By Gene Markel
It is
a common mistake to look at a vehicle’s chassis and suspension system
as just components that are either good, bad or broken. It should be
looked at as a system. Understanding basic terminology and vehicle
configurations is important to the technician for maintaining or
returning that new car chassis control and performance.
It is also
necessary to identify the basic suspension types and components before
describing the dynamics of chassis control and their effects on vehicle
performance. The goal is to have a motorist say, “It rides like it did
when it was new,” whether it be luxury or sport. Suspension engineers
spend countless hours designing and tuning a suspension to the chassis
to produce the ride and performance that is expected by the potential
new car buyer. A modification to the chassis or suspension can have a
significant effect on vehicle safety and performance.
Chassis/Frame/Body
The
chassis can be either a frame and body or a uni-body. The construction
of the chassis will locate the vehicle center of gravity or mass. The
center of gravity or mass is one of the most important safety and
performance factors in the design and construction of a vehicle.
Suspension
There
are 16 types of suspensions in use today ranging from solid axles to
struts. The suspension’s main job is to let the wheels move
independently from the body.
One thing that almost all of them have
in common are elastomer bushings. These soft parts of the suspension
are used to dampen road shock before it is transferred to the chassis.
The
friction created by the bushing’s position in the linkage has a
dampening effect when the linkage moves around the bushing, just like
the early friction shocks of the 1920s.
Springs
There are
four types of springs used to suspend the mass of the vehicle: Air,
Coil, Leaf and Torsion. The springs help to support the weight of the
vehicle and absorb and releases energy. This energy is adsorbed by the
shocks or struts.
Shock Absorber
There are two types of
hydraulic valve shock absorbers — dual cylinder and monotube. The dual
cylinder has a piston and base valve. The piston has two sets of
orifices and valves to meter fluid for the compression and rebound
stroke of the shock.
The base valve is used to compensate for the
displacement of fluid caused by the piston shaft movement in and out of
the inner cylinder and meters fluid on the compression stroke of the
shock. The outer tube acts as a reservoir for the fluid displaced by
the shaft and its lower mounting. The upper mounting is attached to the
shaft. The monotube shock uses a piston and pressurized chamber at the
bottom of the shock to compensate for the displacement of fluid caused
by the shaft. The piston valves are the same as the dual cylinder. The
magnetorheological fluid shock is a monotube design with one set of
orifices. The magnetic field generated by the coil in the piston will
control fluid flow in both directions. The shock piston valves are
tuned to the unsprung weight to dampen suspension travel.
The
MacPherson Strut is a combination of a spring and shock absorber and
upper mount that contains a bearing to allow the strut to rotate with
the steering knuckle. An air strut uses a tapered sleeve type air
spring to replace the coil spring.
Measuring Chassis Performance
Measuring
chassis performance begins with the engineer and a computer model
before the vehicle is built. The dynamics of the suspension and chassis
are placed on a three-dimensional set of axes based on the front of the
vehicle. The X, Y and Z axes intersect at the center of gravity. The
center of gravity is the point on the vehicle where you could place a
jack and lift all four wheels and the vehicle would remain level. The Z
or roll axis is at the centerline of the vehicle and passes from the
rear suspension roll center to the front suspension roll center.
The
Y or yaw axis is perpendicular to the Z axis. The X axis is on the same
plane as the Z axis. The roll center is determined by the center point
of the axle. In the case of an independent suspension, the center point
is located between the two lower control arms.
Lateral Acceleration/Cornering
The
steering angles generated by the knuckle and linkage are generally
referred to as Ackerman steering angles. These angles are associated to
low speed turning radius dimensions. As vehicle speed increases,
steering angles tend to move forward toward the center of gravity. As a
vehicle executes a cornering maneuver, the chassis is affected on the
X, Y and Z axes. The X axis is a lateral acceleration that pushes the
vehicle toward the outside of the corner. It is measured in “G.” G or
Gravity is the attraction of a body toward the earth. The resting
weight of a body on the surface is calculated as 1 G. The Z axis is
affected by a rotational acceleration that would tip the vehicle toward
the outside of the corner. It would be measured in degrees per second.
As the rear wheel slip angle increases, it causes the chassis to rotate
at its center of gravity.
The faster the corner, the more the
chassis will tend to roll. During a cornering maneuver, the entire
weight distribution of the vehicle changes. In a right-hand corner, the
most weight is placed on the left front wheel.
The right rear wheel
carries the least amount of weight. If the vehicle is braking in the
corner, even more weight is shifted to the left front wheel. Chassis
roll is determined by an axis running from the rear roll center to the
front roll center of the vehicle. The Z axis determines the rotation of
the chassis during a corner. The inclination of the chassis and center
of mass of a vehicle determine how much chassis roll will occur during
a corner.
Acceleration and Deceleration
The center of
gravity and its mass affects the amount of vehicle pitch during an
acceleration or deceleration. The location of the suspension linkages
and axles are designed to minimize these rotational forces. As an
example, changing the mounting angle of an upper control, arm or
MacPherson strut can reduce the amount of pitch during a deceleration
or stop.
The extremes of vehicle construction can best demonstrate
how the roll center and center of gravity or mass affects chassis roll.
The Corvette is a performance vehicle designed for high-speed
cornering. It has a roll center that is parallel and as close to ground
level as possible. Its center of mass is very close to the roll center
and is located in the center of the vehicle. This produces an almost
neutral position during high-speed corners.
The ground clearance of
the Corvette is minimal, as it is to be driven on maintained paved
surfaces. A van is a utility vehicle designed to transport passengers
and cargo and its ground clearance is much greater. Its roll center
axis is on an incline as the roll center of the rear axle is higher
than the control arms of the independent front suspension.
Its
center of mass is toward the front of the vehicle and can change based
on how the vehicle is loaded. The suspension of the van will receive
more stress under normal service than those in the Corvette.