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Visible Crack in Fork

Old 08-15-21, 03:23 AM
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Visible Crack in Fork

In this thread, I am presenting symptoms. I don't how to assess these symptoms. What say you?

Today, I discovered a cracks in both sides of the carbon fiber fork. The cracks are the same on both sides. The crack is straight, affected the outside face and front of the fork. I am sorry the images are blurry.




To confirm the crack is through the carbon fiber, I removed the paint over the crack. Removing the paint revealed the crack is directly over the seam between the metal slug that forms the drop-out and the carbon fiber.
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Old 08-15-21, 04:58 AM
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Carbon fiber is an extremely strong material, til it ain't. Be glad you found that before it failed.
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Old 08-15-21, 05:12 AM
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Not much more to say, time for a new fork.
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Old 08-15-21, 05:13 AM
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Is it a crack or where two parts meet?
How has it not fallen apart if both sides are actually cracked thru?
Now you have discovered the damage what will you do?
Have you tested it?

​​​​​​
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Old 08-15-21, 05:43 AM
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Originally Posted by blamester
Is it a crack or where two parts meet?
How has it not fallen apart if both sides are actually cracked thru?
Now you have discovered the damage what will you do?
Have you tested it?

​​​​​​
Good question, you could probably tell if this was the case by tapping either side, and seeing if there was a distinct difference in sound, and also doing the same to the other side. This appears to be the case, given how the OP as sanded the paint off the area

Those forks do look well used, with a lot of chipping to the lowers, replacement for a safety critical part would have probably be a good idea, even it they weren't cracked. now with the work that the OP as done to them, defiantly a case of replacement needed.
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Old 08-15-21, 07:01 AM
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Originally Posted by nomadmax
Carbon fiber is an extremely strong material, til it ain't. Be glad you found that before it failed.
but the CF itself hasnít failed - the crack in the paint is where the CF and the metal meet. Itís possible that the metal fitting has become lose in the CF sleeve and that this slight movement has cracked the paint (although the exact same thing happening to both fork legs suggests that this is unlikely). Itís also possible that this is simply the two distinct materials behaving differently - exhibiting different degrees of thermal expansion, for example, that caused the paint at the interface to crack. Regardless, though, if thereís even the possibility that the CF/fork end joint is compromised, the fork is toast. A failure here is a potentially life-changing event.
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Old 08-15-21, 07:12 AM
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Originally Posted by Litespud
but the CF itself hasn’t failed - the crack in the paint is where the CF and the metal meet. It’s possible that the metal fitting has become lose in the CF sleeve and that this slight movement has cracked the paint (although the exact same thing happening to both fork legs suggests that this is unlikely). It’s also possible that this is simply the two distinct materials behaving differently - exhibiting different degrees of thermal expansion, for example, that caused the paint at the interface to crack. Regardless, though, if there’s even the possibility that the CF/fork end joint is compromised, the fork is toast. A failure here is a potentially life-changing event.
You can split frog hairs all you want.

It's a carbon fiber fork.
It's has failed and could do catastrophically if the OP continues to ride it. (this we agree on)

It isn't falling from a plane without a parachute that kills you; it's the deceleration impact once you hit the ground. There is no functional difference except the terror on the way down.
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Old 08-15-21, 09:17 AM
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It's on both sides...the same? It's not structural, I've seen this hundreds of times. I used to have an @Trekbikes.com email address so I have a little experience with it. It's the paint getting brittle and cracking. This is why Trek (and others) only warranty their paint for a year while the frame and fork are lifetime. It happens where dropouts join frames and forks as well as where the OCLV tubes join the lugs. 99% chance this fork is fine and everyone here that has posted otherwise doesn't know what they're talking about. Have an experienced mechanic at a Trek dealer take a look at it.
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Old 08-15-21, 10:00 AM
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Originally Posted by cxwrench
It's on both sides...the same? It's not structural, I've seen this hundreds of times. I used to have an @Trekbikes.com email address so I have a little experience with it. It's the paint getting brittle and cracking. This is why Trek (and others) only warranty their paint for a year while the frame and fork are lifetime. It happens where dropouts join frames and forks as well as where the OCLV tubes join the lugs. 99% chance this fork is fine and everyone here that has posted otherwise doesn't know what they're talking about. Have an experienced mechanic at a Trek dealer take a look at it.
This is the same effect as when a concrete road is paved over with macadam (tarmac); the underlying seams will soon appear in the macadam layer due to the expansion/contraction of the concrete at its joints.
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Old 08-15-21, 11:44 AM
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Originally Posted by blamester
Is it a crack or where two parts meet?​​​​​​
That was my first thought. That the "crack" is straight, and perpendicular to the axis of the fork blade makes me think it might be a joint where a metal fork end is bonded to the carbon fiber blade. We saw similar "cracks" in the early bonded carbon fiber frames at Trek, where the carbon fiber tubes were bonded to the aluminum frame fittings. Flex across this joint eventually caused the paint to crack, but the joint itself was uncompromised. Check with the manufacturer to learn what they may think.
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Old 08-15-21, 02:25 PM
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Classic carbon fear porn as always, by the usual suspects. Dissimilar materials with correspondingly different properties of expansion as well as different flex characteristics causing a crack in the paint. Paint is getting more brittle with age. Too bad you sanded the paint off, fork is fine, have seen this numerous of times.

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Old 08-15-21, 04:48 PM
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That fork is fine. Well, it was before you sanded the hell out of it anyway.
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Old 08-15-21, 05:12 PM
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This from the OP "To confirm the crack is through the carbon fiber, I removed the paint over the crack. Removing the paint revealed the crack is directly over the seam between the metal slug that forms the drop-out and the carbon fiber." So more than paint. Maybe he is using the term "crack" to describe a seam/joint but that's not what he said.
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Old 08-15-21, 06:29 PM
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Originally Posted by JohnDThompson
That was my first thought. That the "crack" is straight, and perpendicular to the axis of the fork blade makes me think it might be a joint where a metal fork end is bonded to the carbon fiber blade. We saw similar "cracks" in the early bonded carbon fiber frames at Trek, where the carbon fiber tubes were bonded to the aluminum frame fittings. Flex across this joint eventually caused the paint to crack, but the joint itself was uncompromised. Check with the manufacturer to learn what they may think.
I had one such trek that formed a crack in the paint between the bottom bracket shell and the downtube. It was like that for about ten years before I sold it.

So I think the fork is probably fine. But ďprobably not going to kill or maim meĒ would not cut it for me in this case.
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Old 08-15-21, 08:11 PM
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it looks by all accounts, as many have said, to be a crack in the paint caused by thermal expansion, slight movements over time, etc, in the joint between dissimilar materials. what i see in the underlying fork is not a crack, but a seam.

nonetheless, now it seems to be new fork time.
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Old 08-15-21, 09:10 PM
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I also think that crack is purely cosmetic. Aluminum and CF have very different reaction to temperature change. The paint has to try to bridge a moving target.

That said, I nearly lost my life to a fork failure and did lose my profession, a great deal of money, years of my life and a lot more. This forks costs what? $200?
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Old 08-16-21, 05:03 AM
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That is not a crack in the fork. It is just what normally happens when two materials that expand and contract at different rates are joined together. The paint cracks at the joining line.
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Old 08-16-21, 07:23 AM
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I would have said it looks fine to ride, but then I saw the picture of after OPgot scared and sanded it. I wouldn't ride it now. Further, even if it was covered by warranty before the sanding, it almost definitely isn't now.
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Old 08-16-21, 08:19 AM
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Agree that it could be perfectly fine. I say could be so I recommend taking it to 2 or more shops for their opinion. A bicycle CF specialist repair place would be best but you could also try other places that work with CF such as boating repair or golf club repair shops etc. I had an experience of my own with a Reynolds Ouzo fork. Took my aluminum bike to a shop to get my headtube faced for the bearings and the owner/mechanic who it turns out was not a fan of CF told me my fork was cracked near the crown. The shop was a little dim so I decided to take a look when I got home instead of buying a Titanium fork he wanted to sell me. Turns out it was just the seam where the CF met the fork crown. 10k miles and 20 years later it's still fine but my aluminum frame is not since developing cracks in the headtube. Lots of fear about CF that is often, but not always of course, unfounded.
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Old 08-16-21, 08:34 AM
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I did formal risk assessments when I was working and learned we all have varying degrees of risk tolerance. Risk is simply likelihood times consequence. I think we all can agree that a failure of a fork can be a very serious consequence. While I suspect this was a very low risk cosmetic condition the OP has taken action that has made it more difficult to evaluate the loss of structural integrity (load carrying and transferring capacity) by the manufacturer or other "expert" so at my risk tolerance I would stick a fork in this fork it is done.
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Old 08-18-21, 08:54 PM
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Originally Posted by blamester
Is it a crack or where two parts meet?
How has it not fallen apart if both sides are actually cracked thru?
Now you have discovered the damage what will you do?
Have you tested it?

​​​​​​
Thank you for taking time to respond.

It is the seam where the two parts meet. I am not certain there is a crack.

When I discovered the crack in the paint is over the seam, I got the idea the fork might be ok. Then again, there must have been movement of the metal with respect to the carbon fiber for the paint to crack. I didn't mention in the OP, I found the same crack in the same spot on both sides of the fork. This had its share of crashes, twice running into curbs, most recently about a year ago. I am sure the cracks in the paint are much more recent, because I look for cracks regularly. I haven't had any crashes since the two I mentioned.

I don't know how to test the bond between the metal and carbon fiber.
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Old 08-18-21, 09:00 PM
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Originally Posted by jimc101
Good question, you could probably tell if this was the case by tapping either side, and seeing if there was a distinct difference in sound, and also doing the same to the other side. This appears to be the case, given how the OP as sanded the paint off the area

Those forks do look well used, with a lot of chipping to the lowers, replacement for a safety critical part would have probably be a good idea, even it they weren't cracked. now with the work that the OP as done to them, defiantly a case of replacement needed.
There is a difference in sound, but the difference seems consistent with one side being metal and the other carbon.

Removing paint does not weaken the part.
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Old 08-19-21, 01:43 PM
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Originally Posted by danallen
There is a difference in sound, but the difference seems consistent with one side being metal and the other carbon.

Removing paint does not weaken the part.
Fit the wheel in the fork. Try to pull it out of it
See does anything move. Bounce up and down on it. Hold the brake on and push it back and forth. Be sensible. Think about what it goes thru when you ride it. Tap test is fine but you want to know will it break when you ride it.
No need to be gentle.
You can't ride a bike if you are not confidant that to fork is going to break.
If you can't be confidant get a new one. You are the one who is going to ride it.
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Old 08-20-21, 08:56 PM
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If this were my fork, I'd replace it.

Not because it's cracked... it isn't. The line is where the aluminum dropout fitting was glued into the carbon fork blade.

Not even because there's been structural damage to the carbon fiber tubing... there probably hasn't been. (Although I have no confidence in my ability to sand carbon fiber without damaging it.)

I'd replace it because, while I often have conversations with the voices in my head while riding, I wouldn't enjoy it if one of them occasionally whispered, "Hey... what about that fork? You know, if it broke right now, you'd totally die..." just as I'm braking into a hard downhill right-hander. (Yeah, yeah, I know. Sometimes they lie, and they're often wrong. But they don't always lie, and sometimes they're right.)

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Old 08-21-21, 09:53 AM
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I would have thought that Step 1 should have been to take some photos of the crack that were IN FOCUS. Step 2 would have been to contact Trek. If Step 2 failed, maybe ask in an internet forum. Do all of this before deciding to sand away all the paint.
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