AUT203
CHAPTER: 1. CHASSIS
FRAME AND BODY
Objectives
• Define the most important parts of a vehicle
• Explain body design and frame variations
• Compare unibody and body-over-frame
construction
• Identify the major structural parts, sections, and
assemblies of body-over-frame vehicles
• Explain the vehicle body repair technology
Introduction
Vehicle construction refers to how a vehicle is made.
Chassis is a French term and was initially used to
denote the frame parts or Basic structure of the
vehicle. It is the backbone of the vehicle. A vehicle
without body is called Chassis. The components of
the vehicle like power plant, Transmission system,
Axles, Wheels and Tyres, Suspension, Controlling
System like Braking, Steering etc,.. and also electrical
system parts are mounted on the Cassis frame. It is
the main mounting for all the components including
the body. So it is also called as Carrying Unit.
Layout of Chassis and its main
Components
Crash Testing
• Vehicles must be light, aerodynamic, and yet
strong and safe
• Computer-simulated crash testing is used before
building a prototype to find weak structural areas
• Critical that passenger compartment is strong
enough to prevent injury
• Certified crash tests are done with a real vehicle
and sensor equipped-dummies
• Crush zones are built into the frame or body to
absorb some of the energy of a collision
Vehicle Classifications
• Vehicle classification relates to the construction,
size, shape, number of doors, type of roof, and other
criteria of a motor vehicle
• To communicate properly in collision repair, you
must understand these basic terms
Functions of the chassis frame
To carry load of the passengers or goods carried in body
To support the load of the body, engine, gearbox etc.
To withstand the forces caused due to the sudden braking or
acceleration
To withstand the stresses caused due to the bad road
condition and To withstand centrifugal force while cornering
Types of chassis frames
Conventional frame: It has two long side members and 5
to 6 cross members joined together with the help of rivets
and bolts. The frame sections are used generally.
• Channel section – Good resistance to bending
• Tabular Section – Good resistance to Torsion
• Box Section -Good resistance to both bending and
torsion
• Integral Frame: This frame is used now days in most of
the cars. There is no frame and all the assembly units
are attached to body. Only disadvantage is repairing is
difficult.
• Semi-Integral Frame: In same vehicles half frame
is fixed in the front end on which engine gear box
and front suspension is mounted.
Various loads acting on the frame
1.Short duration Load- while crossing a broken patch
2.Momentary duration Load – While taking a curve
3.Impact loads – due to the collision of the vehicle
4.Inertia load – while applying brakes
5.Static Loads – Loads due to chassis parts
6.Over loads – Beyond Design capacity
Vehicle Construction
• Part or component refers to the smallest units on a
vehicle
• An assembly is several parts that fit together
• Panel refers to a large removable body part
• Pan refers to a floor-related component
• Chassis includes everything under the body,
consisting of the mechanical systems that support and
power the car
Vehicles have different construction methods according
to how it can attach to the vehicle frame.
The different most construction methods used are:
separate construction, partially self-supporting
construction and self-supporting construction.
(A) Partially self-supporting /Unibody construction welds major body panels
together to form the frame for attaching the engine, drivetrain, suspension, and other
parts. This type of construction is commonly used on cars.
(B) Separate /With body-over-frame construction, a thick gauge steel frame provides
the foundation for holding other parts. This type of construction is commonly used on
large trucks and SUVs. Two very different methods used to construct modern vehicles:
unibody and body-over-frame construction
Full Frames
• Body-over-frame vehicles have separate body
and chassis parts bolted to the frame
• Full frame vehicle is heavier - high amounts of
energy are absorbed by the frame in a collision
• Torque boxes allow some twisting to absorb
road shock and collision impact
• Crossmembers extend sideways across frame
rails to support the engine, suspension, chassis
• Full- or partial-frame construction is used on
most full-size and some small pickup trucks
Unibody Construction
• Unibody construction uses body parts welded and
bolted together
– Uses lighter, thinner, high-strength steel alloys
• Body shell is formed by welding sheet metal into a
box- or egg-like configuration
– Strength is achieved through shape and design instead
of mass and weight
• Stressed hull structure disperses force over the entire
body shell
Space Frame
• Space frame vehicle has a metal body structure
covered with an outer skin of plastic or
composite panels
• Roof and quarter panels may be attached with
mechanical fasteners or adhesives
• After a collision a space frame is more likely to
have hidden damage, or hidden corrosion
• Support members are bolted to unibody bottom
– Needed in high-stress areas to reduce body flex
Joining Parts
• Fastened parts are held together with fasteners
• Welded parts are permanently joined by welding
• Press-fit or snap-fit parts use clips or an
interface to fit parts together
• Adhesive-bonded parts use high-strength epoxy
or special glue to hold parts together
• Composite unibody is made of plastics and other
materials
– Keeping metal parts to a minimum cuts weight
while increasing strength and performance
Note the space frame construction. Composite (plastic) panels fasten to
a metal inner body structure. Composite panels can be made flexible to
resist door dings and small dents.
Major Body Sections
• Vehicle is divided into three sections
– Front, center and rear
• Front section includes everything between front
bumper and fire wall
– Also called nose section, front clip, “doghouse”
• Center section or midsection includes body parts
that form passenger compartment
– Also called a “greenhouse”
• Rear section also called tail section, rear clip,
“cathouse”
This top view of unibody construction shows how
structural members are added to support the engine
suspension and other mechanical systems.
Gaskets and Seals
• Various gaskets and rubber seals prevent air and
water leakage between body parts
• Seals or weatherstripping are often used around doors
and the rear deck lid
• Rubber seal is partially compressed when door or lid
is closed to form a leakproof connection
• Rubber gasket often seals stationary glass where it fits
into body
Anticorrosion Materials
• Anticorrosion materials prevent rusting of metal parts
• Undercoating is often a thick tar or synthetic rubberbased material sprayed onto underbody
• After performing repairs, you must restore all corrosion
protection
Sound-Deadening Materials
• Sound-deadening materials help quiet passenger
compartment
– Insulation that prevents engine and road noise from
entering passenger area
•
•
•
•
•
Roof Designs
Sedan is a body design with a center pillar that
supports roof
Hardtop does not have a center pillar to support roof
Hatchback has a large third door at back, commonly
found on small compact cars
Convertible has a retractable canvas roof with a steel
tube framework, or a removable hardtop
Station wagon extends roof straight back to rear of
body with a rear hatch or tailgate
Vans and Trucks
• Van has a large box-shaped body to increase
interior volume or space
• Full-size van normally is front-engine, RWD
• Minivan is smaller and often uses front-engine
FWD with unibody construction
• Pickup truck normally has a separate cab and
bed, typically with front-engine, RWD
Summary
• Vehicle classification relates to construction, size,
shape, number of doors, type of roof, etc.
• Three main types of frame construction
– Body-over-frame, unibody, and space frame
• Vehicle commonly divided into 3 body sections
– Front section, or nose section
– Center section, or midsection
– Rear section, tail section, or rear clip
• Vehicle sizes: compact, intermediate, full size