In drag racing, elapsed time and trap speed are two different measurements of performance over a fixed distance, often a quarter mile. Elapsed time is how long the car takes to travel from the starting line to the finish line. Trap speed is the car's speed measured near the finish line, usually over a short timing zone.
Together they help engineers separate launch performance, traction, gearing, power, and aerodynamics.
Key Facts
- Elapsed time, or ET, is the total time from launch to finish: ET = t_finish - t_start.
- Average speed over a run is v_avg = distance / ET, but trap speed is not the same as average speed.
- For a quarter mile, distance = 1320 ft = 402.3 m.
- A lower ET usually means better launch, traction, gearing, and acceleration through the whole run.
- A higher trap speed usually indicates more power and stronger acceleration near the end of the run.
- Kinetic energy at the finish is KE = 1/2 mv^2, so increasing trap speed requires a large increase in energy.
Vocabulary
- Elapsed Time
- Elapsed time is the total time a vehicle takes to travel from the starting line to the finish line.
- Trap Speed
- Trap speed is the vehicle speed measured near the finish line over a short timing section.
- Quarter Mile
- A quarter mile is a standard drag racing distance equal to 1320 feet or about 402.3 meters.
- Launch
- The launch is the first part of the run when the car converts engine torque into forward motion through the tires.
- Traction
- Traction is the tire's ability to grip the track and transmit force without excessive slipping.
Common Mistakes to Avoid
- Treating trap speed as the average speed is wrong because trap speed is measured near the finish, while average speed uses the entire race distance.
- Assuming the car with the higher trap speed always wins is wrong because a slower launch can produce a worse elapsed time even with more speed at the finish.
- Ignoring traction when comparing ET is wrong because wheelspin can greatly increase elapsed time without showing the full effect in trap speed.
- Comparing runs without checking distance and timing method is wrong because eighth-mile and quarter-mile results are not directly interchangeable.
Practice Questions
- 1 A car runs a quarter mile in 10.0 s. What is its average speed in ft/s and mph? Use 1320 ft and 1 mph = 1.467 ft/s.
- 2 Car A runs 11.2 s at 124 mph. Car B runs 10.9 s at 118 mph. Which car wins the race, and which car likely has the stronger top-end acceleration?
- 3 Two cars have the same trap speed, but one has a much lower elapsed time. Explain what this suggests about launch, traction, and early-run acceleration.