Wheel Alignment: Caster Angle
The vehicle in the chart is within specifications for camber and toe, but has excessive positive caster. This excessive positive caster will cause a shimmy at higher speeds and excessive steering wheel snapback. This is why vehicles that specify high positive caster usually have a steering dampener attached to reduce any adverse effects.
Caster is read in degrees. It is the forward or backward tilt of the steering axis viewed from the side of the vehicle. Vehicles with SLA suspensions use the upper and lower ball joints to create the angle and vehicles with struts use the center of the upper strut mount and the lower ball joint as the angle. It is compared to an imaginary line down the center of the tire or zero caster.
Zero caster is when the tire is true vertical. Negative caster is when the lower pivot point (ball joint) is toward the back of the vehicle compared to the upper pivot. Positive caster is when the lower pivot is toward the front of the vehicle, and upper pivot leaning away from the front toward the rear. Most vehicle's have positive caster because it provides greater feel and stability, helps the wheels track straight, and provides steering wheel return after taking turns.