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Originally Posted by Vman455
Throttle-stop testing is just one method for measuring drag changes--a very good one, but one that may not work on all cars (e.g. electronic throttle controls). Other methods include measuring fuel economy over long distances/repeated long trips, and coastdown testing. You have to be careful with coastdowns; you can find my attempt to improve coastdown testing on the road here.
But drag changes are just one aspect of aerodynamic measurement. Tuft testing and pressure measurement (of both body panels and heat exchanger flows) will tell you a lot. Ride height sensors will show you whether your car is creating lift or downforce. If you buy Julian's book, you'll see new techniques for measuring lift and trailing vortex strength/direction.
Approach aerodynamic modification like an investigation: Your job is to figure out what happens to airflow as you change the shape of your car. Same as if you were diagnosing a fault code to figure out what's wrong with an engine.
For example, I bought a cheap Hellcat spoiler on Amazon to test. I didn't just use one method because that wouldn't give me a complete picture of what this spoiler does on my car. I taped it on and measured pressures on the rear window and hatch; I tufted the window and spoiler and videorecorded it; I did several coastdown tests and ran a statistical test to analyze the data. Even having done all that, I still don't have a complete picture, as I don't know what that spoiler does to the trailing vortices, for instance (so I'm planning on finding out soon).
Unfortunately, what you'll find on this site and a lot of others is a belief that guessing is better. It isn't. |
I follow your website. Yeah testing is hard. Usually the best way that I’ve been “ testing drag” is “ feel of my car”. I drive so much that I can tell if something is off. Like you know how you can feel that difference in driving power on a wet road versus smooth dry concrete? I can kind of feel the diff with aero drag. I can feel how my car drives with and without my side mirrors on since I put them back on any time I go somewhere for an oil change or tire rotation and balance and so on. I know it’s not scientific, but it’s like muscle memory.
For example, people here are telling me to start my “ boat tail” at the end of my spoiler, and not to follow the roof line of my hatch because it doesn’t “ match the template “
But I’m assuming I can just extend the roofline and then add a spoiler for pressure recovery and maintain attached flow and tuft test it. Obviously I’d be able to “ feel” a diff with a boat tail keeping attached flow even if it’s not the template. I mean a Tesla model 3 has a pretty steep rear roof but a little spoiler and keeps attached flow