Massive air dam on '98 Geo Metro (Firefly), A-B-A coastdown test; Update: version 2

[MetroMPG]

Did some CAD work today (Cardboard Aided Design).



For fun, vote on what you think will happen with this massive cardboard air dam in A-B-A testing....

1) Reduced drag vs. normal grille block
2) Increased drag vs. normal grille block
3) No significant change
4) AAAAAHHH! MY EYES!!

(Multiple answers permitted.)

Notes:

1) Same size openings for cooling air in the grille block vs. air dam.

2) There's just over 8 cm / 3 inches of clearance

3) I angled it slightly to lower the stagnation point

4) the floor of the car has about 15 cm / 6 inches of clearance

5) the lowest suspension points have approx 12 cm / 4.7 inches of clearance

UPDATE! For coastdown test results, see post #29





Compared to stock height/angles:



- - - - -

UPDATE - July 7, 2015: less massive, less ugly, more durable air dam v. 2.0 is now on the car.

See post #89 for details.

[2003protege]

I think the new dam will send more air to the sides and top of the car. Hurry up with the results!

[MetroMPG]

(P.S. I should state proactively that I realize lowering and a smooth belly pan is the way to go, but am looking at this as a quick 'n' dirty mod for "competition purposes". )

[cfg83]

MetroMPG -

I voted "Reduced drag vs. normal grille block" because that's what I want to happen.

CarloSW2

[tasdrouille]

Darin, if you are not completely happy with the results, hack the airdam away in the middle in order to transform it more into wheel deflectors. That way you'll reduce frontal area, but you'll also divert airflow away from the wheels, which play a significant role in drag.

I still vote reduced drag with the full dam.

[MetroMPG]

You hit the nail on the head, Martin: the only reason I'm somewhat reluctant to predict that this will be a success is that it increases frontal area so significantly. It easily adds more than a square foot to the car.

It certainly breaks the "don't make your air dam lower than the lowest hanging underbody parts" rule of thumb that is sometimes mentioned.

Yet I often see OEM drag-reducing dams that extend below the lowest underbody level of production vehicles, which is why I decided to go big or go home.

A-B-A results to come tomorrow.

[Frank Lee]

Reminds me of a college project/paper I did back in the day.

Fully kitted out a Rabbit with airdam, nice sloping grille block, wipers "up" and antenna remove, mirrors folded in, and rear wheel skirts. Found the flattest, emptiest piece of road I could with the calmest conditions I could wait for and made several runs timed with stopwatch.

Wanted in the worst way to report improvements!!!

Buuuuutttt... presented the numbers as they were and if they weren't lying, there was no statistical improvement.

Could be my designs weren't good. Well they weren't all that unique; they followed convention. Does this sort of thing need lots of work to be optimalized?

Could be my testing wasn't accurate enough. But then, if the improvement was that small an increment, what good is it?

[lunarhighway]

i'd be somewhat surprised if this would actually reduce drag in it's current form. but aerodynamics are a complex matter that sometimes go against what one would expect

i'd either reduce the overall height by about half or reduce the height only in the center section.

on the well documented 0.26 Cd calibra it was found that raising the center of the airdam actually reduced drag als although this car had a rough engine cover and some large cavities patched up, it by no means had a perfectly flat underside.

after seeing some new low drag mercedes designs where the lower bumper edge smoothly curves to the undertray i'm even considdering shaveing off the small lip spoiler on my car in the center.

i did some quick calculations and perhaps you might want to substiture more accurate numbers if you have them

but from what i could google up i believe the dimentions of the car are

1.585 meter wide and 1.340 height, according to carfolio

the previous model had a Cd of 0.32 wich looks about right for this car too

so 1.585x1.340=2,1239 sqare meter now from the back of my head i think it's about right to take 81% of this to get the actual frontal area (correct me if i'm wrong... wich i likely am)

that gives a frontal area of 1,72 sqare meter

1.72X0.32 = 0,5504 CdAm² that's pretty good

i estimte the airdam adds about 20cm of height, so that makes for0.2X1.585=0,317 square meter
i almost forgot the dam covers the tires and so the area of the tires have to be substracted as it was already figured into the original value

if the dam is as high as the exposed tires and they're both about 20cm wide than thats 0.2m x 0.2m x2
that's 0.08
0,317-0.08=0,237
wich adds up to a total of
1,957


what we can do now is calculate what the maximum Cd of this new configuration has to be to realize a break even situation the actual Cd has to be lower than that to get an actual improvement over the base car

1.72X0.32 = 1,957xA => (1.72X0.32)/1,957 = A = 0,28

that means the new drag coeficient has to be lower than 0.28 in order to achieve any improvement
that mean that the dam has to improve the Cd by more than 12,5% for it to have any positive effect

anyway it's all a bit generic but i wanted to see if the figure was remotely achievable of not, but honestly i don't know if this is realistic... but it seems a lot...

[Matt Herring]

Looking forward to seeing the "Fly" in Fonda this weekend and all the mods you have on it!

[aerohead]

Darin,you were the one to remind me that CAR and DRIVER's low-hanging Pinto front airdam still registered lower drag( ie.better mpg),so I had to give it a yea.Bondo did get in trouble on his F-150 with a "low" front airdam,and only restored mpg after clipping it back higher.Your clearance violates the SAE recommendations for "approach" clearance angle but you know that and I'm sure your driving technique will compensate for it.---------- If you get in trouble,a pair of scissors will remedy it.Anxious to here about numbers.