Broken Crank

[Benedikt]

I seem to remember seeing a similar post to this a few weeks/months ago but here it is ...

My VO crank that had 13,000 miles on it busted this morning on my way into work. I was at a stop. Pushed down with my right foot, locked my left clip in, pushed down with the right and "clunk" crank arm came right off. Fortunately I wasn't going that fast, hammering down the road. What causes this? Do ALL cranks have a life span? These are an aluminum alloy.

Here's a pretty good picture of the break - https://www.flickr.com/photos/neutralbuoyancy/16320815710/

- Brian in Seattle

[islaysteve]

Have you contacted VO? I can't imagine that they would be too happy with this performance.
Steve

[Wes Ramos]

This is a classic fatigue failure -- looks like a fracture had existed for a while before it sheared off. There's a small write up in RR42 about the exact same failure (and location, probably safe to say this one of the highest stress areas in a crank arm). 

http://www.cyclofiend.com/Images/pdf/RR42_web.pdf  see page 7 

As far as prevention, a better alloy (steel) will allow more load cycles. Aluminum alloys can take a magnitude less cycles before failure, but mostly dependent on composition, load environments etc. As far as shear failure prevention after crack initiation, could have run an NDT inspection and seen the crack... however this is not economic on bike parts. 

[Deacon Patrick]

There is darker discoloration on both pieces (bottom left, top right). My understanding is that indicates where the crack and thus oxidation/exposure/somesuch formed and was ridden for x amount of time. It sure looks like a very hard place to see a crack on inspection! Glad you're OK and weren't hurt!

With abandon,

Patrick

[Eric Norris]

I agree that alloy parts have a finite lifespan, but 13,000 miles for a crank arm seems a little short to me. I would definitely contact VO for a replacement--I've had very good experiences with them in this regard. 

Eric N

www.CampyOnly.com

Twitter @CampyOnlyGuy

Blog: CampyOnlyGuy.blogspot.com

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[Jan Heine]

All cranks do have a lifespan, but it's usually in the 100s of thousands of miles. It depends on the rider – some riders are harder of drivetrains than others, but it also depends on the cranks. Material choice, the way they are made (net shape forging vs. machining to length), and even the surface finish all will affect the fatigue life of the part.

The VO cranks are made from 7000-series aluminum, which is very strong, but also suffers from stress corrosion cracking. Most people consider it a poor choice for a crank for that reason, especially if it's just polished and not anodized.

Disclosure: Compass Bicycles manufactures and sells René Herse cranks.

Jan Heine
Compass Bicycles Ltd.
http://www.compasscycle.com

Follow our blog at http://janheine.wordpress.com/

[Benedikt]

Actually I sent VO an email with a picture of the crank. There response was,"What address do you want us to send your new replacement crank?"

[cyclotourist]

GREAT to hear a response like that!!!

On Thu, Feb 12, 2015 at 7:17 PM, Benedikt <neutral...@comcast.net> wrote:

Actually I sent VO an email with a picture of the crank. There response was,"What address do you want us to send your new replacement crank?"

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Cheers,
David

Member, Supreme Council of Cyberspace

"it isn't a contest. Just enjoy the ride." - Seth Vidal

[Eric Norris]

In my experience, VO is very good about standing behind their products. When my VO bottle cage broke, they sent a new one. When a Nitto cage (bought from someone else) broke, I was out of luck.

Eric N
www.CampyOnly.com
Twitter @CampyOnlyGuy
Blog: CampyOnlyGuy.blogspot.com

> On Feb 12, 2015, at 7:17 PM, Benedikt <neutral...@comcast.net> wrote:
>
> Actually I sent VO an email with a picture of the crank. There response was,"What address do you want us to send your new replacement crank?"
>

[Cyclofiend Jim]

Kudos to VO to having the best of all answers.

I have a couple docmented failures - both exhibit that "minor/easy to miss crack that migrated until it failed" light/dark mode:

https://www.flickr.com/photos/cyclofiend/sets/72157603388190693/

https://www.flickr.com/photos/cyclofiend/sets/72157622446951852/

and I tapped out a long column on the old blog, pining for quality steel cranks.

http://ramblings.cyclofiend.com/?p=399

- J

[cyclotourist]

Moral of story: Don't buy used cranks from Mr. Fiend!

I'd love to have a quality steel crankset BTW.

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[Cyclofiend Jim]

Ha! 

yeah.... but the carbon cranks were new in the box, and the the Ritchey replacements only lightly used.

And like the famous Twilight Zone episode of days gone by, I was forced to be completely honest regarding my experiences....

The purchasers did see pleased. ;^)

- J

[Benedikt]

Tank you Wes Ramos and Deacon Patrick. Those are exactly the sort of answers I was looking for. Wes, the article you linked to was great. I was wondering about that discoloration. Obviously there was a crack for some time slowly growing. Weather it is a flaw in manufacturing I don't know but since VO is willing to send a free replacement, I'm willing to give them a second chance.

[Tim McNamara]

That looks like it may have failed along a machining line, with the crack initiating where there was a step-up from the flat of the back of the arm to the curve up to the spider. You can see another line between the arm and the spider. It may have been enough to cause a stress riser, especially if the alloy was not well-chosen. The VO site indicates it is 7075 but does not indicate whether it is one of the heat-treated varieties; this seems to be a pretty standard alloy for bike cranks.

These sorts of failure are very, very serious and can in fact be fatal, if the crank breaks and dumps you in front of a moving vehicle- they almost always break when you are standing on them. I would examine the replacement crank very closely before installing it to check for machining ridged, lines, etc. Looking at the photos on the VO site, the machining looks quite clean and well-polished. You want to see no abrupt transitions.

All cranks do have a lifespan, in part because that is the nature of aluminum- it cracks after enough repeated load cycles. Once a crack starts on the face of the material, it propagates through the metal. A collection of crank failure photos:

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

[Tim McNamara]

> On Feb 12, 2015, at 3:01 AM, Joe Bernard <joer...@gmail.com> wrote:
>
> The short answer is yes. Aluminum will eventually fatigue and fail, and 13k is pretty good mileage on a crank arm.

13K is IMHO terrible mileage on a crank before failure. Cranks should- and often do- go at least ten times that far. This was a premature failure.

[Benedikt]

The start of the break does look like it's along a machining line. In fact, it almost follow's it perfectly however there is no step up there. There is a step up on the crank just before it but the site of the fracture is a smooth curved transition up to the spider with a little machining. I examined the crank on the left side very closely and (IMHO) it's in great condition. It does look and feel a bit more polished then the crank that failed.

- Brian

[Ron Mc]

I agree, it started right in the middle on the inside of the arm at the machined fillet.  

[Benedikt]

Thinking back over the last 6 months or so I do seem to remember an occasional "tink" sort of sound and feel in the downward stroke of the right crank. Could this have been a tell tale sign of the crank cracking ever so little bit more? I couldn't think of anything else in that area of the drive train that would cause this. Bottom bracket, chain ring bolts, etc. were all buttoned up nice & tidy.

[Joe Bernard]

I'll betcha that was it. It's a bummer that it snapped, but very cool of VO to send you a new one.

[Geoffrey]

That radius cut ends right where the crack begins which is what I'd expect.  That radius is there to relieve stress. Maybe a larger radius would help.  Im more heavy duty applications, that area would be shot peened to relieve more stress.  Glad that it happened at a red light and that VO is giving you great service.

On Thursday, February 12, 2015 at 2:31:07 AM UTC-6, Benedikt wrote:

[Johan Larsson]

I'm curious about how the backside of the arm looks at the crack, the side that is towards the table on the pictures. It's not possible to see the radius of the inner corner otherwise.

Johan Larsson,
Sweden

[David Banzer]

Kudos to VO for sending a replacement, but I think that should be the appropriate response from any manufacturer when a product fails prematurely. Did they mention anything else in their response? Such as a history of the particular crank failing or that they changed tooling due to these stress points?
It is nice that you'll have a replacement, but I'd be wary of a product that has failed. While you didn't get hurt this time, whose to say the replacement won't fail as well, and could have more damaging results.
Just being a skeptic this morning.
David
Chicago

[Brian Pickart]

David, VO said they'd never seen anything like that before.

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[Tim McNamara]

Well, that’s good news (that it’s an anomaly). I would think they’d want to examine the broken crank just in case this is the front end of more such failures, however. Anyone using one of these cranks should be inspecting them closely

When you think about it, the stress on that end of the crank is significant. Imagine a 200 lb rider standing on the pedal- the stress is focused on where the crank meets the spider- on the VO crank, exactly where this one failed. Many Campy Nuovo/Super Record cranks failed there, too, because of the way in which aluminum fails combined with a tiny design problem (that could be prevented with 30 seconds of time and a round file- Campy actually sanctioned customers doing that little bit of filing once the problem became known).

The other most common place for failure is the pedal eye. This is also a poorly designed spot in all cranks because the pedal is allowed to rock slightly in the threads. If the shoulder of the pedal threads was shaped conically like a lug nut mating with a chamfer in the crank, that problem would be solved and braking a pedal eye would be a very scarce event indeed.

There had been a fundamental issue throughout the history of the bike industry which is that bike parts have been designed by mechanics rather than mechanical engineers. While this has mostly worked out OK over a century and a half or so, it has also resulted in significant problems (brakes that pivot in such a way that the pads can end up in the spokes, breaking cranks, cracking rims, etc.) when basic mechanical engineering principles are ignored.

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[Benedikt]

I added a few pictures for those of you who wanted to see the other side of the crank including some "mating" the two up.

https://www.flickr.com/photos/neutralbuoyancy/16529273621/in/album-72157607896493013/

- Brian

On Thursday, February 12, 2015 at 12:31:07 AM UTC-8, Benedikt wrote:

[Johan Larsson]

On Sat, Feb 14, 2015 at 10:34 PM, Benedikt <neutral...@comcast.net> wrote:

I added a few pictures for those of you who wanted to see the other side of the crank including some "mating" the two up.

https://www.flickr.com/photos/neutralbuoyancy/16529273621/in/album-72157607896493013/

- Brian

 Thanks, that was very helpful. It didn't look like I thought it would at all, judging from the first photos - I thought the big gentle inner curve was a flat machined surface. Even if there's a gentle curve, the very transition to the arm is abrupt and sharp, and it's a definitive stress riser. And the crack started exactly there. Even so, I would not say it's a poor design - it could improve in that detail to have more margin of error, but it's likely a materials problem in manufacturing. I have decades of experience with dogsled runners made of 7075 aluminium, and they are subject to an extreme amount of flex and corrosion compared to a bicycle crank, and they mostly break because of severe mechanical damages and cut-outs that initiates the break.

I don't know how the original French cranks were shaped there, I use an old Stronglight 49A crank myself daily in summertime but the bike is in storage now and I can't take a look.

Johan Larsson,

Sweden

[Zack B]

Does anyone make quality steel cranks?

Seems like a perfect "Riv" project.

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-Zack