Am I the pessimist here ? While most seem to have good results in modifying the aerodynamics of their cars, my experiments show no or little improvement ! I did three modifications :
1- Grille block with a polyethylen film covering tightly the two grille openings and bumper. Only one pass B.
2- Smooth cover hubcaps made with coroplast at the rear and light cone shaped cardboard at the front. AABB.
3- Front skirt, made of a flat piece of 1/8" veneer and 4" lawn edging, from the bumper down 2" to the ground. Covering the lower grille. One passB
The experimental road is a fast downhill 1 km section coming to a flat section for a total of 2,4 km to a stop sign. There is a slight up portion 10 meters from the stop. I had to turn left at the stop sign for a few hundreds feet. You can see it on GPS map:
The test were all done as follow: From a standstill at the top, engine off, accelerating downhill without power ( gravity only), and measuring the MAX speed and the distance before the car stops.
Wind : Strong side wind (90?), and then a head wind after the stop sign.
Acuracy: Rounded up to the whole number ( no decimals), which is about 1% variation on 100 km/h. Speed taken on Scangage and GPS. Distance on the GPS but rounded to 25 feet. These experiments were not intended for scientific acuracy but for gross evidence of + 1%. I didn't want to write down acurate numbers to the hundred'th (0.00) because of the wind factor which is not constant.
The pass A ( without modifications) ( 3 times) was executed at 99 kmh avg. and for all 3 ( so no noticeable standard deviation) and distance = stop sign + 300 feet. So:
0- Stock car: 99 kmh max speed and distance = stop sign + 300 feet.
1- Grille block : 101 kmh max speed and distance = stop sign + 400 feet.
2- Smooth hubcaps: 100 kmh max and distance = stop sign + 400 feet.
3- Front skirt : 97 kmh max and distance = stop sign + 0 foot.
Conclusion: The gross measure give about 1% increase for the first two tests and a decrease of 2% for the last one.
The skirt test surprised me a lot. It might be because the skirt was flat and the stock car is rounded at the front. The undercarriage is also covered with plastic except the engine compartment.
That's a tough aerodynamic case. It's almost taller than it is long. But it is so light that aerodynamic improvements (especially @100kph) are real critical to improving your glide times. I think you have demonstrated that, and being able to glide another 100 feet into a headwind is nothing to sneeze at.
I would also speculate that the air dam had minimal effect due to the underbody, just speculating.
Like Bill says, any wind while testing can completely screw up everything. And it sounds like you had a terrific wind on top of that. I would not get too excited about results till you can get a real test. I would never even attempt a test in the conditions you described, and surely not make any determinations of the value of the mods.
If you have a smooth underbody, you don't need an air dam to reduce drag. Sports cars have an air dam to increase downforce (which increases drag), even when they have a smooth underbody. Cars with a rough underbody can use an air dam to divert the air away from the rough underbody, counterbalancing the frontal area increase in drag from the air dam by the drag reduction in keeping the slipstream away from the draggy underside. In a car with the proportions of a Smart (tall and short), most of your drag is being generated by the vacuum from the relatively huge eddy (wake) generated off the back of your car. Anything you can do to reduce the size of that wake would help immensely.
This nice sunday morning I took my optimism with me and back to my prefered downhill testing road. No wind or just a little air movement from the NW ( front of the car). I did the same 2 last test in this order :
A1 : Stock car. Air temp 16°C. Pavement T° 16°C.
B : Moon caps ( smooth cover hubcaps).
C: Moon caps + Front air dam ( front skirt).
A2 : Stock car. Air temp. 18°C. Pavement T° 22°C
A1 : Max speed SG=94, GPS=93.8, minus - 83 feet of the stop sign.
B : Max speed SG=96. GPS= 95.54, plus + 3 feet of the stop sign.
C : Max speed SG=94, GPS= 94.04, minus - 62 feet of the stop sign.
A2 : Max speed SG=96, GPS= 94.74, plus + 25 feet of the stop sign .
0 - Test were done as the first serie but with more acurate measurement ( with a measuring tape), and measures were taken at 5 kmh mark before stopping.
1 - GPS is not of a great help in its great acuracy since it is an instant readup every second or so, but it is a plus to the SG (scanguage).
2 - Wind might have had some influence, not measurable, but felt stale air.
3 - Temperature at the end might explain the difference in stock measures, but seems a big difference.
4 - I am surprise the previous strong side wind test gave faster speed and distance. ( hotter weather too).
These data confirm my previous test. Moon caps seem to improve speed and distance. Front air dam (flat) decrease speed and distance. Even combine with the moon caps, it was about the same as stock A1. It is probably even worse since according to the A2 test with higher temp, the distance should have increase.
Moreover, the speed zone being pretty short, the % increase would be higher on a 5 km constant downhill. My test road is from a standstill, acceleration, max speed and then deceleration on the flat. For example, if I get a gain of 2% on the whole run (2.4km) then I could have 5-10% gain for the portion of fast speed ( not even half km).
Your comments would be appreciate like the first ones.
I think it might be possible to give your airdam a more aerodynamic shape, and get better results.
The datalogging function of the GPS could be useful for plotting time vs distance. That might show some things that are masked in the current way you are measuring. Plus, it's always fun to make graphs. :-)