Amateur Aerodynamics

It’s that time of year again: opening weekend at Watkins Glen, which means last Friday nearly 40 people took to the track to drive (slowly) around it in the annual Green Grand Prix. Although, not as slowly as you might think. If you do it right, you won’t touch your brakes; my traction control was cutting in on the back sweeper on the Boot and Turn 1. I’ve been making the trip out to New York from Illinois for a few years now, and I always find myself thinking about the efficiency of my car not just during the competition but also during the long drive (I removed the stereo several years ago, so I have a lot of time to think!). This year I had also read something thought-provoking in Carlo Rovelli’s Helgoland , a book about the discovery of quantum mechanics: “Abandoning assumptions that seem self-evident can lead to greater understanding.” What assumptions have I made about energy efficiency that might be holding me back? Well, the only way to answer that is to abandon everything an...

Cicero, in his letter On Moral Duties , wrote, “The definition of terms must in fact form the basis of every scientific exposition if the scope of the argument is to be clearly understood.” Terminology in aerodynamics is often not well understood even by people who think they have a good grasp on it, let alone those mostly or wholly unfamiliar with the field. You will see people throw around words with apparently little or no understanding of what they actually mean all the time or use vague terminology in an attempt to impress or obfuscate (I was guilty of this for years, aping what I saw modeled in online forums). To help dispel some of this uncertainty, here’s a primer on words and phrases you’ll see used in forum discussions and online articles about aerodynamics, often incorrectly—with accurate definitions pulled straight out of car aerodynamics textbooks . Aerodynamics : “Aerodynamics is simply the study of the forces involved in the movement of an object through the air. Vari...

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...

I haven’t had any time to test aerodynamic modifications over the past several weeks because I’ve been busy dealing with other maintenance issues—completely new brakes on the truck (if you’ve never seen inside a 30-year-old drum with all its original hardware and shoes, hoo boy) and bad wheel bearings on the Prius. But I have had time while I worked to think about lessons from these other aspects of vehicle engineering that might apply to how we think about the process of aerodynamic modification.   If You Want It Done Right, Do It Yourself   I last had my Prius aligned several years ago. At the time, the tech said they had to shim one rear wheel to get it in spec and I thought nothing of it. Fast forward to last week, when I had to replace that wheel bearing: the shims (two stacked on top of each other) were spacing it out from the axle, and as a result the bearing wasn’t even seated in the housing—it fell off in my hand when I removed the four retaining bolts.   I put t...