Carbon cranks - info on construction?

[Pastashop]

So I’m getting curious about the construction and materials used for bicycle cranks, including the newest carbon cranks. (Aluminum, steel, titanium are interesting, too, but I haven’t seen (m)any examples that deal adequately from a sound engineering perspective with fundamental shortcomings in the various failure modes.) It seems like for most there’s a metal spine over which the carbon is wrapped or molded, but I’m curious how the spindle is fixed, what material is used for the spindle, the spine, and pedal hole. Anyone have further info or sources for the info?

TIA!

[lhuerta]

…I would be more curious to know more about the failures u allude to. I comb through many bicycle newsgroups almost daily and have heard of almost no carbon crankset failures. Last failure I remember reading about was a few pedal thread inserts come loose on older Campagnolo cranksets…but I suspect those were due to hamfisted install and not manufacturing defect. BTW… most high quality carbon crank sets are hollow construction and lower end stuff is just alum wrapped in carbon.

[unterhausen]

The only failures I am aware of are the pedal inserts coming out.

I should put the carbon cranks I have on my gravel bike.

[pdonk]

I had a pair of zipps fail. Broke about 20mm up from pedal hole. They probably had about 20,000 km on them.

They were a 3 piece crank that used the Isis standard with the interface molded into the crank.

I have pics of the failure someplace. If I can find, I'll post.

[Pastashop]

Thanks for the construction info, pdonk.

I’m interested mostly in knowing the details of construction and materials used, though any documented history on experienced failures certainly would be helpful.

Thx!

[prototoast]

My understanding of sram carbon cranks is that each arm is molded in two halves, hollow in the middle, then bonded together after the carbon has cured. When the two halves are bonded together, aluminum inserts for the spindle and pedal are also bonded in.

[zap]

Google Luescher Teknik Campagnolo Super Record 11 crank arm. Very detailed look at construction and materials used.

[pdonk]

Here is a pic of my broken carbon crank. Took some hunting, as it happened 6 years ago.

And here is a pretty good set of photos of a complete set from one of our members.

http://www.bikerecyclery.com/zipp-30...sis-420g-rare/

[wallymann]

here's how campy super-record UT cranks are constructed

https://www.youtube.com/watch?v=Pi0i1DeVvzE

[Pricci09]

as previously mentioned, it's typically the pedal insert that goes.

[JWB475]

On Easton/Race Face cranks the insert is bonded in the crank arm.

Mine have failed twice, both were the drive side crank arm. Easton replaced the first one under warranty (~3 years old), waiting to see what they do with this one (~ 1 1/2 years old).

The first time the pedal flew off into the ditch and I couldn't find it. This time I noticed it before it failed, and was trying to take it easy, but I didn't make it very far. At least this time I was going really slow and the pedal landed in the road.

Here you can see how it fails, I think the insert cracks (part of it is still bonded on the back of the crank), with only the front portion of the threads supporting the pedal it eventually fails...

[NHAero]

That's fascinating. It's an aluminum failure to begin with!

Originally Posted by JWB475

On Easton/Race Face cranks the insert is bonded in the crank arm.

Mine have failed twice, both were the drive side crank arm. Easton replaced the first one under warranty (~3 years old), waiting to see what they do with this one (~ 1 1/2 years old).

The first time the pedal flew off into the ditch and I couldn't find it. This time I noticed it before it failed, and was trying to take it easy, but I didn't make it very far. At least this time I was going really slow and the pedal landed in the road.

Here you can see how it fails, I think the insert cracks (part of it is still bonded on the back of the crank), with only the front portion of the threads supporting the pedal it eventually fails...

[mcteague]

Originally Posted by Pricci09

as previously mentioned, it's typically the pedal insert that goes.

That is what happened to my 2006 generation of the Chorus compact crank. The insert never came out, just started getting loose. My 2013 Chorus and 2019 Record cranks are working fine. Hopefully they will continue to do so.

Tim

[berserk87]

My Zipp cranks broke at the BB interface, by cracking and I couldn't tighten one of the arms after that. Funny enough, it was the aluminum internal interface that cracked.

I've ridden carbon cranks from Zipp and FSA (2 different sets). Also ridden aluminum from 105, Ultegra, Dura Ace, Athena, Chorus and Record. I couldn't tell a difference between carbon and aluminum in terms of ride dynamics. I got my carbon stuff inexpensively and wanted to try it out. I'm not into aesthetics much but the carbon cranks did look cool.

[Pastashop]

Thanks for the shared photos and the links! I’ve learned a lot.

The Luescher video of a sectioned Campy SR cranks was interesting from a composites fabrication standpoint! (His discussion of the mechanical attributes was a bit shallow.) Noticed in the picture that the pedal eye is actually a well done junction, at least from the standpoint of stress dissipation, using a 45 degree angle between the threaded insert and the carbon “housing”, metal-metal interface notwithstanding. (http://pardo.net/bike/pic/mobi/d.ped...int/index.html)

I found a few more examples and discussions of crank designs and failure modes here:

http://pardo.net/bike/pic/mobi/

and here:

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

where the Hollowtech II style spindle failures also appear… The Campy carbon cranks in PowerTorque and Ultratorque formats have their own design issues…

Wonder if it might be useful to have a sticky thread on various equipment failures to learn / improve?..