carbon fiber undertray?

babak said:

Is the undertray flat and flat or does it have curves and channels to stream the air properly and create downforce?

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Flat. The air channels are the diffuser in the rear.

When it comes to automotive aerodynamics, you can create actual down-force (wings, race car like underbody channels, etc.) or you can create "imaginary down-force" buy eliminating any lift that might otherwise be created.

Generating real down-force also creates an increase in drag. Eliminating the lift usually doesn't generate drag (or at least creates less). Rear lip spoiler are very good at reducing lift, while not inducing drag (they actually tend to reduce drag too). Same thing for front air dams, although they do generate some drag, they reduce lift by not allowing the airflow to enter the area under the car and become compressed (generating lift).

From looking at the Elise (not by actual measurements), and applying what I know about aerodynamics, in the Elise's case, I tend to believe that "down-force" is created by the elimination of lift. The spoiler shape of the rear of the car, combined with the front, below the grill (including the "air strakes" toward the sides) are designed to reduce lift. The undertray, greatly smooths out the air flow under the car, greatly reducing drag, and at the same time eliminating some of the lift. The diffusers at the rear would tend to do just that, defuse the air back into the air stream - reducing the induced drag at the rear of the car - while possibility creating a small amount of down-force.

Tim Mullen

Here's a drawing of the undertray.


We have no plans to produce a cf version for many of the reasons mentioned above.

Ah yes, finally a useful application of those aerospace classes I took. First of all, something that seems a little hard to understand is that the faster the air moves, the lower pressure it is. So, when you keep the bottom of your car perfectly flat, it has very little drag and stays moving very fast, thus keeping pressure low and holding the car to the road better.

The rear spoiler on the bottom is also quite essential, because the interesting things about air foils is that the exit of the air is at least as important as the entrance. But if it just went to the back of the car flat and stopped, you'd get a lot of turbulance at that point. Turbulance is bad, and creates back pressure. So, in order to have a smooth exit and help keep the pressure low under the car it curves up to help keep turbulance down.

Ok, and as it's already been explained the front spoiler lets less air under the car thus keeping the pressure lower on the bottom of the car than the top.

So, to extend this even more, the rear top spoiler is curved up to create a pressure on the front, and less pressure on the bottom by forcing the air on the bottom to travel farther to reach the same exit point at the back of the spoiler. Faster air has lower pressure so, it gets forced down.

So, theoretically, if you stick your hand straight up out the top when the top is off, you should get more drag on the top, and assuming the same drag under the car, you'd actually stick to the road better

So, in short, flat spoilers don't do much even at an angle, because there is a lot of turbulance and the speed below and above remains about the same. For best results use a nice professionaly formed air foil, which is essentially what Lotus did for us. Flat bottoms are awesome!

End of Lesson. Please let me know if you think I got something wrong. I don't want to misinform people.

usofrob said:

So, to extend this even more, the rear top spoiler is curved up to create a pressure on the front, and less pressure on the bottom by forcing the air on the bottom to travel farther to reach the same exit point at the back of the spoiler. Faster air has lower pressure so, it gets forced down.

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Actually, this part is slightly wrong. And it's hard to explain without pictures...

Air flowing off the rear of a car continues in the same path (relatively) for a distance. This has a tendency to create a low pressure area (vacuum) behind the car, which increases the drag dramatically. The air flow also tends to create lift at the rear of the car (the fast moving air over the rear). The air will flow smoothly past the rear of the car for some distance, and then suddenly become violently turbulent - this is creates the drag.

A rear lip spoiler "spoils" the air flow off the rear of the car. It causes the air to go mildly turbulent as it leaves the rear, and this turbulence causes the air to gently "tumble" into the "void" behind the car. The result is a decrease in the drag, and a reduction in the lift generated by the air flowing off the rear.

Some lessons learned from some of my "wind tunnel" classes:

We had to measure the drag on several shapes, all the same diameter. The shapes were a flat plate ( | ), a sphere ( O ), half a sphere ( (| ), a cone ( <| ), and a cone/sphere ( <) ). Not surprising, the flat plate had the most drag. The sphere, had significantly less. You would tend to think that the cone (with the cone pointed toward the air flow) would have less drag, then the sphere, since we all "know" that pointed things have less drag - but this isn't correct. The cone actually has more drag than the sphere, and only slightly less than the flat plate. Add the half sphere to the cone (again, with the cone toward to the air flow), and the drag is about the same as the drag of the sphere. However, turn the cone/sphere (cone away from the air flow)around, and the drag drops tremendously. The drag of the cone by itself (pointing away from the air flow) is only slightly more than the same thing with the rounded front end.

The objects that have a flat or curved "backside" cause the air to become turbulent behind the object. The smooth taper of the cone allows the air flow to gently join together and keeps the air flowing smoothly - smooth air flow equals less drag. The shape of the front of an object isn't as important - air "hitting" a flat surface just builds up a high pressure area that causes the air flow "upstream" to flow around the high pressure area. That's why the half sphere and the full sphere have nearly the same drag.

Bottom line is, that contrary to common belief, the aerodynamic shape of the front of an object is much less important than the shape of the rear of an object.

Remember the old "aerodynamic" car designs of the '30s and '40s? The big flat front ends that tapered in the back almost to a point? They were actually much more aerodynamic than the "wedge" designs with the pointed front ends, and flat back ends of the '70s. Now days, most cars are tapered and rounded at both ends.

Tim Mullen

jay: it would actually not cost nearly 3000 dollars. CF goes for about 100 bucks a yard (depending on the weave and how much you purchase but its around there.) Veing a relatively flat peice it would probably be about 2-3 yards used layered running different directions, then vacuum/heat hardened with resin and little recoup required for a form. Figure on a total cost of no more than 500 bucks each including 100$ profit. Your intake costs more because it is not a flat surface. The molds and processes for making an intake with good solid multilayer CF are very complex and take lots of time. Add into it the R+D that would not be required for this peice, and you have probably 1000 worth of materials 1000 worth of expertise and 1000 worth of profit/basic tooling repayment. Make 10 of them and you are probably close to paying back the cost of the molds...

Back to the underbody tray and as I said before it would be trashed the first speedbump you go over or pothole you hit. Or imagine a board sitting in the road that you run over that slams into the bottom of the car. If the CF shatters and throws nice shards up into your car/engine bay that would NOT be good. Hoses would definitely be cut, pulleys screwed up, etc.

As others have put it, this would be a completely useless addition on your car. If you were track only, go for it, but not on the street.

Now a CF exhaust, that might be interesting! (Though a bit loud)

Scot