Quote:
Originally Posted by Sven7
^^ thanks for the info! Do you have the detailed results (%'s, etc) posted anywhere?
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Just did a brief search for that info. It was all handwritten notes at the time, and I don't believe I posted them anywhere. I was unable to come across the notes for the air dam tests just now, but I did find them for the more recent rear-end height tests.
Guess I didn't remember the tests exactly; my notes say these were coastdown times from 105 to 50, not 65 as I wrote previously. I guess the idea was to let it coast as long as possible, hoping it would magnify any differences. But I know I was also thinking that by keeping it above 65, the aero would be more isolated from rolling resistance, sporadic crosswinds, etc. Guess I went with the longer duration idea in the end..
In re-reading my notes, I did 2 northbound and 2 southbound tests per rearend height setting. I summarized my test into a graph, the results I'll list here;
R. Hgt. - - Low - - High - - Avg.
25" - - - - 54.2 - - 54.9 - - 54.6
23.75" - - 54.25 - - 57.1 - - 55.7
23.0" - - - 56.15 - - 57.05 - 56.6
21.0" - - - 56.55 - - 58.2 - - 57.4
R. Hgt. = height above ground to a marked point on rear bumper in inches
Low = the minimum coastdown time recorded in seconds
High = the maximum coatsdown time recorded in seconds
Avg. = the 4 run (2 n.b. & 2 s.b.) average coastdown time in seconds
The front bumper height stayed between 16.75" and 17.25"
As you can see, there was a pretty clear trend towards longer coastdown times with the lower rear end. Between each set of 4 runs, while adjusting the rearend height, I also added about a gallon of gas to the tank to try to keep the weight the same. The tires were set at 70 psi. Speeds were measured by a GPS speedometer & coastdown times with a stopwatch.
I set 2 orange traffic "cones" out a mile apart on a straight, flat section of road. I had 2 "turnaround areas" about a mile beyond each cone (end of the course). I'd take off and accelerate southbound to about 110 or so before the north cone, then throw it into neutral and watch the GPS speed. When it hit 105 (hopefully really near the cone), I'd start the stopwatch. When it dropped to 50 (hopefully near the south cone), I'd stop it and roll to the turnaround area on the far end, stop, and record the time. Then I'd take off and repeat the process the other direction (northbound), aiming to be at 105 while passing the south cone so I'd be coasting along that same stretch of road each time.
That would be one set. Then I'd do that all again before pulling off to the parking area to put a gallon of gas in and change the rearend height. Each run being 3 miles each way, that's 12 for all 4. Add another one for going over to the "staging area" and back, and that gets us 13. Guessing the car was probably getting about 13 mpg during this, adding one gallon each time should have kept the weight pretty much the same. The fuel level may have dropped a little overall, but it wasn't too much.
When I did the air dam tests, the methods and results were similar. Though I can't remember the exact numbers (will look again), what I DO remember is that the "barn door", or "snowplow" air dam had definite longer coastdown times than the more "streamlined-looking" air dam (second best), or no air dam at all (the worst).
2002 VW Jetta TDI 5-speed(completed 01M-5-speed swap at 155K miles) 45 MPG City with the 01M, 5-speed 60+ MPG City. Nokian Entyre Low RR Tires. Experimenting with the "Hybrid" 205 Deg F T-stat:
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