Pick up almost any import tuner type magazine these days and you'll see ads for aerodynamic devices for our del Sols. There's also seems to be lots of pictures of girls in skimpy clothing too, but I digress. Aerodynamic add-ons are the rage. Not only can a 'Soler make their car look unique in a world of stock, but with the right choice of these devices they can significantly improve various aspects of their vehicle's performance. The performance issues concerned about in this tech article are acceleration, braking, cornering speed and stability, and top speed and straight-line stability. The reader of this tech article should bear in mind that the comments about how these devices relate to del Sols is founded in known aerodynamic principles and a general knowledge of aerodynamics. However, without having wind tunnel test data for each device, these comments should not be taken as absolutes.
The principle aerodynamically produced forces concerned about here are lift and drag. Lets have a look at what they are and what causes them.
Lift causes less pressure to be exerted downward at the contact patches where the tires and road surface meet, than there would be if the vehicle were sitting still. It counteracts to some extent the static1g loading caused by the earth's gravity, and artificially lessens the vehicle's weight in a downward direction. The result is less adhesion of the tire to the road, which translates to less grip and less stability at higher speeds.
What causes this lift? Our del Sols, like most production cars when viewed in profile, have a shape that's similar to an airplane wing. The bottom surfaces are generally straight from front to rear, but the upper surfaces start relatively low at the front, curve up higher and then drop back down again in the rear. By the way, if this sounds a little like Ann Elk's dinosaur theory to you, you're as old as I am (for those of you would like to delve further into this obscure reference, go to Ann Elk's Theory of the Brontosaurus). The following illustrates:
Airflow over Airplane Wing
Airflow over Brontosaurus
Hatchbacks, station wagons and true convertibles are exceptions to this shape to a degree. In all cases though, air traveling over the vehicle has a longer distance to cover than air traveling under the vehicle. Airplane wings affect airflow that way by design. As the wing is propelled through the atmosphere, the air it encounters has to go over, under or around it. Air going over the wing accelerates. Air going under the wing slows down. Bernoulli's principle (widely recognized as the definitive principle of fluid dynamics by four out of five dentists) asserts that air traveling at a higher velocity has a relatively lower pressure, and that lower velocity air has relatively higher pressure. Its this pressure differential that causes lift. Most production cars (including our 'Sols) generate lift when going fast for the same reasons an airplane wing does. The stock ride height of a del Sol does little to help keep air from getting to the underside of the car, as does the lack of a factory supplied front air dam. The amount of lift generated is made even worse by the the air going under our cars getting bounced around by the all the engine, suspension and exhaust pieces, becoming turbulent and slowing the airflow down even more. The lift that is caused by all these factors is detrimental to our above mentioned performance issues.
Because the upper surfaces of most cars (wings and brontosauruses too) eventually drop back down and stop, the accelerated air coming over the roof or the back of the vehicle tends to separate from the surface of the vehicle when the vehicle is at speed. Not only does this result in a lower pressure area at this location, but it results in turbulence as well. Turbulence in this case is when the air no longer flows in a laminar (layered) fashion, but instead tumbles about in whirlpools and eddies in the general direction it was already going. You've probably noticed this to a great degree when passing a tractor-trailer vehicle on the highway (especially when you have the roof off). Approaching the 18 wheeler from behind, all that air that's trying to blow your brim off gets progressively more violent until you've passed the vehicle. This same kind of turbulence above the trunk and behind the rear of the vehicle causes drag, and to some extent tries to hold the vehicle back from its direction of travel. The roof structure of our 'Sols is not very aerodynamic compared to most sedans, hatchbacks, station wagons, sports cars and dachshunds. The recessed rear glass presents an almost vertical surface for the air coming over the roof to try and follow. The two center pillar trim panels to either side of the rear window area keep air flowing along the side of the vehicle from helping to fill in that low pressure area too (see the above "Airflow over del Sol" illustration). All of this conspires to create turbulence and low pressure directly above the trunk lid. Fortunately, our 'Sols have a long, relatively high trunk lid, that helps to a great extent to smooth out that turbulence and try and get the air flowing in a laminar fashion again. There is not a production vehicle made that does not have some measure of drag at some speed, and usually the higher the speed, the more pronounced the drag. The higher the drag, the lower the top speed, all other things being equal.
The only way any of these things can improve the performance of our del Sols is to somehow offset or eliminate lift, and/or somehow offset drag. Here's a list of what these things are and their purpose to that end, and what it takes for them to be effective. Bear in mind that these descriptions are relative to how these devices would be used on a street driven del Sol of medium to high performance, and not an Indy car or Top Fuel dragster!
Automotive wings are inverted airfoils usually located at the rearmost top part of the trunk lid. They can be made using any of the usual go-fast materials such as fiberglass, plastic, carbon fiber or metal (usually aluminum). They usually span at least the width of the trunk lid, and are usually supported by two pedestals. These pedestals can also be made of those materials and are placed equidistant from the centerline of the vehicle. The pedestals can even be an integral part of the airfoil itself (see the Supra and Mid style wings below).The airfoil cross section can be of several different shapes, but to be effective must generate downforce. True inverted airfoil designs act just like an upside-down airplane wing, causing a lower pressure area to be formed beneath the wing and a higher pressure area to be formed above the wing. Some styles though may simply have an oval cross-section, which creates no pressure differential at all (but may increase drag). For a true wing to work, there needs to be enough area beneath the airfoil for a low pressure area to form. In a regular airplane wing, the sky's the limit above the airfoil where you want the low pressure to be. On a del Sol though, how effective the area below the airfoil where the low pressure area needs to be is dictated by how far the airfoil is located above the trunk lid. The height that the airfoil is placed over the trunk surface can vary from none to a couple of feet depending upon the style. To be a true wing these airfoils must be separate from the body of the vehicle. If they just sit on the trunk lid they become a spoiler or at the very least an interesting conversation piece fo' yo' buds, guaranteed to generate lots of finger-pointing and chuckles. The higher the properly designed wing, the more effective it will be in offsetting lift. Once a wing is located above the path of turbulent air, there's no performance advantage to placing it higher. The distance from the leading edge of the wing to the trailing edge of the wing varies by style, but the greater this distance, the more effective the wing is at producing downforce. By the way, its generally considered that the higher the wing's downforce producing capability, the greater the amount of drag that wing will produce. An aerodynamic engineer's dream is to produce a high-lift, low-drag wing.
Since del Sols are front-wheel-drive, one thing a typical rear-mounted wing won't help the car do is accelerate by providing better traction. The only way it will hurt acceleration though is by possibly adding weight to the car. There might be an extreme case where a very effective or very large wing would produce enough actual downforce at speed to push the car's rear end down on its suspension and give the car a "nose high" attitude. Frankly, there's probably not a del Sol on earth though that has enough torque at the speed where such a condition would cause a traction problem.
Given all that, lets look at some of the more popular styles
of wings available for the del Sol.
Many times wings have been called spoilers, but its simply not the case. Like wings, spoilers are usually located at the rearmost top part of the trunk lid (hatchbacks of course, are an exception). Unlike a wing however, the base of a spoiler attaches directly to the vehicle, usually along its entire width. Spoilers can be found made of plastics, fiberglass and rubber. A spoilers function, at least on the street, is to be an extension to the body of the vehicle. As moving air detaches itself from the rear of the car, it does so in a turbulent manner. The longer the air can stay attached to the car in an organized airflow, the less drag the car will have. The spoiler can be designed to manage that airflow either by simply extending the rear of the car a few inches, or by presenting itself to the airflow slightly above the trunk lid to try and take turbulent air and smooth it somewhat before it detaches from the rear of the car. In the del Sol's case, not only did the factory never offer an optional spoiler (like it did on earlier Preludes), but the aftermarket does not presently offer one either. Although a spoiler would be a useful item for the del Sol, automotive fashion has dictated only wings will be offered. It would be nice to see a del Sol at one of the many import show-offs show up with a nicely designed and executed spoiler, instead of the usual wing de jour. Below you see artist's renditions of what a spoiler for a del Sol might look like.
An air dam's primary purpose is simple, and that's to keep as much air from getting under the vehicle as possible by directing air either above the vehicle or around it. To be able to do that though and weather all the curbs, road junk and little woodland creatures that gets thrown at it without coming apart is tough. Even though some air dams are still made of fiberglass, many are now being made of nice, flexible polyurethane. Since aftermarket bumper covers are marketed as air management items in the same vein as air dams, they will be discussed together here. As far as performance goes, a bumper cover should only route as much air to internal components as necessary, and direct all other air above or around the vehicle. Bumper covers in the recent past were styled with that kind of air management in mind. The trend lately seems to be styling more weighted towards the appearance end of the spectrum, and away from the performance end. Many of the newer styles have huge openings that direct most of the frontal air directly into the body cavity. Although these fronts ends look "aggressive", they're a poor choice for increasing performance unless specifically used to direct air to intercoolers, oil coolers or brake ducts. Even then some of their performance may be traded off by the parachute-like affect these orifices have on actually moving the car through the air. Fortunately, for performance sake, there are still some aftermarket manufacturers out there that achieve balance with their designs, offering aerodynamic advantages while allowing the del Sol owner to personalize their ride.
By keeping as much air as possible from getting underneath the car at the front end, air dams and bumper covers seek to eliminate the major contributor to lift. With less air flowing underneath the vehicle, the air moving over the top of the vehicle now becomes less of a relatively low pressure area. This greatly enhances most performance issues discussed here. Other than lowering your 'Sol, the installation of a functional air dam or bumper cover is probably the most effective aerodynamic modification you can make to your 'Sol.
Here's some of the more popular air dams and bumper covers available for the del Sol. Bear in mind that many of these do not have a "style" name, but only a brand or product name. For the sake of fairness to all manufacturers whose products may have been excluded here, no product names are given.
If you like keeping your del Sol OEM and don't want to go the aftermarket route in getting a air dam, Honda offers a Civic air dam that fits just right, and on a lowered 'Sol is probably as effective performance wise any of the aftermarket offerings at blocking airflow to the underside of the car. See Slater's excellent "Add a Civic Lip..." article in the TDS "How to" section.
While a front air dam helps to lower the air pressure under the vehicle by blocking airflow, that lower pressure now offers an invitation to higher pressure air around the sides of the vehicle. Side skirts, if they actually drop lower than the stock lower sill, help to keep the higher pressure air from rushing into that area. Even though side skirts are offered separately by some manufacturers, they're most likely purchased in a complete body kit. Examples of some side skirts follow.
The purpose of a rear skirt is to try and smooth turbulent air coming from underneath the rear of the vehicle. Some rear skirts use a spoiler-like flat edge to do this, where others incorporate a separate wing-like airfoil very low beneath the rear bumper cover. Our del Sol's actually come with the spoiler type that's an integral part of the rear bumper cover, although its placed high enough above the ground to where its probably not very effective. The placement of this flat edge or airfoil is a compromise. Too high and it doesn't see enough turbulent air to be effective, too low and ground clearance becomes an issue. Aftermarket rear skirts for the del Sol are also most likely purchased in a complete body kit.
Don't believe the hype. If you're interested in improving the performance of your del Sol through aero mods, choose those mods wisely. Very few of the devices available for a del Sol have ever seen a wind tunnel test. If a manufacturer says their product will make your car go faster, handle better, or stop on a dime, ask that manufacturer for hard data to back it up.