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Old 05-03-2020, 08:06 PM   #6 (permalink)
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Originally Posted by M_a_t_t View Post
Most of these methods are new to me, hopefully I have time to do some reading later. What method (or combination) would you suggest aeromods be tested with? Especially the ones that take awhile to make, such as boattails and the like. Currently the most common method for checking is based on coastdown testing. This provides some actual numbers for comparison, even if they might be skewed, to see if a mod works.

If tuft testing is not indicative of lower Cd, and coastdown testing not reliable* both of which are the least cost to do some testing what would be the next best bang for the buck?

*In one of your other threads you suggested you never had any luck with coast down testing. Could you expand on why you feel that way? I was just assuming unreliability in the data.
In my experience of coastdown testing, the data tells you nothing. For example, I couldn't measure a statistical difference in windows up/down testing. (That testing was done over many runs.) I did this a few years ago when I was preparing material for my book. That followed coastdown testing I did in the late 1980s when I found that the results were all over the place - such a contrast to (say) 0-60 mph testing that was consistent within tenths of a second.

When writing the book, I found two SAE papers on coastdown testing and looked carefully at what they had done to get good results. It was all way beyond the technology available to an amateur eg specially calibrated anemometers on probes out the front of the car constantly logging data on things like yaw angles of the oncoming air, and sophisticated computer modeling of driveline inertia, etc, etc.

I have seen people using coastdown testing to purportedly find tiny changes in aero drag and, frankly, I don't believe it. If you were doing coastdowns from (say) 100 mph down to 80 mph, it may start developing useful data - but otherwise, I can't see it. If you are attempting coastdowns, you should at minimum test the results from something like windows up / windows down and expect to see a calculated 10-12 per cent (or similar) change in drag.

All testing techniques - tufts, pressures, downforce/lift, pitot tube, mileage, throttle stop - can give you extremely useful information. But you need to:
  • ignore changes that are tiny (because they're probably statistically inconsequential)
  • do the testing with rigorous methodology
  • understand the significance of what you are actually seeing (tuft testing a good example)

Frankly, I believe that most of the aerodynamic modifications that I see people undertaking do very little, especially those that attempt to decrease drag. However, I can see some major modifications (full length belly pans / undertrays, large boat tail extensions) as giving results above noise.

In my aerodynamic modification, I first do a mock-up out of cheap and easy materials, and if I cannot get a measurable positive change in behavior, I take it off. If I get a positive change, I make a proper one. Never invest time and energy in the 'proper' version when you can't be sure if it works. Cardboard, coroplast and tape first.

For most accuracy in measuring changes in drag, long distance (thousands of miles) mileage tests are best. I am hoping that the throttle-stop measurement technique I have developed will be next best.
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