Carbon repair buildup, DIY or otherwise

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DurianGrey
Posts: 56
Joined: Tue Jun 13, 2017 9:57 pm

by DurianGrey

I've been taking a look at some professional carbon repair jobs, and they come out looking perfect. But looking at how these are done via DIY videos I don't understand how are they able to keep these repairs flush.

For a small crack, for example, the process is always sanding through the paint but not the underlying carbon, then adding carbon patch layers with resin and curing, aligning with direction if possible. Exaggerated drawing below.

Doesn't adding those patches add to the thickness of the tube? How do professionals keep things looking flush?
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tmr5555
Posts: 356
Joined: Tue Jan 28, 2014 12:13 am

by tmr5555

From personal experience, carbon repair is never flush, depending on the severity and style of repair the tube ends up having somewhat of a bulge, sometimes very obvious, sometimes you'd need to know what you're looking for.
Also from personal experience the cracked carbon section is cut out to stop the crack from advancing post repair. If I understand correctly the shop I use first plugs the hole with some kind of epoxy then proceeds to wrap the tube at 0-45-90 degrees et cetera...
They then cure the repair at 50 degrees celcicus by turning the heat on and off for 18 hours and finish off by sanding the outermost level down a little. Disclaimer, I'm not a carbon repair professional and everything written above is anecdotal.

by Weenie


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DurianGrey
Posts: 56
Joined: Tue Jun 13, 2017 9:57 pm

by DurianGrey

Makes sense. I guess another question I have is if you can sand through part of the carbon.

Here's an example of what I mean, while they don't show the actual patching process it's in an area where I would think any bump would be pretty obvious:

https://ruckuscomp.com/news/featured-re ... be-repair/

usr
Posts: 889
Joined: Thu Mar 25, 2021 5:58 pm

by usr

No expert either, just hearsay, but the way I understand it the ideal repair would be sanding down to zero thickness at the point where it's broken, with a *very* gentle slope between zero and full thickness for maximising the contact area between old material and new material. The contact area is the weakest link (all resin, no fibre), so you want forces distributed as wide as possible. That very flat valley would then be filled up with new carbon exactly to the former thickness. Reality can only approximate an infinitesimal ideal like that, so most real life repair jobs are a blend between the hypothetical ideal and a patch layer on top. If the end result looks flawless it's because both patch are and neighboring paint are sanded down until flush and then it's filled up with another set of paint layers.

CamW
Posts: 301
Joined: Mon Jan 17, 2011 10:26 pm
Location: New Zealand

by CamW

Basically what USR said. You need to remove all the damaged cracked/delaminated fibre and replace it with new fibre. Often called a "scarf repair" or something to that effect.

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tmr5555
Posts: 356
Joined: Tue Jan 28, 2014 12:13 am

by tmr5555

DurianGrey wrote:Makes sense. I guess another question I have is if you can sand through part of the carbon.

Here's an example of what I mean, while they don't show the actual patching process it's in an area where I would think any bump would be pretty obvious:

https://ruckuscomp.com/news/featured-re ... be-repair/
The sanded part photo here is probably “exploratory” sanding they did to asses damage. Pretty sure they would proceed to cut out the damaged section. Also see how the upper most layer is referred to as “cosmetic”? This is the layer that gets sanded down somewhat during paint prep.

Let’s assume a hole of 5 mm ( sorry , metric) the first carbon patch is going to be 7 mm with the following patches being incrementally wider, say 9,11,13,15, 17 mm amounting to 5 layers of which the uppermost is cosmetic ie, structurally more than needed, so is fine to compromise by sanding down somewhat. (Above numbers are made up) so how does the repairer know how many layers to use in any spot? I guess they could count the original amount of plies in the bike at that point.
The staggered layup must help to increase “friction” exponentially so the bond is secure.

Here a repair I had done on my bike:
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GeoffS
Posts: 183
Joined: Wed Feb 05, 2020 12:52 pm

by GeoffS

It's pretty much what everyone has said above. With the right tools, jigs, materials and expertise you can very impressive results.

Have a look at these guys. Some nice quality repairs are shown on their "Our Work" page.
https://carbonbikedoctor.com/carbon-repair/
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DurianGrey
Posts: 56
Joined: Tue Jun 13, 2017 9:57 pm

by DurianGrey

Thanks all, I think that answers my question. It looks like you can sand through the patch as necessary to flatten things out; I wasn't sure if sanding through fiber was ok. Appreciate the help!

DurianGrey
Posts: 56
Joined: Tue Jun 13, 2017 9:57 pm

by DurianGrey

And to add for anyone looking later, this is a video demonstrating an opposite but probably easier approach, just putting a patch over the tube and having the repair bulge.

https://www.youtube.com/watch?v=DHWsTg9mkTg

Marin
Posts: 4035
Joined: Wed Jan 22, 2014 11:48 am
Location: Vienna Austria

by Marin

The point is that you have to sand away the crack- that leaves you with low/zero thickness in the center of the repair. Then fill back up.

by Weenie


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