Step 1: Understanding the Concept:
This question asks for the term that describes the difference in distance traveled by a drive wheel when it is under load versus when it is not under load (or rolling freely). This phenomenon is a key factor in traction performance.
Step 2: Detailed Explanation:
Let's define the terms:
(A) Rolling resistance: This is the force resisting the motion when a body (like a tire) rolls on a surface. It's caused by the deformation of the tire and the surface. It is a force, not a relative movement or distance ratio.
(B) Rim pull: This is the term for the tractive force that a vehicle can generate at the rim of its drive wheels. It is a force, not a relative movement.
(C) Wheel slip (or just Slip): This is the relative motion between the tire and the surface it's contacting. It is defined as the difference between the theoretical distance the wheel would travel in one revolution (based on its circumference) and the actual distance it travels. The question describes this phenomenon: the wheel moves a certain distance at no load (theoretical travel) and a different (usually lesser) distance under load (actual travel). The difference, expressed as a percentage, is the wheel slip. For example, 10-15% slip is often optimal for maximum traction in agricultural soils.
(D) Tractive efficiency: This is the ratio of the power available at the drawbar to the power delivered to the axle. It's a measure of how efficiently the engine power is converted into useful pulling power, and it is highly dependent on wheel slip. It is an efficiency ratio, not a relative movement.
Step 3: Final Answer:
The relative movement of the wheel due to the difference between its theoretical and actual travel distance under load is known as wheel slip. Therefore, option (C) is the correct answer.