Understanding Vehicle Dynamics

To begin this training program, you need to possess some very basic information. The chassis is what connects the tires and wheels to the vehicle's body. The chassis consists of the frame, suspension system, steering system, tires and wheels.

The frame is the structural load-carrying member that supports a car’s engine and body, which are in turn supported by the suspension and wheels.
The suspension system is an assembly used to support weight, absorb and dampen road shock, and help maintain tire contact as well as proper wheel to chassis relationship.
The steering system is the entire mechanism that allows the driver to guide and direct a vehicle.

The side to side distance between the centerline of the tires on an axle is called track. The distance between the centerline of the front and rear tires is called wheelbase. If the vehicle is in proper alignment, the wheels will roll in a line that is parallel with the vehicle’s geometric centerline.

You should also understand that tires and wheels make vehicle motion possible. The amount of grip or friction between the road and the tires is the major factor that limits how the vehicle accelerates, maneuvers through corners, and stops. The greater the friction, the faster the car can accelerate, corner and stop.

The tire to road contact of a vehicle is affected by several forces. Vehicle dynamics is the study of these forces and their effects on a vehicle in motion. Our discussion will concentrate on how these forces affect handling with some consideration given to how they affect acceleration and deceleration.

FUNDAMENTALS OF HANDLING

Vehicle geometry, suspension, and steering design all affect the handling of a vehicle. To better understand the term “handling,” we can address the following fundamentals that contribute to good handling:

Road Isolation
Road isolation is the vehicle’s ability to absorb or isolate road shock from the passenger compartment. The degree to which this is accomplished is controlled by the condition of the suspension system and its components.

A properly functioning suspension system allows the vehicle body to ride relatively undisturbed while traveling over rough roads. This is accomplished through the combined use of bushings, springs, and hydraulic dampers.

The springs support weight as the vehicle travels down the road. When a vehicle encounters a bump in the road, the bushings receive and absorb the inputs from the road, while the springs compress and store kinetic energy. This energy is then released, causing a rebound in the vehicle’s weight. The rate at which the springs compress and rebound is controlled using a hydraulic damper, such as a shock absorber or strut. The result of this action is to limit the amount of road input felt in the passenger compartment.

Road Holding
Road holding is the degree to which a car maintains contact with the road surface in various types of directional changes and in a straight line. Remember that the vehicle’s ability to steer, brake, and accelerate depends first and foremost on the adhesion or friction between the tires and the road.

Tire force variation is a measure of the road holding capability of the vehicle and is directly influenced by shock absorber or strut performance. Shock absorbers and struts help maintain vertical loads placed on the tires by providing resistance to vehicle bounce, roll and sway during weight transfer. They also help reduce brake dive along with acceleration squat to achieve a balanced ride.

Worn shocks and struts can allow excessive vehicle weight transfer from side to side and front to back, which reduces the tire's ability to grip the road. Because of this variation in tire to road contact, the vehicle’s handling and braking performance can be reduced. This may affect the safe operation of the vehicle and the safety of those riding inside. Therefore, shocks and struts are safety components.

Tire loading changes as a vehicle's center of gravity shifts during acceleration, deceleration, and turning corners. The center of gravity is a point near the center of the car; it is the balance point of the car.

The size of the four contact patches of traction at the tires also varies with the changes in tire load. As the vehicle brakes, inertia will cause a shift in the vehicle’s center of gravity and weight will transfer from the rear tires to the front tires. This is known as dive. Similarly, weight will transfer from the front to the back during acceleration. This is known as squat.

Consistently controlling vehicle weight transfer and suspension movement enhances the road holding capability of the vehicle and ultimately its safe operation.

Cornering
Cornering is defined as the ability of the vehicle to travel a curved path. It is also referred to as cornering power or lateral acceleration. Many things can affect the cornering ability of a vehicle, such as:

Tire construction
Tire tread
Road surface
Alignment
Tire loading

As a vehicle turns a corner, centrifugal force pushes outward on the car’s center of gravity. Centrifugal force is resisted by the traction of the tires. The interaction of these two forces moves weight from the side of the vehicle on the inside of the turn to the outside of the car, and the car leans. As this occurs, weight leaves the springs on the inside and that side of the vehicle raises. This weight goes to the springs on the outside, and that side of the vehicle lowers. This is what is known as body roll.

When the cornering requirement of a particular maneuver is less than the traction that can be provided by the tires, the car will go where it is pointed and steered. However, if the cornering force exceeds the available traction from the tires, the tires will slip across the road surface and they will skid.