Rotor 3D+ broken crank

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highdraw

by highdraw

Relatedly wang and speaking of the industry you come from which has very stringent standards, rampant speculation that the Russian plane that just went down in Egypt had a repair to the tail section many miles back that ultimately gave way to fatigue and failed mid flight.

Rotor seems to employ a different manufacturing process comparing the 3D+ that the OP owns to say compared to the heavier 3DF crank. The 3D+ is not forged like Rotor's 3DF crank resulting in a lighter crank with the 3D+ because it is machined from extruded Al and many know forgings tend to be denser and hence heavier relative to specific volume and of course stronger but this strength difference can be compensated with greater web thickness...or...perhaps not enough in the the OP's case...same weight/strength dynamic comparing cast versus forged wheels on an automobile...castings displace more volume because they aren't as strong but forgings are denser but greater volume loses the strength tug of war and cast wheels are heavier and less preferred.

Rotor's description of the 3D+ :

ROTOR developed the 3D crank to be extremely stiff in order to meet the demands of Thor Hushovd and the sprinters of Cervelo TestTeam. But it also needed to be lightweight enough to satisfy the needs of Carlos Sastre and the climbers as well. ROTOR utilizes a special manufacturing process, named the “Trinity Drilling System. An extruded aluminum bar is intricately CNC machined with three drilled holes through the length of the crank. The result is a unique triple hollow crank arm that enables Rotor’s engineers to remove the excess aluminum in the core while still maintaining the structural strength of the crank. Available in road compact double (110 bolt-circle diameter) and road standard double (130 bolt-circle diameter) with either a steel or titanium spindle.
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Of course the integrity of the crank comes down to the integrity of the bar stock the crank arms were machined from. My sense other than an effort to keep weight down thereby making crank arm attaching web to the spider as minimal as possible, this failure could be the result of deficient metallurgy.

But it is notable that the manufacturing process for the 3D+ is substantially different than what Rotor employs for the 3DF...the latter being a forging and perhaps stronger at the end of the day...or less sensitive to fatigue.
Last edited by highdraw on Tue Nov 03, 2015 11:28 am, edited 5 times in total.

maquisard
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by maquisard

That very issue happened with Japan Airlines Flight 123 in which an incorrect repair of a tail strike caused a failure that lef to explosive decompression of the aircraft. https://en.wikipedia.org/wiki/Japan_Airlines_Flight_123

I'm actually a pilot as well ( CPL ME/IR ) and the MEP aircraft I have a share in has an oldish Lycoming engine which whilst not limited on hours, is limited on time. So now every annual inspection we have to get a penetrating dye test done on the hollow crank shaft. I might ask the guy to do my crankset on the bike next time!

I need cheaper hobbies!

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highdraw

by highdraw

You may need cheaper hobbies but you know how to live maquisard. As engineers we both constantly think about about 'what if' scenarios when up in the air at the mercy of the flying machines we are riding in designed by those with our training. Metallurgy fails and fatigue is a real issue then it comes to repetitive flight. Good odds though. 1 in 8 million crash I believe. :) Just don't want to be that 1 in 8 million.
Last edited by highdraw on Tue Nov 03, 2015 11:36 am, edited 1 time in total.

highdraw

by highdraw

wangsanegara wrote:Sorry to hear about what happen to you. I have been using Rotor Crank since 2009 and nothing happen to me.

Looking at the picture, I am quite sure that the crank was cracked half way for some time. The blackened part was oxidized, which means air was present. The white part just happen split second so that no oxidation occur. I suspect that the crank has a "hairline crack" not long after production, which propagates during repetitious load before 16k Km. Until one day, it cannot handle anymore load it cracked under load. Without a hairline crack to start, it almost impossible things like this to happen. By bringing the crank to the material science lab, you can determine if the crank has a hairline crack or not.

Working at aerospace industry, and doing Non-Destructive Test on parts is part of my job. I need to catch "hairline crack" right after production so that it will not go into Assembly.

A.W

Wang,
Here is the problem with the notion about the crack being present as this has been mentioned before. Chicken and egg as it turns out...which came first. You see cracks are or rather can be a function of fatigue. The 3D+ cranks is a CNC machined crank from extruded Aluminum. Aluminum isn't homogenous perfectly and therefore has mild inclusions on the microscopic level which can set up stress risers and sensitivity to fatigue aka cycling under load over time. So cracks in the early stages that aren't discernible by the naked eye become discernible over time due to fatigue and fatigue failures aren't even linear and can be precipitous...what happened to the OP after the crack reached a particular proportion.
Just to keep in mind that a crack lingering for a long period of time as noted by others here...that crack and this failure can still be due to fatigue. Fatigue causes cracks. Hope that makes sense.
You see your thesis about hairline crack to start isn't sound because it doesn't take a hairline crack in manufacturing to create this crack and subsequent failure is the point. A crack can start at the microscopic level and be indiscernible and emanate into a hairline crack and then grow bigger causing failure.

Some scholarship about the emanation of fatigue cracking below:
Attachments
Fatigue Cracks.jpg
Last edited by highdraw on Tue Nov 03, 2015 11:52 am, edited 1 time in total.

maquisard
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by maquisard

This thread is proving very educational, in my ignorance I thought that an extruded and machined part like the 3D+ would be stronger than a forged and machined part like the 3DF. That was actually my reasoning for buying the 3D+ over the 3DF this time around, I thought it would be stronger! I'll ask on here next time.

highdraw

by highdraw

maquisard wrote:This thread is proving very educational, in my ignorance I thought that an extruded and machined part like the 3D+ would be stronger than a forged and machined part like the 3DF. That was actually my reasoning for buying the 3D+ over the 3DF this time around, I thought it would be stronger! I'll ask on here next time.

This relationship is in fact blurred masquisard. Will give you an example. In Rotor's zeal to make the 3D+ as light as possible say compared to their 3DF crank...weight being an inducement to marketing the advantage of the 3D+...it is clear based upon the variation in Al metallurgy used and based upon your +3 sigma power output on the Gaussian curve you mentioned, that Rotor simply didn't design in enough section modulus to reduce stress impact due to cyclic loading for worse case. If Rotor had increased web thickness aka section modulus aka moment of inertia at the interface of crank arm to crank spider where your failure occurred...which is inherently a stress riser by definition due to differential section modulus...which btw, would have added weight and detracted from Rotor's marketing and potentially lost sales...reading their marketing boilerplate about the 3D+'s low weight advantage for climbers for example...understanding the impetus behind it...then your issue would have never occurred. It always comes down to section modulus relationship with fatigue. In fact would love to hear from the masters grad student who contributed the mathematical relationship of fatigue crack propagation on this very subject as a boundary condition of design...would be educational.
To use a simple analogy, a beer can may fatigue in the hands of a strong man. But one 1/2" thick won't. Same metallurgy, same loading, same no. of cycles.

Marin
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by Marin

I wonder if carbon parts are subject to similar fatigue scenarios?

highdraw

by highdraw

Marin wrote:I wonder if carbon parts are subject to similar fatigue scenarios?

Carbon natively isn't as susceptible to fatigue as aluminum. But at the end of the day, stress is also a function of geometry and geometry can easily trump metallurgy when it comes to strength but of course at the detriment to weight...the theme of this forum. Carbon cranks have also fallen victim to high watt riders. This all goes full circle back to the rather heated discussion here about the light weight stem discussed a while back. Stress goes way up when section modulus is reduced to minimize weight in an effort to market this advantage to sell more parts. That is the slippery slope at the end of the day and why there are no 9lb bicycles and sub 1000g wheelsets. Buyer beware...especially if you are a very strong guy like the OP. Strongest guys expose design weakness when designs are mostly focused on average strength in an effort to reduce weight and market this advantage.

erik$
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by erik$

An important thing to note about carbon is that the product you end up with isn't carbon on it's own but a composite material, which is a combination of the carbon fibers and a matrix of some sort of resin (often epoxy) to hold everything together. We then have a carbonfiber reinforced polymer (CFRP). While carbon fibers on their own may seem to have very good fatigue properties, which is what is exploited in marketing, the composite itself usually has a given fatigue limit. The main reason is the cracks forming in the matrix which in the end means that you get local zones where you don't have a composite, only fibers which is less stabile [this is taken from the top of my head so don't take this as textbook stuff, maybe someone wants to pitch in]. To generalize we can say that CFRP is way better than aluminum when it comes to fatigue but it is worth mentioning that composites also can degrade over time. The difference is that the degradation is so small compared to aluminum so in most practical cases there are other reasons of failure.

Anyone sitting on information regarding composite materials with high strength vs high modulus fibers in relation to fatigue? Seeing as lower stiffness fibers would seem more beneficial in applications where flex is wanted, e.g. seatstays, this would be cool to see.


Mechanical engineering student

highdraw

by highdraw

Good thoughts eric and best of luck in school.

goodboyr
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by goodboyr

You should hire yourself out to bike companies to torture test their stuff. You've gone through two Rotor cranks, and I see you've also broken some Look pedal axles. I know that a bunch of experts have weighed in here, but let me add my two cents, since this is a discussion forum. So far, many possible explanations, mostly centered on design flaws. But in my experience, stuff fails due to three possible causes (and they are not mutually exclusive, so it could be a combination). They are mis-operation, poor or lack of maintenance, and of course flawed design or construction. My experience with materials failure analysis, is that it is necessary to physically examine the parts under magnification, before drawing conclusions. Of course this is the internet, so its fun and interesting to try to conclude via photographs. I'm just as guilty as others. But without a proper physical examination of the failed parts, the conclusions are just theories and hypotheses. And we are ignoring mis-operation and maintenance issues. Doesn't sound like your power numbers are that out of bounds, and as others have pointed out, there are pros on Rotor cranks that have not failed them. So, once again, here we are dissing a manufacturer about their design, without that manufacturer coming on here to comment. That seems to be a pattern too. Shimano DA 11 speed cassettes, zipp hubs, praxis bb's. It would be great to hear from these companies, because each of these issues seem to affect some small percentage of users.

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mythical
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by mythical

@maquisard: Sucks to have your crankset fail but thankfully you're only mildly injured. It could've been so much worse and I hope your recovery is going accordingly. Just out of curiosity....what will your next crankset be?

Anyway, it seems this thread has also fallen into the trap of an indeterminable engineering debate and I’m gonna make an attempt to chip in at the risk of sounding cynical. I also broke my share of cranksets during riding, with my last fail being a heavy duty forged 175mm Shimano XT FC-M750 where the drive side arm broke straight in half. The last cranks that failed me were a pair of Zipp VumaQuads due to a deformed axle spline. Then again, I’ve broken handlebars, road and MTB suspension forks, BB’s and countless chains, never ending up badly hurt or with anything more than perhaps some road rash.

I'm the kinda person who shrugs it off and doesn't feel the need to vent on forums though, out of respect of the manufacturer. No, I am not a Rotor fanboy (far from it) and won't defend their crankset design (which they blatantly ripped off). In fact, I would never ride or recommend any of their parts in the first place, also after receiving various firsthand accounts from (some even sponsored) riders whose Rotor products failed them. I do, however, respect Rotors’ business successes and the execution of their IMHO unexciting products. They’ve gotten quite far, considering think they started out with a crankset with springs and a BS marketing story with a claimed 16% improvement in power output.

And then the title-slinging engineers step in and throw words around like section modulus and crack propagation, claiming superior physics and metallurgy kung fu. Contrary to the gospel of highdraw, low weight has little to do with why parts fail, plus Rotor cranks aren't that light anyway. Many weight weenies wrongfully believe that a lot of material makes something strong and that a heavier part will always be stronger, or that endless debate about forging versus CNC-machining. None of that holds much weight (pun intended) in the science of surface stress, as one contribution aptly pointed out.

Let's suppose that Rotor 3D cranks are built to pass the EN14781 protocol, which they have, and (like the UCI) consider it a benchmark for being used in professional bike races. Riders have won their share of cycling events using these cranks since came out in 2008, such as, I'm not mistaken, Thor Hushovd, who won the Worlds on a set.

This EN-14781 certification involves 100K cycles of 1800N of force on a pedal axle end at a frequency of >25Hz. This is what such a setup looks like:

Image

1800Nm of force is basically the same doing a one-legged squat or leg press with 183 kg or 404 lbs, and this test is times one hundred thousand. The thought alone of having to do all these reps tires me. Good luck with that, sprinters claiming to have a >1500W power output! Therefore, rider weight and power output are irrelevant to this discussion, and can we conclude that Rotor did their homework. No self-respecting engineer can legitimately claim otherwise.

Ever done leg presses or squats? So these particular cranks, and all cranks that pass this kind of rigorous testing, are supposed to be good enough and not fail like on the photos of the OP, let alone two different models from the same brand! Yet it happened...

For the record, I’m 1m83 with a race weight of ±68kg and a modest FTS of >400 watts, as witnessed by a respectable professional who has tested the likes of AIS athletes and Cadel Evans. During my first road bike ride ever, many a moon ago, I sprinted all out on a flat road (Holland, duh) and the bike computer said 74km/h. Once during a race in Ronse (Belgium), I took a single turn up front up an incline, thereby decimating the entire peloton and reducing it to a mere 14 riders. After the race, my buddy told me that I apparently went 68km/h. So effing what?!

In my untrained guesstimate as someone who has dabbles in crankset design, and also having made cranks with arms with 3 alternating longitudinal drillings before Rotor ever did theirs, the break on maquisard's crank is is at the exact spot where it’s supposed to fail from the highest surface tension during the heaviest load point of the pedal stroke.

It’s interesting that the aluminum alloy Rotor specs for their 3D+ crank arms is AL7055. Specification of this material include an higher UTS and yield strength and an increased resistance against fatigue and fatigue crack propagation compared to other commonly used 7000-series alloys. Rotor 3D cranks are CNC-machined from round bar. It may be a fault in the billet or an irregularity originating from the CNC-machining process leaving some unwanted residual stress, a repeated yet erratic stress peak induced by a sudden burst in power output and metal fatigue that caused the failure of maquisard’s crank arm.

cobrakai wrote:To be fair googling rotor crank failure brings up this thread, your previous thread and dcrainmaker's post, and that's pretty much it.
As for the DC Rainmaker Rotor fail article, since no photo of the failure was published, the fail might have been due to a faulty crank bolt, which is a known issue. A replacement bolt and reinstallation probably fixed it.
Last edited by mythical on Wed Nov 04, 2015 1:22 am, edited 1 time in total.
“I always find it amazing that a material can actually sell a product when it’s really the engineering that creates and dictates how well that material will behave or perform.” — Chuck Teixeira

Qman
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by Qman

A slight correction to what you said Mythical:
Based on the information that I have, the EN-14781 test calls for 1800 N of force, offset 65mm from the crank face. As I'm sure you're aware, the unit N-m is for moments and torques, not forces.

Some general comments:
Components that have passed the tests but fail in the field can fail for numerous reasons. It could be a flaw in manufacturing, or in the material. It could be a design change that was not tested. Or, it could be that the test didn't capture all the stresses that it is actually seeing in the field. For example, the EN-14781 test has the loads applied vertically with the cranks at 45 degrees. Real-world loads aren't vertical and aren't only applied with the crank at 45 degrees. Some cranks designs could have a sensitivity to other loading directions. They could pass the test, but fail under certain riders because of their pedaling style. A good testing program will look at all possible causes of failure, and devise a test or series of tests accordingly. If you start seeing multiple warranty issues then you either have a manufacturing problem, or your tests didn't capture one of the stresses. A stress is not just a load, it could be temperature changes and exposure to water, salt, and cleaning chemicals. Combinations of stresses can result in failures.

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mythical
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by mythical

Qman, my information says a force of 1800N, not torque in Nm. I can share it with you via email, if you like. Btw, the specified 65mm has been corrected to a shorter length, since pedal spindles are shorter and many cranks failed the test with the specified pedal length. You can see also discern this from the photo with Rotor cranks in the test setup. If I remember correctly, the distance was changed to 58mm. Anyway, EN14781 certification has already been superseded by ISO 4210, although many of the protocols remain the same.

Good point about field-testing though. What struck me back in 2008 was how Rotor quickly released their 3D cranks without much real-world testing. The reason Rotor developed their 3D cranks in the first place is because of being the main sponsor for the now defunct Cervélo Test Team, and their Ágilis cranks failed EN-testing in December '07, plus none of the team riders wanted to use them.

Then again, what team riders break never gets published in magazines, and brands are keen on marketing results such as wins on their product that can be attributed more to the rider and race conditions than to one single component.
“I always find it amazing that a material can actually sell a product when it’s really the engineering that creates and dictates how well that material will behave or perform.” — Chuck Teixeira

highdraw

by highdraw

mythical wrote:Qman, my information says a force of 1800N, not torque in Nm. I can share it with you via email, if you like. Btw, the specified 65mm has been corrected to a shorter length, since pedal spindles are shorter and many cranks failed the test with the specified pedal length. You can see also discern this from the photo with Rotor cranks in the test setup. If I remember correctly, the distance was changed to 58mm. Anyway, EN14781 certification has already been superseded by ISO 4210, although many of the protocols remain the same.

Good point about field-testing though. What struck me back in 2008 was how Rotor quickly released their 3D cranks without much real-world testing. The reason Rotor developed their 3D cranks in the first place is because of being the main sponsor for the now defunct Cervélo Test Team, and their Ágilis cranks failed EN-testing in December '07, plus none of the team riders wanted to use them.

Then again, what team riders break never gets published in magazines, and brands are keen on marketing results such as wins on their product that can be attributed more to the rider and race conditions than to one single component.

Mythical...are you an engineer? Reason I ask is because you misinterpret what Qman stated which is largely accurate and honestly quite simple. If you read your two posts, they contradict themselves.

Here is some simple math. Qman stated 1800N, not 1800 Nm. You stated Nm in one post and N in the other...lol. 1800 N offset by 65mm creates a torque aka Moment = F X D. Simple math is...a force of 1800N applied 65mm from the face of the crank applies a Torque of 1800N X .065m = 117 N-m

As to your comments about my assertions...they dovetail your inability to comprehend what Qman wrote...lol. I do find your posts entertaining tho and thanks for that. :D
Last edited by highdraw on Wed Nov 04, 2015 12:14 pm, edited 1 time in total.

by Weenie


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