Adams Assignments – Problem 4 – Steering geometry 2

AIM :-

To perform kingpin geometry optimization to achieve the following targets,

• Caster angle = 4 deg.
• Caster moment arm = 21 mm.
• king pin inclination angle = 13 deg.
• Steer axis resultant offset = 85 mm.
• Steer axis longitudinal offset = 0 mm.
• Scrub radius = 20 mm.

Compare the values and state how the changes affect the system. Explain if all the parameters can be controlled independently.

MODEL AND SIMULATION :-

The model studied is the double wishbone suspension assembly.

The hardpoints are checked to ensure that the default coordinates are present. Parallel wheel travel simulation is conducted with 250 steps and 100 mm of suspension travel in jounce and rebound. After the simulation, the following values are obtained.

• Caster angle = 5.36 deg.
• Caster moment arm = 8.58 mm.
• Kingpin inclination angle = 10.085 deg.
• Steer axis resultant offset = 85.77 mm.
• Steer axis longitudinal offset = -18.47 mm.
• Scrub radius = 33.8 mm.

These are considered as the baseline values and the hardpoints are modified to achieve the targets.

OPTIMIZING THE ASSEMBLY :-

The hardpoints of the upper and lower control arm balljoints are modified. The variation is performed in small increments or decrements and through trial and error, the following hardpoint coordinates are obtained.

The x coordinates of the ball joints are the same to obtain a straight geometry as opposed to the default geometry which is shown below.

The image on the left is the original geometry and the image on the right is the modified geometry. The simulation is run again with the same number steps and suspension travel and the following values are obtained,

• Caster angle = 4 deg.
• Caster moment arm = 21.24 mm.
• Kingpin inclination angle = 12.05 deg.
• Steer axis resultant offset = 85.77 mm.
• Steer axis longitudinal offset = 0.88 mm.
• Scrub radius = 20.52 mm.

The above values are the targets.

OUTPUT PLOTS :-

The output plots and the effect of variation is provided below.

Caster angle,

A lower caster angle would affect the straight line stability of the vehicle.

Caster moment arm,

A higher caster moment arm increases the self aligning torque but would make the steering quite heavy.

Kingpin inclination angle,

A higher kingpin inclination angle will produce a larger vertical displacement of the vehicle during steering in an upward direction. The lifting effect produces a self centering torque similar to that of a caster.

Steer axis resultant offset,

A lower resultant steer axis offset would reduce the effect of torque steer.

Steer axis longitudinal offset,

A lower longitudinal steer axis offset would increase the self aligning torque and improves straight line stability.

Scrub radius,

Lower scrub radius decreases the braking disturbances of the vehicle.

CAN THE PARAMETERS BE CONTROLLED INDEPENDENTLY? :-

No, the parameters cannot be varied individually, because they are interdependent. Tuning the hardpoints to achieve on target will affect the other target. Which is why the parameters cannot be controlled independently.

CONCLUSION :-

Kingping geometry optimisation is performed and the target values are achieved.