+x

 

Lester Populaire - 6/12/2020 3:20:18 PM

Hello tomBIKE

I'm an avid biker myself and recently built a frame as well. In fact I'm sitting in the train home after the first really long ride with it!
In my opinion it is unfortunately a little more complicated than you probably envision. there is a number of issues:

Carbon fibers composites are a lot stiffer than aluminium, which means that when you deform a hybrid part with aluminum and carbon the carbon will take the majority of the load. This is especially problematic as carbon has a lower elongation at failure than the aluminum. This difference in stiffness creates big shear stresses at the interface.

Carbon and aluminum are quite hard to bond. A halfway decent bond can be achieved quite easily with proper roughing and scoring of the aluminum, but to get a really good bond you need to acid etch the metal which is nasty.

And last but not least the area of the crack is really tight and achieving proper fiber alignment will be hard. It certainly was the area i struggled the most on my frame.

Oh and one more point actually once covered in carbon you do not know what is going on underneath. So the crack might be growing and delaminating from the composite layer without you knowing until it fails.

So to sum it up: a repair is certainly possible but i would not recommend it as a first project. I personally would drill a 1.5mm hole at either end of the crack (and make sure it really is the end of the crack...) to get rid of the stress concentration and then ride it and keep an eye on it to see if it keeps spreading. If it spreads i would repair it.

If you want to attempt that, give us a heads up and i and the others will talk you through. Now you know the problems that come with such a repair and hopefully can make a more informed decision...