The Frame: Backbone of Your Vehicle's Chaasis

Topics covers here -

1] Introduction to Frame

2] Functions of Frame

3] Construction of Frame

4] Types of Frame: Conventional , Ladder , Semi-integral , Integral , Space frame , Perimeter frame and Supperleggera frame.

5] Material used in Frame

6] Design consideration in Frame

7] Loads acting on chassis Frame

8] Defects in Frame

9] Maintenance or Repair of Frame

10] Alignment of Frame  

11] SubFrame   and

12] Types of Frame Sections.

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Introduction to Frame

A frame is the main structural component of the chassis of a motor vehicle. All other remaining components of chassis are fastening to it; a term for this design is body-on-frame construction.

Does the Frame & Chassis same things? 

Frame was earlier name of chassis; the lighter and rigid to make it your car will perform better. But technically it totally different from each other. Frame is only one of the major component of the chassis. And remain parts fitted to it, to make a complete skeleton of the chassis.

It is the supporting component or called under-body of the automobile. The automobile frame is the skeleton (structural backbone) of any vehicle chassis. It is the foundation for carrying the engine, transmission system, steering system, brakes by means of spring, axle, rubber pads etc.

Frame is the basic frame work of the automobile chassis. It supports all the parts of the automobile attached to it. All the systems related to automobile like power plant, transmission, steering, suspension, braking system etc. are attached and supported by it only. 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.

Functions of Chassis Frame:

To carry load of the passengers, goods or cargo carried in the body.

To support the load of the body, engine, gear box etc.,

To withstand the forces caused due to the sudden braking or acceleration

To withstand the stresses caused due to the bad road condition.

To withstand the centrifugal force caused by cornering of the vehicle.

To withstand torsional vibration caused by the movement of the vehicle

To withstand bending stresses due to rise and fall of the front and rear axles.

Construction of Automobile Chassis Frame:

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. The frame is supported on the wheels and tyre assemblies. It is made up of drop forged steel. And all the parts related to automobiles are attached to it only.

Fig 1.42: Frame

A simplified diagram representing the frame, (Fig. 1.42) shows the longitudinal (side) members A and the transverse (cross / vertical) members C. The frame is narrowed down at the front called Inswept, to have a better steering lock which provides space for pivoting & swinging of the front wheels, which also gives a smaller turning circle. The frame is Upswept at the rear and front to accommodate the vertical movement of the axles due to springing as it travels over road bumps & other road inequalities. It also keeps the chassis height low.

The frames for these have only straight members without taper towards the front or upsweep at the front or rear. F are the brackets supporting the body. i. e. called as a Body brackets. D are the dumb irons to act as bearings for leaf spring Shackles. They also take the Bumper brackets. Brackets E are meant for mounting the springs. Spring brackets are provided for mounting the body of the vehicle.

Extension of the chassis frame ahead of the front axle is called ‘Front Overhang’. Extension of the chassis frame beyond the rear axle is called ‘Rear Overhang’.

The engine clutch and the transmission are all bolted together to form one rigid assembly which is mounted usually on the front end of the frame. It is supported on the frame at three places by means of rubber blocks. This helps to isolate the engine from road shocks and the body from the engine vibrations. Moreover, this method accommodates any misalignment between the engine or the transmission relative to the frame or the body.

Types of Automobile Chassis Frame:

The major types of frames are:

1. Conventional frame,

2. Ladder frame,

3. Semi-integral frame,

4. Integral frame (or frameless or unit frame or unibody),

5. Space frame, and

6. Perimeter frame

7. Superleggera.

1. Conventional Frame:

The conventional frame is also known as Non-load carrying frame. In these types of frame, the loads on the vehicle are transferred to the suspension by the frame which is the main skeleton of the vehicle.

The body is made of flexible material like wood, aluminium sheet metal and isolated frame by inserting rubber mountings in between. Fig 1.43 depicts a conventional frame with x-member in the center to resist twisting force.

Application: - Used on old Daimler vehicles.

Fig 1.43: Conventional frame

2. Ladder Frame - The Foundation:

A ladder frame is one of the oldest forms of automotive frame. It is the body-on-frame construction. It is clear from its name that ladder chassis frame resembles a shape of ladder having two long longitudinal rails inter linked by lateral and cross bars.

This design offers good beam resistance because of its continuous rails from front to rear, but poor resistance to torsion or warping if simple, perpendicular cross members are used. Also, the vehicle's overall height will be higher due to the floor pan sitting above the frame instead of inside it.

Examples: Trucks, Long Buses & SUV’s. The ladder frame was gradually phased out on cars around the 1940s in favour of perimeter frames and is now seen mainly on trucks & some SUV’s.

While most passenger vehicles  have moved away from the body-on-frame construction,  it is still used today for trucks & SUV's, as  it allows for easier access to the engine.

A Fig 1.44 shows ladder frame. L₁ L₂ - Longitudinal Member, C1 C2 C₃ C₄ C₅ C₆ - are Cross Member. It has two long side members (L₁ , L₂) and 5 to 6 cross members joined together with the help of rivets and bolts. The frame is made of channel section or tubular section of box section.

Fig 1.44: Ladder chassis frame

3. Semi-integral Frame:

In some vehicles half frame is fixed in the front end on which engine, gear box and front suspension is mounted and another half is in under-body. It has the advantage when the vehicle is met with accident the front frame can be taken easily to replace the damaged chassis 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.

Application: This type of frame is used in FIAT cars and some of the European and American cars.

Fig 1.45: Semi-integral frame

4. Integral Frame or Frame-less Construction:

In this type of construction, there is no frame. It is also called unitized frame-body construction or say Unibody. In unit construction, the body shell and under-body are welded or bolted together into single unit assembly.

Uni-body construction, (fig 1.46) assembled by 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.

Fig 1.46: Integral frame or frameless construction

This type of body construction that doesn't require a separate frame, it provide structural strength or support for the car's mechanical components. 

A unitized body can employ monocoque construction, or it can utilize strong structural elements as an integral part of its construction.

Unibody construction uses the body assembly itself to create the infrastructure of the vehicle and is constructed in most cases by spot welding together hundreds of smaller metal assemblies.

On a modern assembly line you may see automated spot welders sparking away on hunks of sheet metal, eventually forming a car body.

The under-body is made of floor plates and channel and box sections welded into single unit. This assembly replaces the frame.

Therefore the frame is always hidden or it eliminates separate frame construction. But the frame is always there in the form of unibody. Sometimes referred to as a sort of frame.

This decreases the overall weight compared to conventional separate frame and body construction. And gives more strength and rigidity or durability.

There is no frame and all the assembly units are attached to the body. All the functions of the frame carried out by the body itself.

Due to elimination of long frame it is cheaper and due to less weight most economical also. Therefore it is mono or frameless or chassis-less. Only disadvantage is repairing is difficult.

Application: This frame is used now-a-days in most of the modern cars. Today, nearly every car and many modern trucks or busses are built on the "unibody" concept, for reasons of weight and cost.

Purpose for this construction: if an unibody is damaged in an accident, getting bent or warped, in effect its frame is too, and the vehicle undrivable.

If the body of a body-on-frame vehicle is similarly damaged, it might be torn in places from the frame, which may still be straight, in which case the vehicle is simpler and cheaper to repair.

5. Space frame:

Space frame has a particular kind of tube frame that consists exclusively of relatively short, small diameter tubes. The tubes are welded together in a configuration that loads them primarily in tension and compression.

A metal body structure covered with an outer skin of plastic or composite panels.

Showing by extruded view of fig 1.47. 

Roof and quarter panels may be attached with mechanical fasteners or adhesives. The space frame chassis frame has all components attached to a skeletal frame of tubes and the body panels have limited structural function.

Fig 1.47: Space frames

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. Composite (plastic) panels are fastening to a metal inner body structure. 

Composite panels can be made flexible to resist door dings and small dents. Space frames construction are lighter and stiffer than steel.
Advantages: 

It gives proper vehicle geometry, rigidity & maintains precise controls. It is a lightweight rigid structure constructed from interlocking struts and is strong because of the inherent rigidity offered by the shape of triangle. 

Application:

Notable examples of cars using a space frame are Audi A8, Audi R8, Ferrari 360, Lamborghini Gallardo and the Mercedes SLS AMG. All are Lighter cars.

Audi A8 was the first mass production car to feature an aluminium space frame (ASF) chassis. The claim is that the ASF chassis is 4 per cent lighter and stiffer than the conventional steel monocoque.

Disadvantages:

The issue a space frame can create is that it encloses a lot of the car and can make access for the driver and to the engine difficult.

Fig 1.48: Space frame ( Audi R8 & A8 )

6. Superleggera:

An Italian term (meaning "super-light") for sports-car construction using a three dimensional frame that consists of a cage of narrow tubes that, besides being under the body, run up the fenders and over the radiator, cowl, and roof, and under the rear window; it resembles a geodesic structure. 

The body, which is not stress-bearing, is attached to the outside of the frame and is often made of aluminum.

Superleggera is an automobile coachwork construction technology developed by Felice Bianchi Anderloni of Italian coachbuilder Carrozzeria Touring Superleggera in 1936. 

The first superleggera coachworks were made by north of Milan, near Alfa Romeo, Italian Citroën, and the former Isotta Fraschini companies.

Fig 1.49: Superleggera

The superleggera system consists of a structural framework of small-diameter steel tubes that conform to an automobile body's shape and are covered by thin alloy body panels that strengthen the framework. 

Aside from light weight, the superleggera construction system allows great design and manufacturing flexibility, enabling coachbuilders to quickly construct innovative body shapes.

The superleggera tubes were brazed to shape on a jig and the panels were then fitted over this. The panels are only attached at their edges, mostly by swaging the panel edges over angle-section strips on the steel framework. 

Most of the panel has no rigid or metal-to-metal contact with the framework, it merely rests on it, with the tubes wrapped in hessian or with a rubber spacer.

The superleggera system is no longer used in high-volume automobile production for a number of reasons. 

Primarily, a superleggera body cannot meet modern impact resistance standards, and the cost of manufacture and galvanic corrosion between the aluminum body panels and the steel tubular frame are also prohibitive factors.

Additionally, the frame tubes used to construct a superleggera body are too small and of unsuitable material for mounting suspension components, so a chassis is required, a disadvantage not found in space-frame and other chassis systems.

In 1948 Bristol car maker introduced Superleggera construction on the Bristol 401. Bristol, which had aircraft industry experience, were more successful in countering galvanic corrosion than other manufacturers.

Several other manufacturers created automobiles using superleggera construction technology. Notable examples include:

Alfa Romeo 8C 2900 Mille Miglia, Alfa Romeo 1900 Super Sprint, Alfa Romeo 2600, Aston Martin DB4, DB5 and Lagonda Rapide, BMW 328 Touring Roadster, Bristol Cars, Ferraris 166, 195, 212 and 340 models, Lamborghini 350GT, Lancia Flaminia Convertible, Maserati 3500GT, Pegaso Z-102. etc.

6. Perimeter Frame:

Similar to a ladder frame, but the middle sections of the frame rails sit outboard of the front and rear rails just behind the rocker panels or sill panels. This type of frame offer lower floor pan and better safety in the event of a side impact.

Fig 1.50: Perimeter frame

This type of design lacks stiffness, it reduce beam and torsional resistance in transition areas from front to centre or centre to rear. To overcome this, some required additions are in design like the use of torque boxes, and soft suspension settings.

Perimeter frames used in passenger cars in the U.S. but not in the rest of the world, until the uni-body gained popularity and is still used on US full frame cars. These types of frames see a lot of 1950s sales of U.S. passenger cars.

Types of Frame Sections:

The frames are made up of different sections, they are; Box, Tubular channels, U-shaped section etc. which are welded or riveted together to build a chassis frame.

Three types of steel sections are most commonly used for making frames:
   1. Channel section,
   2. Tubular section, and
   3. Box section.

Fig: 1.53: Frame Sections

Different sections are used for longitudinal & cross members. Generally channel section & box section are used for longitudinal (side) members & other sections like I-section, hat section, tubular section are used for cross members. 

Channel sections are preferred for heavy-duty applications. The channel section (in heavy vehicle) is used for long members and box section for short members. Tubular section is used now-a-days in three wheelers, scooters, matadors and pickup vans. The different types of frame cross-sections are shown in Fig 1.53.

Channel sections have good resistance to Bending, Tabular sections have good resistance to Torsion, and Box sections have good resistance to both Bending and Torsion.

Material for Frame:

Composition of nickel alloy sheet steel having content carbon 0.25-0.35%, Manganese 0.35-0.75%, Silicon 0.30% max., Nickel 3%, Phosphorous 0.05% max., Sulphur 0.5% max.

‘Alpax’ is an aluminium alloy has also been used for frame material. While material used for different frame brackets are; mild steel, medium carbon steel, hard steel, 1% nickel, soft steel, Iron (black heart).

Design consideration in frame:

Before design, first selection of material their properties and shape or type of cross section are primary requirements. Selection of material depends on, Type of vehicle (heavy or light), Chassis design (engine locations), manufacturing company etc. 

The design of the Chassis frame with adequate stiffness and strength is necessary. Along with the strength an important consideration in the chassis frame design is to increase the stiffness (bending and torsion) characteristics. Adequate torsional stiffness is required to have good handling characteristics. Normally the chassis frames are designed on the base of strength and stiffness.

The frames should be strong enough to bear load while sudden brakes and accidents.

Loads on chassis frame, maximum shear stress and deflection under maximum load are the important criteria for design and analysis.

In the conventional design procedure, the design is based on the strength and emphasis is then given to increase the stiffness of the chassis with very little consideration to the weight of the chassis.

One such design procedure involves the adding of structural cross member to the existing chassis frame to increase its torsional stiffness.

As a result weight of the chassis increases. This increase in weight reduces the fuel efficiency and increase the cost due to extra material.

Loads acting on Chassis Frame:

The frame experiences loads of different nature during motion of the vehicle. These loads in turn, produce stresses and strains of various kinds. Generally, the frame sustains the following types of loads.

1. Stationary (static) loads namely the loads of permanent attachment like all the parts of the chassis, body etc.

2. Inertia loads while turning, braking etc.

3. Momentary loads while quick acceleration, sudden braking etc.

4. Short duration loads applied while crossing roads of irregular and uneven surfaces.

5. Impact loads caused by impact of wheels with road obstacles.  It may result in distortion of the frame or its collapse. Sudden accidents, head on collusions etc. Bumpers and over -riders are used to sustain impact loads, thereby providing safety and longer life to the frame.

6. Over loads caused by irregular and overloading of vehicle.

7. Flexural (or bending) load:  It is produced in a vertical plane of the side members due to –
   a. dead weight of the vehicle,
   b. weight of the passengers,
   c. Engine torque,
   d. braking torque.

Flexural load also develops in lateral plane of the side members due to –
   a. road camber,
   b. cornering force,
   c. side wind.

The effect of flexural load is to cause flexural stresses which can be either tensile or compressive in nature. The compressive stress is resisted by placing thrust -taking members between the frame and the back axle. The tension is resisted by using material of sufficient strength.

8. Torsional load (or twisting moment): It is caused due to vertical load when the vehicle comes across a road bump. The twisting induces a shear stress in the frame. The frame is designed strong enough to resist torsion by providing:
   a. torque -resisting members,
   b. cross -members,
   c. radius rod,
   d. Benzo-frame type torque members.

Defects in Frame:

Generally frames are defects proof but the only prominent defect that usually occurs in the frames due to accidents is the alignment fault. This may be checked by means of a plumb line method.

The possible cause of defects, then, may be any one of the following:

1. The dumb irons or side members may be bent.

2. Cross members may be buckled.

3. Some rivets may be loose or broken.

Maintenance or Repair of Frame:

Frames require little, if any, maintenance. However, if the frame is bent enough to cause misalignment of the vehicle or cause faulty steering, the vehicle should be removed from service.

Drilling the frame and fish plating can temporarily repair small cracks in the frame side rails. Care should be exercised when performing this task, as the frame can be weakened.

The frame of the vehicle should not be welded by gas or arc welding unless specified by the manufacturer. The heat removes temper from the metal, and, if cooled too quickly, causes the metal to crystallize.

Minor bends can be removed by the use of hydraulic jacks, bars, and clamps. If the damage to the frame members is small, they can be repaired by means of a hydraulic jack and wringing irons. If the damage is more, the bent frame member may be heated to straighten it. Another alternative may be to cut the damaged part and weld a new one instead. Some accidental vehicle cannot be repair easily or it should replace once.

Accident, harsh driving and or overloading etc. are causes for bend, cracks in frame members and also alignment is another parameter for defects in frame. Weather it will wheel alignment or frame alignment problem. This would is another cause to damage the frame. If vehicle wheel alignment is not in good condition then alignment is necessary.

Alignment of Frame:

Alignment checked by means of a "simple plumb line method". The vehicle was placed on a level surface and by suspending a plumb line from four different points on each side of the frame and then their position on the ground was marked.

The vehicle is then taken away and the diagonals are measured between corresponding points. These should not differ by more than 7 or 8 mm. If any of the corresponding diagonals do differ by more than this amount, the frame is out of alignment i.e. misalignment. 

Sub-frame:

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