Amateur Aerodynamics

Measuring radiator airflow is easy and cheap. For almost as long as I’ve had the Prius, I have blocked off part of the grill opening—the theory being that reducing flow through radiators reduces the drag associated with pushing air through them, what we call cooling drag . On my truck, my first throttle-stop tests showed that covering the grill openings completely reduced drag around 4% . Obviously, we can’t drive around like that all the time; most manufacturers today use shutters in front of radiators that can open or close to allow airflow when needed and reduce drag when not. Most of them use internal shutters, but a few (such as Ford and Hyundai) have started using visible, external shutters. This is the front end of the Ford Mach-E; the slats rotate to an open position when cooling air is needed. I didn’t want to get that involved in designing an active system so I blocked part of the truck’s grill with plastic covers, leaving the center part of the upper grill open. But what d...

After my last round of testing , I made a permanent version of the air dam extensions I tested: This brings my truck’s aerodynamic drag down to around 84-85% of what it was before I started this project:   Modification Percent Change mirrors removed, grill blocked -4.0% 9” air dam -8.6% air dam extensions -3.8% Total -16.4%   I didn’t expect to get that much with these changes, especially because I haven’t even tested anything on or around the bed, rear undercarriage, tailgate, rear wheel housings, or rear bumper.   Tailgate   Many websites claim that lowering a truck’s tailgate increases its drag. This was even explored on an episode of the popular television show Mythbusters ; the show’s hosts put a model truck in a water tunnel to illustrate the recirculation that happens in the bed (and ran some fuel economy tests with the tailgate down, removed,...

So far, I’ve lowered the aerodynamic drag of my 1991 Toyota Hilux by removing its mirrors, blocking part of the cooling air opening , and fitting a large front air dam . Together, these reduce drag by more than 10%--which means I’m about halfway to my goal of cutting the drag of this truck by 20% or more .   Moving on from the front surfaces, I thought I would trial some modifications to the airflow around the wheels next.   Keep It Simple   Right off the bat, I have a conundrum. The front wheel housings have a metal inner fender and a narrow plastic cover which seals the gap between inner and outer fenders. But in front of the tires, there are effectively no wheel housings; it’s open to the bumper, stock valance, and now air dam. According to various textbooks , wheel drag is typically reduced when wheels are enclosed in housings, with drag decreasing the smaller the ratio of the wheel housing volume to wheel volume. This may or may not hold true on my truck; I hav...