Wool Tuft test


Header picture

On the autospeed.com site there’s an article that was one of the reasons for developing my rear window spoiler. It’s an article about aerodynamic testing on a Volkswagen New Beetle. They are using short pieces of wool (tufts) taped to the car at certain strategic points. See this article: Autospeed Aero Testing – Part 3.

I’ve always wanted to do the same to my beetle. So when I finally had some time ( the last day of my holiday.. ) I gathered my family and got to work…

I cut a bunch of wool tufts and taped them to the rear of the car (with the help of my daughter Smile ).


The plan was to first test it with the spoiler on. Then remove the spoiler and do the test again. The test track was a piece of highway
between my hometown and a city nearby, about 10 kilometers long.


  1. Good job!
    This is something I know I’ve wanted to try and never made the effort 🙁

    “The more I learn about (basic) aerodynamics the less I know”… So much to learn. So little time 🙂

    No question -IMHO- that the airflow had detached at the point of your spoiler, as expected. The yarn looks like it’s doing a great job of visualizing (for us) the air flow. It’s really slick to see a low-tech-low-cost method of understanding the hidden air flow.

    Thanks so much for taking the time to share! Really appreciate it.

    Long Beach
    Ca. USA

  2. In the 1971 my family bought a Super Beetle wit AC. my mother & I picked it up at the Porsche plant in Stuttgart. I was 14 & we drove it around Europe for a while before bringing it home to Charleston, South Carolina (USA). One of the AERO tricks that we tried was a thin, sheet metal, matt black spoiler which mounted on thin, sheet metal stands on the body vent below the rear window. This made the car more stable when passing semi-trucks at about 130 KPH. The highway gas millage also increased 3-4%. This, being at the top of the rear window, seems like it would do even better.

    • Hi Eddie,

      Very cool, that must have been quite an adventure driving through Germany in 1971. Germany has a lot of very beautiful areas.
      Once a year we do a weekend holiday with our beetle in Germany (we live close to the German border). We really like Germany as a holiday destination.

      The spoiler you are talking about sound like a “Herrod Helper”:

      A lot of people have had the same experience as you with the Herrod Helper. Better gas millage and more stability.
      Thanks for your comment!


  3. Something that I wonder is being factored in here. remember that a New Beetle is a rebodied Vw Golf; a front engine watercooled car. The “old” Beetle is of course air cooled and relies heavily on air drawn in through the vents below the back window.
    Surely, therefore, we want to direct air into this area rather than away from it (airflow separation). The suction from the vent area is going to reduce the boundary layer significantly anyhow I should think? In terms of rear airflow, the engine and it’s cooling fan are a major factor: Air goes in through the rear vent, through the engine, and is expelled at pressure through the rear beneath the apron and guards, reducing rear drag. This is an effect that wind tunnel tests of the Beetle rarely take into account because they are static tests with the engine switched off!
    It would be interesting to play with spoiler shapes above and below the cooling vent to see what effect it has on overall airflow dynamics.

    • Hi Martin,

      I think you over-estimate the amount of air the engine is sucking in. Compared to the fast amount of air passing over the car at, say, 80 km/h, it’s probably only a small percentage.
      If it was comparable, you wouldn’t be able to stand behind a beetle without experiencing a storm-like wind coming from under the car.


  4. Um
    if your trying to increase mpg the Tufts pointing up mean there’s more drag therefor decreasing mpg.
    You want the air as close to car as long as possible on the rearend. That’s why the best aerodynamic shape is a teardrop type shape.

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