Caster Angle

Caster angle builds mechanical trail between the tire contact patch and the projected steering axis. With positive caster, the tire follows behind that axis like a shopping cart wheel, which creates a self-aligning torque. You feel stronger on-center stability, better high-speed tracking, and a natural return to center after a turn.
This improves confidence on highways and reduces driver fatigue. If caster is too high for the steering assist level, effort increases and the vehicle can follow ruts. If caster is too low, the wheel may feel light and the car can wander. Matching caster to tire size, assist curve, and vehicle weight gives you balanced steering.

Caster angle is a side view property of the steering axis, while camber is the inward or outward tilt of the wheel when viewed from the front, and toe is the inward or outward pointing of the wheels when viewed from above. Caster mainly influences self-centering and straight-line stability, camber primarily affects cornering grip and shoulder wear, and toe has the largest impact on tire scrub and tracking at low angles. All three interact.
For example, adding positive caster can create beneficial camber gain during steering, which can reduce the amount of static negative camber you need. A correct alignment sets each angle to complement the others.

Caster angle is set by the relative positions of the upper and lower control arm pivots or the strut top and lower ball joint in a MacPherson strut. Service adjustments depend on the design. Some vehicles use slotted strut towers, eccentric bushings, or adjustable control arms to change caster. Others have fixed geometry and require aftermarket arms or bushings to shift the axis.
During alignment you center the steering wheel, set ride height and tire pressures, then measure caster with a sweep procedure on the alignment rack. You loosen the relevant fasteners, adjust the hardware to reach the target, then tighten to spec and recheck to ensure repeatability.

Caster changes subtly with ride height because the body and control arm angles move the projected steering axis. Heavy cargo, towing, or lowering springs can tilt the chassis and shift effective caster. Bushings and ball joints also deflect under braking or cornering, which adds compliance steer effects. If the geometry is well tuned, these changes stabilize the vehicle as speed and lateral load increase. If not, you may feel brake shimmy, tramlining, or vague response over bumps.
Align the car at the specified curb weight and tire pressure, and verify bushing health. After any ride height or bushing hardness change, remeasure caster to keep handling consistent.

Follow the OEM nominal and allowable range for both left and right sides, and control cross-caster, which is the side-to-side difference. Many platforms allow a small positive cross-caster on the left to compensate for road crown. On the line, fixture datums and robust hole patterns help hold caster within tolerance through welding and paint. In service, use a level rack, calibrated heads, and a full caster sweep.
Tighten bolts to the correct torque with the suspension settled at ride height to prevent shift. If values change during torque, replace worn bushings or fasteners. Document before and after measurements for traceability and quality control.