general questions .. dams, pans, skirts, spats, tails etc.

[dmac257]

OK .. I have been looking for recommendations for placement of air dam, belly pan, side skirts, wheel spats, wheel boat tails with respect to ground height.

The only thing I found for air dam is "as low as the lowest under belly part .. or lower" However I have seen several vehicles with air dams almost touching the ground. Is there an ideal ground clearance as far as aerodynamics is concerned?

Belly Pans also nothing about an ideal height and I could not find anything about the contour of the pan itself (ie can it curve from the bottom of the door panel under the vehicle to the other side or is it better to be parallel to the road) .. but I found a suggestion that if full belly pan .. higher is better than lower.

If both air dam and belly pan the air dam doesn't need to go below the pan as everything behind the dam is smoothed by the pan?

Side skirts along the side between the front and rear wheel. Would these extend down below the center of the wheels and if so how far down? Also same question for front of wheel spats and behind wheel boat tails. Is the goal to part the air in front of the front wheel .. guide it past on both sides with the skirt to the rear wheel .. then recombine behind the rear wheel.

Would the ideal shape of the underside of the vehicle be a smooth tunnel parallel to the road that begins to rise up towards the rear bumper just before recombination behind rear end (small turbulant wake).

I am thinking to do as much as I can .. and would like to know how these all fit together for end product so I can plan how to proceed

Don

[aerohead]

Originally Posted by dmac257

OK .. I have been looking for recommendations for placement of air dam, belly pan, side skirts, wheel spats, wheel boat tails with respect to ground height.

The only thing I found for air dam is "as low as the lowest under belly part .. or lower" However I have seen several vehicles with air dams almost touching the ground. Is there an ideal ground clearance as far as aerodynamics is concerned?

Belly Pans also nothing about an ideal height and I could not find anything about the contour of the pan itself (ie can it curve from the bottom of the door panel under the vehicle to the other side or is it better to be parallel to the road) .. but I found a suggestion that if full belly pan .. higher is better than lower.

If both air dam and belly pan the air dam doesn't need to go below the pan as everything behind the dam is smoothed by the pan?

Side skirts along the side between the front and rear wheel. Would these extend down below the center of the wheels and if so how far down? Also same question for front of wheel spats and behind wheel boat tails. Is the goal to part the air in front of the front wheel .. guide it past on both sides with the skirt to the rear wheel .. then recombine behind the rear wheel.

Would the ideal shape of the underside of the vehicle be a smooth tunnel parallel to the road that begins to rise up towards the rear bumper just before recombination behind rear end (small turbulant wake).

I am thinking to do as much as I can .. and would like to know how these all fit together for end product so I can plan how to proceed

Don

Don,The Society of Automotive Engineers have some minimum criteria for ground clearance for 'fixed' suspension cars.
Ahead of the front wheels they recommend nothing below a line projecting forwards and up at a 16-degree angle.
Behind the front tires and in front rear tires they say to respect a 10-degree angle.
And behind the car,also a 10-degree angle.
Anything hanging lower is at risk of being scrunched at the nearest driveway ramp.
Many states have laws forbidding any portion of the car to be below the bottom of the wheel rim bottom.If say,you made wheel fairings which were lower,the first pot-hole might rip them right off.
The belly pan should maintain the highest allowable ground clearance.
The diffuser portion at its rear will be determined by where it begins.
I will be able to tell you more about the airdam/bellpan question in a few weeks.On my Civic,even with a smooth belly,the car got better mpg with an airdam blocking air from underneath the car.All the lowest-drag,highest mpg concept cars block the underside even with perfect bellypans.
Side skirts should be even with the belly.

[Big Dave]

Aerohead is right about the SAE recommendations, but there are always exceptions.

My experience:
I made an air dam out of nearly indestructible (and hard to fabricate) 3/8" ripstop conveyor belting.

I got greedy and (first iteration) left it 3/4" above grade. From a fuel economy standpoint, it worked like a champ. But it dragged a lot. No damage, just that annoying sound.

Next try, trimmed it back to 1.5" above grade. Again, excellent MPG performance but it still dragged.

Next try 3" above grade. More or less same result: Good MPG, dragged a lot. No damage, just annoyance.

This year I trimmed it back to 8" above grade in front of the wheels and 12" above grade inside the hanging-down suspension parts. Big mistake. MPG dropped off but at least it doesn't drag or look so dorky.

If it doesn't improve MPG, what good is it?

[dmac257]

Originally Posted by aerohead

Ahead of the front wheels they recommend nothing below a line projecting forwards and up at a 16-degree angle.
Behind the front tires and in front rear tires they say to respect a 10-degree angle.
And behind the car,also a 10-degree angle.
Anything hanging lower is at risk of being scrunched at the nearest driveway ramp.
Many states have laws forbidding any portion of the car to be below the bottom of the wheel rim bottom.If say,you made wheel fairings which were lower,the first pot-hole might rip them right off.
The belly pan should maintain the highest allowable ground clearance.
The diffuser portion at its rear will be determined by where it begins.
I will be able to tell you more about the airdam/bellpan question in a few weeks.On my Civic,even with a smooth belly,the car got better mpg with an airdam blocking air from underneath the car.All the lowest-drag,highest mpg concept cars block the underside even with perfect bellypans.
Side skirts should be even with the belly.

So these angles are based on the bottom of the wheels to prevent ripping parts off the underside of the vehicle?

My state (Connecticut) states minimum ground clearance is 4" and I am plenty high enough for that. Are you saying that the air dam should go down as low as either 4" (state limit) OR the 16degree angle, whichever is higher?
Would spats also follow this 16degree angle forward from the 4" clearance height? Would boat tails following the wheels angle up at 10degrees from 4" also?

As for the belly pan, you say highest allowable ground clearance. Ok, would this be as flat as possible from side to side or would curved higher on edges work better/worse? In conjunction with skirts it looks like you are saying flat accross and build down with the skirts to level of the pan. Is that right? And if I do full belly pan should the air dam in front extend below the pan or stop even with the pan?

And part I dont understand at all:
"The diffuser portion at its rear will be determined by where it begins" Heh, I dont even have a clue on WHAT to ask about this comment ;-)

I am going to go see if I can pick up some Clorplast tomorrow and will begin trying to figure out how to attach the big pieces so they are easily removed.

Don

[dmac257]

Originally Posted by Big Dave

My experience:
I made an air dam out of nearly indestructible (and hard to fabricate) 3/8" ripstop conveyor belting.

I got greedy and (first iteration) left it 3/4" above grade. From a fuel economy standpoint, it worked like a champ. But it dragged a lot. No damage, just that annoying sound.

Next try, trimmed it back to 1.5" above grade. Again, excellent MPG performance but it still dragged.

Next try 3" above grade. More or less same result: Good MPG, dragged a lot. No damage, just annoyance.

This year I trimmed it back to 8" above grade in front of the wheels and 12" above grade inside the hanging-down suspension parts. Big mistake. MPG dropped off but at least it doesn't drag or look so dorky.

If it doesn't improve MPG, what good is it?

State law here says min clearance for anything attached to car is 4" but I have seen dropped vehicles MUCH lower than that. I think they have air bags to lift them up for inspections or something. I dont want to go to that much expense. Just a little Clorplast in front to see if it works or not.

You didnt say if you also have a belly pan underneath as well as the air dam?

Don

[euromodder]

Originally Posted by dmac257

And part I dont understand at all:
"The diffuser portion at its rear will be determined by where it begins" Heh, I dont even have a clue on WHAT to ask about this comment ;-)

It's a physical limitation, depending on the structure and stuff underneath the car.
You can't start angling up the bellypan if some part of your car gets in the way.
The car's structure also limits how much you can angle it up from the belly pan to the very rear of the car.

[dmac257]

Well, I ordered two 4X8 sheets of black Coroplast to be included with the next order from the sign shop. I will try and make an air dam and belly pan between the dam/bumper and front axel. I don't know what I can attach it to at this point but will worry about that when I actually start cutting the pan itself. Anyone use Dzus Fasteners before?



I was thinking a couple of angle brackets and Dzus fastener to make the pan easily removed. Same for rear wheel skirts.

Don

[BamZipPow]

Don't be afraid to make yer own frame to make attachment points to. That's what I did fer making my belly pan. Fer my airdam...I just extended the valance down using thread all and sched 40 1/2" pipe.

[dmac257]

Originally Posted by BamZipPow

Don't be afraid to make yer own frame to make attachment points to. That's what I did fer making my belly pan. Fer my airdam...I just extended the valance down using thread all and sched 40 1/2" pipe.

I am not afraid to make a frame and I realize that I will need to support the under pan to keep it from flapping around. The back of the bottom of the air dam will also need to be supported.

I did a google search for thread all and got lots of sewing thread sites .. is that just threaded rod? got any pictures of your dam? My front bumper angles back about 80 degrees from vertical. Should I make the dam vertical or continue the 80 degrees down to clearance height?

Don

[BamZipPow]

Extended/dropped air dam thread and Belly Pan thread...

Yes...thread all is just threaded rod.