Strange pattern visible under paint on carbon frame — cosmetic or structural?

[irie]

Hi everyone,

I went to check out a brand new Cube Stereo Hybrid ONE44 AT, model year 2026 at a local bike shop.
The bike has a matte black carbon frame, but I noticed something unusual on the top tube.

Under the paint, there is a strange irregular pattern visible.
It looks like darker lines or marks beneath the surface, almost like folds, wrinkles, or some kind of irregular structure or cracks showing through the finish.

I'm surprised that such a cosmetically 'second' frame made it past quality control.

 

[TQFreak]

I agree with your analogy, you made an interesting comparison finished wood has two grades:
“Stain grade” is used when a natural or transparent color is clear-coated.
“Paint grade” is smooth wood without grain transfer, used for opaque finish.
If that frame was just clear coated carbon you would indeed see patterns, (personally, wouldn't mind that but I wouldn’t want the physical, topographical textures/folds) if its an opaque finish those irregularities are unwanted… as an example, no one wants a physical wrinkle in the middle of their car hood.

The surface of the frame has no wrinkles, the wrinkles are in the compacted fabric laminate reflect the light the way you can see those fabric fibers wrinkles as darker or lighter areas at sertan angles.

Those wrinkles are not avoidable (at least not the cheap way) and located all over the frame, not only in one perticular area. Since those wrinkles are random and are all over the frame and do not affect the performance of the frames the manufacturers left it without painting it over (raw carbon finish is more durable than paint) and now it is one of the frame finish options. If you do not like it just do not purchase and look for other finish.

There are other finishes. Painted - it is less durable finish, but you see no wrinkles. Cosmetic carbon fiber - top layer is cosmetic carbon fiber fabric, usually with cosmetic weaves pattern, keeps the durability of raw carbon finish but this is more expensive finish.

 

[sledgy]

@sledgy: This bike does not have a clear coat, it's a matte black paint, as you can see on the rest of the frame. (a bit higher on the top tube for example, closer to the display). Nowhere else on the frame it has the same wrinkles/folds pattern... If it's really common, can you show me some other examples of bike frames with this kind of pattern?

Sorry, I don't keep files of carbon fiber pics handy I just have experience working with CF for 20y and I understand the MFG tech involved in producing CF bike, moto, automotive parts. My understanding is that some folks didnt like the weave of plain or 2k 3k CF fabric and on occasion it can get misaligned in molds and look funky as the pattern is disturbed, so it is often used as the base layers of CF fabric to build the frame and the Unidirectional final layer of fabric, is basically to hide any fabric layup/alignment issues as it basically produces a plain solid matte black surface.

As for the paint/clear coat or matte,... often a satin clear coat on a naturally black unidirectional CF frame can look exactly like a matte black paint when the unidirectional CF fibers under the epoxy matrix are aligned and straight
when there is some movement as the layup is installed in the mold before it is baked and cured, unidirectional cf strands close to the surface can shift "out of alignment" so to speak, and can be visible at the surface yet that surface is perfectly shaped and smooth.
With out inspecting the frame closely I cant be 100% correct, just sharing what I understand of the tech involved. There are MANY frames and parts out there where you see patterns of Unid. CF in the parts, the issue you are concerned about is not a defect in the frame in my opinion.

NOW,...If that frame is indeed entirely matte black painted AND you are seeing some pattern in the frame..? but no surface defects that would seem to be a paint/finish issue..Vs a frame/manufacturing defect.

Maybe THAT as an issue, means you can get it for less than retail..? But patterns & lines like this are usually common artifacts of the CF manufacturing process when using unidirectioinal carbon fiber

Good luck!

 

[Pauliemon]

Top of my S-Works Levo looked like that.

My S-Works has the same thing.

 

[copier-tunnel6n]

Since I know that not all users have the same screen brightness or settings, I felt that the subtle creases or irregularities in the frame didn’t show up clearly on all screens and devices. That’s why I adjusted the lighting on the photos in the opening post so they should be more visible to everyone.

 

[copier-tunnel6n]

I’ve got a purplish Cube, can’t remember what they call the colour, but it is covered in these patters and is the carbon layer grinning through. Looks cool, if I can remember what cool means.

@Old Codger : Can you post some pictures of your frame for comparison?

 

[copier-tunnel6n]

I’m curious, in the photos it also seems to show darker spots in the paint coating in addition to the surface irregularities… is that correct when viewed with the naked eye or is it just something from camera in the photos or posting of them?

Which dark spots are you referring to exactly? Can you point them out in the photos?

 

[Downhillr]

Which dark spots are you referring to exactly? Can you point them out in the photos?

In his top photo you see longitudinal darker lines. If it’s a clear coat over carbon then it’s probably in frame surface. If it’s an opaque paint coat it should have uniform color. Thats why I mentioned not sure if it’s from camera?

 

[copier-tunnel6n]

In his top photo you see longitudinal darker lines. If it’s a clear coat over carbon then it’s probably in frame surface. If it’s an opaque paint coat it should have uniform color. Thats why I mentioned not sure if it’s from camera?

In my opinion, it's an opaque paint

 

[johnnystorm]

I have to admit that I’m actually a bit surprised that there are users here who consider this frame to be normal.

I’ve been actively involved with bicycles for 30 years (road bikes, city bikes, mountain bikes, etc.) and I have to say that I’ve never seen this kind of irregularity in a carbon frame before.

Even when you search Google Images for carbon frames, 95% of the results show the regular, typical checkered pattern of woven carbon fibers.
Occasionally, you might come across parts made of flaked carbon or forged carbon. But the photos of this frame here don’t even remotely resemble any of those.

It really looks like some kind of misaligned structure beneath the surface. It looks as if the carbon mats used in the manufacturing process were crumpled at that point. Something that looks like it should never have passed Cube’s quality control.

I'm surprised that in those 30 years you haven't been aware of unidirectional carbon fibre that has been the norm for bike fabrication for ages. There is nothing wrong with the bike in the OP's image.

Some info here on unidirectional versus woven carbon.

Understanding carbon bike frames: what does unidirectional, woven and high-modulus fibre all mean?

So you've got a carbon bike, but do you know much about it? Read this article to find out about different terms when it comes to carbon bike frame manufacturing, and how different techniques and types of carbon affect the stiffness, strength and compliance of your bike.

road.cc

 

[Eliadn]

Defective or not, it looks hideous and shouldn't have passed quality control if the look wasn't intended that way. I wonder how costly would be respraying it to your preferred finish.

 

[TQFreak]

Defective or not, it looks hideous and shouldn't have passed quality control if the look wasn't intended that way. I wonder how costly would be respraying it to your preferred finish.

If you are willing to pay more or compromise durability sure, they can offer different carbon finishes or paint it for you. It all comes to money. They do what customers pay for.

I got a Markhor KunLun V2 frame with forged carbon finish. That is one of the cosmetic finishes, it is more expensive than structural unidirectional carbon without cosmetic carbon layer. I personally do not care about the finish and I would gladly purchase a frame with raw structural unidirectional carbon just like the one was posted here if they would offer such option for a little less money, but Markhor did not offer such option.

Forget carbon finish is next level it terms of expensiveness after raw structural unidirectional carbon, but it is cheaper than weaved carbon finish because of cosmetic weaved carbon requires even more labor for layup. Forget cosmetic carbon layer is a little bit easer for layup.

But regardless of the finish any raw carbon finish will have parting line. You can see it here on my frame

But even on the forged carbon some people may come here and tell what are those flakes and call the forged carbon finish defective lol.

The raw unidirectional carbon finish has other advantage though. Besides it got durability and it is cheaper than other finishes, it is also slightly lighter then other finishes. So if you need the most strength with least weight and save money the raw unidirectional carbon finish is the way to go.

The wrinkles on the unidirectional carbon are not avoidable and they are random and not in good control (at least not for the price customers are willing to pay for a bicycle frame). If you want the factory to reject what you see on the picture OP posted then prepare to pay for rejections. But no one is willing to pay more, that's the problem, everyone wants to pay less for more performance. No one wants an expensive heavy not durable eye candy, everyone wants cheap lightweight strong stiff durable frame.

 

[johnnystorm]

Defective or not, it looks hideous and shouldn't have passed quality control if the look wasn't intended that way. I wonder how costly would be respraying it to your preferred finish.

It's what carbon looks like when you make it this way, if you don't like it, pick one with an opaque paint finish. To reject it would mean disposing of a perfectly good bike frame.

 

[copier-tunnel6n]

But regardless of the finish any raw carbon finish will have parting line. You can see it here on my frame

View attachment 181028

I agree that it's normal to have some wrinkles in a unidirectional carbon frame and sure, the parting line that you can see on your frame is normal. But that's a clean and straight parting line. That's normal.

But the visible cracks on the pictures in the opening post are very far from a straight and clean parting line. They are completely different things. That shouldn't have never passed quality control