Monroe's Technical Support will help you find
answers to the most frequently asked product and installation questions
and provide invaluable technical training information. If you need
additional information or assistance, Monroe's Ride Control Technical
Assistance Team can assist you.
UNDERSTANDING VEHICLE DYNAMICS BASIC TERMINOLOGY
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.
