LEGO carbon fibre monocoque

[JimV8]

C0A6D6AD-A729-434F-AC98-A04C1A4A5F36.jpeg

I’ve done a couple of small scale CF projects and like the rigidity and incredible lightness of the finished product and been thinking how to make a complete chassis using CF. The first thing that jumps out would be the myriad of shapes and bends making it impossible to do it from one or two moulds.

So I thought why not for instance, build two side rails extending from the front to the rear suspension points, then a floor with all the various brackets and a centre tunnel, then a front bulkhead and a rear bulkhead and bracing. There are great adhesives to bond the parts but I’m not too keen on putting all the forces into shear and wondered, why not build the parts so they have male and female joints ala, LEGO blocks putting the joined forces into tension and compression.


Your thoughts please?

[RichardSIA]

My first thought is that you will have more time and effort into the several molds than building the car.

CF strands do not 'Flow" well into complex shapes, they resist corners, pulling away from the mold.
Strand orientation and avoiding voids are very important to final strength characteristics.

I want to do a CF chassis for one of my projects, but co$t dictate that if I do a monocoque it's going to end up being plywood.

[Driven5]

CF strands do not 'Flow" well into complex shapes, they resist corners, pulling away from the mold.
Strand orientation and avoiding voids are very important to final strength characteristics.

As part of that, CF really gets its great strength specifically from the individual strands that are in tension. Every bend in a fiber is a reduction of strength. I don't see how you would make the male/female features themselves without adding complexity to the strand path. So while the joint design may be a bit stronger, it may be making the joint features substantially weaker.

[JimV8]

Thanks, great inputs. This what I’m hoping to do. The LEGO idea was a method for joining but may be over complicating things. Page 41
http://www.mate.tue.nl/mate/pdfs/10019.pdf

[KinFung]

see this lego go kart
https://www.youtube.com/watch?v=Ae7XLg3RFWY

https://www.youtube.com/watch?v=9af8c2zHGqw

[Warren Nethercote]

Going back to the first post, why are you reluctant to put glued joints in shear? I haven't chased down the numbers but a quick reference to Gougeon Brothers (WEST System epoxies) publications indicates that shear loading is their preferred orientation for glued joints. Joint configuration is typically designed to maximize shear area for glue, not minimize it, and to avoid peeling loads.

[Lasso]

I would probably do something like this -> https://www.slideshare.net/ThomasAyres/ ... eport-2010

[Omaha Vette Graveyard]

I'd probably first want to see or personally do testing beyond what that Formula SAE paper described. The strength of the material was well established, but strength of the construction technique was not. Perhaps there's other established info on the strength of the folded joints?

[KinFung]

My first thought is that you will have more time and effort into the several molds than building the car.

CF strands do not 'Flow" well into complex shapes, they resist corners, pulling away from the mold.
Strand orientation and avoiding voids are very important to final strength characteristics.

I want to do a CF chassis for one of my projects, but co$t dictate that if I do a monocoque it's going to end up being plywood.

CF strands do not 'Flow" well into complex shapes>>>>how about forged carbon?

[RichardSIA]

For a Locost at home project?
Some folk can weld Titanium too, but that does not make it practical for most of us.

[LoadOS]

A guy on YouTube (super fast Matt I think) did a series of videos about mold-less fiberglass layups for a land speed racing car body.

He did resin infusion but I think a standard vacuum bag wet layup could be done using similar methods. The thing that impressed me was how he used his peel ply to get the parts to release.

If you make a male buck for the passenger compartment you could do a layup in that style, then add a foam core and another layup to create a monocoque.

Doing the layup inside out will give you great access to areas like foot wells so you don't struggle as much with inside corners and you could reinforce weak looking spots before you add the foam core and exterior glass.

Not much of a Locost monocoque would be visible on the exterior but if you were smart you could oversize your foam core, hot-wire cut it to size and sand it to create sides that are close enough to flat that you won't need too much filler to make it look nice when painted.

Any way, composites are all about prep work and strategy. If you do it anywhere close to right you will get a chassis that is massively stiff and lighter than a steel chassis after you skin everything. Carbon fiber probably isn't worth the cost compared to E-glass so save some money there and spend it on good spray foam and resin.

You will need bulkheads or inserts to mount your suspension subframes and roll bar/cage to. That's the most critical thing probably.

I would not trust a monocoque made with fiberglass/CF shapes glued together. It might be fine but composites derive their strength from the fiber reinforcement, not the resin. Folded honeycomb seems neat but foam is a lot cheaper.

Also a foam core is nice but it does not need to be 100% foam core. I actually prefer it when some areas have no core and the interior/exterior layers are bonded to each other.

I think the trickiest part of this is designing the male buck so that it can be disassembled and removed after the interior layers have hardened.