Track Exchange Method
Is Exit-Speed Over-rated?
Most drivers consider corner exit speed the paramount objective in maximizing lap times. It’s a concept that’s been hammered into them by driving instructors and racing literature for decades. Obviously exit speed is important, but is it overrated?
In the excellent Competition Driving, authors Alain Prost and Pierre-Francois Rousselot state: “Exiting a corner at 125 mph rather than 120 mph gives you a 5 mph advantage down the following straight. Supposing that the speed remains constant, after half a mile the difference will be 0.6 second, or around 35 yards.”
This statement depends on a seemingly logical but invalid supposition. The speeds do not remain constant. Aerodynamic drag increases as the car accelerates. Indeed, aero drag increases as the square of speed. Gear ratios also change as the car goes faster and reduce the thrust at the wheels. The faster car faces more aero drag and generates less thrust and thus accelerates less. The slower car accelerates harder and reduces the difference in speed. The race cars approach their maximum speed where engine output is matched by aero drag and the car can no longer accelerate. The faster the cars go the less the difference in speed.
In reality, The 5-mph advantage Prost and Rousselot mention is worth about 0.2 seconds for a high-powered car and maybe a tenth more for a less powerful car.
Considered the U.S. road racers’ bible, Driving in Competition provides a template for analyzing racetracks. Author Alan Johnson suggests that a track is characterized by three types of turns: Type 1 turns feed onto a long straight: the Type 2 turn immediately follows a long straight; while Type 3 turns compose the rest of the lot.
Johnson declares Type 1 turns of primary importance, with the turn leading onto the longest straight being the most important. Logical and seemingly scientific, this notion has survived for more than 30 years, including a mention as recently as a 2003 NASCAR telecast from Pocono. But is it flawed?
When the SCCA Runoffs were held at Road Atlanta (1970 to 1993) Turn Seven provided the classic test of Johnson’s “most important turn” theory. Indeed, Road Atlanta was the primary racetrack analyzed in Driving in Competition. Turn Seven is a slow (50-70 mph) turn of about 100 degrees leading onto a 5,000-foot straight - dubbed by Sports Car publisher Paul Pfanner “the SCCA car-classification straightaway”.
Paul White, a partner in the ADF and later Swift Formula Fords, once told me that no matter how much a driver messed up Turn Seven it only cost about a quarter of a second at the end of Road Atlanta’s monster backstraight.
White’s comment stuck with me as a flaw in the face of Johnson’s classic analysis. It was the impetus that led me to develop the Track Exchange Method.
The Track Exchange Method
The Track Exchange Method is an analytical tool for better understanding sector times. Consider two identical racecars exiting a turn onto a 5,000-foot straight. Car A exits the corner at 60 mph, while Car B exits at 55 mph. Determining both car’s sector times requires a complicated series of calculations. But we really only need the difference in sector times, and the Track Exchange Method answers this question with a minimum of math.
Plowing through the mathematical calculations yields a sector time of 23.623 seconds for Car A and 23.939 seconds for Car B. The difference is 0.316, essentially the same as the Track Exchange Method, but without the simplicity and visual impact of the Track Exchange Method.
The lesson is that exit speed does not gain as much time as you might think. With identical cars there is more potential for gaining or losing time in turns, not on straights.
For details of the Track Exchange Method,
and to apply it to your racecar,
consult the Driving Simulator.
This screen is just part of the Track Exchange Method.
The actual form gives an animated display of the analytical process.
This will give you a whole new way to look at race track performance.