Beetle Aerodynamics / Roofspoiler

 

Drag measurements

 

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:

Autospeed new beetle airflow drawing

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:

Drag measurements with

 

Now, let’s see how much the beetle looks like a teardrop:

VW beetle on teardrop

 

This shows that the rear of the beetle is to steep. This next picture of a windtunnel test performed by Volkswagen proves it:

Volkswagen windtunnel test

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:
Mercedes in windtunnel
Merdedes in windtunnel wake
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:

Holzapfel 1303 race beetle
(more on this car here and here)

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.

 

15 Comments

    • Hi William,

      The beetle in stock form has no air flow into the engine compartment. The engine relies on the fan to suck the air in.
      The rear of the beetle (without a spoiler) has a turbulent air area at the rear decklid and above it.
      See this picture:
      http://images.gerrelt.nl/roofspoiler/beetle_windtunnel_test.jpg
      And apparently the fan can suck enough air in from this area to cool the engine.
      My spoiler doesn’t change much about this. It enlarges the turbulent air area a bit, making it start at the top of the rear window, instead of the bottom of the rear window.
      I don’t think it matters much for engine cooling.
      My car is now watercooled, and it get it’s cooling from the radiator in the front of the car, so maybe my car is not a good reference point. But I haven´t got any messages about this from people driving with my spoiler with aircooled engines.

      Greetings,
      Gerrelt.

    • Hi William,

      In theory: yes, this should provide greate stability. But I don’t know with what percentage.
      I’ve driven 100 mph (160 km/h) in my beetle, and it felt pretty stable. But I don’t know how it felt without the spoiler.
      Furthermore, my car is equiped with a Kamei front spoiler and a thicker swaybar, which helps stability too.

      Greetings,
      Gerrelt.

  1. Hi Gerrelt,
    I went to an airfield here today and went here with the idea to gather info on vortex generators. You will find it on F1 cars and planes. They stir the air on the higher places on the wings to force the air to follow the surface of the wing and to separate and move away like you get at the back of the beetle creating and low pressure volume. And from this low pressure area you must try and cool and feed an engine. Fans cant suck air if there is no air. I am of the opinion that vortex generators will cause the air flowing to the back of the Beetle will follow the shape of the roof flowing down over the bonnet and supply air for cooling and breathing for the engine.
    Vortex generators are little wings that stir the air in spirals this disturbing the direction of flow to follow lower pressure volumes of air or areas on wings. Study it on Google .Vortex generators and design dimensions . I am going to fit a golf engine to mine . Your opinion please
    Good luck.

  2. Hi there, I was reading through this about beetle Aerodynamics as it seems to have been a Hot top for years and I was looking at designs of spoilers for the purpose of stability if anything. With most of the wind tunnel tests I have seen the vehicle isnt running and part of me feels that it becomes a null test if you have an aircooled car. Having owned a beetle for many years and driven it alot in the rain, you get to see a similar effect to aerodynamic testing as the water sprays off the car, which in my experience was 2 perfect spirals off the back! I have seen that there is the area of lower pressure at the bottom of the rear window, where the engine draws its air from, it then moves this to the underside of the car exiting under the rear valance, which makes me think that playing with the aerodynamics without some better testing may not help cooling and may worsen the actual aero effect. I equally have been looking at fitting a watercooled (polo TDI) engine in my 1302s and something needed to be considered about air flow here is to actually direct some at the engine for the intercooler. I have found that having a porsche whale tail does little for the beetle, but I did find adding a rear window louver actually caused the engine to run hotter. Essentially the top of the louver acting like a small spoiler above the window. Would be interested to see how the aerodynamics are affected by the engine actually running at 3000rpm though to simulate 70mph.

    • Hi Dale,

      For what I have read, it is indeed better to have a running car, but also the road underneath the car should be moving.
      However, I don’t think the aircooled engine’s fan moves enough air to really make a difference aerodynamic wise. The low pressure area at the bottom of the rear window is caused by airflow seperation.
      That’s also the reason that a spoiler shouldn’t affect cooling (i’ve never had people with my spoiler complain about less cooling with my spoiler).
      A standard beetle does not have airflow into the engine room. The fan is taking it’s own air, and is not dependent on airflow.
      On the Porsche whale tail: I’ve had people tell me it did improve stability. Which theoretically could be true, by creating an extension after the beetle (it almost makes it into a 3 box car design) the airflow is forced to follow a more shallow angle. The shallower angle will create less drag and less lift.

      I’ve looked at the 3 cilinder VW TDI engine as a candidate for an engine swap. It would fit quite nice because it’s shorter then the 4 cilinders. But Dutch legislation and engine cost prevented me from doing it 🙁 . That was quite a few years ago, the engines will be quite a bit cheaper, but the Dutch legislation hasn’t chagned sadly.

      Greetings,
      Gerrelt.

  3. Hi Gerrelt.
    Have you tried modified running boards lowering the outside edge preventing air entering from the sides? Your opinion and experience on this aspect please.
    William

    • Hi William,

      No.
      But I have thought of installing “wheel spats”. And I don’t mean those things that cover half of the wheel on the side (also called side skirts), but the little spoilers just in front of the wheel. These are seen ofte on the newer cars. These little spoilers prevent air from rushing up into the wheel well.

      Greetings,
      Gerrelt.

  4. There is some question about the impact such a spoiler would have on engine cooling for a stock aircooled engine. Contrary to what Gerrelt has said, I’ve found it actually is problematic. I built a Holzapfel-style spoiler based on Gerrelt’s instructions on another page, making it’s length part way between that of the regular Holzapfel and Gerrelt’s small fiberglass one, if I remember right it was about 20cm long, and made of plastic, possibly ABS (this was a few years ago). Fortunately, at my Mom’s suggestion, I didn’t screw it down, held it on with Gorilla tape initially. I drove with it on the freeway at about 75mph (120 km/hr), didn’t really notice much difference in handling or power need, but the oil started getting really hot, monitoring oil. It got so bad I had to pull over and rip the spoiler off. For completeness of this data, I should include that this was at higher altitude, around 5,000 feet (1500 m), so there is less air – drag and stability improvements less significant, and the engine already struggling to get enough cooling air. I’m still thinking of trying to find a way of improving drag, possibly new air scoops on the front edges of the rear fenders (would look sort of like fighter jet air intakes) so that a spoiler won’t cook the engine. Or possibly the more common “Herrod Helper” type spoiler below the back window, possibly combined with vortex generators above the back window, with a gap between spoiler and body to allow cooling air to get into the louvers. Would like to mess with Computational Fluid Dynamics (if this engineering school ever gets around to teaching me something useful that is…) to see what design has the most benefits, and how much, how much work and expense it’s worth.

    • Hi Quill,

      I haven’t had any of the people that use my spoiler reporting me that they had cooling issues. There are even race-beetles using them, and they get abused quite a bit.
      The air doesn’t flow into the rear vents, the engine fan is sucking air in. If you look at the windtunnel picture from Volkswagen, you can see that the airflow doesn’t reach the vents without the spoiler either:
      http://images.gerrelt.nl/roofspoiler/beetle_windtunnel_test.jpg
      Maybe it was the high altitude in combination with the spoiler which made it harder for the engine fan to draw air in.

      I once saw some pictures of somebody who had made a hole in the front engine plate (the one that closes the gap between the engine and the firewall), and connected an air duct to it on the underside of the car. The end of the duct was somewhere close to the torsion bar, scooping up cold air from under the car. He said that it improved cooling quite a lot. Which make sense. Getting air from the low pressure area behind the rear decklid is harder then getting air from the high (but messy) pressure area under the car.

      edit: re-found the page: https://www.thesamba.com/vw/forum/viewtopic.php?t=361406

      Greetings,
      Gerrelt.

  5. On a road trip, in my (new to me) 2004 TDI Beetle, with the rear “wale fin”, I found my car handles very well (smooth as silk) at under 100 mph. Then I noticed I had 4 snow tired and the heat was eating them alive. I ordered new rims and “Z” Toyo radials. I have been on another road trip and found that at over 110 mph the car became squirrely. I wanted to go faster but thought I should get some educated opinions.

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