CHASSIS AND BODY

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15 Νοε 2013 (πριν από 3 χρόνια και 8 μήνες)

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CHASSIS AND BODY

CHASSIS AND BODY

CHASSIS AND BODY

CLASSIFICATION OF CHASSIS

CLASSIFICATION OF CHASSIS

CLASSIFICATION OF CHASSIS

FRAME

FRAME

TYPES OF FRAME

TYPES OF FRAME

Integral and
chassisless

construction

The terms
integral and
chassisless

construction are often confused, but the

difference is simple. Integral construction is that in which a chassis frame is

welded to, or integrated with, the body. It was the first stage in the evolution

of the
chassisless

form of construction, in which no chassis frame can be

discerned. The first two quantity
-
produced vehicles in the latter category

were almost certainly the Saab 92 and the Austin A30, the design of which was

very fully described in
Automobile Engineer, October and December 1952,

and March and April 1954.

TYPES OF FRAME: INTEGRAL AND

CHASSISSLESS FRAME

The first two of these four references describe the vehicle itself, while the second pair
elaborate on methods, developed by the author, for use in its structural design.

The details of
chassisless

construction are much too numerous, varied and

complex to be described here. In principle, however, its advantages stem

from the facts that beams formed by the body panels may be something like

50 cm deep, whereas a chassis frame for a car is only about 8 to 13 cm deep,

and the area enclosed by a complete body is similarly vastly bigger than that

enclosed by the cross
-
section of a frame side or transverse member. Since

the strength and stiffness of a beam are proportional respectively to the

square and cube of its depth, while both the
torsional

stress and stiffness of

a box section are proportional to the area enclosed by it, it follows that the

strength and stiffness of a body shell are potentially much greater than of a

chassis frame.

TYPES OF FRAME: CHASSISLESS FRAME

The Austin A30 was almost certainly the first car of truly
chassisless

construction to go into
quantity production anywhere in the world. Virtually all the panels of 0.914 mm thick steel,
the principal exceptions being some 1.299, 1.626 and 2.032 mm brackets carrying the front
and rear suspension, the 1.626 mm front apron and two inverted channel sections on each
side of and parallel to the engine large load provided that it is
stabilised



supported against
buckling or other forms of distortion

CHASSISLESS FRAME

Backbone
-
type frames have also been used. The advantages of the backbone frame include


high
torsional

stiffness at low cost, and light weight. A disadvantage is the length of the

outrigger arms needed to carry the body sides. These arms tend to introduce
torsional


vibration problems because of their bending flexibility.

TYPES OF FRAME

BACK BONE TYPE FRAME:

SUB
-
FRAMES

Sub
-
frames are employed for one or more of three basic reasons. The first is to
isolate the high frequency vibrations of, for example, an engine or a

suspension assembly, from the remainder of the structure. In this case, rubber

or other resilient mountings are interposed between the sub
-
frame and main

structure.

Secondly, a sub
-
frame can isolate an inherently stiff sub
-
assembly such as

the engine or gearbox from the effects of the flexing of the chassis frame.

This is done generally by interposing a three
-
point mounting system between

the sub
-
frame and main frame, one of the mountings being on the longitudinal

axis about which the main frame twists, and the others one on each side.

Thirdly, a sub
-
frame may be used to carry, for instance, the front and rear

suspension sub
-
assemblies, where to
utilise

the front and rear ends of the

body structure for this purpose would increase unacceptably its complexity

or cost, or introduce difficulties in either manufacture or servicing, or both.

A good example of such sub
-
frame usage is the BL
Mini, the front and rear

sub
-
frame assemblies of which have been used by some kit car manufacturers

because of the ease with which the engine and front suspension, on its
subframe
,

can be bolted to the front, and the rear suspension, similarly on its

sub
-
frame, bolted to the back of a different body designed to receive them.

MATERIAL OF FRAME & LOADS ON FRAME

Mild steel


easily pressed and welded


used to be the invariable choice for all frames, but
modern heavy commercial and even some light vehicles frequently have frames of carbon
manganese steel with a yield stress of about 3620 kg/cm2.

With the introduction of independent front suspension, chassis frames were called upon
to take much higher
torsional

loading. This was because, whereas the
centres

of semi
-
elliptic leaf springs on a beam axle have to be well inboard of the front wheels to leave a
clearance for steering them, the effective spring base


distance between spring
centres



with independent front suspension is approximately equal to the track. In these
circumstances, when a wheel on one side only rises over a bump, the upward thrust it
exerts on the frame has a much greater leverage about the longitudinal axis of the car

The transverse members most heavily loaded in torsion are of course those that
support the independent front suspension. This is partly because of brake
-
torsion
reaction which is applied by the rearward thrust of the road on the
tyre

contact patch
and transmitted through the brake disc or the drum brake
backplate

to the stub axle,
and thence through the suspension links to the frame

LOADS ON FRAME

. Additionally, an entirely different
torsional

loading arises in this transverse member as a
result of single
-
wheel bumps


when the wheel on only one side rises. Such a bump,
lifting one side of the front end of the frame, leaves the far side and the rear end down
in their original positions, thus causing the side members to tend to twist the front
transverse member and, incidentally, all the others. Hence, heavy gusseting is needed
between the transverse and side members. Sudden local changes in stiffness at or near

the junctions between the transverse and side members have to be avoided, otherwise
trouble due to fatigue failures will be experienced.

Tubular sections of any shape


round, oval, triangular, square, rectangular, etc


are inherently very rigid
torsionally
. Such sections therefore began to be used for
both longitudinal and transverse members on car frames. A selection of sections

that have been used is illustrated in Fig.

CROSS
-
SECTIONS OF FRAME