Aerodynamic coefficients and downforce effects on racing car performance

Aerodynamic Coefficients Influence on the Performance of LMR1 Car

Car aerodynamics is governed by the same principles governing aircraft flights; the main focus being to produce down force instead of lift. In car aerodynamics, there is need to force more high speed-low pressure air to go under the car creating negative lift; down force (Rapid Racer, 2012). This results in higher grip levels for the tires and more traction, especially speeding around corners. For high performance cars, aerodynamics is achieved in several ways. The basic hypothesis is that faster driving increases down force thus pushing car's tires down resulting in higher grips and tractions during races. The downward force also known as drag is the square of the car's velocity; which is double the speed (Unlimited Performance Products, 2007). However, this increased down force reduces top speed resulting in more engine power to propel the car forward as shown below.

Nomenclature

In aerodynamics, there are several terms used unique to aerodynamics while some are used in unique ways when discussing aerodynamics. These include airfoil nomenclature, and glider axis among others.

The shape formed by the cross-section of a wing is known as airfoil and the round sides of the airfoil are leading edges (Rapid Racer, 2012). As the airfoil moves through the air, it experiences a relative wind and since the airfoil's gliders have forward and downward movements, the relative wind comes from the front and below the airfoil.

There are three axes of rotation intersecting at the glider's center of gravity with the glider's mass evenly distributed around the center of gravity. Movement about the pitch axis makes the glider's nose moves up or down while as it moves about the yaw axis, the nose moves from side to side. Additionally, whenever the glider rotates about the roll axis, one wing moves up, the other down.

Racing Car Aerodynamic

Drag is the force acting opposite to the path of the vehicle's motion. This force results in increased fuel consumption as it hinders vehicles top speed. According to manufacturers, low drag vehicles have these characteristics; streamlined shape, low frontal area, and minimal openings in the bodywork for windows or cooling ducts.

The drag performance of vehicles is characterized by the drag coefficient (CD) equated as: CD = FD / (1/2 • •V2•AF). From this, FD is the drag force, ? is the air density, while V is the free stream velocity, and AF is the frontal area of the vehicle.

Lift is the force acting on vehicles as they move and may be manipulated to enhance the performance of a race car and decrease lap times as opposed to drag (Buresti, 2004). However, by manipulating the race car geometry it is possible to create negative lift, known as down force. Down force enhances vehicle performance by increasing the normal load on the tires. This increases the potential cornering force which results in the ability of the vehicle to corner faster and reduce lap times.

The lift of the vehicle is characterized by the lift coefficient (CL) and is calculated as; CL = FL / (1/2• •V2•AT) (2). In this equation, FL is the lift force; AT is the area of the top surface of the vehicle and the other. A negative lift coefficient shows a vehicle is experiencing down force.

LMR Series

LMR1 cars are designed to be champions in street racing competitions. The vehicles feature Late Model Racecraft package with approximately 481 rear wheel horsepower and stock 20-inch rims and OEM clutch. The cars in addition, have Corsa exhaust systems complete with custom grind camshaft. The other features include MSD 8.5mm wires, under-drive pulley and NGK spark plugs (Katz, 2006). Besides, the vehicles have 160 degree thermostats and maximized factory mufflers. The LMR cars dominate the street races due to their powerful engines, reducing drag, and increased amount of air sucked in to their cylinders. LMR1 racing rules are initiated to ensure vehicles have bigger and larger tires of approximately 250mm to 300mm. The fuel tanks and chassis of the vehicles are supposed to the heavier and larger to increase down force while minimizing lift during racing and turning around sharp corners.

Lap Time Simulations

The LMR1 vehicle concept is developed after conducting an acceleration sensitivity analysis using simple constant acceleration equation lapsim seeking minimum lap time (Taleb, 2007). The lapsim result suggests the vehicle should be conceptually biased towards lateral acceleration over longitudinal acceleration (Haug, Choi, & Komkov, 1986). Whilst being a very simple lapsim, it is effective in quantifying the conceptual trade-offs. To bias the design towards lateral acceleration, a 50:50 weight distribution is chosen with four equal sized tires in an attempt to provide similar dynamic vertical loads on outside front and rear tires.

Conclusion

It can be concluded that aerodynamic coefficients greatly influence the performance of an LMRI car. In car aerodynamics, there is need to force more high speed-low pressure air to go under the car creating negative lifts. This would ensure cars have lower lift forces during acceleration to thus sucking the car into track with increasing speed. In designing racing cars, superior dynamic competiveness is required, affordability, reliability and maintainability (The University of New South Wales, 2010). The vehicle must require minimal maintenance and additionally have wider wheels to provide higher drag and traction.