Steering Axis Inclination

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Steering Axis Inclination (SAI) is also known as King Pin Inclination (KPI).

The ‘Steering Axis’ is the axis around which the wheel assembly swivels as it turns to the right or left. It’s formed by drawing a line through the upper and lower ball joint pivots. This axis is not necessary centred on the tyre contact patch. If you view this as from the front it is inclined  inwards at the top. ‘Steering Axis Inclination’ is the angle in degrees between the ‘steering axis’ and vertical.

Steering axis inclination (SAI) causes both front wheels to gain positive camber as they steer away from center. This acts with caster to provide self-centering of the front wheels. This gives the car straight line stability.

With the wheels in the straight-ahead position, the stub axles are almost horizontal.

As the wheels turn to either side, SAI causes the stub axles to decline and the ends. In order for this to happen the inside end of the stub axle must rise and this in turns raises the front of the car.

The mass of the vehicle pushes down on the stub axles, so when the steering wheel is released, the wheels are pushed back to the central position.

Without any inclination, no self centering would occur. The mass of the vehicle would not resist the wheels from being pivoted by the bump (Bump Steer). The steering would be hard to control and straight line stability would be very low.

Steering Axis Inclination Effects :

  • With positive ‘Spindle Length’, the car will be raised up as the wheels are turned. There is an increase in the steering moment at the steering wheel. Regardless of which way the front wheels are turned, the larger the Steering Axis Inclination angle is, the more the car will be raised. Without any caster, this effect is symmetrical from side to side. The raise of the car has a self-aligning effect of the steering at low speeds.
  • SAI affects the ‘Steer Camber’. This is when a wheel leans when the wheels are tuned (positive camber for positive SAI angles). The amount of this is small but not to be neglected, especially for a vehicle making tight high speed turns.
  • If the driving or braking force is different on the left and right side (due to wheel spin or skidding), there will be an induce steering torque proportional to the scrub radius. This will be felt by the driver at the steering wheel. Under acceleration this is often called ‘Torque Steer’.

The distance from the ‘Steering Axis’ to the wheel centre plane measured horizontally at axle height is called ‘Spindle length’.

Steering axis inclination brings the pivot point close to the center of the tyre contact patch at the road surface. It intersects with the camber line drawn through the tyre and the wheel.

  • If these 2 lines intersect at the center of the tyre, at the road surface, then the vehicle is said to have zero offset, or zero Scrub Radius.
  • If they intersect above the road surface, then it has negative offset or Scrub Radius (shown above). The angle between the steering axis inclination and the camber line is called the included angle. It is a diagnostic angle.
  • If they intersect below the road surface, then it has positive offset or Scrub Radius (shown below).image: Positive Scrub Radius

Steering axis inclination is set by the dimensions of the upright and therefore not adjustable once defined.

Steering Axis inclination is usually kept below 8° since too much SAI causes a lot of rising of the front axle when steering.

Included Angle:

The sum of the camber and SAI angles in a front suspension. This angle is measured indirectly and is used primarily to diagnose bent suspension parts such as spindles and struts.

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