Under-Car Diffuser Questions

General Composites Discussions Composites Talk
#1

I am curious if anyone has any experience making under-car diffusers with composites. I am somewhat new to composites and this is a project I would like to do down the road, but would like to start doing my homework now. I do some racing events as well as lots of car shows and feel a diffuser would help me out on both fronts, as well as improve gas mileage while driving. Looking for input on the project.

  • Do any special considerations need to be taken with the resin used? It won’t be exposed to much, if any, direct sunlight, however, road debris will probably take a toll on the underside of the car.

  • I am planning on using 3 layers of Kevlar. Do I want more? Less?

  • Should I sandwich the Kevlar layers around a core or just leave it straight Kevlar?

  • Will I want to use vacuum infusion with pieces this big or what method would make the best pieces?

Again, this is just a “dream project” of mine, but would like to start working on the logistics of it all. Thanks in advance.

#2

IMO, it seems you’re getting into 2 new fields in a big way.
Aerodynamics is quite a science, and it’s tough to design parts that work.
Composites manufacturing can be tricky. Learning is by trial and error, and throwing away lots of expensive materials.
I’d suggest doing some small wet layup fiberglass projects to get familiar. Then try bagging and infusion.
To help with your questions:

  • A diffuser by itself won’t make big (positive) changes to a vehicles aerodynamics. Even then, only a very good one will improve MPG and stability. We do a lot of CFD and full scale testing even for small gains.

  • Pretty much any resin will work. If you’re infusing, it will need to be something with low viscosity.

  • A kevlar only piece is a bad idea. Go with either carbon or glass, maybe with kevlar as the second top and bottom plies, or hybrid carbon/kevlar on the surfaces. You’ll most likely need 6 or more layers. I’d recommend reinforcing large flat area with core material 1/4"-1/2" thick.

  • Vacuum infusion makes good parts, but it’s not necessary for your application. Bagging will give you good results and is much easier.

#3

i have read about flat helping MPG and dimples like a golf ball,as far as the track thats alot more info to read up on,add under car pics

#4

[QUOTE=TET;20541]

  • A kevlar only piece is a bad idea. Go with either carbon or glass. QUOTE]

Out of interest, why do you say that? Lack of rigidity?

#5

Yup. Kevlar by itself is not ridged. it might not handle the air pressure changes at race speeds. Carbon/Glass are MANY times more stiff. Kevlar is mostly only good for abrasion/shatter resistance.

#6

Thanks everyone. This is exactly what I am looking for. I have done some small composite pieces and am looking at building bigger, more complex pieces. Even with marginal MPG gains, it would be worth it for me because of the “show” aspect of it. Of course, the more functional, the better, but having an under car tray will really help score points with the judges.

If I go with 4-6 layers of glass between Kevlar layers, will that add a lot of weight compared to using a core material? I haven’t dealt with using cores before in any of my projects.

#7

i think the new BMW have some under tray work od them ,may look there

#8

Ah, but if you were exceptionally clever and had a multimillion dollar wind tunnel then you might just turn the flexibility to your advantage. :wink:

#9

Yeah, the glass will add weight. Core material is anything from balsawood, foam (of ALL sorts of densities. look at it wrong and it deforms, or one you need a hammer to deform!), and honeycomb. I have 1/4" nomex honeycomb core between 4 layers of carbon, and it’s bugger strong!

the diffuser you want to do is just that front lip wing thingy on the front right? or the entire underbody covering?

#10

Can I ask where you got the nomex honeycomb from? I am leaning more towards a “cored” piece and am thinking anywhere from 3/8 to 5/8 of an inch thick core material.

I want to start with a front splitter, but ultimately, have the entire underbody covered.

#11

Sorry, I can’t remember. It was ages ago. Check out northern fiberglass supply…they might have an idea on where ot get that stuff.