In light of renewed economic and political interest in petrochemical conservation and an upswing in the popularity of aftermarket aerodynamic devices, let us take a look at our old friend the ’02 series BMW. Despite being four years out of production, these cars constitute a 47% plurality of our club membership and an even larger majority of BMW’s on the roads of the United States. The “two box” design of the ’02 body shell dates back to the mid sixties and was directly derived from the hundred series (1600/1800/2000) four cylinder, four door sedans. As such, it was designed as a space efficient two door sedan of modest potential, at least in its initial 1600-2/1602 guise and staid aesthetics. Initially there was little regard for aerodynamics as demonstrated by its drag coefficient greater than 0.40 (C(w) = 0.42).
During the early seventies a “shovel type spoiler” was popular. These items also reduce air flow under the car and achieve a general flow pattern similar to their more vertical contemporaries but do so in part by generating a stagnant zone of turbulent air in the crease between the nose and the “spoiler.” For this reason, such spoilers are less efficient than contemporary designs.
But, before we consider the functioning of any given body shell, a brief review of fluid flow, with air being the fluid in this case, is called for. The above mentioned drag coefficient is a relative index (less than 1.0) of the ease with which a body will move through a fluid; the lover the C(w), the less the resistance. The C(w) of contemporary vehicles ranges from about 0.20 (Mercedes CIII/IV diesel speed record car) to 0.65 (multi-axle trucks) with most current road going automobiles in the 0.35 to 0.50 range. Already we can see that the ’02 body is nothing to rave about. However, drag per se is not the only concern. How that drag is generated is important. Current racing cars, with their various wings and trim tabs are rather “dirty” designs. Yet, these devices are constructed to generate lift (in this case, negative or downforce) and as a by-product produce considerable drag. It is up to the designer at his table and the driver at the track to weigh the trade-off of downforce and increased cornering speeds against drag and decreased straight-away speeds.
Such decisions are grossly made by road car manufacturers and there is little the owner can do to radically alter the aerodynamic profile of his/her car. Interestingly, the later designed Touring and Baur Cabriolets have a more steeply raked windshield; although exact figures are unavailable, the Touring might be the slickest stock body of the series. But, it was not until the final development of the ’02 series, the factory 2002 Turbo, that the engineers in Munich altered the aerodynamic profile of the body shell by fitting a fiberglass air dam and a polyurethane rear spoiler. Also used were fiberglass “eyebrow” fender flares, but these were added for tire clearance and increase, rather than decrease, drag. Nevertheless, there are a number of sore spots that the road car driver can improve upon. Foremost among these is the nose profile. Befitting its mid-sixties, Amphibicar-era design, the nose of the ’02 would make an excellent lifting (planing) hull. Even at current interstate speeds, the steering lightens as air pressure tends to lift the nose. Cross country touring with a full tank of gas and a full trunk exacerbates this nose-high attitude
The simplest way to reduce front end lift is to prevent high pressure air getting under the car. This is accomplished by that most popular aftermarket aerodynamic device, the air dam or chin spoiler. These items, usually constructed of fiberglass or ABS plastic, extend the nose profile downward and deflect air around, rather than under, the car. During the early seventies a “shovel type spoiler” was popular. These items also reduce air flow under the car and achieve a general flow pattern similar to their more vertical contemporaries but do so in part by generating a stagnant zone of turbulent air in the crease between the nose and the “spoiler.” For this reason, such spoilers are less efficient than contemporary designs.
But, if one is to alter the suspension ride height, it should be done for handling reasons because the aerodynamic concerns would dictate ride heights resulting in terrible handling.
Let us now look more closely at some design and function parameters of the average air dam. Its foremost function is to prevent airflow under the nose and direct it around the sides of the body. Thus, it partially prevents incident air. from being deflected downward. In doing so, some additional air is directed at the grill openings. This, coupled with lower pressures under the car, increases air flow past the radiator matrix and into the engine compartment. (Realize that the majority of this air is exhausted under the car.) Thus, in cars with marginal cooling capacity, operating temperatures may be reduced. Unlike the 3-, 5-, 6-, and 7-series cars, which have air ducted to the front brakes by integral ducts in the below bumper air intake, the ’02 series relies on under car air for brake cooling. Therefore, any ’02 air dam should have incorporate brake ducts or else the owner should be prepared to duct cooling air in from elsewhere, such as behind the grill. (Brake cooling obviously presents more of a problem at driver’s schools, time trials and such than in sedate day to day driving.) Some air dams have a horizontal slot in their center, a la 2002 Turbo. On the Turbo, an oil cooler was mounted in this vulnerable position. Unless an owner is going to do likewise, such an orifice does nothing but partially defeat the purpose of the air dam. A small, forward facing horizontal “lip” along the lower edge of the dam will increase its efficiency by preventing air from rolling off the edge and heading under the car. Efficiency also increases. as the road-air dam distance is reduced – but ground clearance concerns come into play; some air dams have a flexible plastic skirt to enhance their function yet yield should they encounter a solid object. Lastly, it is a good idea to “test fit” any dam to your car prior to purchasing it as the accuracy of fit can vary widely with manufacturer or prior damage to the front end of the car. The front bumper will also reduce the efficiency of the dam by breaking up the flow pattern before the dam; however, the facts of real world motoring suggest that it is wiser to leave the bumper in place.
Air flow within the body shell is also important although more difficult to alter. The primary sore spot is a grill design which traps more air than can pass through the radiator matrix. Blocking off the outer two thirds of each side grill and carefully ducting the remaining opening would not compromise radiator air flow and would reduce turbulent air trapping. (As an example, examine the front “grill” of a Volkswagen Rabbit.) Blocking off the openings above the headlights between the nose and front bulkhead will force a bit more air past the radiator if the radiator is not ducted, but watch the cool air intake on carburetted cars. An interesting oddity is an opening of unknown function in the front bulkhead below the battery carrier. This can be used to duct fresh air to the engine intake or as a less vulnerable location for a small oil cooler. Air flow into the cockpit ventilation system, if altered, should reflect a concern for bettered ventilation rather than aerodynamics. The ventilation intake is already fed off a high pressure zone at the base of the windshield.
Having considered the front portion of the car, we now move to the rear. Here the options are rather restricted. First of all, unless you are going to extremes, a la Porsche’s 930 whale tail, do not use any form of rear wing. The smaller ones rarely leave the slow moving boundary layer about the body surface and end up functioning much as a “lip” or “ducktail” spoiler. Larger functional rear wings are overkill and leave you with an unbalanced down force distribution unless you take equally radical measures at the front. Thus, the most viable device is some form of lip at the rear of the trunk. These tend to be either fiberglass or polyurethane add-ons or replacement fiberglass trunk lids with the lip molded in. They generate downforce by forestalling the flow break up into low pressure turbulence and the Kamm effect tells us that their sudden rear contour tends to cause less drag than the gentle fall off of the stock trunk lid. A useful adjunct would be a CSL-like basket handle over the rear window to keep the rear window clear and direct more air at your rear spoiler. However, I do not believe that such an item is commercially available for most cars.
Overall, most body shapes tend toward less drag as their real angle of incidence is made negative by lowering the nose and raising the rear. The ’02 series is no exception. But, if one is to alter the suspension ride height, it should be done for handling reasons because the aerodynamic concerns would dictate ride heights resulting in terrible handling. However, the virtual incidence angle can be altered by “lowering” the nose profile with an air dam and “raising” the rear profile with a rear spoiler. This also serves to demonstrate how the two work in a synergistic manner. Many owners add an air dam and are pleased with the results at the front end without realizing that enough front downforce, acting in lever fashion through the center of gravity, tends to lift the tail. Adding a rear spoiler alone forces the tail down and accentuates nose lifting (hear that Camaro owners?). As an aside, recall that it usually requires less engine horsepower to run your air conditioner than it does to overcome the drag horsepower of an open window or sunroof. Also, anything which increases frontal area (i.e., wide tires or fender flares, sunroof wind deflectors) will increase drag.
If one were to undertake some or all of these modifications, what sort of return could one expect on his investment? Immediately noticeable would be subjective improvements in high speed stability and in resistance to side winds. You might notice some reduction in wind noise. The objective drag reduction is harder to predict. Laboratory work in a wind tunnel suggests that gains of up to 20% drag reduction are possible but problems of scale and scale modelling make me reluctant to extrapolate this figure. Another variable is the capital outlay for these parts. Car and Driver demonstrated that workable aerodynamic devices can be fashioned from sheet metal at home for very low cost. At the other extreme are brand name German imports. Also to be factored in are installation costs (painting, fiberglassing in parts, etc.). The bottom line is that there are too many variables in gas and parts costs versus benefits to predict a general break-even figure in terms of miles driven. Thus, any decision must be based on personal cost-benefit evaluations and personal aesthetic evaluations of what your car looks like, how you view it, and what you would like it to be. Consumer advocate types might find the expense of additional aerodynamic devices unwarranted if they do little highway driving or find a low front air dam incompatible with their driveway, while hard core “boy racers” may find that they can’t get enough of the devices if they improve their lap times slightly.
Editors Note: The information presented in this article is based in part on the author’s personal experience with scale testing in a subsonic wind tunnel (Duke University School oi Engineering, Durham, NC) and his 1972 2002 tii.
Original article by Jeff Mulchahey
Last update: 2007-01-02 11:59
Author: Tii Register Archives