FRAME AND CHASSIS
INTRODUCTION
The automobiles such as cars, buses and trucks, etc. are
generally considered to be consisting of two major assemblies, chassis and
body.
CHASSIS
Chassis is a French term which is now denotes the whole
vehicle except body in case of heavy vehicles. In case of light vehicles of
mono construction, it denotes the whole vehicle except additional fittings in
the body.
“Chassis consists of engine, power train, brakes, steering
system and wheels mounted on a frame”.
FRAME
The frame is the main part of the chassis on which remaining
parts of chassis are mounted. The frame should be extremely rigid and strong so
that it can withstand shocks, twists, stresses and vibrations to which it is
subjected while vehicle is moving on road. It is also called underbody.
The frame is supported on the wheels and tyre assemblies.
The frame is narrow in the front for providing short turning radius to front
wheels. It widens out at the rear side to provide larger space in the body.
TYPES OF FRAMES
(a) Conventional frame,
(b) Semi-integral frame, and
(c) Integral frame (or unit frame).
CONVENTIONAL FRAME
It is non-load carrying frame. The loads of the vehicle are
transferred to the suspensions by the frame. This suspension in the main
skeleton of the vehicle which is supported on the axles through springs. The
body is made of flexible material like wood and isolated frame by inserting
rubber mountings in between. The frame is made of channel section or tubular
section of box section.
SEMI-INTEGRAL FRAME
In this case the rubber mountings used in conventional frame
between frame and suspension are replaced by more stiff mountings. Because of
this some of the vehicle load is shared by the frame also. This type of frame
is heavier in construction.
INTEGRAL FRAME OR FRAME-LESS CONSTRUCTION
In this type of construction, there is no frame. It is also
called unitized frame-body construction. In this case, the body shell and
underbody are welded into single unit. The underbody is made of floor plates
and channel and box sections welded into single unit. This assembly replaces
the frame. This decreases the overall weight compared to conventional separate
frame and body construction.
TYPES OF SECTIONS USED IN FRAMES
(a) Channel section,
(b) Tubular section, and
(c) Box section.
SUSPENSION SYSTEM
FUNCTIONS OF SUSPENSION SYSTEM
In Automobiles
1. To prevent road shocks from being
transmitted to the vehicle frame and its
Parts.
2. To preserve the stability of the
vehicle in pitching or rolling while in motion.
3. To safeguard the passengers and cargo
from shocks.
4. To provide good gripping on road
while driving, cornering and
Braking.
5. To maintain proper steering geometry.
Front End Suspension: In order to permit
the front wheels to swing to
one side or the other for steering, each
wheel is supported on a spindle
which is part of a steering knuckle.
The Front-end suspension are of two
types:
1. Rigid front axle suspension
2. Independent front suspension
Rigid front Axle suspension: In this type of suspension, the front wheel
hubs rotate on antifriction bearings on
steering spindles which are
attached to the steering knuckles. In
this suspension either two longitudinal
leaf springs or transverse springs
usually along with shock absorbers are
used.
Independent front Suspension:
In independent front suspension, each
front wheel is independently
supported by coil spring torsion bar or
leaf spring along with shock
absorber.
There are three types of coil spring
arrangements. In the first type,
the coil spring is located between the
upper and lower control arms. The
lower
control arm has one point attachment to the frame.
In the second type, the coil spring is
located between the upper
and lower control arms. The lower
control arms has two-point attachment
to the frame.
In the third type the coil spring is
between the upper control arm
and spring tower in the front end of the
body.
TYPES OF SUSPENSION SPRINGS:
1. Steel Springs
a) Leaf Spring
b) Tapered leaf spring
c) Coil spring
d) Torsion Bar
2. Rubber Springs
a) Compression Spring
b) Compression shear spring
c) Steel reinforced spring
d) Progressive spring
e) Face shear spring
f) Torsional shear spring
3. Plastic Spring
LEAF SPRING:
Leaf spring consists of a number of
leaves called blades. Every
successive blade is shorter than the
main leaf. The lengthiest blade in
main leaf (Master leaf) which has eyes
on its both ends. All the blades
are clamped together. The spring is
supported on the axle. One end of
the spring is mounted on the frame with
a simple pin, while on the other
end, connection is made with a shackle.
When the vehicle comes across
a projection on the road surface the
wheel moves up deflecting the
spring. This changes the length between
the eyes of the spring. If both
ends are fixed, the spring will not be
able to accommodate the change
of length. Therefore, the shackle is
provided to hing the Leaf spring unit.
Generally, the leaf springs used at rear
part kept longer than front leaf
spring unit. The leaver of leaf spring
require lubrication at periodic
intervals
except in some vehicles like Hindustan Ambassador Cars.
HELPER SPRINGS:
Helper springs are provided on many
commercial vehicles in
addition to main leaf springs. These
helper springs will only come into
operation when the load is increased.
These are used for rear suspension
only.
SHACKLE: The Front end of Leaf spring is
fixed with the bracket of the
frame while its rear end is connected
with the shackle and the shackle is
connected to the frame. The shackle is
hinged with the frame. It can
swing forward or backward.
COIL SPRING: The coil springs are mainly
used with independent
suspension system. The energy stored per
unit volume is almost double in
the case of coil springs than leaf
springs. The spring takes the shear as well
as bending stresses but cannot take
torque reaction and side thrust.
SHOCK ABSORBERS:
The shock absorber is a damping device
and is used to damp the
vibrations of the spring. It prevents
excessive flexing of the spring and
enhances riding comfort. It is used
along with leaf spring or coil spring.
The shock
absorber controls the vibration of spring by damping it down.
The frame and body of an automobile are mounted on front and
rear axles through springs and shock absorbers. If it is mounted directly on
axles, all the socks and vibrations will be transmitted to body causing
discomfort to the passengers. The springs and shock absorbers are used to damp
the shocks and vibrations. The suspensions system includes all those parts
which are used to perform the damping action. Besides, springs and shock
absorbers, a suspension system includes other mountings also. The suspension
system of a vehicle is divided into front suspension and rear suspension.
FUNCTIONS OF SUSPENSION SYSTEM
(a) The main function of a suspension system is to prevent
the socks to transmit to car or vehicle body so that passengers may ride
comfortably.
(b) To maintain the stability of vehicle during pitching and
rolling actions while the vehicle is in motion.
(c) To provide better road holding at the time of driving,
braking and cornering.
(d) To allow proper steering geometry.
SPRINGS
Different types of springs are used in the suspension system
of an automobile. Springs absorb the energy which is generated due to force
which comes when vehicle moves over bumps and trenches. Springs are required to
absorb the energy of shocks very quickly and release it slowly and slowly. For
this a absorber is also used. Coil springs and leaf springs are used in the
automobiles. Besides this some other devices are also used such as torsion bars
and shock absorbers. Description of these devices is given in the following
sections.
LEAF SPRINGS
These springs are made by placing several flat strips one
over the other. These are made of steel plates. One flat strip is called a
leaf. Lowest leaf is of smallest length and the length of other leaves placed
above this keeps on increasing progressively. In this way, the length of top
most leaf (main leaf) largest. Main leaf has eyes at the ends. All the leaves
are clamped together at centre and sides by the centre bolt and side clamps
respectively. The centre portion of the leaf springs is connected to the axle
with the help of U-bolt. A simple sketch of leaf springs is shown in Figure
8.2.
Spring eye is used to attach spring to the body frame by
passing a bolt through one eye. Other end of leaf spring is attached to a
shackle through its eye. Shackle is in turn attached to chassis. The shackle is
used to accommodate any change in length of spring due to its expansion and
contraction. The contraction and expansion take place when the vehicle passes
over road surface irregularities. Semi-elliptical springs are generally used in
all the vehicles particularly n trucks. In case, leaf springs were used in rear
suspension and independent suspension in the front. But, leaf springs are not
used n cars also.
COIL SPRINGS
Coil springs are in the form of helix. These are made from
special steel. It is made from steel wire in the form of a coil. The coil springs
absorb energy when this spring is compressed while vehicle moves over road
bump. The coil springs are mainly used in independent suspension. However,
these can also be used in the conventional rigid axle suspension. Coil springs
are capable of resisting shear and bending stresses but not torsion and side
thrust.
When coil springs are used in the suspension system, other
arrangements are made to bear torsion and side thrust.
SHOCK ABSORBERS
If only springs are used to absorb shocks, the oscillations
of springs continue even after the vehicle has passed over a bump. The
oscillations cause the wheels the jumps up and fall down till the oscillations
die out. Thus, dampers or shock absorbers are used to arrest the oscillation of
springs after the vehicle passes over irregular road surface.
Shock absorbers are necessary used with coil springs. In
case of leaf springs, the friction between leaves provides some dampening
effect. However, this is not sufficient sometimes, depending upon friction
between leaves. Hence, shock absorbers are necessarily used as additional
damping devices.
TYRES
Tyres are mounted on the rims of wheels. They enclose a tube
between rim and itself. Air is filled at a designated pressure inside the tube.
The tyre remains inflated due to air pressure inside tube. The tyre carries the
vehicle load and provides cushioning effect. It absorbs some of the vibrations
generated due to vehicle’s movement on uneven surfaces. It also resists the
vehicle’s tendency to over steer or turn which cornering. Tyre must generate
minimum noise when vehicle takes turn on the road. It should provide good grip
with the road surface under all conditions.
TYPES OF TYRES
(a) Tube tyres, and
(b) Tubeless tyres.
Tube Tyres
Tube tyres encloses a tube which is wrapped on the wheel
rim. Air is forced into tube which inflates the tube and tyre. The outer side
of tyre which comes in contact of road is made from rubber. It is called tread.
Tread provides resistance to slipping. It is very thick at the outer periphery.
Beads are made at the inner bide by reinforcing it with steel wires. Beads are
very strong which have good resistance to wearing against the wheel rim. Rayon
cords are also formed into a number of piles. Beads are cords provide good
strength to tyres.
Tubeless Tyres
These tyres do not require any tube. The air at pressure is
filled into the tyre itself. The construction of tyre is same as that of tube
tyre. For filling the air, a non-return valve is filled in the tyre itself.
CAUSES OF TYRE WEAR
(a) Lower or Higher Tyre Pressure: If the tyre pressure is
perfect, there will be full tread contact with the road. If tyre pressure is
lower than required, severe flexing of tyre piles, and side walls take place
(b) Tyres wears out more one on side than the reason is
incorrect caber setting.
(c) Toe-out causes remarkable wear on tread inner end of
both front wheels.
(d) High speed of vehicle is also the cause of more tyre
wear and failure.
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