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STEERING AXIS INCLINATION (SAI)
The SAI angle is specified in degrees, but specifications are not provided for some vehicles. SAI is used primarily as a diagnostic angle (see reference chart) to verify the angular relationship between the steering axis, wheel spindle and rest of the suspension. But it also affects steering. With MacPherson strut suspensions, SAI is more
important than caster in determining steering wheel return. The inward slant of the steering axis lifts the car as the wheels turn.
When the steering wheel is released, the weight of the vehicle helps to recenter the wheels.SAI also reduces the need for excessive positive camber. SAI works with camber to maintain vehicle load toward the inner wheel bearing. This reduces steering feedback and harshness. MacPherson strut suspensions typically run a much higher SAI angle (12 to 18 degrees) than SLA suspensions (6 to 8 degrees) because a FWD suspension needs the extra leverage for directional stability and to counter torque steer. If the SAI angles are unequal side-to-side (even if static camber is within specifications), it can contribute to torque steer, brake pull during sudden stops and sometimes even bump steer.
Checking SAI when doing an alignment can tell you several things. It will tell you if the upper strut towers are located properly, if the lower control arms are bowed or if the center cross member has shifted. Comparing the SAI angle, the included angle and camber can help you identify bent parts. All too often the mistake is made of trying to adjust camber back to specs without determining why it is out of range in the first place. Page
