Automotive Automoti ve Vehicles Vehicles Lecture 4 Chapter 2
Body and Chassis Cha ssis (Frame) (Frame)
Dr. Dr. Kiran D. Mali Assistant Professor Department of Mechanical Engineering
Body and Chassis
2
Body and Chassis Frame Chassis
and Frame (structure) supports various components and body of the vehicle in addition to loads, it is supposed to carry.
o f auto body construction 2 principle types of 1. Uni Unibo body dy or In Inte tegr gral al Co Cons nstr truct uctio ion n
2. Bo Body dy an and d Chas Chassi siss Cons Constr truc ucti tion on Pay Load
Dead Weight
Power units
Occupants
Transmission
Goods
Suspension
Support sys system tem
3
Body Work Terminology
4
Terminologies
Purpose of the body structure To provide the mounting points for the vehicle’s rear
suspension and final drive, front suspension and
steering,
engine and gear box, tank for fuel and seats for occupants. It requires rigidity to maintain accurate handling, lightness to
reduce inertia, toughness to sustain fatigue loads from road, power unit and drive. Design is based on deflection basis rather than the stress
basis. 6
Body structure The car structure classification is as follows 1.
Integral: Main loads are carried by the members making up the structures. All body work, except doors and hatches.
2.
Semi – integral: Separate chassis frame but the remainder of the structure contributes substantially to the overall stiffness.
3. ‘Punt’ structures:
Floor is a box section with adequate torsional stiffness so that open sports or light weight body work can be fitted. This is usually of sheet metal construction, in which the floor members (cross-members,etc.) are of large closed section, with good joints between members. It is thus a grillage structure of members with high torsion and bending properties locally. In many cases (but not all), the upper body is treated as structurally insignificant. The punt structure is often used for low production volume vehicles, for which different body styles, or rapid model changes are required. 7
Body Work Requirements
8
Body structure Dynamic
measurements on test vehicle have shown peak
acceleration of 3g recorded in the vertical direction Horizontal plane, transverse cornering forces and longitudinal
braking forces are limited by the adhesion of tyre to the road and limiting retardation of 1g has become acceptable. Garret suggests multiplying these accelerations by 1.5 as a factor of
safety in design to arrive at the corresponding maximum load. Thus maximum accelerations are 1)
± 4.5g vertical
2)
± 1.5g fore –(Braking and accelerating)
3)
i
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Integral body construction •
Around 1934, the all steel body construction was introduced so that a separate frame could be eliminated.
•
This frame less construction provides a stiff, light construction, which is specifically suitable for mass produced vehicles.
•
When suitably designed the body shell is capable of withstanding the various stresses.
•
Floor and roof panels resist the sagging effect caused by the weight of the occupants. 11
Unibody or Integral Construction •
•
•
•
Individual metal parts are welded together to make up the body assembly and provide overall body rigidity trough an integral all steel welded Construction. In a fully integrated body structure, the entire car is a load-carrying unit that handles all the loads experienced by the vehicle This is sometimes also referred to as a monocoque structure, because the car's outer skin and panels are made load-bearing This design provides weight savings, improved space utilisation, and ease of manufacture .
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Body- Integral Construction 2. Body and Chassis Construction
Unibody or Integral Construction 13
Integral construction Integral Body construction
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Integral body construction Sheet metal thickness 0.915 mm for large area parts 0.765 – 0.66 mm for curved contours 1.00 – 1.25 mm cross members, pillars, rails, sills 1.00 – 1.65 mm Local reinforcements A body shell is normally fabricated either by spot welding
the panels, pillars and pressings together to form a strong box, or by building a skeleton or space frame.
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Integral body construction Car front end section Figure
demonstrates the triangulation in elevation and plan designed to provide rigid connection between front suspension and bulkhead.
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Integral body construction Crumple zone 1. Vibrations of the panels, which produces an unwanted noise called drumming, is avoided by fixing a sound damping material on inside of the panels. 2. Front and rear end of the rigid compartment are designed to concertina on impact. 3. Crumple zones of the body absorb the shock of a collision so that the rate of deceleration experienced by the occupants is reduced
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Chassis Chassis frame supports the various components and the body,
and keeps them in correct positions. The frame must be light and sufficiently strong to withstand
the weight and rated load of the vehicle without having distortion Chassis design includes the selection of suitable shapes and
cross section of chassis members. •
Materials: Cold rolled open hearth steel, treated alloy steels. 19
Chassis Chassis are classified based on controls
Conventional chassis- In which engine is mounted in front of the drivers
cabin. This arrangement avoids full utilization of space.
Semi forward chassis- In which engine is so mounted that half of it is in driver’s cabin whereas the other half is in front, outside the driver’s cabin.
Full forward chassis- In which the engine is mounted completely inside the driver’s cabin. Obviously maximum utilization of space is achieved in this
type of arrangement, •
Materials: Cold rolled open hearth steel, treated alloy steels. 20
Chassis Frame construction
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Chassis operating conditions Chassis generally experiences four major loading
situations that include. 1. Vertical bending
2. Lateral bending 3. Longitudinal torsion 4. Horizontal lozenging 22
Chassis operating conditions Vertical bending
Considering a chassis frame is
supported at its ends by the wheel axles and a weight equivalent to the
vehicles
passengers
equipment,
and
luggage
is
concentrated around the middle if its
wheelbase,
then
the
side
members are subjected to vertical bending causing them to sag in the central region.
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Chassis operating conditions Longitudinal torsion
When
diagonally
opposite
front and rear road wheels roll over bumps simultaneously, the two ends of the chassis are
twisted
in
opposite
directions so that both side and
cross
subjected
members to
are
longitudinal
torsion 24
Chassis operating conditions Lateral bending
Chassis is exposed to lateral force
that may be due to camber of the road, side wind, centrifugal force while turning a corner or collision with some object.
The adhesion reaction of the road wheel tyres opposes these lateral
forces. As a net result, bending moment acts on the chassis side members
so that the chassis frame tends to bow in the direction of the force. 25
Chassis operating conditions Obstacle Reaction
lozenging
A chassis frame, if driven forward or backward, continuously
subjected to wheel impact with road obstacles such as pot holes, road joints, surface humps and curbs while other wheels produce the propelling thrust. These conditions cause chassis frame to distort to parallelogram shape, known as lozenzing “
”
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Chassis frame design
A frame suitable for a light truck is shown Frame uses a non independent suspension system and is consisted
of 2 channel shaped side members, which are joined together by cross members These cross members are placed at points of high stress. Made up of low carbon steel Since the load varies at each point of the frame, so to reduce its
weight either the depth of channel is decreased or a series of holes will be drilled along the neutral axis in the regions where the load is 27
Chassis frame design •
•
•
Frame tapers from rear to front to permit adequate movement of the steering wheel. Longitudinal members, by sweeping upwards at the rear end , allows for vertical movement of the vehicle
Torsional rigidity of the frame is increased by providing tubular or box section cross section.
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Chassis frame design •
Frame shown in figure does not have sufficient rigidity against torsion. Back bone frame
1. Two longitudinal box section members are welded together at the center and separated at the front and rear to 2. A series of rigger frame members are accommodate the welded to the spine to support the main components floor of the body 29
Chassis frame design •
•
Chassis in older designs were made very stiff in order to improve safety for the occupants of a car when involved in collision.
Energy absorbing frame
Modern frames are manufactured as front and rear end of the frame in a manner so that it crumples in a concertina manner during collision and absorb the main shock of the impact.
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Body shapes A carbody with the aerodynamic shape passes
with least resistance through the air, as a consequence fuel economy is improved. For a vehicle without aerodynamic shape of the
body, a lot of engine power is required to drive through the air.
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Body shapes
Air flow and pressure distribution around a typical saloon car
Fig. shows a profile of a car
assumed to have not front air intake. As air approaches the body, it
has to divide to flow around it. Point of diversion is termed
as stagnation point, where the dynamic head is (1/2)ρV2 When the air flows around
the body, pressure distribution is developed as indicated 32
Body shapes When the air flows around the body, pressure distribution is
developed as indicated. As the air velocity is increased, over the roof pressure drop
occurs to cause a suction zone. At the stagnation point, as the air moves around the curved
nose it speeds up and pressure falls until at B atmospheric pressure is reached. Drag force is the summation of horizontal components of the
arrows around pressure distribution. 33
Body shapes Aerodynamic resistance of the vehicle is obtained from
3 sources. 1.
From drag, caused by the turbulence in the wake of the vehicle. This depends on shape of the vehicle body.
2.
Skin friction, caused by the shear force exerted on the vehicle surface by the air stream.
3.
Resistance due to air flow through the radiator system, interior cooling and ventilating system. 34
Force
required for overcoming air resistance
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Aerodynamic lift RL acting on a vehicle
To reduce drag 1. 2.
Recessing of protruding items such as door handles and the shaping of the body below the front bumper to form an air dam. Air flow control devices are sometimes fitted in to the rear of the vehicle. These devices smooth out the air flow to reduce the disturbances or act as a spoiler to deflect the air upwards so that the adhesive force acting on the rear wheels are increased.
Aerodynamic pitching moment M a
CM Coefficient of pitching moment Lc Characteristic length of the vehicle
s l e d o m s u o i r a v r o f s e i d o b f o s e p y t t n e r e f f i D
Chassis Types
Sedan Cars
Convertibles
Lift back
Station Wagon (Sumo, Qualis type)
Pickups
Vans
Sport utility vehicles
