On high-speed oval tracks, IndyCars spend much of a lap fighting air resistance. At speeds above 200 mph, aerodynamic drag becomes one of the largest forces slowing the car, so even a small reduction can change a race. Drafting happens when a trailing car lines up behind a leading car and rides in the disturbed wake.
This lets the trailing car use less power to maintain speed or use the same power to go faster.
Key Facts
- Aerodynamic drag increases with the square of speed: Fd = 1/2 rho Cd A v^2.
- The power needed to overcome drag increases roughly with the cube of speed: P = Fd v.
- A wake is a region of slower, lower-pressure, turbulent air behind a moving car.
- A trailing IndyCar in the wake experiences reduced drag because the air speed relative to the car is lower and pressure drag is reduced.
- A slingshot pass happens when the trailing car gains speed in the draft, then moves into clean air to pass.
- Drafting can reduce drag, but turbulent air can reduce downforce and make steering less stable.
Vocabulary
- Drafting
- Drafting is driving closely behind another vehicle to reduce aerodynamic drag by using its wake.
- Aerodynamic drag
- Aerodynamic drag is the force of air resistance that acts opposite to a car's motion.
- Wake
- A wake is the disturbed, slower, turbulent region of air left behind a moving object.
- Downforce
- Downforce is the aerodynamic force pushing a race car downward to increase tire grip.
- Slingshot pass
- A slingshot pass is a move where a trailing car builds speed in the draft and then pulls out to pass the leader.
Common Mistakes to Avoid
- Thinking drafting removes all air resistance is wrong because the trailing car still moves through air and still has drag, just less than in clean air.
- Assuming the trailing car can stay safely inches from the leader is wrong because reaction time, turbulence, and sudden speed changes make very close following dangerous.
- Confusing low drag with high downforce is wrong because the wake can reduce drag but also disturb airflow over the wings and reduce grip.
- Using drag as a linear force at racing speeds is wrong because aerodynamic drag depends on v^2, so small speed increases cause much larger drag increases.
Practice Questions
- 1 An IndyCar has a drag force of 4200 N in clean air at 90 m/s. If drafting reduces drag by 18 percent, what drag force does the trailing car feel?
- 2 A car needs 378 kW to overcome drag at a certain speed in clean air. If drafting reduces the drag force by 15 percent at the same speed, what power is needed to overcome drag while drafting?
- 3 A trailing driver gains speed in the draft before pulling out to pass. Explain why the car may speed up in the wake but become harder to control when it moves through turbulent air near the leader.