Rotational Talk… The Australian Automotive Aftermarket is a constantly changing field, especially in the area of chassis components. Longer warranty periods, increased repair costs to older cars, better manufactured products and new vehicle designs are all affecting this area of the market.
One key area of change in steering parts is due to Steering Axis Inclination (SAI) settings in modern vehicles.
Steering Axis Inclination (SAI) is the angle formed by a line that runs through the upper and lower steering pivots with respect to vertical. On a vehicle with standard suspension, the line runs through the upper and lower ball joints. On MacPherson strut suspension, the line runs through the lower ball joint and upper strut mount or bearing plate. Viewed from the front, SAI is also the inward tilt of the steering axis. Like caster, it provides directional stability and also reduces steering effort by reducing the scrub radius.
Basically, SAI urges the wheels to a straight ahead position after a turn.
In the past, an aid to return to centre steering was achieved by caster settings, now, as SAI settings change, self centreing is more reliant on other factors, including lower friction ball joints.
What does this mean to the steering and suspension components? - there is a need to manufacture ball joints with smoother operation and lower rotational torque – due to increasing demands on self centre capabilities and requirements of steering geometry.
In the past the mechanic was used to experiencing a ‘tight’ ball joint – these days however, due to new vehicle designs, a low friction ‘looser’ style ball joint is more in fitting with requirements.
For the older Holdens and Falcons you would expect the ball joints to be quite tight in operation and with their existing steering geometry that would be quite acceptable.
Today, the ball joints for the later models have to operate in a way with much less friction. This allows an easier rotation of the stub axle and subsequently more effective self centering of the steering.
This means that today’s mechanic will begin to see ball joints and tie rod ends alike that appear ‘already worn’ and with a very free operation.
One current example of this new design requirement is the AU Falcon Lower Ball Joints. Rotational Toque on this particular ball joint should be between 5-7nM. Rotational Toque much tighter than this can lead to self centering problems.
This does not mean that excessive up and down movement of the ball stud in the joint is acceptable but rather that from new, these joints will have a smoother freer movement.
With the ever-changing market, we will continue to experience change in parts design subsequently the need to keep up with current design requirements and constant product development has never been more pertinent.