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:

http://ecomodder.com/forum/showthrea...etro-5403.html

http://ecomodder.com/forum/showthrea...-0-a-5462.html

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.

[orange4boy]

I have been tossing this around in my mind for a while now trying to figure out why your front skirts didn't reduce drag and I have a theory.

Let's see If I can explain.

Short version: They would probably show an improvement in combination with a front air dam.

Here is my attempt at the why. High pressure air flowing under a car is forced by the body overhang around the wheels and between them. The wheel wells are a path of least resistance escape to the lower pressure area on the sides of the car. If you reduce the amount of air coming into the crunch zone under the overhang to the area left between the wheels with an air dam, the rest of the air will go over the top and sides of the car.

If you just close off the wheel wells without changing anything else, you are just increasing the pressure gradient under the car and closing off an escape route, albeit a dirty one. I think this is the reason all the newer front ends you see have the higher middle section of the air dam. Block off the air that would have hit the wheels and leave enough to go between the wheels.

Look at the picture of the Opel Calibra in this report.(Thx lunarhighway) The smoke shoots out from the middle to the sides toward the wheel at a 45˚ish angle and ends up going through the wheel wells and around the wheels into turbulence on the sides. It wants to go from the higher pressure to the lower but the wheels are in the way. Send it over and around rather than trying to ram it under the car.



That's my attempt at an explanation of my thoughts on front wheel skirts and air dams.

[NeilBlanchard]

Yes, I was wondering essentially the same point: the Blackfly has a sloped surface under the bumper (IIRC) and so it has a fair bit of air flowing under the car. Some of this air may be escaping out through the front wheel opening? If so, then the front skirts may be obstructing it and causing more drag than they save. Maybe with a full underpan and/or a front chin spoiler, the front wheel skirts could then lower drag?

[Rokeby]

The smoke path shown in the photo confuses me to no end.

Just as it gets to the bumper, it veers sharply to the right, directly behind
the front tire air deflector -- the black inverted eyebrow shape. ( I take it
that the Coanda effect is at work here once the air stream contacts the back
of the deflector.0

OK. So what in the heck is the air deflector doing? I would have guessed
that it was intended to delfect air to both the outside and inside of the tire.
Apparently not for this configuration of deflector.

Somewhere, probably here on EM, I have read that the front tire deflector's
size and distance from the tire is hyper critical. The deflector looks grossly
oversized to me, it as least twice the size of the one on my Prius, and
considerably farther forward, away from the tire.

Is it possible that this picture was originally intended only to show the
negative effect of an improperly sized/placed deflector?

Also, I see no evidence of rollers under the wheels. I take it then that the
wheels are not turning. Wouldn't that further minimize a widespread
applicability of the pictured air flow?

[orange4boy]

Also, I see no evidence of rollers under the wheels. I take it then that the
wheels are not turning. Wouldn't that further minimize a widespread
applicability of the pictured air flow?

It may be a conveyer type rolling surface. The wheels would look stationary at a the shutter speed to capture the smoke detail.

Just as it gets to the bumper, it veers sharply to the right, directly behind
the front tire air deflector -- the black inverted eyebrow shape. ( I take it
that the Coanda effect is at work here once the air stream contacts the back
of the deflector.0

The deflector looks like it is parallel to the airstream. If the smoke trail was further to the right it would probably flow in front of the deflector.

This picture is so helpful in visualizing under car airflow. My point is that there is only so much space between the wheels for all that air to go once it's under the overhang so it wants to go sideways into the wheels and ends up going through the wheels and wells. It's pressurized at that point and moving more slowly as well so once it goes between the wheels it has to try to accellerate to fill the low pressure areas behind the car. All that slowing down and speeding up wastes energy. If it was directed over and around the path is smooth and easy in comparison.

I also got to thinking of a car as if it were an air plow. A snow plow would never be designed to leave a compressed swath of snow under the bumper and between the wheels to drive over. It would drag on the undercarriage and slow it down and the wheels would ride up on it adding more drag. A snow plow gets all of the snow out of the way right now, right up front. Not an exact example but you get the idea.

An optimal air dam would leave just the right volume of air to go under the car. Enough to calmly go between the wheels. The wheel skirts would then have the job of smoothing the air as it passed over them around the sides. It seem like to me now that the air dam is doing the heavy lifting and the wheel skirts are cleaning up after. An active air dam would be the best so it could be lifted when needed.

[moonmonkey]

so maybe front skirts hurt in applications that dont have the front end modification like basjoos? do we have any 100% proof that they do reduce cd, in any application where they are an afterthought, im just asking because i want to know if i should go ahead with making some for my echo. maybe tire spates and skirts working together like has been suggested, but how big and what shape/distance from the tire for the deflectors/spates, my brain hurts. I will make and test the rearskirts first anyway.