Start Here

So, you want to modify the aerodynamics of your car? Great! These posts explain how to get started testing and measuring things.

A Practical Guide to Aerodynamic Modification
Identify goals and make a plan
Don't Guess--Test!
Measure, not guess (you're probably wrong anyway)
How to Test
Goals, effective testing, and analysis
It's easy and cheap to measure pressures on your own car
Learn what yarn taped to your car can tell you about the airflow over it
Want to lower your car's drag? Here's how you can measure it
A problematic method for measuring drag--with my suggestions for improvement
A Glossary of Aerodynamics: Part 1
Terms and phrases, and what they mean
Coefficients in Aerodynamic Engineering
Just what is a "drag coefficient" anyway?
No, Cars Are Not Wings
A common internet (and print magazine) myth
Understanding Significant Figures
Basic engineering practice to keep your calculations realistic

Popular Posts

How Spoilers Work

Image
Buckle up, this one's long. After hanging around Ecomodder for years, I had a pretty messed up idea of the function of spoilers because of what I had read on that forum. You might too.   See, the prevailing theory there—based entirely on posts by one forum member who is widely seen as an aerodynamic “guru”—is that spoilers “reach out” to an ideal “template” profile . By so doing, the theory goes on, a “locked vortex” is captured and the airflow above the spoiler follows the “ideal” template line as the air just ahead of the spoiler recirculates. So, simply position a spoiler and extend it until it touches a template profile overlaid on a side-view image of your car and, voil à ! Lower drag—because the spoiler isolates negative pressure in front, higher pressure in the wake behind it, and the outer flow follows the “template” shape. Someone might want to tell this person that the "perfect match" RS spoiler was, according to Burst himself , supposed to be 15 to 20 mm long...
Read more

Tuft Testing: A How-To Manual

Image
Updated September 13, 2023 Tuft testing is an incredibly powerful technique for visualizing airflow. Tuft testing is one of the easiest, cheapest, and most enlightening methods for measuring airflow over your car. Tuft testing is so effective because the behavior of the tufts tells you something about both the characteristics of the boundary layer and the velocity at any spot on your car. Background and Theory To see why tufts are such a great way to test, we first have to look at the paths that air can take over a car and the various ways we have available to visualize them. Aerodynamicists typically use three different illustrations of airflow: pathlines, streaklines, and streamlines (there are more, but we won't get into those here). If we divide the air into tiny “parcels” of constant volume (the technical term is Lagrangian parcels ), the path that a single parcel follows as it moves around and over the car forms a pathline , a record of its movement over a certain elapsed...
Read more

Optimizing Aerodynamics of a Truck: Part 5

Image
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,...
Read more