Wheel alignment — definition of caster angle Caster refers to the:

Introduction / Context:
Caster is one of the three fundamental wheel alignment parameters (the others are camber and toe). While camber concerns inward/outward wheel tilt, caster is about the tilt of the steering axis when viewed from the side of the vehicle.

Given Data / Assumptions:

• Steering axis may be a kingpin on SLA suspensions or a MacPherson strut axis.
• Angle is measured relative to vertical in the longitudinal plane.
• Sign convention: top of axis rearward = positive caster; forward = negative caster.

Concept / Approach:
By definition, caster is the fore–aft inclination of the steering axis. It can be either forward (negative) or backward (positive). Positive caster enhances straight-line stability and self-centering, while negative caster reduces those effects. Therefore, a correct definition must allow for both directions rather than only one.

Step-by-Step Solution:
Identify parameter: steering axis tilt in side view.Recognize two possible senses: backward (positive) and forward (negative).Choose the option acknowledging both possibilities.

Verification / Alternative check:
Alignment specifications will list caster as a positive or negative value; vehicles are commonly set with small positive caster for stability.

Why Other Options Are Wrong:
Options a and b give only one direction, not the general definition. ‘‘None of these’’ is incorrect because option c fully captures the concept.

Common Pitfalls:
Confusing caster with camber (front-view tilt) or toe (plan-view convergence). Misinterpreting sign conventions between manufacturers.

Final Answer:
either ‘‘a’’ or ‘‘b’’ (the steering axis tilt can be forward or backward, corresponding to negative or positive caster)