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A Practical Guide to Aerodynamic Modification

Tuft testing shows the streamlines on a car as the yarn aligns itself with airflow while you drive. Gas prices have recently reached their highest level in nearly a decade. You may find yourself looking at your car, wondering if it’s possible to use less fuel on your long commute and keep some money in your pocket. You may have heard of people who modify their cars to get better fuel economy. You might have even seen cars like the Aerocivic, a weird-looking contraption that was reported on in mainstream media articles during the gas price spike of 2008-09. Would doing something like that work on your car? Can you modify the aerodynamics of your car at home? The good news is, you can! The better news is, you don’t have to (and shouldn’t) make your car look like the Aerocivic. Air drag has an influence on the fuel economy of cars, and that influence is greater the faster you typically drive. You can also do a lot more with airflow than just reduce drag. Many people have modified the ae

Garage Lessons

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 the new bearing

Testing and Improving Stability: Part 3

In my first two posts on testing directional stability, I used first a cellphone with angle measure and then a Scangauge reporting steering angle from my car’s OBD system to try and measure changes in its stability. That worked okay but I had no way of logging data to verify that what I thought I saw on the gauge reflected a real change.   Not anymore. I bought an OBD Bluetooth scanner that connects to an app that can log hundreds of different parameters through the car’s computers. In this first test, I’ll see if I can use it to record steering angle in a crosswind as I’ve been doing. To see just what it can show, I’ll first move my car’s center of gravity around and record the differences.   How to Calculate the Position of Your Car’s Center of Gravity   Figuring out where your car’s center of gravity sits longitudinally (that is, its location between the front and rear axles) is quite simple. All you need are front and rear axle weights, something you can get at any truck scal

Optimizing Aerodynamics of a Truck: Part 5

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, and replaced with

Spoiler Roundup

Over the past several months, I’ve tested eight different spoilers on my car. I could test more, but I’ve reached a point where I would rather install something permanently and move on to other things, leaving this spoiler project behind. Hence this Spoiler Roundup. Let's meet the competitors: Straight Gurney flap Sinusoidal Gurney flap Commercial lip strip 10 ° board 20 ° board 20 ° board with slot 30 ° board Hellcat. I want this one to win only because I enjoy saying "Hellcat." Which one should I use, if any? That depends on a few things, including my goals , my design limitations, and how much I want to permanently alter the car.   Goals: I want to reduce lift and improve stability as much as possible without increasing drag or, if possible, even decreasing drag.   Limitations: I don’t want to extend the factory spoiler backward at all. Upward is fine, but I use a hatchback tent when I roadtrip that won’t fit if I elongate the spoiler.   Alterations: I don’t care