Steering Geometry of a Utility Vehicle: Chassis

Steering Geometry of a Utility Vehicle: Chassis

The steering geometry of a utility vehicle is a complex system. Castor, steering kinematics, toe-in, camber, and kingpin inclination ensure that the wheels always remain under control.

Depending on the application area, a truck may roll on more or fewer axles. These do not only have to carry the load of the truck. They are also part of the steering system. The wheels of the front axle are usually suspended in a steerable manner. However, demanding application areas require several steered axles. Complex axle geometry with accurately defined angles is required to suspend all these potentially unstable wheels in a safe way.

The purpose of the suspension geometry is to transmit all forces that act on the tires onto the superstructure. This includes, for example, the steering forces of the tires and the forces that occur during acceleration and braking. When the driver turns the steering wheel, the steering geometry determines the wheel position and transfers the forces between tire and road to the steering linkage.

In order to avoid that the wheels on the steering axle—which must not be suspended rigidly but need to remain steerable—do their own thing and flutter uncontrollably, they have to be controlled, and this control is accurately defined by the castor, toe-in, camber, kingpin  inclination, and kingpin offset.

Castor Sets the Wheels Straight

The castor ensures that the wheels always point straight in the driving direction as long as the vehicle is moving. This is best described by referring to teat and shopping carts. The wheel is mounted so that the kingpin is offset diagonally towards the axle. Its central line hits the floor before the middle of the wheel. The actual wheel axle is therefore behind the rotation point of the wheel construction.

Due to this construction, the wheel is virtually pulled forwards. It therefore trails and automatically aligns in a driving direction. This effect is generated when a lateral force that acts on the tire while driving around a curve is transferred onto a lever. The resulting restoring torque acts to align the wheel in a straight direction. The castor angle describes the position of the middle line of the steering axle relative to the middle of the tire support surface. The castor angle of trucks is usually approx. three degrees.

Toe-In Prevents Flapping

The tilt of the wheels is also important for stability. The toe-in ensures that the wheels do not flap on the axle in an uncontrolled manner. The wheels are therefore not parallel to each other but tilted towards the inside when pointed in a driving direction. The toe-in is measured by the difference in distance at the edges of the wheel rims at the height of the middle of the wheel. The toe-in has another important function. While running straight forward, the rolling resistance generates a force that pushes the left wheel towards the left side and the right wheel towards the right side. The toe-in counteracts this and prevents the wheels from rolling away from each other. This is different when the front wheels are powered. The opposite effect is generated when the wheels are exposed to torque: the propelling forces press the wheels towards the inside, not towards the outside. The wheels should in this case be tilted towards the outside. This is referred to as toe-out.

However, the wheels can also take other positions as part of the steering geometry. The camber describes the position of the wheel relative to the driving surface. The camber originated in coach construction. Its purpose was to prevent the wheel from jumping off the axle when the wheel nut is loose. Positive camber means that the wheels tilt outwards while negative camber means that they tilt inwards. Positive camber reduces the effect of the bumps in the road on the components of the steering, ensures even wear of the tires, and reduces the steering forces required. Negative camber means that the wheels are tilted towards the inside. This position increases the cornering forces of the tires. Trucks normally have a positive camber of one to two degrees.

The Tilt of the Kingpin Determines the Steering Effect

The kingpin offset also has an important role in the axle geometry system. It determines the tilt of the kingpin towards the longitudinal vehicle plane. It is usually approx. five degrees. The kingpin offset and the camber jointly determine the steering effect. It also determines the steering force required as well as the restoring torque and ensures that the wheels automatically return to their initial position after driving through a curve. The driver would have to turn the steering wheel back after each curve if this resetting momentum were not present. This would obscure any feeling for driving performance and cornering speed. It would also add an additional risk: the truck would go off the road if the steering were not reduced in due time while driving around a curve.