Automobile chassis frame and body

1.5 Automobile Chassis, Frame & Body

In previous section of this unit, we covered automobile industry along with its history over the world as well as Indian automobile segments. We discuss how automobile is born and make a history over the world; it helps to overview of automobiles that makes life much more comfortable, simple and ease of transporting. 

Now in the this section we discuss on chassis, frame & body. We cover the system that forms the base for every automobile. Whether it's the simple ladder frame or a carbon fiber monocoque, the chassis goes a long way to defining an automobile. This time, we're turning our gaze inwards. We're looking at a very basic structure, a skeleton on which the automobile is built – the chassis. Let's find out how?

Introduction to Chassis:

“Chassis” is a French term and it was initially used to denote the frame or main structure and or basic structure of a vehicle. Now a Chassis is denotes to the complete whole vehicle except body in case of heavy vehicles. 

In HMV, the chassis and the body are each made as a separate unit and then bolted together. But in case of LMV (i.e. Cars) of monocoque construction or integral body construction (unibody), it denotes the whole vehicle except additional fittings in the body. And say, it is the body integral with frame. 

A vehicle without body is called as ‘Chassis’. It is the backbone of any vehicle. It is conjoing or assemblage of no. of components (except Body). “Chassis” which means the complete automobiles without body and it includes all the systems like Power plant and Running gear. If body is also attached to chassis then it is known as the particular type of automobile vehicle as per the shape and design of the body.

The chassis carry all the major units necessary to propel the automobile, direct its motion, stop it, and allow it to run smoothly over uneven surfaces. It is a main mounting for all the components as well as the body. Therefore it is also known as “Carrying Unit” or “Carriage Unit”; it includes the instrument panel, hood & amp; the cowl section.

Chassis and body makes a complete vehicle. Vehicle can be driven after placing a driver seat on the chassis. 

One more thing the chassis is not a part; it is a skeleton of any automobile vehicle (except body). Automobile chassis ensures less noise, vibrations and harshness throughout the automobile.

For a good chassis design and for its good performance it must have fast pick up, Strength, Safety, Durability, Dependability, Ease of control, Quietness, Speed & power accessibility, Economy of operation, Low center of gravity, Stability, Load clearance, Braking ability, Good springing, Simplicity of lubrication and Great resistance to bending, twisting etc. (i. e. the chassis frame and front and rear suspension systems without abnormally raising the frame or transmitting shock to the frame due to road irregularities.)

The fundamental construction of chassis:

The chassis includes the following components (as shown in Fig. 1.20): 

   1. Frame. 

   2. Engine or Power plant. 

   3. Suspension system.  

   4. Steering system. 

   5. Braking system. 

   6. Wheels and tyres. 

   7. Transmission system- clutch, gear box, and final drive-line. 

  8. The auxiliaries- all necessary electrical equipment. All moving parts are fitted to chassis.

Above all components are to be supported on a rigid structure, providing a skeleton to hold the component together called ‘Chassis’.

The chassis is general is equipped with “I– beam” construction front axles, it is made of steel metal. To give it extra strength it is provided with bolted, riveted or welded cross-piece.

Fig 1.20: Chassis Layout

To withstand the shocks, blows, twists & vibration coming during operation, chassis are provided a cross bearing to increase the rigidity. The whole chassis is now fastened to a frame of channel or “U” shaped section.

The frame, suspension, axles, wheels and tyres are the only remainder units required for built the basic structure. Basic structure is the unit to turn into a power-operated vehicle.

Steering system and brakes are the controls parameters of the vehicle.

Propeller shaft, slip joint / half shaft, universal joints, Differential and axles are come into the final dive-line transmission.

The chassis is sub -divided into the running gear and the power plant. The running gear includes the frame, steering system, suspension system, brakes, wheels and tyres. The power plant includes the engine assembly and power transmission assembly. The electrical system is part of both chassis and body or say it is a common player or system.

Classification of Chassis:

A) According to controls:

1. Forward chassis – ex. Cars.
2. Semi-forward chassis – ex. Tata 407 pickup.
3. Fully-forward chassis or Bus chassis.

B) According to fittings / position of engine:

1. Conventional drive.
2. Front engine drive (Cross engine fitting).
3. Rear engine drive.
4. Center / middle engine drive.

C) According to number of driving wheels:

1. 4×2 wheel drive chassis.
2. 4×4 wheel drive chassis.
3. 6×2 wheel drive chassis.
4. 6×4 wheel drive chassis.

D) According to construction of chassis frame:

1. Conventional chassis.
2. Ladder chassis.
3. Integral or chassisless. 
4. Monocoque.
5. Unibody or Unit-body or Unitize construction.
6. Backbone chassis.

Now above those chassis are discussing below.

In the very early days of the automotive industry, automobile designers leaned heavily on construction techniques that had been in use for centuries or longer. 

Over the years, basic structure has undergone a number of changes, and like everything else, it has evolved. It's widely known that the chassis plays a major part in vehicle dynamics. Advancements in technology brought in the possibility of using different materials and creating different types of chassis. Before we run through the various types, let's make it clear that we're not following a chronological development of chassis types. Automobile manufacturers experimented and created what they could with the means available to them. 

As a history result, there were many different types created during various times between the 1930s and today. A lot of type’s co-existed in the early days and the same continues today. Let's check out some of the popular types.

1. Forward chassis:

Fig 1.21: Forward control chassis

Fig 1.21 shows the forward chassis. In which engine is fitted in front of the driver cabin.

In this type of arrangement, the driver seat is far enough from the front wheels and is not able to see just in front of vehicle properly. This type of arrangement avoids full utilization of the space. And forward type chassis generally found in light motor vehicles, like Mahindra jeeps or Cars. It is general construction for automobile cars.

As per safety concern, this type of chassis provides more safety. In case of accident or direct impact from front side, the total impact sustain by front beam and engine. The driver cabin is far away from the direct impact.

2. Semi-forward chassis:

Fig 1.22: Semi-forward control chassis

In semi-forward chassis (Fig 1.22), half portion of the engine is in the driver cabin and the remaining half is outside the cabin. Like "Tata 407" pickup vehicle is the one of the type of semi-forward chassis. This type of chassis arrangement gives better visibility of the road to the driver.

3. Fully-forward chassis:

Fig 1.23: Fully-forward control chassis.

Fig 1.23 shows the fully-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. It provides an increased floor space in the vehicle. The driver seat is just above the front wheels or engine and he can see the full front road sight from the cabin/front wheels.

This type of arrangement mostly used in trucks & Buses, thus fully forward chassis also called as a Bus chassis.

4. Conventional drive:

In most off the vehicle, engine is fitted in front portion of the chassis. The engine power / drive is given to the front wheels. It is general arrangement for final drive, as shown in fig 1.24.

Fig 1.24: Conventional (Rear wheel) drive

5. Front engine drive:

Fig 1.25 shows the front wheel drive chassis. This type of chassis mostly used on sedan type cars. The engine is fitted (cross engine fitting) or mounted on front side space and final drive or say engine power gives direct to the front axles, which becomes a live axles.

Fig 1.25: Front engine drive (cross drive).

6. Rear engine drive:

The engine placed at back side of the chassis, this arrangement does not require long propeller shaft. Gearbox and differential is one combine unit. Rear engine drive mostly used in luxurious vehicle like Volkswagen of West Germany, Leyland bus of England.

Fig 1.26: Rear engine drive.

7. Center / middle engine drive:

Fig 1.27 shows middle engine drive. This type of chassis utilized full floor space by fitting of an engine at middle of the chassis.
For examples – the Royal tiger, World master buses of Delhi Transport; the engines are fitted center of the chassis.

Fig 1.27: Middle engine drive.

According to number of wheels fitted in the vehicles and the number of driving wheels (Fig 1.28 Shows), the vehicle chassis are following types:

8. 4×2 drive chassis:

It consists of 4 wheels out of which 2 wheels are driving wheels.

9. 4×4 drive chassis:

It consists of 4 wheels and all of them are driving wheels. Commonly say, it is Four Wheel Drive.

10. 6×2 drive chassis:

It consists of 6 wheels out of which 2 wheels are driving wheels.

11. 6×4 drive chassis:

It consists of 6 wheels out of which 4 wheels are driving wheels.

[Point should be note that, one side of the axle may consist of either one wheel or two. It is considered one unit.]

Fig 1.28: Different wheels drive frames

12. Conventional chassis:

In which a separate frame is used and all components or systems of automobiles fitted on it. This type of construction is being used presently only for heavy vehicles. Just like conventional drive, in case the engine fitted at front and power / final drive gives to rear wheels. Refer fig 1.24 as conventional chassis.

13 Ladder chassis:

Early automobiles were built using body-on-frame construction, which was the exact technique that had been used to build horse-drawn carriages, and that technique proved to be very popular. In fact, it’s still in use todays commercial vehicles.

Fig 1.29: Ladder Chassis

Traditionally, the chassis is the internal framework, or the under part of an automobile. The running gear consisting of the wheels, suspension, engine, transmission, driveshaft, differential and steering system is attached to the chassis.

Together, this combination of the chassis and the running gear is referred to as the rolling chassis. The body is then mounted on the rolling chassis and this would complete the vehicle.

The type of chassis that is typically used for this procedure is called a ladder frame and the term for this construction is body-on-frame. The inspiration for ladder frame came from carriages that literally ran on horse power.

Today this combination is mainly used by commercial vehicles and SUVs like the Mahindra Scorpio.

It’s common practice for manufacturers to set up a rolling chassis and leave the body work to custom coach builders.

It's very rugged, easy to design, build and modify this form has its disadvantages. Vehicles made-up by ladder chassis are heavy, performance and fuel efficiency may affected.

They are also far less resistant to torsional flexing & thus compromising handling and road grip.

Due to the lack of a crumple zone in this design also causes higher rate of injury in case of accidents. But there have been advancements in the traditional ladder frame chassis.

The New Scorpio and the Aria have aluminium ladder frame chassis made by a process called hydroforming which gives a higher stiffness -to -weight ratio.

14. Integral and Chassis-less Construction:

The terms integral and chassisless construction are often confused, but the difference is simple.

Integral construction, fig 1.30 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 chassis-less form of construction, in which no chassis frame can be discerned.

Fig 1.30: Integral or Chassis-less construction

Integral construction is a primary factor in the safety, durability and resale value of today’s modern coaches. Originally developed in the 1930's in two different versions, integral construction has been modified, improved and blended continuously.

Integral design is responsible for the durability and longevity of modern intercity coaches. The several coach improvements and passenger items such as high quality suspension, large underfloor compartments and passenger safety.

The first two quantity-produced vehicles in the latter category were almost certainly the Saab 92 and the Austin A30, Chevrolet Camaro, Rolls-royce cars.

Why called Chassis-less…….? 

Old days, the body is separate from chassis. The chassis, frame and body combines together to build automobile. Now-a-days, this is totally different; a vehicle chassis, frame & body are together a skeleton for additional body part fittings on it. It just looks like body-shell. This body-shell is made up from pillars, rails, sills, and panels all welded together, and a reinforcing channel-section under-frame with an extended sub-frame at the front is provided to replace the chassis. It is nothing but chassis-less or frame-less construction. 

The integrated frame and body type of construction (fig 1.30) combines the frame and body into a single, one-piece structure. This is done by welding the components together, by forming or casting the entire structure as one piece (elimination of separate chassis-frame), or by a combination of these techniques. 

Simply by welding a body to a conventional frame, however, does not constitute an integral frame and body construction. In a truly integrated structure, the entire frame-body unit is treated as a load-carrying member that reacts to all loads experienced by the vehicle-road loads as well as cargo loads. 

This type of construction allows increase the amount of noise transmitted into the passenger compartment of the vehicle, that’s creating harsh in drive. Weight of chassis is also more. However, this disadvantage is negated by the upgraded unibody construction.

15. Monocoque chassis:

Monocoque is a French term for “single shell” or “single hull”, and its structure reflects that.

A monocoque chassis frame is basically a “skin” that supports its load by distributing tension and compression across its surface. It lacks a load carrying internal frame.

Fig 1.31: Lunica Lambda monocoque (1922)

Monocoque is a design method where the exterior skin supports the structural load. Monocoque is ideally supposed to be a one-piece structure which defines the overall shape of the car which made by welding several pieces together.

The floor pan, which is the largest piece, and other pieces are press-made by big stamping machines. They are spot welded together by robot arms (some even use laser welding) in a stream production line. After that, some accessories like doors, bonnet, boot lid, side panels and roof are added. Most commercial cars, though, are never monocoques.

As always there's some technology that captures exactly what everyone requires and achieves near universal adoption. In automotive chassis technology, this was the monocoque. 

It is a structural approach that supports loads through the external body, like an eggshell. It's almost opposite to the space frame or the ladder frame, which were covered with cosmetic body panels.

Monocoque construction techniques were first used as early as 1912 on aircraft called Deperdussin monocoque. The purpose is to prove aerodynamic shape and built a light-weight structure for aircraft compartment. But they didn’t catch on for a number of reasons.

After that in 1922, the Lancia Lambda was the first production automobile car to adopt monocoque construction techniques (as shown in fig 1.31).

Monocoque construction is now an automobile design technique that uses the exterior shell of the vehicle to provide structural support. This can be contrasted with traditional body-on-frame construction, which relies on the frame to provide structural integrity.

Advantages of such chassis are Cheap for mass production, Inherently good crash protection, Space efficient. Disadvantages are It's very heavy, Impossible for small-volume production.

Fig 1.32: Lamborghini – monocoque carbon fiber

Monocoque chassis frames have been also used sparingly in racing and luxury vehicles. Like the Lamborghini, Cooper mini & McLaren F1 was designed with a carbon-fiber monocoque. Fig 1.32 shows modern lamborghini monocoque chassis.

The modern monocoque chassis or unibody construction is usually made by welding several pieces together. All the pieces that form the unibody are press -made by large stamping machines and welded together by robots. The whole process usually takes minutes on an assembly line.

Fig 1.33: Monocoque chassis cars built by Honda

Today, a modified type of monococque construction called “unibody” or “unit body” is very popular. Marketers use it to describe unibody vehicles which have a single skeleton under the body panels which supports the vehicle. At the present, most of Honda’s cars are unibody construction. Some of those are Honda Jazz and Honda City (As shown in fig 1.33).

Fig 1.34: Modified monocoque - citroen > Unit-body.

Citroen was an early adopter of the modified monocoque “unit body” construction technique in the 1930s.

Honda Technology:

Fig 1.35: Honda CR-V Monocoque Skeleton

significant advantage of the monocoque and the unibody is safety. Crumple zones can easily be built into them. The total most of Honda’s cars built by unibody constructions, having a Crumple zones. This is not possible with the Ladder frame chassis. But there's a catch – in case of an accident, cars with monocoque or unibody construction are safer, but cars with ladder frame chassis are easier to repair.

Fig 1.36: Crumple zones for safety

The other advantage is space efficiency. Since the whole structure is an outer shell, engineers and designers can experiment with the placement and layout of certain parts. A good example of this would be how Honda has cleverly worked out the construction and layout in a way that maximizes efficient use of space in its cars. By challenging accepted norms, it has managed incredible advancements in automotive technology. 

Honda technology moved the location of the fuel tank from behind the rear seat to below the center of the car. For the New Jazz and the City, it has added further refinement to the center tank layout and made significant reduction in vehicle weight. The refined chassis design has also helped create an incredibly roomy interior that greatly exceeds expectation for these classes of cars. As always, the chassis continues to contribute to vehicle dynamics and this refinement ensures that the cars retain Honda's original fun -to -drive aspect. 

Application of efficient light -weighting technology is also a contributing factor to delivering outstanding fuel economy. This comes from years of fine-tuning its chassis philosophy and development process. 

There are two main types of automobile designs: structural shell and structural frame. Monocoque vehicles fall into the former category, while traditional body-on-frame construction makes up the second. Today, structural shell construction techniques have become dominant, especially for smaller vehicles, but very few vehicles are truly monocoque in nature.

16. Unibody chassis or Unitbody or Unitize construction:

Unibody chassis are the preferred construction for most cars on the market today. The most common design is in use today, sometimes referred to as a sort of frame.  In a unibody design, the frame, the floor plan, and the chassis of the vehicle are made as a single structure. This combination allows for a lighter and more rigid frame than the body-on-frame construction that was used in the past.

Fig 1.37: Unibody – (mini cooper)

Unibody chassis vehicles which have a single skeleton under the body panels which supports the vehicle. If a unibody is damaged in an accident, getting bent or warped, in effect its frame is too, and the vehicle undrivable. But it provides more safety to the passenger compartment during crash.

Some advantages of this type of chassis are substantial weight reduction, which is possible when using a well-designed unitized body. Lower cargo floor and vehicle height. Protection from mud and water required for drive line components on amphibious vehicles. Reduction in the amount of vibration present in the vehicle structure.

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 than unibody. It is one of big disadvantage of unibody.

First mass-produced unibody construction in US: by Nash Motors in 1951. The Ford Consul introduced an evolution of monocoque chassis, called unitbody or unibody, it is the modified construction of monocoque. 

Now-a-days Honda Technology provides much more of its cars Unibody construction. As shown in above fig 1.37.

Monocoque Vs. Unibody: The main difference between monocoque construction and unit body construction is that a monocoque vehicle consists of a single, structural component where the exterior skin of the vehicle provides the bulk of the structural integrity. In unit body construction, most of the structural integrity is provided by metal tubes, bulkheads, and box sections. There is no frame, but the stressed skin of the vehicle doesn’t provide any real structural support like in monocoque construction. 

Among automakers developed own chassis and gave to name on its construction base. Integral or frameless, chassis-less, monocoque, and Unibody or Unitize body are the variant of modern automobile chassis. These all same but upgraded from one by one with small but crucial changes. Chassis is develop time-to-time and give to name as per development, specification and application.

17. Backbone chassis:

Similar to the body-on-frame, the backbone tube chassis didn't have a two-dimensional ladder type structure. It consisted of a strong tubular backbone that connected the front and rear suspension attachment upon which the body was placed. A number of well-known sports cars like the TVR St. Lotus Esprit and Lotus Elan have used this system.

Fig 1.38: Backbone chassis (make- Lotus Elan)

One, the "DeLotean DMC-12", was even capable of time travel, but only in movies. While the backbone tube chassis has a complicated design and is more costly to build. it lends itself readily to building vehicles that require between two and six axles and those of various wheelbase.

There's also a thick tube that covers and protects vulnerable parts of the drive tube and is overall, a reliable system. But when problems occur, repairs are more complicated. Also, this structure doesn't provide any side protection in case of collision so the body has to be designed and built keeping this factor in mind.

A strong tubular backbone (usually in rectangular section) connects the front and rear axle and provides nearly all the mechanical strength.

Inside which there is space for the drive shaft in case of front-engine, rear-wheel drive layout like the Elan. The body is built on the backbone, usually made of glass-fibre. 

Colin Chapman, the founder of Lotus, invented backbone chassis in his original Elan roadster. 1962 Lotus lunched the Elan.

Advantages of backbone chassis are Strong enough for smaller sports cars, Cheap for low volume production, the most space-saving other than monocoque chassis.

Disadvantages are Not strong enough for high-end sports cars. The backbone does not provide protection against side impact or off-set crash.

Backbone chassis – Acura RL
Acura is the luxury vehicle division of Japanese automaker Honda Motor Company. The Acura RL is a full-size luxury sedan manufactured by Honda and sold under their Acura division 2009, released in 2013-14.

Fig 1.39: Backbone chassis (make: Acura RL_2009)

Main Features of Acura RL Chassis:

A highly rigid unit body – by Advanced Compatibility Engineering

Suspension (independent)- at front - double-wishbone and at rear - multi-link.

Stiffer subframe mount bushings, more performance minded coil springs, revised dampers, larger rear stabilizer bar,

Transmission - Active Torque Transfer System (ATTS).

Steering system - Electric assist power steering (EPS) assembly- rack & pinion, improved steering rack components, a new compact noise isolation (CNI) steering joint,

Braking system - at front- disc brake- ventilated type & at rear- Drum brake- solid type. With ABS and EBD system.

Wheels & Tyres - Larger diameter aluminum alloy wheels, and more advanced all seasonable tyres.

Quick handling response, nimble performance, high dynamic capabilities, outstanding ride comfort,

Safety features and a fun-to-drive character,

Numerous aluminum components that reduce unsprung weight to provide better handling, more consistent traction, improved suspension response, and a suppler ride-even on rough roads.

As always, ride quality and road noise isolation were also the focus of substantial engineering effort, so even with its elevated sporting performance, the RL remains quiet and composed.

18. EV platform chassis:

An EV platform chassis suits for a wide range of electric applications. EV platform is made up of a pressed aluminium box from 3mm thick aluminium sheet that doubles as the main structural spine and the battery box.

The approach keeps fabrication very simple, maximises rigidity, keeps the weight low down and concentrated in the centre of the car and keeps the battery pack away from damage in impacts.

One of the disadvantages of using a thin walled box section is that large concentrated loads cannot be applied directly to the aluminium walls. This is easily overcome by using tubular steel subframes or bulkheads to spread loads. The tubular steel subframes are relatively small and are not difficult to fabricate.

Vehicle Platform – Its chassis frame looks like a platform. A vehicle platform is basically rolling chassis with drive-train installed. It is derivable and just needs some bodywork to complete the car.

Fig 1.40 shows the EV platform chassis – Trexa made by Tesla Motors.

Tesla Motors is one of the Electric Vehicle (EV) automakers in USA.

Fig: 1.40: EV platform chassis_ (Trexa by Tesla Motors)

Fig: 1.41: EV platform chassis_ (Tabby EVO)

"TABBY EVO" EV CAHSSIS SPECIFICATION: - 

Chassis frame is made up of steel with 105kg (2 seats) 130kg (4 Seat) weight.
Steering system: EPS rack and pinion arrangement.
Braking system: all four wheel Disc brakes with ABS and regenerative system.
Suspension system: Front & Rear – McPherson independent type.
Wheels & Tyres: Rim wheels and Pirelli P1 “green” Tubeless Tyres. 
TABBY EVO is an electric mini car which open source platform for electric vehicles. Free to use, available to everyone (as company say).  

EVOLUTION IN COMPOSITES CHASSIS: – 

The automotive industry is in a constant quest for efficiency. One way forward is reducing vehicle weight. Incremental improvements in steel and aluminium chassis designs are being implemented regularly. But incremental improvements are probably not enough to meet the world’s demands for cleaner and frugal cars. The answer today lies in composites. With five times the strength of steel and one third its weight, carbon fibre is the lightest and strongest material available to us. Prohibitive costs keep it from mainstream production and currently carbon fibre monocoques are only seen on prototypes or exotics. Car manufacturers are experimenting with ways to incorporate this into the mainstream. Maybe one day, when costs are brought down, well see them in the cars we use every day.

The chassis and the body makes the complete vehicle being the super structure of any car. In larger and heavier vehicles, the chassis and the body are each made as a separate unit and then bolted together. The body is usually made from a large number of steel pressings which are welded together. The body is bolted to the chassis at numerous points, rubber or felt strips being interposed to damp-down vibration and noise.

The Body:

The Frame: