It is time to introduce a new component to motorsports engineering - the driver. SAE papers rarely deal with the cognitive control system which fills the space between the steering wheel and the seat. It seems that only the safety papers admit the presence of a driver, and they treat the driver as a passive object to be protected.
It is the driver who controls the race car. It is the driver who utilizes, or misuses, the capabilities of the car. It is the driver who chooses a path for the vehicle. It is the driver who decides how to use the traction circle to negotiate a turn in hopes of optimizing lap time.
The traction circle is a G-G diagram of longitudinal acceleration as a function of lateral acceleration. It defines the capability of a vehicle to combine acceleration with cornering while exiting a turn or deceleration with cornering while entering a turn.
Combining acceleration with cornering is universal because it offers greater control than driving a constant arc at maximum lateral acceleration. Combining deceleration or braking with cornering, often called trail braking, is much more difficult and controversial.
The traction circle describes vehicle acceleration and thus determines the path a vehicle follows. This path is usually called a racing line.
This paper will explore the subject of trail braking and test the concept on a simple oval. It will include a literature review of driving books. A mathematical simulation will suggest several lines. These lines will be evaluated on a test oval at the Transportation Research Center in Ohio.
The results will show the effect of different lines and demonstrate effective ways to analyze driver data. The traction circle is a useful analytical concept but it can be deceptive if misused. The traction circle can be augmented with additional information.