Wheel alignment — caster angle purpose What is the principal reason for providing a positive caster angle on the steering axis of road vehicles?

Introduction / Context:
Wheel alignment parameters—caster, camber, and toe—shape steering feel and stability. Caster is the fore–aft tilt of the steering axis as viewed from the side of the vehicle.

Given Data / Assumptions:

• Positive caster means the upper ball joint or strut top is behind the lower joint contact point relative to the ground contact patch.
• Self-aligning torque increases with positive caster.
• Small angles typically used in passenger vehicles for stable tracking.

Concept / Approach:
Positive caster creates a “trail” effect, like the caster wheels on a trolley. As the vehicle moves forward, tire forces act behind the steering axis, generating a restoring torque that makes the wheels self-center and track straight. This enhances directional stability and control, especially at speed.

Step-by-Step Solution:
Relate caster to steering-axis trail and pneumatic trail.Positive caster → stronger self-centering → better straight-line stability.Therefore, the main purpose is to maintain directional control.

Verification / Alternative check:
Road tests show vehicles with insufficient caster wander and show poor return-to-center after turns; specification compliance restores stability.

Why Other Options Are Wrong:
Uneven tyre wear is more sensitive to camber and toe.

“Bring the contact under the load point” conflates scrub radius/KPI with caster.

Caster does not compensate for component wear; parts must be replaced.

Rolling resistance is minimally affected by caster compared with tire compound and inflation.

Common Pitfalls:
Overly high caster can increase steering effort; power assist and KPI geometry are designed to balance feel and stability. Always measure caster with the proper turn-plate procedure.

Final Answer:
maintain directional control