BikeTim42

Looking at the second photo would seem to indicate that it is perhaps only a paint fracture. I learned something interesting this weekend about carbon fiber and that the paint is usually more brittle than the carbon and thus an indicator of stress even if it isn't a break. (search YouTube for "Repairing a Huge Crack in a High End Carbon Frame…" by Jourdain Coleman. (sorry, still not allowed to post links) He is being shown both how to damage and repair a carbon frame by a professional. The carbon is considerably less fragile than I imagine but...

Now, would I ride that fork after seeing that? No. Not unless I got a professional opinion that it is only paint or sanded off the paint (carefully) to inspect.

mpetry912 

if it is "only a paint fracture" then we'd have ask - what caused it

clearly a deflection or strain in the underlying substrate

are we still going on about this ? retire that fork

and get a new one

/markp

Harold74

Recognize that this was never a "should I retire the fork?" thread. Folks are welcome to comment on that aspect of things but those comments are of little value to me. I've reviewed a zillion of these threads and fully understand both the risk proposition and how most responders will tend to react to that.

The purpose of this thread is to discuss the nuances of carbon crack diagnosis, crack causation, and likely modes of failure. To the extent that responders continue to "go on about this", those people are providing precisely the help that I am seeking here. I encourage people to continue to "go on about this".

shelbyfv

Could you summarize what you've learned from the thread so far?

Harold74

You bet teach. You want that double spaced? 1500 words? I live to serve.

Atlas Shrugged

Really?

Harold74

Yes, really. I started this thread by asking people to share their knowledge on this subject. Why would I then turn around and discourage folks from... sharing their knowledge on this subject?

@BikeTim42's response is an excellent case in point. I find his recent contribution to be super interesting. And I wouldn't have that if I'd just shut down the conversation after the 420 th time that somebody recommended that I replace the fork.

jlaw

Here's the last guy who rode a bike with a cracked carbon fork.

Don't be him.....

mpetry912 

and can I get fries with that ?

/markp

icemilkcoffee 

No, the adapter sleeve is not a good solution for the stem spacer rings. If you look closely at the threadless headset, the top bearing has a thin wedge that needs to be pushed down by the stem spacers. That floating sleeve inside won’t press down on the wedge. If you are lucky the 11/8 spacer might contact the wedge sufficiently. But I don’t like to rely on luck.

you can easily find 1” stem spacers on ebay:
https://www.ebay.com/itm/292400422820

ljsense

I'd guess with about 90 percent confidence that that is not a crack in the carbon. It looks much more like a scratch.

The overall bias I've seen is to suspect carbon of cracking. Metal is crystalline and cracks. Carbon is different, and when it cracks, it's not like metal cracking. It has layers, fibers -- these strands are not easy to snap. The resin may crack similarly to a metal, but how that plays out in a layup of carbon fiber is a different deal.

Paint cracks in general are usually very faint. They seem more common in carbon frames at layup overlaps or seams. Paint failures on metal are usually due to oxidization. Think of a carbon fiber frame as a papier-mâché project. The paint may crack where one layer is slightly stiffer. A metal frame is more like a clay project. It's one crystalline solid.

It looks like the scratch started toward the bottom of the fork and then moved up and over toward the shoulder.

A move toward unpainted carbon could be due to fears that damaged finishes mean damaged carbon. Or it could be aesthetics, styles, etc.

I'd look at the manufacturer and where it was made, with high points to any company doing the layup in the U.S. or Europe.


I cracked a carbon fork in a crash. It was very obviously cracked, with fibers protruding from the break. I took it off the bike, put a different wheel on it and used to transport lumber over a bumpy, rocky forest path to a treehouse site. Despite the wicked break, it held up just fine, carrying hundreds of pounds of lumber.

Carbon doesn't break like crystal or glass. It's a bunch of tough fibers hopefully laid up in a well engineered fashion. It breaks, but not in the way that a lot of people imagine.

mpetry912 

Right. I was going to get into a discussion of concentration of stress loads, eleastic deformation vs. catastrophic failure, but why bother. Take a minisub ride and be happy

/markp

ljsense

One idea about how the scratch happened: bike computer mounted with a couple zip ties. Guy uses a box cutter to remove, goes a little hard. Blade bites into the paint in a straight line up the fork, then slides off toward the shoulder, leaving fainter damage to the paint in that area.

If stresses caused the carbon to break underneath the paint, the damage wouldn't look like it was coming from the outside.

Harold74

Bother because that's the designated subject of this conversation. Seriously, I'd love to hear what you have to say about stress concentrations etc. Please share. Fracture mechanics for the people!

Harold74

Thank you very much for your detailed, methodical response. In particular, thanks for the confidence level that you expressed above. It takes some moxy to put a number to a contrarian opinion online.

Say hi to Madison for me. I worked there my first decade out of college and miss it a great deal.

I was more wondering if, somewhere along the line, the industry simply figured out how to do a better job of painting carbon such that potential stresses or discontinuity in the substrate would be less likely to telegraph through to the paint. Maybe a less brittle paint? Or a top coat on the carbon that would kind of smooth over the joints in the layup? That said, if internal stresses simply do not telegraph up to the pain in this manner, perhaps this is a non-issue. My wife rides a Norco Valence that neither of us realized has a carbon fork until I did some work on it. The paint matching is that good. On my Lemond, if there's any difference between the paint used on the frame and the paint used on the fork, I'm unable to discern it. It's all kind of a bedazzled, sparkly laquer-ish look / feel.

Thanks for that as well. I'd wondered about similar things. I'm pretty sure that the bike had been on a trainer for a while and not receiving much in the way of love. It was also wired up pretty good with various sensors. It came with a sensor on the other fork arm that had it's own, built in zip tie-ish strap. I struggled to remove it, fiddled with it a bunch, and ended up using box cutters as you said. And I ended up scratching the fork blade paint a bit on the process just by way of dragging some of the strap hard spots around accidentally. I really could see that being the cause of the damage that we've been discussing here. That, particularly, if it was done by someone who no longer prized the bike.

mpetry912 

I have better things to do than write a failure analysis document and RCA based on a couple of cell phone pics. Having said that I would say, compared to steel and titanium, composite materials "tend to" fail catastrophically, especially if subjected to concentrations of stresses, and / or when the epoxy is penetrated and causes a compromise or damage to the fiber proximate to said concentration of stresses.

On a component where the loads are distributed broadly, like an airplane rudder, this is less of a concern than it is on a component like a bicycle fork where static and cyclic loads are concentrated near the fork crown and steerer tube. Good practice for a critical structure (like a bicycle fork) where failure would result in a crash and possibly injury to the rider would be to replace it at the first sign of damage or compromise.

In aviation, we have non destructive testing tools such as eddy current analyzers to determine the extent of a noted crack or flaw. On Bike Forums, we have a couple cell phone pics and pages of speculation.

Having said that, composite rudders fail too. See American 587. Strictly speaking that is the vertical stabilizer. The rudder came off.

but what do I know ?

/markp

Harold74

That's fine, of course. But, if that's the case, then why do you keep interjecting yourself into a conversation topic that you clearly have no interest in? Nobody is forcing you to be here. Or did you simply not read the originating post and mistook this for a routine "should I replace my fork" question where you could just drop a wildly insightful "replace the fork" and drop the mic?

A great deal, apparently. I appreciated your last post. Unfortunately, it's been difficult to figure out just what you do know because it's been like pulling teeth to get you to actually share anything meaningful.

If you're an expert who's willing to actually share their expertise, then welcome. If you're an expert who has "better things to do" then, please, go occupy yourself with the doing of those things and leave this conversation in hands of the folks who are actually interested in discussing it.

mpetry912 

Right. And here I thought the purpose of the post was to "gather opinions on failure modes of carbon fiber" or whatever you said up above. Not to "determine what I know".

One of the things that smart people know is the boundaries of their own knowledge or expertise. In the case of this thread it's impossible to give a definitive judgement on the state of the fork based on the pics and description posted at the top of this thread. So I rendered my opinion.

and I won't waste any more time doing that, at least on this thread !

/markp

Harold74

Recapping our interaction here:

1) In the OP, I made it clear that I didn't require "should I replace the fork" advice. You proceeded to tell me to replace my fork.

2) In the OP, I made it clear that what I was looking for was a detailed discussion of causation and diagnosis. Despite your clearly possessing relative expertise, you gave me none of that until I dragged it from you kicking and screaming, with a bunch of "what do I know?" attitude thrown in along the way.

3) At one point, you actually discouraged people from having the very discussion that I requested in the OP. I had to clean that up.

And you think that it has been your time that you have been wasting??

From where I'm sitting, you owe me some billable hours for forcing me to compensate for your not having read the OP properly before chiming in. You write well. I assume that you also read well when you put your mind to it.
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tFUnK

All speculative:
1) based on this one photo I am 90% confident it's a crack in the carbon, but it may not be a deep crack with functional consequences (0% confidence in speculating how many layers/mm deep).
2) the paint layer(s) is thin and I do not expect the paint crack to expose that much "blackness" underneath unless a black base coat was used.
3) no, due to carbon being more flexible than paint so the brittle paint can chip without carbon being damaged underneath.
4) I think the crack was the result of poor manufacturing such that repeated cyclic stresses from riding caused a weak area of the fork to fail.
5) my guess is horizontal tension.
6) yes I think modern painting involves many layers/stages so that one can better discern the depth of a surface crack.
7) I expect modern forks to be stronger but that is assuming that the testing standards have evolved to be more stringent (it may will have evolved in the opposite direction, or not at all).
8) jumping off of curbs or hitting a proper pothole at speed can cause that blade to buckle but the remaining blade should theoretically still hold it up (hopefully).

Harold74

So I am considering sanding the fork to check out the potential crack. Some questions on that:

1) Is there a better or worse way to do the sanding? Steel wool? Super fine grit? OralB waterjet?

2) Is there any reason to fear that the sanding itself would compromise the integrity of the fork if it turns out that there is no crack?

Lest anyone get too excited about the risk potential here, I ordered a replacement fork from Germany several days ago. My continuing use of the fork will be limited to:

A) Sanding and posting the result here for interest sake.

B) If the sanding does not indicate carbon cracking, I'd like to ride the fork until I get around to switching the new on in.

ljsense

Skip steel wool; it's not very good versus paint. My advice would be to start with a fairly coarse paper, like 150-200, and lightly work your way through the clearcoat and top coat of paint. You can go straight down to the carbon, too. Up to you how much time you'd like to save.

When you go to finer paper, like 300-400, you've got to keep sanding until the carbon in the area is smooth. If you keep seeing a crack, well you've got your answer. But if you smooth it out, it was likely a scratch.

To be sure, move up to finer grits and polish to make the carbon really smooth and glassy, which will show the absence of a crack.

But again, don't make the mistake of not sanding to the bottom of a scratch before going for the glass finish.

Looking forward to seeing the results!

Harold74

Me too. I've got a replacement fork coming from Germany and I'm going to hold off on the sanding until I see the whites of its eyes. You know, get it through Canadian customs, make sure it's the right size etc.

Would you -- or anyone -- happen to know how long a thread can remain inactive here before it gets closed? On another forum that I frequent, that space of time is three months.

ljsense

Has that fork come over from Germany yet? looking forward to the big reveal.

ljsense

In case you don't get notified unless you've been quoted, here's a reminder to get sanding when you can.