The wheels on a drag racing car have a higher diameter at the end of a race
than at the beginning. Which of the following is the least important factor
contributing to this?
The weight of the car is reduced due to fuel consumption.
The tires heat up so the air inside expands.
The tires rotate rapidly so the centrifugal force stretches them.
Due to rubber fatigue the stiffness of the rubber decreases.
Drag racing tires noticeably increase in diameter from the start to the end of a race due to dynamic physical effects during high-speed runs. The least important factor is the weight of the car reduced due to fuel consumption, as this has negligible impact on tire dimensions compared to direct tire-specific forces.
Tire Heating and Air Expansion
High-speed friction generates intense heat in drag tires, causing the air inside to expand and increase internal pressure. This thermal expansion bulges the tire sidewall and tread, directly enlarging the overall diameter for better top-end speed. Drag slicks are designed with soft compounds that amplify this “growth” effect under load.
Centrifugal Force Stretching
Rear tires spin at extreme RPMs—often exceeding 3000—creating outward centrifugal force that stretches the flexible rubber sidewall. This dynamic growth acts like a variable gear ratio, reducing effective gearing for higher terminal speeds without mechanical changes. It’s a key reason drag cars use tall sidewalls on small-diameter wheels.
Rubber Fatigue and Stiffness Loss
Under repeated high-G launches and sustained heat, rubber experiences fatigue, softening and losing stiffness over the run. This allows greater deformation and bulging under centrifugal and aerodynamic loads, contributing to measurable diameter gains. While present, it’s secondary to heating and spin effects in short drag races.
Why Weight Reduction Ranks Least
Fuel burn in drag cars is minimal—races last under 10 seconds with tiny tanks—causing insignificant mass loss that doesn’t alter tire loading or geometry. Suspension weight transfer dominates grip changes, not fuel weight, making this irrelevant to tire growth. Other factors directly deform the tire rubber and air volume.
