You may have seen my earlier post,
Suzuki Swift (1994 Sedan) Coast Down Numbers, where I reduced my Cd from 0.32 to 0.28 (a 12.5% reduction). I took things a bit further this time, adding a SedanKamm (the beginning of a boat tail). I did some tuft testing to see if flow is attached, and did some coast down testing.
I built it out of a 4'x8' sheet of coroplast that I had gotten a couple Novembers ago. It is 4' long at it's longest, over 6' wide, and made out of one piece. The back is supported by a 2x6 and the windows are made out of some very thin very clear acrylic. The acrylic is too thin to be used as a grill block (as we found on the Prius, rocks just punch holes in it), but it seems to have done a good job as windows (other than the reflectivity). The windows would be better bigger, but this was all of the acrylic I had left. It would also be better with a dark, non-reflective interior, but this is just a prototype. For the most part, everything is held in place with duct tape, other than the 2x6. No holes were punched in my car's body.
The tuft testing showed a couple things, first, there IS attached flow on the SedanKamm
. Second, the AeroSpike Mirror Deflector I had made does little if anything, so I took it off. It was good for starting conversations, but I have something new for that
.
The configuration would be the same as the previous coast down, except windows up, both mirrors in stock configuration and folded out, and the addition of the SedanKamm.
The coast down testing shows that I am now hovering somewhere south of Cd = 0.245 (another 12.5% reduction), or around where the Prius or original Insight would be. Funny, more or less the same shape, more or less the same Cd... This is the effect I had predicted, seeing as how over 4 square feet is cut off the trailing edge of the car (about a 20% reduction, and I knew I couldn't take full credit for the reduction). I did 5 bi-directional runs on a very flat stretch, and this time I had an automated GPS logging program catch my speed on ~1sec intervals.
Here you can see it along side our Prius, first from an angle, then a comparison of the trailing edges. The angle for the SedanKamm was picked by the height of a 2x6, and it seeming to be about the correct angle. You can see how close it matches the Prius.
