Hi all,

I'm just starting down the route of building a large oven for use of prepregs etc but for the time being I need to epoxy wet lay projects such as this and for that matter I'm not sure I would even be able to get hold of a low temp cure prepreg from the materials I need to use.

The project is small sidecar fairing much like an F2 Racing Sidecar only a kart sized one that is raced on kart tracks. Its about 1.5m long and has a fair amount of shape. Top end speeds are 60-70mph on larger kart circuits. There tends to be a some body contact and your average polyester/gelcoat fairing tends to look crap after only a few meetings due to the cracking nature of the gelcoat as the bodywork flexes and takes knocks from the other tricycle bumper cars! 

The bodywork needs to have some flex and 'give' at the extremities whilst being fairly sturdy around the middle part and mounting points. I guess the nearest other subject would be a white water racing kayak or similar. I will achieve this by use of multiple materials (Glass, Diolen, Carbon) where appropriate. I know I cant expect something that gets abused like this to look great forever or last forever but keeping it looking fresh for as long as possible would be preferable.

Sorry if that was all a bit long winded but once people have quizzed me about my intended use I would have ended up writing it all in the end anyway.

So...... I want a black surface finish, dont want to paint it and dont want carbon on the outer skin (looks great but is dangerous in a heavy accident). First thought it to use black Diolen for an all over first ply. I will never get this layup to stay down everywhere so will bag it, being a fairly big job I know that I'll end up with dry bits/air bubbles here and there no matter how careful I am partly due to not being able to get a coat of resin to stay on the released mould first. I have some GC50 epoxy compatible clear gelcoat but after doing some tests I'm sure it will de-laminate/crack after some impacts.

Next thoughts are to use fumed silica to spray some thickened resin onto the mould surface as a thin epoxy gelcoat which will most likely end up looking milky? Then I'm thinking maybe just add black pigment as well as the silica or will this go grey rather than black?

Any got any experience with this or have any thoughts or ideas how else I could achieve a decent looking but hardy  black surface on my mobile battering ram?



My mould and pattern

I love a bit of sidecar action, one of my favourite parts of being at the iomtt.
What kind of weight are you going for and is it a monolithoc or cored laminate?

If you did go up to a prepreg vac bagged cure system you could look at the kraibon rubber system that is compatible with epoxy prepregs.
I have used in and outside of laminates for its dampning and impact protection benefits in motorsport and other sports applications.

Glass fibre and kevlar are always better for impact protection, i would look into the semi flexible resin systems that ec sell it might be worth using with a black pigment?

Thanks for the reply, I've just had a look at the Kraibon system and it looks very interesting, definitely something I will investigate in due course, thanks for the heads up  

For now I will try and keep things simple as possible and stay monolithic. Being able to achieve a good surface finish without post layup dressing is the primary goal and playing with the varying needs of different areas of the fairing robustness (is that a word?) along the way. The Diolen seemed like a good cost effective alternative to Kevlar with its elongation and abrasion qualities, comes in Black too! There is also black Twaron I guess.

Carbon where the structure needs stiffness and linking up the mounting points thickening the laminate internally to  3mm or so.  Three or four plies of 200/300g diolen/glass/aramid in other areas....... still need to do a few more test pieces before deciding. I'm open to debate and trials.

Just wondered about the aspect of making an epoxy gel coat to be applied sparingly, how cloudy it gets with the silica  and if black pigment will just make it dark grey?. I want proper black or clear with black Diolen/Twaron showing through.

You have an interesting problem.

If your goal was to have bodywork with strength/stiffness to withstand “normal contact”, with no deformation (which is what is causing cracks, rather than scratches in the finish - structure deformation underneath the finish), that would be achievable with some sort of Kevlar/carbon composite, and carefully designed sandwich structure (alum. honeycomb would be good material), maybe with some internal ribs/reinforcement to transfer impact energy toward structural mounting points.
I would recommend against this. The trouble is that now the impact entergy from even a small contact gets transferred directly to your tire/contact patch, while your competitor just gets cosmetic damage and drives on. Good for saving bodywork, not good for winning races.

The other direction is what auto manufacturers do with the bumpers. Basicslly, this would be a combination of flexible panel material, flexible paint/finish, and energy absorbing structure underneath. Now the whole thing deforms “together” and you avoid cracks, and absorb some/most of the impact energy.
I do not know of a laminate that would provide this degree of flexibility, though. There are somewhat flexible epoxies, sure, but directional laminates would not give you un-directional flexibility required to absorb impacts without permanent damage (keep in mind that “elongation strength” refers to individual fibers, not completed laminate structure).

Anyway, what I would do in this situation, based on what I think is the most common incident in this sort of racing, is make “nose cone” into a separate, easily replaceable section, built as thin outer skin, perhabs with crushable foam structure underneath. The rest of the shell gets to be stiff/rigid structure coated with hard finish (so most of the energy is dissipated in the cone, and deformation to the rest of the bodywork is minimal). Treat that nose cone as disposable/consumable part (make/carry a spare).

As far as black color - you can make nearly anything into black coating by mixing in powdered graphite. Power tools are recommended for mixing...

I would consider fglass/carbon and kevlar like the white water kayaks may use and keep it thin and flexible enough to bounce better. 3mm thickness max, maybe a thin 1mm or 1.5mm soric if you were to consider infusion?
Use epoxy resins which are far more flexible than a typical polyester auto fairing.
Using a black epoxy in your mould for colour can work but epoxies mostly have no UV and the surface will lose gloss quite quick when exposed to sun.
If you use a foam core it will become much stiffer and less flexible, if the skins are just 1-2 layers then your body will get dents from incidents.

I have built lots of light boat hulls like my avatar and these are 1-2 layers of 200g pre preg carbon with 100kg 4mm foam core. They survive reasonably well but will dent when put to the test.

Sorry for the delay in replying, its a bit hectic round here.

Thanks for your thoughts and opinions guys, you make some very interesting points, some of which have crossed my mind also, some that hadn't. Agreed, keeping in mind that once in varying layups the performance of the reinforcement changes somewhat and I reckon they'll be a lot less in some places than others. One thing for sure is that I can never expect a component that 100% ticks all the boxes but the nearer I can get to that the better. To some degree it's going to have to be best guestimation for now and an element of ip-dip dog s**t. I like a bit of R & D and if I end up building a few fairings on the way to get the best recipe then so be it. It's all a good learning exercise, I'll be doing some more test pieces and going for the main lay up asap. I've left it too late to order some extra materials I was going to play with so I'll just  dive in and make 'something' as a starting point from what I've got. I have black and white diolen, glass and various carbon cloths. No pigment. I will either have a punt later today or maybe order a few more bits monday if I dont progress is that far today. I need to get on though as time is short and the clock is ticking.  

I have toyed with the idea of a removable nose and I reckon it's a great idea, it will also aid the transport situation as well so definitely on the cards but probably not for this first one, grafting repair sections in could be done if needed. I would prefer to break bodywork than bend the mounts on the chassis.

One thing I'm finding pretty annoying is that on a couple test pieces there has been alot of air entrapment within the black diolen I fancied as a top ply. This has been on a piece with a decent result on surrounding glass and carbon  areas. The glass is a loose plain weave 200g, carbon tight plain weave 120g and the diolen is a reasonably loose twill 200g. I wonder if its something to do with diolens polyester based fibre?

Sun exposure will probably be less than 20hrs per year so the gloss finish dulling is not a biggy for me. Are your boat hulls painted Fasta?

A quick test made in a battered mould thats had stuff stuck to it and all sorts.

Yes my hulls are just painted after moulding, I find my pre preg work from fglass moulds mostly has some surface/weave type porosity but I have ways and tricks of filling these to make quite nice paint finishes in the end.
Your fairing has so much curvature and shape that it may need just 4-5 layers of 200g carbon plus some extras around the edge etc. No cores at all?? Don't mix carbon and glass in a pre preg part as it may twist or change shape to some degree.

I guess I'm asking a bit too much from a wet lay really. There's a guy on another thread trying to wet lay a front splitter with no bag at all and his problems are pretty bad! I'm just trying to get rid of some slightly dry and non uniformed weave. I just can't deal with making stuff that looks like crap and I'm trying to avoid post moulding processing of any sort.

I hear what your saying regarding mixing different types of reinforcement. Its a funny old game, I'm aware of quasi-isotropic layup and the like and I am mindful of this.With shapes like this fairing it's not such a big problem. Seems care needs to be taken when expecting flexing of the finished component also. You cant expect carbon to stretch around the outside of a laminate thats being deformed that contains more elastic types of fibres. Putting the carbon in the middle of the layup seems like the best chance so the more elastic fibres can move around it. There won't be any carbon in places that I expect to see any significant deflecting though.

NB: the test piece shown above is purely an experiment to judge the surface consistency of different reinforcements applied with the exact same manufacturing process, nothing more.

I keep a layer of kevlar inside any body panel exposed to damage. It ensures that even carastrophically cracked piece would not separate into multiple sections (makes field repairs easier).

To re-iterate what I mentioned earlier - with a cone/cupped shape like that, any laminate structure based on a fabric will be extremely strong, because of geometry (any "push" into it causes most fibers to be excersized in elongation direction). If you would like to have flexibility in there, I would try non-directional fiberglass chop in a flexible epoxy matrix, or some sort of polyurethane cast. Anything based on a woven fabric will be very stiff in that section.

Make a mini-mold in 1:4 scale for that area and experiment with simple layups, you will see what I mean.

Don't mind that other thread, they have some sort of process/contamination/material issue. I have seen more complicated shapes done with hand layup with no problems