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

[tasdrouille]

So, are you gonna leave it at that or are you willing to make a few cuts to improve the design? I'm betting reducing frontal in key places is gonna help.

[wagonman76]

Looks like I'm in a bit late, but I figured the air dam would give some noticeable gains. Based on my experience with an air dam.

[MetroMPG]

Martin - I may trim up the middle section a little bit, but not very much because it would only be a guess without more testing. Shot in the dark! And I don't have time to do more runs/number crunching before the weekend.

Quick calculation: at 50 km/h (30 mph), 1.8 additional seconds of coasting time equals 25 more meters (82 feet). Of course that won't be the actual additional coasting distance from the air dam, since (a) the average speed of the 100-50 coast will obviously be higher than 50 km/h, and (b) the air dam's effectiveness decreases with speed. But I don't feel like trying to figure out the math. Math is hard.

[lunarhighway]

i found some details on the calibra bumper with with numbers about the size of the cutout

of course it did not have such a large dam as your can but it's more detailed than what i've found about other cars

the original article is in german
but i've ran it trough yahoo bablefish wich makes it somewhat readable in english with some goodwill and imagination.

interestingly the research diagrams show that they did a great deal of optimising, increasing the size of the cutout untill they noticed an increase in drag again.

perhaps it's doable to make a large cutout in the center, than tape a flat piece of cardboard on it that you can raise and lower, first in a few large steps to see where drag gets worse and than refine it untill you get the optimal height

[Big Dave]

Your cardboard air dam looks a lot like the conveyor belt rubber one on my truck. Big, ugly, and mine was good for about 0.75 MPG.

Mine was down to 1.5" above grade. The road trimmed it to where itwouldn't drag. I may cut out the middle a little bit and increase clearance to 3 inches.

[MetroMPG]

Big Dave - what did that 0.75 mpg represent as a percent increase?

[Big Dave]

About 3%.

[robertwb70]

I've never subscribed to the "not lower than the lowest under-hang" air-dam guideline, just seems counter intuitive to me. Probably that rule is intended to prevent damage to an air-dam since if it isn't lower than stuff that's already under the car and has never contacted the ground the air-dam should be safe. But we all know that where these low points are in relation to the wheels is very important as well.

I've been thinking about making my air-dam larger like that since on my car the bumper cover actually directs quite a bit of airflow under the car because of its shape. Good to see it works well.

[MetroMPG]

Thanks, Big Dave. 3% is pretty much what my fuzzy math estimate tells me this cardboard would be worth on my car.

Interestingly, it's possibly better than the 2% improvement I measured with my A-B test for the full belly pan.

Of course, still on my to-do list is testing lowering the suspension. In combination with a smooth undertray and tire deflectors, that may be the best total solution.

[IsaacCarlson]

It starts at the bumber in the middle and then angles back is it makes its way to the wheels!!!!! I KNEW IT!!! GOING TO TRY AN ANGLED AIR DAM!!!


Ike AND THANKS FOR THE PIC!!!!!!