Rear wing molding - Sandwich molding?

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#1

I posted this on the FibreTalk forum, but no replies. I am hoping either Evan or someone else might be able to give me some advice. It seems like folks are busy with SEMA now.

I am trying to figure out what is the best way to reproduce the upper wing (see attached pic). Thoughts?

1 - Foam core with upper and lower CF laminates: To do this I was going to make two molds of the wing each split down the center line. With the first mold, drill a couple holes in the top for pouring the foam and clamp the lower and upper pieces together. I would probably have to put a plastic liner in the mold so that the foam did not stick to the mold. Separately, I would create a upper and lower SF skin from the second mold. I was then thinking that I would bond the two skins to the foam core and seal the seam with CF tape. This would seem to best approximate the original shape and form (thickness). The clean up of the seam might get messy and time consuming though to get it real clean.

2 - CF upper surface mold only with foam or honey comb core: This would make the wing thinner than the original, but I was thinking that the upper surface was probably the most important for funneling the air flow. Also, it would probably be easier to do this than #1 above. I would only take a mold of the upper wing surface. Then I would lay in CF, S glass, honey comb, S glass, CF using VER. To then make sure that the underside had a finished and smooth surface, I was thinking that I would have to use a sandwich mold using a second mold of the upper surface and squeezing the laminate in between. Don’t know much about sandwich molding, but that seems to be the only way to get both sides with a smooth finish. If this is used, can you use infusion or just vacuum bagging with a resin rich lay-up squeezing out the excess?

3 - CF upper surface only with no core: Given the shape and width of the wing this would not seem to give it enough strength, but I thought I would mention it.

The original wing tends to flex a lot in the middle, especially when subjected to high pressures. Remaking in CF would make the piece stronger and lighter since the original is also fairly heavy. Any thoughts would be greatly appreciated. Oh, and the lay-up would be done with either just vacuum bagging or with resin infusion which might present its own problems

Thanks

#2

Actually both sides of the wing are critical to producing down force. The bottom is slightly more important than the top. If you just had two parallel surfaces at an angle, you end up creating a lot of turbulance with the leading and trailing edges. The lower surface needs to have more surface area to speed the airflow to decrease the pressure relative to the upper surface in order to generate downforce.

[rant on]Although I’m not an aeronatical engineer, I’m a mechanical engineer, I think wings are stupid on regular production cars. Guys that stick big rear wings on a front wheel drive car are just plain dumb. All it does is increase drag, which results in slower acceleration, speed, gas mileage. The biggest is it makes the car handle worse. FWD cars, especially production cars, understeer. By adding more down force to the rear, you cause the car to understeer even more. FWD race cars have the suspension set up to produce oversteer and they run minimal rear wing setups to allow for higher top end speed. The other thing is that no one drives a production car at high enough speeds to require the use of a wing.
[rant off]

The hardest way to make the wing is to do it in one shot by using an internal air bladder. Just have the inlet for the bladder located at the attachment points.

If you want it flex less in the center, use lots of unidirectional carbon fiber. Orient the fibers in the direction you want to resist the flex. Too many people make carbon fiber products for looks and forget about the mechanical properties. Structual carbon fiber parts are almost always done completely with unidirectional fibers.

#3

Hey Scooter,

Thanks for the reply. The car has actually been heavily modified and the wing is the factory unit. While I agree that most wings are just there for looks, this particular one actually has pretty good ratings at increasing rear down force at high speed. The rear of the car tends to get light as the speed increases. As far as oversteer, this car’s suspension has been changed and the car’s weight has been rebalanced placing more weight in the rear of the car.

Again, thanks for the comments.

#4

Right on Leon (Scooter)!!!

What is your level of expertize in fiberglass or composites?

#5

Hey, no piling on! :smiley:

Since you are all so anti-wing, I guess thats why there has not really been any response to the question on the other BB. If you set aside you opinion on the wing itself, any recommendation on the best approach to make the piece?

#6

If you are going for looks, the best way is still to use an air bladder. It’s also the most difficult to get right. You’ll need to make a two part mold. Lay all the material w/resin around the air bladder. Put the overlap seam on the outer layer down on the bottom side of the wing. Make the air bladder so that the inlet is in them middle. That can get covered up by that triangle thing. By doing it this way, you have the top layer covering from top to bottom with the seam on the bottom side where it can’t be seen.

BTW, you WANT oversteer in a FWD car if you want it to corner fast.

#7

will help you when I get back. I haven’t left for SEMA yet as some parts are still being laid up now. I haven’t slept since Tuesday so my thought process is shot to hell.

#8

Thanks again Scooter/Leon 8)

If you want to discuss the suspension setup we should take it offline, but I never said that oversteer was eliminated, but that we re-balanced the car so that the nose was not sooo heavy. We place the car on a four wheel scale to measure the weight at each wheel. We are developing a set of coil overs for the car with which we can then adjust the load.

So, yesterday before I read the posts, I made a foam core that turned out pretty good. I took a two piece mold and drilled three holes in the top piece. I put a plastic liner in the mold to prevent the 2lb foam from sticking to the mold and removal easier. I then clamped the two mold pieces together and poured the liquid foam in the holes. Once the foam hardened, I removed it from the mold and trimmed the edges. I was going to make another (cleaner) top and bottom mold today. From that mold I was going to layup the top and bottom halves and seal over the foam core. However, the air bladder approach sound much cleaner withouth the open seam. The down side is that I would still need to clean, sand, and polish the two-piece mold seam either way. I have been using a clear gel coat with my other pieces and would need to do “gel coat repair” on the seam so that it matched the other surface.