Looking for Reinforcement recomendation

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#1

I have a part I need to make that is a combination undertray and front splitter for a sports car (fairly large). I made a prototype out of PER and e-glass and now want to move up to a full production part. Here is a picture of the plug:

The mould is 10 layers of chopped mat and 3 layers of 18 oz. roving.

My question is, I was planning to use an Shell DPL 862 Epoxy and use a layer of 2x2 twill carbon on the inner and outer but don’t know what to use as a core material. This part has some sharp contours. I was planning to use a wet lay up and a high elongation vac bag.

Any help would be appreciated.
Thanks,
Jeff

#2

first off…why use expensive epoxy for a part that doesnt require it?

Youre obviously making a functional while being cosmetic part…Id just use the PER resins and gelcoat.

How stiff does the part need to be? with the complex contours and arcs a relatively thin part will be more than stiff enough to divert airflow and not sag or drop off.

If I was making that part Id use some 6oz plain weave glass sandwiched. Then again Im not sure why youre even going to add the carbon to the “backside” if you do its going to be heavy if you try to add enough resin to make it pretty.

Good luck with your part…it looks like it was a complicated build and Id actually would like to see a summary of your building process. It looks to have taken a long time and must have been tedious.

#3

If your dead set on using a core I would only use it in specific areas (larger areas). Like hybrid said, looks like most of your curved areas will support themselves strength wise with out a core.

Divinymat is what I would use, it will contour to most of your larger curves. Obviously there are going to be a LOT of different opinions/options.

And good job on the plug. I like it.

#4

I’ve made similar splitters to this for Grand-Am Sports cars.They were structural and supported the loads from the nose downforce as well as the radiator.My choice would be 1/2" aluninum honeycom as a core, between 6 - 8 layers of 5.7oz twill. The honeycomb would have to be spliced to fit around some of the shapes you have. I would cure the outer skins first, then bag-cure the honeycomb, then cure the outer layers. It sounds a bit heavy, but would be very rigid. Glassfiber could be substituted for some of the layers if cost was a concern. Nomex honeycomb could also be used instead of the aluminum or a mixture of both. Which side of the part will be the finished side, I assume the top? It looks a very nice looking plug, what paint did you use?

#5

The finish on the plug is Duratec surface primer. This picture was taken before final sanding and polishing…

I want to make the part as light and stiff as possible. That was my reasoning for epoxy resin. The PER part I made as a prototype (1 layer 5.7oz carbon 7 layers 6781 S glass) is ok but it needs to hold up to air flow at 150mph+ and accelerated air speeds under the car of 215mph+ with down forces up to 85 ft. lb.

Even a recomendation for a glass reinforcement?..

This piece is both cosmetic and functional. Much R&D went into the overall shape. I just want to be able to make the part quickly and efficiently.

Thanks to all for the input
Jeff

#6

Here are some more pics of the mock up:

#7

That looks really good. Glad to hear its not going on a street car though.

#8

Actually, it’s both. But in no way would I condone racing on the street. The only racing this car does is on the track.

The other part I forgot to mension was that the under tray also replaces a factory metal body brace. I think the tensil strength of the epoxy would be more suited for this… Again, I am no expert. I am looking to you Guru’s for assistance.

So would 2-3 layers of 6oz plain weave cloth sanwiched between carbon twill do the job? My thought for using carbon on both sides was for stiffness. Am I wrong in thinking that? I would keep the resin as close to 50/50 by weight as possible. The underside would be dry and that is fine. I was going to add 2-3 extra strips of carbon in the structual areas anyway.

#9

Like I said…My guess is that it really wouldnt need much.

If youre truly worried about it, the honeycomb is about all Id use. you can get it super thin and just place it in your large flats to help eliminate any chance of a problem.

I still wouldnt bother using epoxy. I make much smaller parts and have stood on them (200 pounds) and while they deflected and went completely flat we are talking about a part less than 1.5 sq ft with my entire weight on it.

What did you use to develope the design and how are you calculating the down force? Im a motorcycle guy but Ive studied engineering…so Im curious as how you came up with approximate numbers or if youre using a general math formula versus the drag area or if you actually tried to strain gauge with a tattle tale.

Interesting stuff.

Again…maximum for me building it YMMV.

2 layers of 5.7 carbon
several layers of 6.0 plain eglass (up to you how thick and heavy you want it)
1 layer of honeycomb
several layers of 6.0
2 layers of carbon

Youll also find that 2X2 twill is not the strongest weave on the planet. Unfortunately the stronger weaves are really kind of ugly.

Id also infuse the hell out of it…resin rich= weaker parts.

#10

Great, Thanks.

I have a friend who is CFD engineer. We ran some analysis on the part. All the downforce (90%) is generated by the upsweep of the radiator duct. The air volume was determined by the opening of the bumper and using a secondary foil on top the the splitter paralel to the splitter in transition. The underside has a “mouth” that captures air and accelerates it by forcing it down slightly and spreading it left/right. The rear undertay then has an upward arc to create vacume/downforce over the rear axle… all in theory anyway…:slight_smile:

I haven’t had a chance to shake it out on the track yet.

#11

Good stuff…Ive been really busy designing and testing “ram air” intakes and air boxes and the thing Ive learned is that the math is good but I learn alot about mother nature with nothing more than basic tools and no math.