el es tu

Im in the middle of making a diffuser - have got most of the core pieces made in 24oz fiberglass (surprisingly turning out pretty lightweight), but will have to make a decision on the outer layers

right now Ive got a few yards of plain weave carbon fiber and kevlar




because this is not intended to be for a dedicated track car getting used solely on smooth surfaces (it will see use on backroads that will be crowned along with rocks and debris that could potentially tear up an all or mostly carbon part), Im tempted to limit the use of carbon and keep the majority kevlar for better reliability

At this point I can totally cover the fiberglass cores in carbon or mix it up:

Possibly only using carbon on the outer parts of the outside fins, then kevlar for the inside facing sections of those fins, kevlar for both sides of the fins in the middle. Then kevlar for the main body thats facing the ground.

either configuration is going to get thin aluminum heatshielding bonded to the top since the exhaust will be very close

for those that are wondering - the glass cores are very strong to begin with - the kevlar provides abrasion and impact resistance, with very little in terms of stiffness. Conversely the Carbon would stiffen it up and make it "stronger" in a way but wouldnt offer much in terms of impact or abrasion resistance.


input is appreciated!

mgarfias

Why not one of the hybrid carbon/kevlar weaves?

el es tu

That would have been ideal, but it wasnt available at my local shop unless I bought an entire roll on special order. Online prices are about twice what they should be (50 to 80 bucks a yard) so I stuck with what was available.

spazegun2213

could you post more about how you are making this? if I had the time, I'd love to try making my own parts like this.

el es tu

started with foam board, tape, and coat hanger wire to make this mold:



then coated it with primer and then a clearcoat.

originally those side sections that look like wings were going to be used (they extend to the sides of the bumper to close off the open areas) then I decided to go a bit more traditional in my diffuser design (and get an even larger venturi area).

The way its being built is in sections - the fins are separate of the main panel that way if somethign breaks the whole thing doesnt need replacement.

The fins are molded out of the same mold and cut to size. They have also been curved outward a few degrees than in the picture above to allow for more expansion.

right now theyre made of 2 layers of 24oz heavy weave fiberglass and polyester resin. Ive found that after getting a thorough wet-out of the material, I can force more resin out with a scraper like you use for bondo and get a light but strong part. The glass is about 130 bucks for roughly 70 yards by 50inches vs 35 to 50 bucks a yard for carbon (unless you buy it online or from a boat shop where they upcharge like crazy). The resin is 35 bucks a gallon vs 100 for epoxy.

Right now I have 2 fins cut out and 2 more that just finished curing. Main panel was done but it rained on it when I had applied a third layer, and the cure was messed up so Im going to have to trash it and build another.

specs on the main panel are 40 by 24 inches with a 7 degree angle (optimal for a rear diffuser) with very light camber (not sure exactly but only a couple degrees to prevent separation issues with the air).

Im going to rebuild the main panel to probably 50 inches and extend it further. This will have the exhaust blow down it and potentially help the efficiency further. I have tested this part in a simulation and it created significant downforce at highway speeds and enough downforce at 150mph to bottom the suspension out (however I dont anticipate anywhere near the downforce Ive found in the computer program, but its nice to know Im on the right track...).

The outside fins are the same as the inside - but basically longer on the mount points - Just filled the entire mold with glass and will cut so they have enough length to extend out 4 to 5 inches from the main panel and main fins. These will use air from the wheel wells but not the dirty air coming right off the tires. The main section will just get clean air from the center of the chassis.

final specs should be 34 inches by 50 inches, 7 degree angle for the part, and maybe 2 to 4 degrees of camber.

383

Looks good keep us posted. I too am trying to make carbon parts and its a pain! Try the "talk composites forum" I post there for help too.
The Kevlar helps with abrasion but rock chips would be in the layer of resin right? If you ran over huge rock and it scraped the bottom it would help but I think carbon will hold up to rock chips just as well. Maybe I'm wrong but that's how I thought Kevlar worked.
Good luck
Chris

z06801

looks great so far, thats how I built my carbon surround ducting for my radiator was planning the same for a diffuser

moespeeds

Be careful layering carbon and kevlar sheets where you are going to have extreme temperature variations. I've had issues with delamination, I assumed it was from the 2 different materials expanding/contracting at different rates. I'm not 100% sure that was the reason, but I vacuum bagged my parts and never had any issues but the one instance where I layered them. Do a little research before you spend the time. Also keep in mind that the kevlar is going to do little to help with rocks if it's layered under the carbon. It will need to be on the outside to do you any real good. Rather than Kelvar, I'd do a heavy epoxy coating or better yet a gelcoat.

VetteDrmr

Your technique of using a plastic squeegee to pull the excess epoxy/polyester resin to the sides is good, I did that a bunch while building my Long EZ back in the 80s.

One thing to consider is the ambient temperature your part will be seeing. Room temperature cure resins are only good to around 130 degrees. If you think you need to survive higher temps then you need to post-cure your part. You pull it out of the mold, then set it back in the mold to give it support. Then put it out in bright sunshine under black plastic sheet for several hours. The goal is to elevate the cure temperature and let it cool down while in its desire shape.

Or, you could use your local bazillion $$$$$$$$ autoclave to do all the above. Sadly I don't have one in my neighborhood.

HTH, and have a good one,
Mike

fatbillybob

Wrong resin! FG can be PE resin. CF and KV use Epoxy. Especially for undertrays you want durability and you get delams with weak PE resin which does not stick to aramid fibers well. Consider after the squeege putting the whole thing in a vaccum bag which will suck out more resin and the part will be way lighter and denser and more durable.

383

I got my resin from illstreet composites it was relatively cheap 1.1/2 gal for like $60 bucks

el es tu

Update: I took some time yesterday to get the main panel done - 3 layers of 24oz glass vs the 2 layers on the fins.

Im hoping to have some time to get the cores all prepped for the final layers of material. Still havent been able to decide all carbon or carbon and kevlar - not sure if the yellow is going to look hideous on a street car (obviously the outer layers of carbon are more vanity than structural, but the functionality of having that kevlar there is very tempting...)

I think the only reason one would use the kevlar internally is so that if the part broke, it wouldnt shatter and you could repair it instead of losing it.

If the kevlar gets used it will be facing the road. If I chose to do all carbon I may still coat the edges of the fins with kevlar just to give them little more longevity.

I tried this last night with the main panel - during the cure time (after it hardened and had partially dried) using a heat gun and then covering the part with aluminum, shooting more hot air in and sealing it off.

I cant wait to see how it turns out!

I think Im still going to bond aluminum shielding to the main panel just to be sure its doesnt distort.

An autoclave would be nice - Ive heard a large oven will help do the trick on prepeg carbon... maybe one day

Ive actually read that before, but have had success using glass carbon and kevlar with polyester resin before (apparently some have done this with boats and it also worked out) the kevlar is a big pain to wet out but if youre persistent it will work. Id love to use the west 105 epoxy, but the cost is just so high and Ive had to go through so much resin just to make several prototype parts (that Ive thrown out) that Id be broke if I used the expensive stuff. For this first one cost is a big factor. If I make another, Ill have a better idea of what exactly Ill need and will be able to go that route along with a pure carbon part. I wish Id taken more notes on how much material Ive used but next go around I think I could get away with just using 2 gallons of epoxy and 5 to 6 yards of carbon (probably 2 yards of heavier weave twill for the main panel, and 3 to 4 yards of lighter material for the fins) - roughly 500 bucks.




Everyone: Thanks again for the replies!

moespeeds

I would definitely stay away from Polyester or Vinylester resins. When laying up high quality sheets epoxy is the only way to go.

Don't rule out vacuum bagging, it's not that hard to do. I made a pump from an old refrigeration compressor and the rest of the materials are not that expensive. It will greatly improve the quality, durability, and weight of your parts. I don't know if I posted this somewhere else, but www.shopmaninc.com is a great source.

el es tu

I might try to vacuum bag on the next one I build. Is this process correct?:
lay out and wet out the carbon like normal - remove as much excess epoxy as possible
then lay up the release film (does it need to be perfect? Do I need to use a roller to get it perfectly smooth on the wetted out carbon?)
lay down the absorbing mat on top of the release film
bag it (does the material matter?) and seal the thing off
vacuum out the air and let it cure

also if the release film is a mesh, is there a tendency for the epoxy to harden in between the holes of the film and hold it to the finished part?

moespeeds

The release film pulls away no problem. Wet out parts witha brush or squeegee and I also used to use a studded roller. The only issue you might have is you may have to modify the mold to accommodate the bag. You want to seal all around the edges of the mold and suck down against it, you want to be sure you don't deform it. For large parts like that you will seal the edges with a material similar to silly putty, and you will want to T off the vac lines and suck from a few different locations. The bag doesn't matter much as long as it's not too stiff. If it get's heavy wrinkles they will transfer to the part. It's a bit of a PIA to learn, but once you do you'll never go back. Good luck!

383

Is this going to be attached to the stock diffuser or will you mount it to the bumper somehow?

el es tu

I removed the valence (its just a big parachute that causes drag and looks ugly) and will either trim the rear fender liners or make some shorter ones from kevlar and glass to evacuate more air from the rear fenders. The front facing end of the diffuser will mount to the front facing end of the rear subframe with bolts and washers, then the other end of the diffuser will mount with turnbuckles to the actual frame. It should extend a few inches past the exhaust system when its done.

I also plan to make a fiberglass closeout panel to prevent as much air as possible from getting into the rear bumper.