What instrument is used to measure caster on a straight beam steering axle?

• A. Ruler
• B. Dial indicator
• C. Protractor
• D. Caster gauge

Answer: (D)

Measuring caster angle on a straight beam steering axle requires a tool that directly reads the tilt of the steering axis relative to vertical. A caster gauge is designed for this job, with mounting references on the axle and calibrated angle scales so you can read the caster angle quickly and accurately. A ruler only measures linear distance, not angle, so it can't give you caster. A protractor can read angles but isn't practical to align and mount on a straight beam axle for accurate, repeatable measurements. A dial indicator measures small linear displacements and runout, not the caster angle itself, unless paired with additional fixtures and procedures, making it less direct than a caster gauge.