Carbon Fiber Chassis repair

[JosephS]

A while ago I bought a cut down and cracked graphite chassis off ebay.

I though this was a good change to learn how to do proper carbon fiber repair since I have should have everything needed to do it.
The laminated layers split. I cut a groove in the top layer about 1/4" from the split and used a utility knife to remove the layer.

I then worked backwards peeling off the laminate from the break to the end of the tail.

Looking inside I can see that there were tracker present on the inside layers. That leads me to think that this was a wet layup.

I used a fabric scrap for the inside layer, but looked for a match to the fabric used for the chassis. I did my best to cut everything so the gap between the original and the repair was minimal.

I then used some epoxy on the two later of fabric. I then got some 2 part epoxy mixed to to fill the bulk of the space that needs to be filled. I just stuck some fabric on top to give it a better chance to blend in.

It's not fully cured since it's been a bit too cold for the process, but so far it looks promising.

This should be a pretty strong repair since the damaged area were cut away and new layers were bonded in.
Once it's all cured up I'll remove the extra epoxy and see how well it cuts down.

[JosephS]

I switched up methods. The 2 part epoxy I was using wasn’t working so I switched to some jbweld 5 minute epoxy. One side was carbon fibers and graphite powder mixed with the epoxy and the other was compressed carbon fiber fabric.

The side made of epoxy, carbon fibers and powdered graphite is fully stiff. The side with carbon fiber fabric and the same epoxy is a bit flexible, not solid stiff. I’ll give it a few more days to cure and see then.

[JosephS]

I got some time to rough in the shape and trim this up.

I like how the carbon fiber fabric repair blends into the top of the chassis.

The repair is fully cured and quite strong. I built up the edge repair areas quite thick. However I got the middle just right 3.0mm with the new layers. I sanded down the top to leave the rebuilt areas at about 3.2 mm.

[JosephS]

Started working on the front end repair.
One of the holes was blown out and looking closely the other side hole was in the process of tearing out.

I had been working on my other carbon and mixed some extra resin with captured dust from sanding the carbon fiber.

I mixed the resin, carbon dust and fibers I to a gel and applied them to the front. I am using thin epoxy meant for laminating, so it wicks into cracks in between the layers of the chassis.

This can also be done with graphite powder and regular epoxy, though it’s thicker.

I waited until the epoxy mix soft set. Then I trimmed up the edges with a hobby knife.

After it cures I’ll re-drill and counter sink the holes and use regular epoxy and graphite to fill any small holes.

[JosephS]

I have most of the mechanical repairs sorted out.

I'm looking at some of the reasons why carbon fiber breaks.

For the most obvious breaks, the force from a jump or a crash exceeds the material strength. Carbon fiber fails layer by layer. This could be why the chassis I started with was only broken on 2 layers. The rest of the layers will retain their strength. It could have been driven, though not hard when cracked.

It looks like the yellow cast and coat that some of the older chassis have are from material breakdown and oxidation. It indicated a material weakening of the part
https://compositeenvisions.com/document/yellowing-or-clouding-of-epoxy-how-to-protect-against-it/

2k clear coating will protect carbon and the epoxy resin
https://compositeenvisions.com/wp-content/uploads/2022/08/Prepping-Your-Parts-For-Clear-Coat_v2.pdf

The blow out of the screws though the side is a material failure.

blowout.png (44.47 KiB) Viewed 630 times

blowout.png (44.47 KiB) Viewed 630 times

I watched a youtube video so now I am an expert.
https://www.youtube.com/watch?v=XLzTB4KLCxU&list=LL&index=9

The screw is supposed to hold the suspension mount clamped to the chassis. The as long as the force applied to the joint is less than the clamping force the whole of the material will absorb the force, not just the nut and bolt. If the clamping force is exceeded the spot strength of the material comes into play. Since the Nylon of the parts is what become threaded it is basically the nut. If the nylon , or screw, stretches out over time and stress the joint would lose preload.

In the shear direction the force holding the joint in place is friction. Nylon on aluminum or resin is non-ideal since they have a low coefficient of friction. That is the exact reason that nylon on metal gears don't need external lubrication.

It seems that all suspensions mounts should be torqued down as much as possible then have a jam nut applied with some thread locker to keep the preload on the bolt, even if the nylon wears out.

I was going to use jam nuts anyway but it's nice to understand why the failures seem to be so common on carbon fiber chassis. It also help me see other places it happens. Wear patterns on an aluminum tub under suspension mounts show indicate that the mounts were insufficiently attached to the chassis.

Screen Shot 2023-04-25 at 2.59.12 PM.png (400.32 KiB) Viewed 627 times

Screen Shot 2023-04-25 at 2.59.12 PM.png (400.32 KiB) Viewed 627 times

[JosephS]

Front is drilled out and counter sunk. The edges are trued up and sanded flat.

Now to do the same for the rear.

[JosephS]

Torque specifications
Looking for torque specs for an

Aluminum fastener
2021-t4 for a #8-32tpi Bolt 10.8 lb.in ref

Nylon 6/6
#8-32tpi Bolt 2-4 lb.in deformation ref
#8-32tpi Nut 5 In. Lbs. RECOMMENDED TORQUE BEFORE DEFORMATION ref

It seems that the max torque should be around 5 in•lb
-
It looks like a #2 phillips driven by hand can deliver 67Nm (592 in lb)of force ref

[JosephS]

Are all of the graphite rc10 chassis the same except for the late model coffin?
I'm trying to figure out if this was a team car chassis, or a graphite chassis that was drilled for a stealth. From the 25 years of rc10 thread it all the chassis look the same to me except for the drilled holes.

[scr8p]

If the stealth holes are drilled and countersunk properly, there's no way to know what it started as. Especially if they countersunk the motor guard holes too.

[JosephS]

If the stealth holes are drilled and countersunk properly, there's no way to know what it started as. Especially if they countersunk the motor guard holes too.

Thanks. So the chassis blanks are the same and only the pattern that changed.