Tested: front wheel skirts, A-B-A-B

[MetroMPG]

Based on basjoos' comments, and seeing several concept cars with front wheel skirts, I tuft tested and then even went ahead and built one front articulated skirt last fall. See:

Video: tuft testing front wheel skirts (Geo Metro)

Designing/building front wheel skirts v.1.0 (Geo Metro)

The tuft testing video certainly seems to show the potential benefit of skirts.

However, today I went out and did some coastdown runs, A-BBB-AA-BB with a pair of easily attached/removed cardboard skirts, and the results seemed to suggest that the style of skirts used actually increases drag slightly. EDIT: but the difference compared to the standard deviation of the data sets is so close as to be statistically meaningless.

Bummer, but based on this, into the waste bin they go!

I'll post the full data tomorrow.

[2000mc]

before trashing them, might try chopping off the rear 1/4 ...or 1/3 or 1/2 ...or just enough to cover the front and top of the tire, but let air trying to get out of the wheel well out the back.

[MetroMPG]

I had to throw out one A run because of shaky camera work on the speedometer: I missed one 100 km/h start, but caught it at 95 km/h.

So I'm going to re-measure all the recorded runs from 95 km/h and see if the additional run changes anything in terms of results (they were close) and standard deviation of the numbers.

I'm prepared to believe the original results, but since they're somewhat unexpected I want to make the effort to be sure they're as accurate as possible. Second opinion!

[MetroMPG]

No dice.

Even re-timing the recordings of the runs to include the A run I had to omit earlier, the wheel skirts do not come out ahead:



In this case, adding that previously thrown out non-skirt "A" run causes the standard deviation for the "A" set to be higher than the difference of the average of the A & B sets. Which means I can't really claim any statistical difference between the two sets.

In conclusion! Here are some possibilities:

1) The skirts, as I constructed them, really don't help even though they appear to show improved airflow in the tuft testing video. This could be explained because they increase frontal area in order to reduce Cd.

2) The testing was flawed. The amount of variability, particularly in the A (non-skirt) set, is pretty high.

One thing I can say for sure: trying to do accurate testing is hard and time consuming!

I think it's also fair to say: the front skirts (as I made them) certainly don't appear to be a slam-dunk aero band-aid for this car. Based on these results, it's probably reasonable to say they're so close to making no difference one way or the other that time would be better spent on other aero mods.