Recently I bolted a kamei front spoiler to my beetle. I was impressed with the results, which got me interested in aerodynamics. Then I stumbled on this article from the excellent website autospeed: Flowing the Porsche 993 twin turbo 4WD and the New Beetle. That article got me thinking.
Especially these parts and picture caught my eye:
"So the problem must be at the back? And it is. But it's a different problem to that which we've seen before. Here the flow remains attached right down to the line of the guard/boot opening. And this results in a very small wake for the (it's larger than you'd think) size of the car. But it also means that the airflow wraps in one long curve from the base of the windscreen right around over the top of the car to nearly the rear bumper. Aeroplane wing, did you say? And not only will this shape have major lift (without an undercar ground-effects tunnel, anyway; and I looked under and didn't see that), but much of the force will be upwards and rearwards - creating that monster drag"
and:
"In fact, after the wool tuft session on the Beetle had finished, we gathered around the car commenting on what we'd seen. In that discussion we suggested that a spoiler - which could be used to break up that attached rear window flow and create a clean separation point - would be best placed at the top of the rear glass. It might even reduce overall drag - despite the much larger wake that would then result."
The picture explains it all:
So, at first glance, you would think the beetle has a nice aerodynamic shape. But that is not true. The rear of the beetle drops down to fast, the angle is wrong.
The best aerodynamic shape is a raindrop shape, but that's not a suitable shape for a car. So you'll have to cut it in half, creating a teardrop shape:
Now, let's see how much the beetle looks like a teardrop:
This shows that the rear of the beetle is to steep. This next picture of a windtunnel test performed by Volkswagen proves it:
If you would want to modify the shape of the VW beetle to create a better aerodynamic shape, you would have to extend the rear a lot! It would make the car very impractical.
Luckily, it is possible to get the same effect with placing a spoiler in the right place. It's called the Kamm effect, after a German engineer. Read this wikipedia article: Kammback
By placing a spoiler in the right place the airflow is detached from the car, the airflow breaks free. Then the airflow can flow in a nice teardrop shape.This method is used a lot on modern cars. See these excellent pictures of a Mercedes in a windtunnel:
On the left picture you can nicely see the airflow is detached from the car. On the right the wake that is created behind it is demonstrated. A lot of modern cars have a little spoiler like this mercedes on the back. If you look around you will notice that especially small light cars have a roof spoiler.This is because aerodynamics have more effect on light cars then heavy cars. The heavy cars have no problem bashing through the wind resistance, that's the reason that trucks mostly aren't very aerodynamically shaped.
The rear of this mercedes is also sloped similar to a beetle, and a beetle is pretty light. I think the beetle would benefit from a rear spoiler too.
I've found this picture of a Holzapfel race beetle with a rear spoiler:
This is a little too extreme for a road-beetle, I think. It's quite a large spoiler, but that's probably because this car spends a lot of time at very high speeds. For a normal road car, the spoiler could be smaller.
I've decided to create my own roofspoiler, see: Creating a roof spoiler.
