Air dam or no dam - and height?

[sallen]

Morning all,

I just got my new car and it does 20mpg (US) - so, improvement is required!

I want to add an air dam on the front, but the front is not a consistent height. The bottom lip of the bumper is divided into thirds.

The outer two thirds have a ride height of 20cm (7.9in) and the centre third is higher at 24cm (9.4in).

Now, I wonder whether the centre third being higher is actually helping air get under the centre of the car at a lower pressure (and so less drag).

I also wonder whether an air dam going as low as I can 10cm or less, depending on local roads. is the answer?

[ChazInMT]

The less air under your car, and the more air you can get to go around the sides, the better. Over the top of the car is 2nd best, with sides being preferred. Can you post a picture of what you're talking about? Preferably at about ground level around a foot above looking at the spoiler area in question.

[Ecky]

In the simplest terms, total drag is coefficient of drag (Cd) times area (A). The underside of a car is very aerodynamically dirty, and an air dam increases frontal area (A) to direct air away from the underside, significantly improving the coefficient of drag (Cd). A belly pan can reduce Cd without increasing area. I imagine the less air going under the car the better, but it might not actually work out that way on some vehicles with huge ground clearance.

[BamZipPow]

Sometimes, manufacturers design things just to look sporty so it will influence the buyers in buying the vehicle. Look at the rear diffuser designs on modern sedans as an example.

[oldtamiyaphile]

Originally Posted by sallen

The outer two thirds have a ride height of 20cm (7.9in) and the centre third is higher at 24cm (9.4in).

Now, I wonder whether the centre third being higher is actually helping air get under the centre of the car at a lower pressure (and so less drag).

IIRC the Opel Calibra which had a very low drag for it's time had such a set up (0.27cd?).

Perhaps @Aerohead can explain, otherwise I'll check if Hucho says anything about it.

[oldtamiyaphile]

Hucho says Opel saved 0.003CD by cutting a ~930mm section from the centre of the Calibra airdam. Drag increased either side of this.

Hucho doesn't seem to go into detail as to the hows and whys of this.

[Vman455]

Originally Posted by oldtamiyaphile

Hucho says Opel saved 0.003CD by cutting a ~930mm section from the centre of the Calibra airdam. Drag increased either side of this.

Hucho doesn't seem to go into detail as to the hows and whys of this.

He does a little:
-"Apparently, by the former measure [the cutout in the airdam] the obliqueness of the flow to the front wheels was attenuated."
-"The central cutout in the spoiler may improve the flow at the end of the underbody, so that a diffuser can work more effectively."

[freebeard]

Consider also that a spat shielding the front of the tires is beneficial, but too low a center results in 'high-centering' on dirt and gravel.

[sallen]

Thanks, so I guess I will have to test and see. I've measured up the length of plastic requred and will see if that matches my donor bumper, it's from a smaller car so maybe I'll have to patch it together at the ends or somthing.

@ChazInMT I've attached a picture as you asked. It's probably a Calibra-esque shape as it's from the same styling era.

I should also say that spats are factory-fitted in front of the front and rear wheels on this car.

[aerohead]

Originally Posted by oldtamiyaphile

Hucho says Opel saved 0.003CD by cutting a ~930mm section from the centre of the Calibra airdam. Drag increased either side of this.

Hucho doesn't seem to go into detail as to the hows and whys of this.

A guess would be that the sides are acting as 'spats',limiting air access to the tires and hour-glass belly openings.
I don't have any specific data.
The other thing is that when you enter a driveway from the street at an angle,the reduced center height of the airdam may protect it from a ground strike when going over the vertical apex of the driveway ramp.A practical concession.