindful of wanting to downshift in that situation and do so.