MECHANICS OF PNEUMATIC TIRES

Aside from aerodynamic and gravitational forces, all other major forces and
moments affecting the motion of a ground vehicle are applied through the
running gear–ground contact. An understanding of the basic characteristics
of the interaction between the running gear and the ground is, therefore,
essential to the study of performance characteristics, ride quality, and handling
behavior of ground vehicles.

The running gear of a ground vehicle is generally required to fulfill the
following functions:

• to support the weight of the vehicle
• to cushion the vehicle over surface irregularities
• to provide sufficient traction for driving and braking
• to provide adequate steering control and direction stability.

Pneumatic tires can perform these functions effectively and efficiently;
thus, they are universally used in road vehicles, and are also widely used in
off-road vehicles. The study of the mechanics of pneumatic tires therefore is
of fundamental importance to the understanding of the performance and char-
acteristics of ground vehicles. Two basic types of problem in the mechanics
of tires are of special interest to vehicle engineers. One is the mechanics of
tires on hard surfaces, which is essential to the study of the characteristics of
road vehicles. The other is the mechanics of tires on deformable surfaces
(unprepared terrain), which is of prime importance to the study of off-road
vehicle performance.

A pneumatic tire is a flexible structure of the shape of a toroid filled with
compressed air. The most important structural element of the tire is the car-
cass. It is made up of a number of layers of flexible cords of high modulus
of elasticity encased in a matrix of low modulus rubber compounds,. The cords are made of fabrics of natural, synthetic, or metallic composition, and are anchored around the beads made of high tensile strength steel wires. The beads serve as the ‘‘foundations’’ for the carcass and provide adequate seating of the tire on the rim. The ingredients of the rubber com-
pounds are selected to provide the tire with specific properties. The rubber
compounds for the sidewall are generally required to be highly resistant to
fatigue and scuffing, and styrene–butadiene compounds are widely used
The rubber compounds for the tread vary with the type of tire. For
instance, for heavy truck tires, the high load intensities necessitate the use of
tread compounds with high resistance to abrasion, tearing, and crack growth,
and with low hysteresis to reduce internal heat generation and rolling resis-
tance. Consequently, natural rubber compounds are widely used for truck
tires, although they intrinsically provide lower values of coefficient of road
adhesion, particularly on wet surfaces, than various synthetic rubber com-
pounds universally used for passenger car and racing car tires [1.1]. For tube-
less tires, which have become dominant, a thin layer of rubber with high
impermeability to air (such as butyl rubber compounds) is attached to the
inner surface of the carcass.

The load transmission of a pneumatic tire is analogous to that of a bicycle
wheel, where the hub hangs on the spokes from the upper part of the rim,
which in turn is supported at its lower part by the ground. For an inflated
pneumatic tire, the inflation pressure causes tension to be developed in the
cords comprising the carcass. The load applied through the rim of the wheel
hangs primarily on the cords in the sidewalls through the beads.
The design and construction of the carcass determine, to a great extent,
the characteristics of the tire. Among the various design parameters, the ge-
ometric dispositions of layers of rubber-coated cords (plies), particularly their
directions, play a significant role in the behavior of the tire. The direction of
the cords is usually defined by the crown angle, which is the angle between
the cord and the circumferential center line of the tire, .
When the cords have a low crown angle, the tire will have good cornering
characteristics, but a harsh ride. On the other hand, if the cords are at right
angle to the centerline of the tread, the tire will be capable of providing a
comfortable ride, but poor handling performance.

A compromise is adopted in a bias-ply tire, in which the cords extend diagonally across the carcass from bead to bead with a crown angle of ap-proximately 40 degree,. A bias-ply tire has two plies (for light-load tires) or more (up to 20 plies for heavy-load tires). The cords in adjacent plies run in opposite directions. Thus, the cords overlap in a diamond-shaped (criss-cross) pattern. In operation, the diagonal plies flex and rub, thus elongating the diamond-shaped elements and the rubber-filler.This flexing action produces a wiping motion between the tread and the road, which is one of the main causes of tire wear and high rolling resistance