The average driver doesn't think about what keeps their car moving or what keeps them on the road, but that's because they don't have to. The average driver doesn't have to worry about having enough downforce to keep them on the road or if they will reach the adhesive limit of their car's tires around a turn. These are the things are the car designers, professional drivers, racing pit crews, serious sports car owners, and physicist think about. Physics are an important part of every sports and racing car design. The stylish curves and ground effects on sports cars are usually there not just for form but function as well allowing you to go speeds over 140 mph in most serious sports cars and remain on the road and in reasonable control. The aerodynamic efficiency is the single most important element in designing a competitive car for professional racing or getting the car model on the front of a Car and Driver or Motortrend. Aerodynamics is the study of the motion of gases on objects and the forces created by this motion. The Bernoulli effect is one of the most important behind car design. The Bernoulli Effect states that the pressure of a fluid, in gaseous or liquid state, varies inversely with speed or velocity and a slower moving fluid will exert more pressure on and object than the same fluid moving slower (Yager). The goal of car designers is to make the air passing under a car move faster than the air passing over the car. This causes the air passing over the car to create more downforce than the air passing under the car creates upforce creating a force additional to the car's weight pushing the car to the road. Large amounts of downforce are needed to keep light cars grounded at high speed and keep to cars from sliding around turns at high speeds. The Venturi Effect is also an important in aerodynamic design. The Venturi Effect states that as a fluid, in gaseous or liquid state passes through a narrow space its speed increases (Yager). This is the reasoning behind keeping cars as close to the ground as they can be safely. The narrow space between the car and the ground increases the speed of the air flowing beneath it causing a decrease in pressure to do the Bernoulli Effect and increase in downforce. The Venturi Effect is the reason for front ground effects, which feature small air ducts or venturi tunnels. Negative lift is the technical term for downforce (Yager). Negative lift is the opposite of the lift used by planes to fly, it forces an object down rather than up (Yager). Negative lift is created by front and rear wings on race cars and by ground effects and spoilers on the average sports car. Most negative lift is used to fight inertia as a car rounds a turn. Inertia is the tendency of an object to remain in the same state of motion (Murphy 77). When a car rounds a turn at high speeds it often needs more force than it's weight to resist the car's tendency to keep traveling straight. The increased downforce puts more weight on the tires helping the tires grip the road. Drag force is the cost of increasing downforce. Drag force is the force acting on an object in motion in the opposite direction the object is moving through a fluid (Yager). To most people drag force is simply known as air resistance. The objective of aerodynamic efficiency is to maximize downforce while minimizing drag force. Acceleration and Speed are obviously the two defining characteristics of a fast car. Newton's three laws of motion are an essential part in determining how fast a car will accelerate and how fast it will go. Newton's second law is the easiest to understand in relation to a car's acceleration. Newton's second law mathematically states Force=(mass)(acceleration) (Murphy 78). This law explains why cars that need to accelerate fast should be relatively light in weight compared to other cars. Removing mass, such as a bumper, radio or fancy upholstery reduces the weight of a car. When the tires create a constant force and the mass is decreased the acceleration will increase. Just for an example if you have a 100kg car and a 250kg car both have a 10,000N force pushing it forward (ignoring outside sources of resistance) the 250kg car will accelerate at 40m/s while the 100kg car will accelerate at 100m/s. So a big force and a little mass will yield a big acceleration. Acceleration also ties in Newton's third law of motion, which states the every force is accompanied by an equal but opposite reaction. The tires apply a force on the road and the road applies and equal and opposite force back, but because the road is essentially part of the earth it's mass is much greater than the mass of a car. The road accelerates very little in it's opposite reaction to the force of the tire because it's mass is so huge and the car accelerates allot from the opposite reaction or push applied by the road because it's mass is so small. While Newton's second law Speed and acceleration both incorporate Newton's first law of motion, which states an object will remain at rest or at uniform speed unless acted upon by an unbalanced force (Murphy 76). This law explains why your car does not keep rolling at a constant speed when you let off the gas. The car faces air resistance, rolling resistance and internal friction in the drive train and wheel bearings (Beckman). A car accelerates by applying a force greater than these three things. In referemce to cars horsepower is used rather than force. Designers and racers want to know who much horsepower is needed to maintain high speeds. There is an equation used for this discovered by Russian Physicists, F=CdApv2 (Beckman). In this equation the variables are as follows: F = force
Cd = coefficient of friction
A = frontal area of the car
p = density of air
v = speed
Using a late model Corvette, which has 240hp and can go 150mph we can determine how much of that horsepower is needed to maintain different speeds (Beckman). Using the top speed of 150mph which is maintained by keeping the gas pedal to the floor using all the horsepower we can determine how much horsepower is used to fight air resistance and how much is used to fight rolling friction and the slight internal friction (Beckman). A late model Corvette has a Cd of about 0.30 and frontal area of about 20 square feet (Beckman). Going through all the necessary conversions and math which is complicated because of converting to and from metric and force into horsepower you determine 145hp is used to balance drag and 95hp is used to balance the rolling resistance and internal friction (Beckman). This explains why fast cars have so much horsepower. As speed increases so does the drag and more horsepower is needed to compensate for this. Tires are an essential part of every car and certain sized tires serve different purposes. As far as handling is and acceleration are concerned "wider is better" as Pontiac would say. The reason you don't see extremely wide tires on cars is because it increases the contact patch or area of the tire that touches the road. If you increase this Bosford 7
area you increase rolling resistance. Most cars have tires wide enough to exceed the average driver's demand but not add too much unnecessary rolling resistance. For top speed and not reaching them fast thin tires are the best. The thinner the tire the smaller the contact patch and smaller the rolling resistance is which means less horsepower is used to balance rolling resistance. Thin tires are how ever terrible for handling because the contact patch is small (Beckman). There is one kind of car that utilizes the best of both worlds for its purpose, the dragster. The Dragster has extremely wide rear tires for acceleration and to keep it going straight. The dragster has extremely thin tires in the front to reduce rolling resistance and because handling is not a concern the car goes in a straight line it does not need to turn.
In order to have a fast and efficient car all these things I have discussed need to be taken into consideration. A fast car should be designed with aerodynamic surfaces for a balance of maximum production of downforce and minimum drag creating surfaces. It should have as small an engine as possible to reduce mass and reduce the necessary size of the frontal area, but a large enough engine to be able to produce enough horsepower to be able to create more force than the resistance the car faces to accelerate and enough to balance with those forces at high speeds. The tires should be wide enough for fast acceleration and good cornering but not so wide it creates large amounts of rolling resistance. Your overall best example of such a car would be formula one races or Indy cars because they have to have good handling, fast acceleration and reach and maintain high speeds.