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Wheel Alignment - Caster Angle/Toe/Steering Axis Inclination (SAI)
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Caster Angle/Toe/Steering Axis Inclination (SAI) |
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Caster is the forward or rearward angle of the steering axis compared to true vertical as seen from the side. Caster is measured as the number of degrees that the steering axis varies from true vertical. |
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If the top of the steering axis is in front of true vertical, the caster is negative. | |
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If the steering axis is parallel with true vertical, it has zero caster. | |
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If it is behind true vertical, the caster is positive. | |
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Caster affects vehicle steering effort, steering wheel returnability, and handling.
Steering wheel returnability refers to the vehicles ability to straighten out and the steering wheel return to center after the vehicle leaves a corner.
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Toe is the comparison of the distance measured between the front and rear of the tires on the same axle.
Toe affects tire wear, handling and steering wheel alignment.
Toe is represented by a measurement in either millimeters or inches calculated by taking the distance between the rear of the tires and subtracting the distance between the front of the tires. This measurement is called total toe: |
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If the front of the tires were closer together than the rear, the vehicle would have positive toe, also called toe in. | |
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If the rear of the tire were closer together than the front, the vehicle would have negative toe, also called toe out. | | |
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Steering Axis Inclination (SAI)
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Steering axis inclination, or S.A.I., is the angle created by drawing a line through the upper and lower pivot points of the knuckle when looking from the front of the vehicle and comparing it to true vertical. Included angle, or I.A., is the combination of S.A.I. and Camber. If the side to side difference in S.A.I. or included angle differ 1.5 degrees or more, it may be an indication that the vehicle has damaged components.
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