Automobile Chassis: Components and Functions

• The chassis serves as the main support structure of a vehicle and is known as the 'Carrying Unit'.
• The chassis bears all the stresses on the vehicle in both static and dynamic conditions.
• "Chassis" is a French term for the complete automobile without the body.
• It includes systems such as the power plant, transmission, steering, suspension, wheels, tires, and auto-electric components.
• The chassis is made of drop-forged steel.

Functions of Chassis

• Supports the vehicle body's load.
• Provides space and mounting locations for vehicle aggregates.
• Supports various systems' weights like the engine and transmission.
• Supports the load of passengers and luggage.
• Withstands stresses from bad road conditions.
• Withstands stresses during braking and acceleration.

Components of Chassis

• Frame

• Front Axle

• Steering system

• Suspension system

• Rear axles

• Transmission

• Brake system

• Engine

• Electrical system

• An automobile chassis is subdivided into a power plant and running gear.

• The power plant includes the engine assembly and power transmission assembly.

• The running gear includes the frame, steering system, suspension system, brakes, wheels and tires.

• The electrical system is part of both the chassis and the body.

Classification of Chassis is based on:

• Its control
• The fitting of the engine.
• The number of wheels and driving wheels.
• Wheelbase size.

According to its control

• Conventional Chassis fitting the engine in front of the driver.
• Conventional chassis are common in cars and previous Tata trucks.
• The driver sits behind the engine limiting road view and cannot utilized for carrying passengers & goods.
• Semi Forward Chassis has half of the engine in the driver's cabin and half outside, often used for extra passengers.
• Full forward Chassis has the entire engine inside the driver's cabin.
• With a full forward chassis, the floor area is increased to accommodate 3 extra seats, where the drivers sit above the front wheels enabling full front road visibility.

According to engine fitting

• Engine at front design fits the engine in the front with drive to the wheels from the rear.
• This design offers advantages like:
  • Enough luggage space behind the rear seat.
  • Balanced vehicle weight on all wheels.
  • Adequate space for steering the front wheels.
• Engine placement at the front also provides drive to the front wheels only.

According to number of wheels

• 4x2 drive chassis has 4 wheels with 2 being driving wheels.
• 4x4 drive chassis has 4 wheels and all wheels are driving wheels.
• 6x2 drive chassis has 6 wheels with 2 being driving wheels.
• 6x4 drive chassis vehicle has 6 wheels with 4 being driving wheels.

Advantages of low floor chassis

• Low floor is available because there is no propeller shaft & differential at the back.
• The clutch + gearbox + differential are in one unit, lowering costs
• This design provides spacious luggage space at the rear.

Disadvantages of low floor chassis

• During acceleration or gradient driving, the vehicle's weight shifts to the rear wheels, which is not good for better adhesion.
• This leads to reduced traction effort on gradients.

According to wheelbase size

• Long Wheel Base Chassis includes a long wheel base and comes in four types:
  • Long Front Overhang
  • Short Front Overhang
  • Long Rear Overhang
  • Short Rear Overhang
• Short Wheel Base Chassis includes a long wheel base and comes in four types:
  • Long Front Overhang
  • Short Front Overhang
  • Long Rear Overhang
  • Short Rear Overhang

Frame Characteristics

• The frame serves as the under-body and skeleton of the automobile's chassis.
• Supports the engine, transmission, steering, and brakes with springs, axles, and rubber pads.

Functions of the Frame

• Carries the load of passengers, goods, or cargo.
• Supports the body, engine, and gearbox.
• Withstands forces from sudden braking or acceleration.
• Withstands stresses from poor road conditions.
• Handles centrifugal forces from cornering.
• Withstands tensional vibrations from movement.

Construction of the Frame

• In construction, the frame is narrow in the front for a shorter turning radius for the front wheels.
• The rear side widens for larger body space, supported by wheels and tire assemblies made of drop-forged steel.
• All automobile-related parts are attached to the frame.
• The frame is narrowed (In swept) to better the steering lock, space for pivoting the front wheels, to give a smaller turning circle.
• The frame is Upswept at the rear and front to accommodate vertical axle movement and keep the chassis low as it travels over bumps.

Frame Components

• The frames have straight members without front/rear tapers, with Body brackets supporting the body.
• Body brackets are used as bearings for leaf spring Shackles.
• Bumper brackets and spring brackets attach/mount the body.
• 'Front Overhang' extends the chassis frame ahead of the front axle.
• 'Rear Overhang' extends beyond the real axle.

Classification of Chassis types

• Conventional Frame
• Ladder Frame
• Semi-integral Frame
• Integral Frame (frameless, unit frame, unibody)
• Perimeter Frame
• Space
• Sub Frame

Conventional Frame

• Also known as Non-load carrying frame.
• Loads are transferred to the suspension by the frame which is the main structure and skeleton.
• This type of frames insert rubber mountings in between the aluminium metal or wood, allowing the body is made of flexible material.
• Commonly used on old Daimler vehicles.

Ladder Frame

• The ladder frame is one of the oldest forms of automotive frame.
• Body-on-frame construction is used, in a shape of a ladder having longitudinal rails inter-linked by cross bars.
• Typically used in Trucks, Long Buses & SUV's.

Semi-integral Frame

• Half the frame is fixed at the front for mounting the engine, gearbox, and front suspension, with the other half in the under-body.
• This design makes it's easier to replace the chassis frame, when the front frame is damaged following an accident.
• Typically used on Fiat, European, and American cars

Integral Frame

• Does not contain a separate structural frame and also called a frameless construction.
• Known as a Unibody construction where the body shell and under-body are bolted and fitted together.
• Uni-body construction uses welds to assemble body panels forming the frame to attach the engine, drivetrain, and suspension.
• Commonly found on modern cars, it provides structural support without needing a separate frame.

Space Frame

• A particular kind of tube frame has short, small diameter tubes.
• Tubes are welded together in a configuration by tension and compression
• Has a metal body structure covered with an extruded view of plastic or composite panels.
• Roof and quarter panels are connected using mechanical fasteners or adhesives.
• Examples are Audi A6, Audi R8, Ferrari 360, Lamborghini Gallardo and the Mercedes SLSG

Perimeter Frame

• Similar to a ladder frame, but the middle sections of the frame rails sit outboard of the front and rear rails.
• Located just behind the rocker panels or sill panels.
• Offers lower floor pan and better safety during side impacts.
• Originally found in the passenger cars in the U.S. in the 1950's.

Sub Frame

• A structural component bolted to the vehicle's basic structure.
• Utilizes rubber bushings/springs to dampen vibration.
• Reduces forces generated by the engine and transmission to not disturb passengers.
• Mostly used on cars with frameless/unibody construction.
• The chassis components are mounted on the sub-frame. Common examples are: Austin Mini (1959), Chevrolet Camaro (1967–1981).

Types of frame section

• Channel section: Used in long frame members, good in bedding.
• Tubular section: Used in three wheelers, scoters, and matadors.
• Box section: Used in short frame members, and can be torsion.

Frame Materials

• Nickel alloy sheet steel contains:
  • Carbon ranging 0.25-0.35%
  • Manganese ranging 0.35-0.75%
  • Silicon ranging up to 0.30%
  • Nickel at 3%
  • Phosphorous ranging up to 0.05% max
  • Sulphur ranging up to 0.5% max
• Aluminum alloy called is also used.
• Materials for frame brackets:
  • Mild steel
  • Medium carbon steel
  • Soft steel
  • Iron (black heart)
  • Hard steel
  • 1% nickel