Civic DX Aero mods

[Regenerit]

Originally Posted by Sven7

You might try the particle board down to about 6" clearance and use lawn edging below that. Or the whole thing in coroplast. Remember if it extends below the lowest chassis/suspension component, it's adding frontal area.

I'm seriously questioning this reasoning, if the air dam is low enough to increase CdA beyond the lowest part, wont it also help to decrease the car's drag by limiting the amount of air that goes under the vehicle, despite it going past the lowest component?

And additionally, isnt CdA measured from the bottom part of the wheels up to the highest point in the car?

[aerohead]

Originally Posted by Regenerit

I'm seriously questioning this reasoning, if the air dam is low enough to increase CdA beyond the lowest part, wont it also help to decrease the car's drag by limiting the amount of air that goes under the vehicle, despite it going past the lowest component?

And additionally, isnt CdA measured from the bottom part of the wheels up to the highest point in the car?

The frontal projected area is measured from the shadow of the car cast perpendicularly on a wall directly behind it,from a parallel light source as the sun,or laser,from directly in front of the vehicle.
If 'protruberances' (mirrors) are on the vehicle when tested,they're frontal area cannot be included in the 'official' frontal area for test purposes according to the EPA rulebook,and the vehicles Cd will be artificially higher do to the unaccounted area.
The Cd will be based on actual test measurements,whether from coastdown,or from wind tunnel.

[Regenerit]

So how come some cars are touted to be aerodyamic when they have their modified bumper (such as in an insight) all the way down to inches off of the ground when in this would theoretically raise the Cd of the vehicle?

[California98Civic]

Wait. "Cd" is coefficient of drag. "Frontal area" is the total area of the car as "seen" by the wind. "CdA" is the product of the coefficient of drag times the frontal area. That's correct, right?

A mod can increase frontal area and yet decrease total drag (CdA) if it reduces the coefficient of drag. An example is a really large and low airdam, as in this thread.

(EPA calculates frontal area only to include the area minus the wheels and mirrors, but including any protruding suspension and such.)

This is correct, yes?

[BrianAbington]

well, I did a quick rebuild wednesday night. I shortened it by about an inch and a half. Only slightly touches on really steep dips but if I take them at an angle it's fine. all drive way approaches are fine now.

It worked really well, averaged mid 40's through the hilly parts of Iowa. The really flat 104 mile stretch from my friends house in Waterloo, to Dubuque I only needed 2 gallons of gas. Thats 50 freakin mpg... The stretch from dubuque to chicago and back to iowa will be lucky to break 40 because you have no choice to put your foot down so much in this freakin city.

[BrianAbington]

I forgot to mention...when I built the new one I cut the top 4" off of the prototype, and glued and screwed these strips to the bottom of the new air dam, and I also added a 14" wide piece that spans from top to bottom where the radiator inlet holes are. This double layering added some weight but it really added ton of strength. No flex at all but it still maintains the break point in the middle if I again have no choice but to hit something. Sitting stopped in chicago traffic, or 65mph on the hwy the temp stays perfectly normal.

[hawk2100n]

Originally Posted by California98Civic

Wait. "Cd" is coefficient of drag. "Frontal area" is the total area of the car as "seen" by the wind. "CdA" is the product of the coefficient of drag times the frontal area. That's correct, right?

A mod can increase frontal area and yet decrease total drag (CdA) if it reduces the coefficient of drag. An example is a really large and low airdam, as in this thread.

(EPA calculates frontal area only to include the area minus the wheels and mirrors, but including any protruding suspension and such.)

This is correct, yes?

Yes. You are correct. The only number that is really meaningful is the CdA. Increasing the A could decrease Cd significantly in the correct setup. And CdA allows you to compare cars (or aircraft where I learned this stuff) of different models directly.

[Sven7]

Originally Posted by Regenerit

So how come some cars are touted to be aerodyamic when they have their modified bumper (such as in an insight) all the way down to inches off of the ground when in this would theoretically raise the Cd of the vehicle?

The ideal ground clearance for car aero is around 5". Land speed racers, NASCARs and other performance vehicles are modified to have a lower, slanting nose in order to increase downforce. This is at the expense of drag. In the hypermiling world, the AeroCivic, at .17 Cd, has no ground-scraping air dam.

You can theoretically lower your CdA with a mod that increases A, but why do that when you can keep A constant and nudge the CdA up even higher? Three steps forward instead of two forward and one back.

Anyway, back to our regularly scheduled thread.

[aerohead]

Originally Posted by Regenerit

So how come some cars are touted to be aerodyamic when they have their modified bumper (such as in an insight) all the way down to inches off of the ground when in this would theoretically raise the Cd of the vehicle?

We probably need to know the context in which the term 'aerodynamic' is used.
Anything which the air blows across,or moves through the air is 'aerodynamic.'
Tornadoes and hurricanes are aerodynamic.So are manhole covers and chewing gum stuck to the underside of a park bench.
If they're implying 'low-drag' then their Cd better be no more than 0.15 to even qualify.
All vehicles will exhibit aerodynamic qualities peculiar to their specific architecture.So it's tough to make general 'one-size-fits-all' recommendations about streamlining.
For those professionals who streamline for a living,they have encountered enough situations where a 'deeper' front airdam has increased drag,that they published peer-reviewed scientific papers addressing their concerns and gave lectures at the annual engineering conferences to warn other engineers and designers of the hazards of going 'too far.'
You'll know if your car responds well and if you can live with the modified driving required to keep the dam alive.
The lowest drag cars never have 'deep' airdams,but rather, active suspension and just lower or adjust rake for the lowest Cd.

[aerohead]

Originally Posted by California98Civic

Wait. "Cd" is coefficient of drag. "Frontal area" is the total area of the car as "seen" by the wind. "CdA" is the product of the coefficient of drag times the frontal area. That's correct, right?

A mod can increase frontal area and yet decrease total drag (CdA) if it reduces the coefficient of drag. An example is a really large and low airdam, as in this thread.

(EPA calculates frontal area only to include the area minus the wheels and mirrors, but including any protruding suspension and such.)

This is correct, yes?

For your second question please see #12(permalink) above