Amateur Aerodynamics

Will this barn door reduce drag? Only one way to find out. In Part 1 of this series , I measured drag reduction on my 1991 Toyota pickup with the grill blocked and mirrors removed. In Part 2 , I did some qualitative flow testing, observing the behavior of wool tufts with a modified A-pillar and a bug deflector. Today in Part 3 we’ll go back to measuring drag reduction as we look at air dams.   How Do I Decide Which Test to Use? Before we get into it, an anecdote. When I learned to drive, I remember my instructor taking us out onto a 4-lane highway to have us practice changing lanes. Look in the mirror, check that the next lane is clear, signal, and move over, she told us. When my turn came, I glanced at the mirror and began to move over. Driver’s ed cars had right-side brake pedals, and she stomped on it. There was a car in the other lane I was trying to move into. “See what you’re looking for,” she said. When testing aerodynamic modifications, see what you’re looking for. ...

  In Part 1 of this series, I showed how to use throttle-stop testing to a) get reliable measurements of changes in drag, and b) how to use this measurement technique to test some simple modifications—blocking the cooling air intake at the front of the truck and removing the side mirrors. Together, those amounted to a 4% reduction in drag, so after the test was completed I made permanent versions of both aerodynamic modifications: Next up, we’ll look at one outlandish modification and one typical change that a lot of pickup truck owners make.   Modifying A-Pillar Flow   When I go to auto shows, I take lots of pictures of details on production cars. One of those fascinating details is the prevalence of what I call “fenced” A-pillars; that is, the windshield is slightly inset below the A-pillar and a sharp edge on the pillar stands proud of the glass: Here's an example, on the 2019 Hyundai Elantra. My truck is an older design and doesn’t have this edge. Instead, the wind...

Now that we've looked at the fallacy of using templates to guide aerodynamic modifications , the necessity of testing , and the complicated nature of aerodynamics , today these all come together in the false idea that airflow can be predicted or intuited. Intuitive Design Is Not an Effective Approach to Reducing Drag The claim: Since airflow can be predicted with complete certainty and behaves according to simple principles, it is similarly possible to predict the aerodynamic performance of any design by using intuition. It's so easy--just absorb these (oversimplified) schematic representations of the theoretical flow over prismatic bodies. Just like a real car! The reality: Along with a severe phobia of testing aerodynamic changes, it seems that most—perhaps all—of us harbor the innate idea that we can ascertain whether a car has low drag or not simply by looking at it. In some cases this works: look at a 1932 Ford Model A coupe, for example, and you don’t need to see any numb...

My truck, wearing its first, tested  aerodynamic modifications. It kills me sometimes to see people going on forums and asking questions about how to aerodynamically modify their cars. Why? Because almost inevitably they get a stream of answers that state, quite confidently and often unequivocally, that such-and-such a change will lower drag by x amount, or that a specific modification will always reduce drag, or what the dimensions of a modification should be for maximum, “guaranteed” drag reduction.   Often the people answering these sorts of questions will point to research on simple bodies (these are frequently used to justify guidelines on tapering of boat tails, for instance) or papers on the development of production cars (data on cooling system drag or optimum backlight angle, for example) or studies of modifications on production cars (the most famous is a 2012 investigation of changes to an Audi A2 run by Tata Motors, Jaguar Land Rover, Warwick University, and others...