Carbon Repair Process Questions

[jgrano]

I'm looking for some tips & tricks regarding carbon repair. I've jumped into a seriously garaged Specialized Roubaix that was left for dead. It had a large and serious DT fracture around the entire tube along with some cracking down the seam of the final carbon layer until the BB junction. It was also damaged on the drive side CS.

Basically we went in and removed the carbon that was damaged and rounded off the sharp point in attempt to kill the stress risers. Not sure if this is common practice or not but it seems appropriate from what I've read & seen online. The DT was vacuum bagged with 5 layers around the entire DT from above to below the removed material with some overlap. I assume that it was overkill but for the first frame I'm tooling around with it was better to be safe than have it explode later. This came out decently and cured well, the CS will be filled and re-wrapped either either the vacuum bag of the electrical tape method (still trying to see what kind of pressure that really puts on the fibers).

The big problem I ran into was a lack of a good structure to build off of. I know there are people who use structural foam for repairs but other than "great stuff" I haven't found any kind of expanding foam that I can profile properly.

Can anyone give me some pointers on the process or some things that have worked well in the past?

I'll get some more photos up in the morning.

Thanks

[progetto]

Sometimes it helps to not try and do it one hit ie; structural layer then cosmetic. If using epoxy, you can just use styrine foam as a backing or filler plug just to get the shape, then lay down a couple of layers of carbon to give you a good platform to work off.
Vacuum bagging is best, shrink wrap is ok on round tubing, both need a solid structure for a good finish. I alway's tend to overbuild repairs as the few extra grams are a small price to pay for a solid job.
Bill

[datas_brother]

To be honest it looks like you're going at this completely wrong. From your description and pictures, it looks like you're treating this as black metal. The repair process for both cracks is similar. You first need to remove the damaged material layer for layer leaving 8 - 10 mm of each layer visible. Leave the last layer alone so you have a basis to build on. Build the area back up exactly as the maker had it (watch the fiber direction!). Here is a short description of how I do it: Carbon fiber frame repair process - Polytube-Cycles.de

Carbon handles force in highly directional ways. When you do a repair, you have to rely on the shear force resistance of your adhesive to transfer forces from the undamaged fibers to the patch and back - working like a bridge. If you just cut the damage out as appears on to be the case on the chainstay, the force applied to the lower layers has to move through the layers above to get to the bridge. This is bad for several reasons. First, it is very inefficient. Second, you don't know what direction the original fibers were, which makes a very big difference. Third, you add leverage to the patch which will cause it to delaminate over time.

To keep the shape of the CS you can use foam from the hardware store. The kind used to mount windows with. Spray in and let harden, sand to shape and cover with a thin layer of fiberglass. This isn't the best way of doing it but will work as a cheap solution.

[jgrano]

Thanks for the info, I had a chance to work on it today at my buddies garage and finished up the CS. It looks pretty good now and should be ready for a good sanding and some strength testing.

When I was sanding down the layers on the DT I took so much material out that I thought it would be best to just do a full wrap job since this was the first time I'd tried to put a crunched frame together. I did manage to sand off most of the top two layers but I didn't have a small enough layup pattern for it. I already have another frame waiting to get put back together so I'll try the expanding foam and then slowly remove the layers in a similar pattern to how they show up when removing the broken fibers. It's nice to hear you can bridge it back together instead of having to wrap the entire tube.

I'll but sure to post some updates.

Thanks again

[crumpton]

chop it up toss that in the bin before someone gets hurt.

"garaged" carbon frames not worth repairing IMO. when you load the entire frame to a point that tubes pop like that, there is just to big of a risk of damage elsewhere that cannot be seen.

i personally limit repairs to obviously isolated impacts. i never touch a frame that has failed due to what i would call "end-to-end" loading.

unless you have the ability to perform and document NDT/sonogram on every square mm of the frame, not worth it.

[jgrano]

Thanks for the sound advice. The roubaix has been fun to mess with and I've learned a lot about the process. So next time I come across a bike that has a busted TT from a handlebar impact I can salvage it.

Is the sonogram testing similar to the thermal imaging systems used for aircraft parts?

[Mike Mcdermid]

Sonagram is ultrasonics
The thermal is also another

The units we use for ultrasound come from a company called sonatest in the uk however it's a German company that supply our Tifr units each has its advantages for thick thin etc its hard to do thermal for example on a brake liner where's a thin tube works nicely