At low speeds the mass (weight), the tractive force (horsepower/tires/track) and the inertia (load transfer) are the dominate determinants of performance limited only by loss of control caused by wheel-stands or smoking (spinning) tires. These problems are diminished with increasing speed.
At higher speeds aerodynamic forces become important, increasing as the square of the airspeed. A flat plate of one square foot area and perpendicular to the relative wind will produce drag force of zero at zero miles per hour, 33 pounds at 100 mph, 133 pounds at 200 mph and 299 pounds at 300 mph. The corresponding horsepower required to overcome the aerodynamic drag is zero, 9 hp at 100 mph, 71 hp at 200 mph, and 239 hp at 300 mph. A drag racer may have drag equivalent to a flat plate 12 feet in area and acting several feet above the ground, requiring 2,868 hp to overcome the aerodynamic drag at 300 mph.
| DRAG FORCE ON A ONE-SQUARE FOOT SQUARE PLATE | ||
|---|---|---|
Speed | Drag Force | Horsepower Required to Overcome the Aerodynamic Drag |
| 0 mph | 0 pounds | 0 hp |
| 100 mph | 33 pounds | 9 hp |
| 200 mph | 133 pounds | 71 hp |
| 300 mph | 299 pounds | 239 hp |
Lift, a force trying to raise the car up off the road, caused by air pressures distributed over the body, is normal for all automobiles and is often of the same magnitude as the drag. However, low drag does not necessarily mean low lift and vice versa. Depending on the body shape the lift may be acting more at the front, middle or rear of the car, affecting steering and/or traction. Crosswinds can cause more lift on one side of the car, lifting one wheel at high speed and thereby creating more lift, etc., leading to the car leaving the tract and rolling over (actual case, Formula 1).
Directional stability is the tendency to weathercock or turn into the wind. Most, but not all, cars are directionally unstable; that is, when pointed out of the wind they will tend to turn more. This phenomenon is also aggravated at higher speeds and may combine with lightened front wheel steering.
A different but similar phenomenon occurs for bodies with rounded blunt rear ends, as seen in plan view from above. It is comparable to the twin vertical dust devil like vortices seen shedding from the rear side corners of semi-trailers. However, in this case, known as Karman Vortex Street, the airflow departs from the rear end alternately from the left side and then the right side at a frequency and force depending on the speed and the width of the body. Again, the aerodynamic forces depend on the body surface area involved and the airspeed. Flow separation fixing stabilizers in the form of strakes can very effectively stop the oscillation.
Body modifications and add-on aerodynamic devices can greatly add to the performance and overcome inherent aerodynamic shortcomings. Downsizing, chopping, ram air scoops, streamlining, air dams, ground effects, rear spoilers, wings, vortex generators and strakes can be very beneficial and sometimes essential for safety.
Production automobiles of the past often present complex aerodynamic configurations which are difficult to characterize short of track trials or model wind tunnel testing. The speeds encountered in racing may far exceed those previously experienced on the road, revealing unexpected aerodynamic behavior. Such cases call for a cautious incremental approach to higher speeds and the attendant aerodynamic forces.
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