Showing posts from April, 2022

Common Misconceptions in Aerodynamics: Part 3

The first part of this series dealt with the erroneous use of "templates" to guide aerodynamic modifications , and the second part with the belief that testing is unnecessary or impossible to do at home or both . Now, we'll take a look at the prevailing belief that aerodynamics is an easy subject. Aerodynamics are Extremely Complicated The claim: The principles of fluid dynamics that govern airflows are simple to understand; therefore, aerodynamics as a whole are actually simple. Anyone who says or thinks aerodynamics are complicated is just muddying the waters. There's so much wrong in this forum post, I'm not even sure where to start. The reality: Hand-in-hand with the idea that airflows can be predicted and low drag is as simple as following a template, the belief that aerodynamics are easy to understand (a prerequisite for their ease of prediction) is just as erroneous. The aerodynamic force exerted on a body like a car arises from the pressures acting on it

The Relationship Between Drag and Fuel Economy

Lined up and ready to head out on track at the 2022 Green Grand Prix , an annual fuel economy competition held the Friday of opening weekend at Watkins Glen International. We all know that reducing drag can improve fuel economy. It’s intuitive but can be proven mathematically: when your car has less force acting against it, it takes less fuel (gas, diesel, or electric) to move. But what is the exact relationship between aero drag and economy? Can we measure it? Or predict it? Figuring this out can help us plan modifications to our cars with the aim of improving economy, as well as tell us if fuel economy is an accurate measure of changes in drag.   Rules of Thumb   You’ll see rules of thumb tossed about online which relate drag and fuel mileage. A common one says, “For every 10% reduction in aerodynamic drag, fuel economy will improve 5%.”   Is that accurate? Well, in a word: no, as we’ll see in just a minute. And its origins are murky. Some people attribute it to GM aerodynamicist Gin

The "Should" Manifesto

I used to talk about aerodynamic modifications on my cars—and remember, I had no idea if they worked or not because I had never tested anything—using the word “should” a lot. “Removing the mirrors should be reducing drag,” for example. “Adding this tail should make it a lot more streamlined.” “Putting these lowering springs on should lower drag.” “Taping vortex generators here should keep flow attached over this panel.”   Banish this word, “should,” from your vocabulary whenever you talk about aerodynamic modifications. The air doesn’t care about what you think it should do, and you’re most likely wrong anyway. This guy got it right. Mindset   That word, “should,” betrays a mindset of hoping that your modifications work without testing them to see if they do or not. If fear—of finding out they don’t work, of learning that you failed—is holding you back, see if you can approach aerodynamic modification with a different attitude. The Last Jedi had its problems, but it did have o

Optimizing Aerodynamics of a Truck: Part 1

I’ve written before about my truck , a 1991 Toyota Hilux . This was my grandfather’s truck years ago; it went to my father and then to various of my brothers before I acquired it last year. It was in pretty rough shape, having sat at the back of my parents’ driveway for several years: I had it hauled down to a mechanic to verify it would start and run and, several weeks later when I was able to fly out, I drove it back up to my parents’ house and set to work.   After a new timing chain, front axle, brakes, suspension dampers, front valance and rear bumper, and other odds and ends, I drove it over the mountains and back to Illinois. Coeur d'Alene, ID, where this truck was originally sold at Parker Toyota. Surprisingly, fuel economy wasn’t that bad! The truck averaged just under 25 mpg. But, I wondered, could I improve that? Is it possible to reduce the aerodynamic drag of a truck like this? Measurably reduce drag?   Making a Plan   Unlike my modifications of earlier cars, where