Drag racing measures how quickly a vehicle can accelerate from a standing start over a fixed straight distance. The two common distances are the quarter mile, 1320 ft, and the eighth mile, 660 ft. Comparing them shows how launch traction, gearing, power, aerodynamic drag, and braking all shape performance.
Engineers use these runs because they compress many physics ideas into a short, measurable event.
The eighth mile emphasizes the launch, early acceleration, and traction because the car spends less time at very high speed. The quarter mile adds more time for horsepower, gear selection, aerodynamic drag, and stability to matter. Tracks may use the shorter distance for safety, limited shutdown space, class rules, or very high powered vehicles.
A car that is excellent in the eighth mile is not automatically best in the quarter mile because the later part of the run has different engineering demands.
Key Facts
- 1/8 mile = 660 ft = 201.17 m
- 1/4 mile = 1320 ft = 402.34 m
- Average speed = distance / elapsed time
- For constant acceleration from rest, d = 0.5at^2
- For constant acceleration from rest, v = at and v^2 = 2ad
- Aerodynamic drag grows with speed squared: Fd = 0.5 rho Cd A v^2
Vocabulary
- Elapsed time
- Elapsed time is the time from the start signal to when the vehicle crosses the finish line.
- Trap speed
- Trap speed is the vehicle speed measured near the finish line, often used to estimate power and high speed performance.
- Traction
- Traction is the grip force between the tires and track that allows the car to accelerate without excessive wheelspin.
- Aerodynamic drag
- Aerodynamic drag is the air resistance force that opposes motion and increases rapidly as speed rises.
- Shutdown area
- The shutdown area is the length of track after the finish line available for braking and safely slowing the vehicle.
Common Mistakes to Avoid
- Assuming a quarter-mile time is exactly double an eighth-mile time is wrong because the car is already moving fast at 660 ft and acceleration changes with speed, gearing, and drag.
- Ignoring units is wrong because 660 ft and 1320 ft must be converted consistently when calculating speed, acceleration, or time.
- Treating acceleration as constant for the whole race is wrong because tire grip, engine power, gear shifts, and aerodynamic drag make acceleration vary during the run.
- Comparing cars using elapsed time alone is incomplete because trap speed, launch quality, traction limits, and track conditions can reveal different performance strengths.
Practice Questions
- 1 A car completes an eighth-mile run of 660 ft in 6.20 s. What is its average speed in ft/s and mph? Use 1 mph = 1.467 ft/s.
- 2 Assume a drag car starts from rest and accelerates constantly over 1320 ft in 10.0 s. Find its average acceleration in ft/s^2 and its final speed in mph.
- 3 Two cars have the same eighth-mile elapsed time, but Car A has a higher trap speed at 660 ft. Explain what this suggests about launch traction, acceleration later in the run, and likely quarter-mile performance.