Automotive body structure 101

The automotive body structure is one the main and most important components of a modern automobile. This system serves as the cohesive link for all other subsystems. It accommodates the drivetrain and primarily providing support and protection for passengers and cargo. A robust body structure is imperative to withstand loads, and pressure, and effectively unify all components.

Furthermore, it must effectively mitigate and absorb the impact of a collision to safeguard occupants. Simultaneously, it should strive to be as lightweight as possible to optimize energy management and overall ride and handling performance. Over the years, various body structure designs have been implemented, each with its own set of advantages and disadvantages.

Quick History of the Automobile

The history of automobiles traces its history back to the 18th century, with commercial production initiated by Gottlieb Daimler and Karl Benz in the 1880s. The four-wheeled motor vehicle was designed for passenger transportation. It is commonly powered by an internal combustion engine (ICE), electric battery (BEV), or a combination of both (Hybrid). The modern automobile, comprised of over 14,000 parts, encompasses various structural and mechanical systems. These systems include the body containing the passenger and storage space, situated on the chassis or metallic (mostly steel) frame. The ICE or battery propels the car using its drive unit either transmission or electric motor, while steering and braking systems govern vehicle dynamics.

The performance of a vehicle, encompassing aspects like driveability, energy efficiency, and crashworthiness, hinges crucially on its body structure. Auto manufacturers grapple with the challenge of ensuring the vehicle body possesses the necessary stiffness and tensile strength while carefully balancing factors such as weight and resistance to impact. Additionally, the body-in-white must exhibit qualities of easy manufacturability, repeatability, and robustness, all while offering an ideal surface for painting.

While several of these requirements have persisted unchanged for years, the rising popularity of electric and hybrid vehicles is significantly influencing the materials, design, and manufacturing approaches employed in vehicle body structures.

How Your Car Is Like Your Body?

The modern automobile is a complex assembly of numerous individual components. Much akin to the intricate organization of the human body. Key automobile systems include the engine, fuel system, transmission, electrical system, cooling and lubrication system, and the chassis, which involves the suspension, braking, wheels, tires, and body. These parts are structured into various semi-independent systems, each serving a distinct function.

Similar to the human circulatory system encompassing the heart, blood vessels, and blood, the automobile features analogous circulatory systems for coolant fluid, lubricating oil, and fuel. The engine, often regarded as the “heart” of the automobile, consists of pistons, cylinders, fuel delivery tubes, and other essential elements. Once the BIW leaves the body shop the first subsystem to get installed is all the wiring harnesses.

The electrical system in a vehicle could be represented as the nervous system in the human body. The automotive body structure acts as a foundation to which all other subsystems of the vehicle mount, such as the steering system or the rear bumper. The vehicle frame is analogous to the skeleton of the human body. Each system plays a crucial role in the vehicle’s functionality, contributing to its operation, noise reduction, and ride comfort.

What is the purpose of an automotive body structure?

The automotive body structure provides many functions. However, the primary function of a car’s body is to minimize the consequences of automobile accidents. It does this by enhancing passive safety, while also contributing to the overall design appeal and ride comfort of the vehicle. The outcomes of accidents and the survival of passengers hinge on the level of human exposure and the spatial requirements necessary for passenger survival. When more robust car body components come into contact with a solid barrier during an impact, the degree of human organism exposure relies on the crimple zone components’ capacity to absorb kinetic energy. This entails the use of carefully designed structural elements made of steel, aluminum, plastic, or composites.

The automotive body structure is key in ensuring a vehicle meets its crashworthiness standard and thus can be sold. Depending on the type of crash, large parts of the frame could act as structural load paths to help keep the occupants inside of the vehicle the same. For example, the B pillar and cross members on the floor pan assembly help keep redirect loads away from the occupant in the event of a side impact crash.

Multi-material automotive body structure

The challenge for automakers has been reducing vehicle mass to improve energy efficiency and minimize ecological impacts while maintaining structural strength. The shift towards alternative, low-density materials, such as plastics and light metals, has been evident as an attempt to address these challenges. Despite the ongoing reduction in the use of steel in car body manufacturing, aluminum remains a widely utilized light metal due to its commendable properties. Nevertheless, challenges arise in joining processes involving aluminum and steel, leading to the formation of fragile intermetallic bonds. The current trend in car body construction emphasizes lightweight design and safety improvement, necessitating a careful balance in material properties.

What material is the automotive body structure made of?

Current vehicles are crafted from an extensive array of advanced materials, including:

• Aluminum
• High-strength steel
• Ultra-high-strength steel
• Boron
• Magnesium
• Carbon fiber
• Plastic.

Raw materials cost about 47% of the cost of a vehicle. Steel, iron, plastic, aluminum, and glass account for 65% of the materials used in vehicles, while other materials make up the remaining 27%.

What is the automotive body structure of a car?

In the automotive industry, the automotive body structure (car body) is also known as the body-in-white (BIW). There are 3 main types of vehicle architecture in the automotive industry, a body-on-frame/ ladder frame construction, a space frame, and a unibody type design. Of these 3, unibody construction is the most common for the vast majority of passenger cars and crossover SUVs. Body-on-frame designs are usually only used for full-size trucks and SUVs.

The vast majority of vehicle bodies on the road today rely heavily on steel. Due to its high strength and low cost it is still the primary material in the structure design. However, the grade of steel can vary drastically throughout the vehicle. For more information on the material used in auto bodies see Common Types of Steels found in Automotive Structure Design.

Automotive body structure: ladder frame design structure

The traditional vehicle structural design, known as the body-on-frame construction, features a frame typically comprised of two parallel connected rails forming a “ladder frame” to which the suspension wheel and tires are attached. The remaining body or shell is positioned on top of this frame. Widely used until the early 1960s, this concept was employed by almost all cars globally. Initially constructed from wood, particularly ash, the frames transitioned to steel ladder frames in the 1930s. Currently, the frame design is primarily reserved for pickup trucks and full-size SUVs, resembling a ladder with two longitudinal rails linked by various lateral and cross braces.

The longitudinal members serve as the main stress-bearing components, managing both the weight and the vehicle dynamic forces resulting from acceleration and braking. Lateral and cross members provide resistance against lateral forces and enhance torsional rigidity. Ladder frames are favored in trucks due to their overall strength and weight-bearing capabilities. However some drawbacks include their significant weight and the need for improvement in torsional body stiffness due to their two-dimensional structure. Furthermore, frames occupy valuable space and raise the center of gravity, compromising safety as the rigid rails do not deform upon impact, leading to a higher transfer of impact energy into the cabin and the other vehicle.

Automotive body structure: space frame structure

The structural design of the house frame body takes advantage of the opportunity for extensive half integration, aiming to reduce manufacturing and tooling expenses while enabling a weight reduction exceeding 40%. Although the assembly of top-notch structural pressure die castings and shaped, machined extruded sections incurs relatively high costs, significant overall cost savings are realized for small and medium production volumes when compared to purely sheet-based body design concepts. Nevertheless, the house frame construction incorporates a substantial proportion of formed sheet components. Particularly, the sheet elements positioned between the frame components play a crucial role in determining the structure’s rigidity.

Automotive body structure: unibody design structure

Most automobile bodies do not adhere strictly to the monocoque design; instead, current cars employ a unitary construction commonly known as the unibody design. This design incorporates a network of box sections, bulkheads, and extruded beams to generate the majority of the vehicle’s strength, with the stressed skin contributing comparatively little to the overall strength or stiffness. The unibody design facilitates a substantial reduction in the weight of the car body, enabling a more compact yet spacious vehicle configuration.

Furthermore, safety is improved by incorporating energy-absorbing deformation zones into the unibody structure. The rigidity of the automobile body may be somewhat compromised since the foundational monocoque assembly consists of sheet panels that are typically spot-welded, providing only localized connections, especially in the case of steel designs. However, it is feasible to enhance the stiffness of the unibody by employing continuous joints, such as adhesive bonding or laser welding, or by integrating beams, closed sections, or other stiffening elements. Conversely, in the event of a severe collision involving a vehicle with a unibody design, repairs may prove more challenging compared to a full-frame vehicle.

What is a body-in-white & define its parts?

Automotive manufacturers such as General Motors, Ford, and Honda, use the term BIW to refer to a fully assembled automotive body structure. This system of the vehicle is usually assembled in a body shop with resistance spot welding (RSW). RSW is commonly used in automotive manufacturing due to its low cost and high repeatability. The Automotive BIW is made up of several assemblies:

• Upper Structures (Roof/ A, B, C & D Pillars/ ext.)
• Lower Structures (Floor Pan/ Rear Compartment/ ext.)
• Closures (Hood/ Doors/ Lift Gate)

Vehicle BIW structures are usually joined together by fusion welding sheet metals together. The only exceptions of parts that are not welded together are:

• moving parts: the doors, hoods, deck lids, fenders, windshield wipers
• trim glass, seats, upholstery, electronics,
• the chassis sub-assemblies and the powertrains

RSW is also not used on any exterior auto body panel due to the poor surface finish. Most vehicle closure parts such as the doors, use hemming to join the door structure to the exterior door skin.

Side and greenhouse members/Upper Structure Assembly:

Upper structure assembly usually consists of the following sub-assemblies

• roof sub-assembly (roof panel/ front and rear roof header and roof bows)
• Rear end sub-assembly ( quarter panel inner/ rear end panel / D pillar inner/ext.)
• body sides sub-assembly ( Body side outer/ B Pillar inner/ hinge pillar/ ext.)

What is a master section in BIW?

A section is a 2D representation or slices through a vehicle. Packaging and BIW Engineers primarily use 2D sections during the initial part of the BIW development, when 3D CAD models are still too premature to evaluate a vehicle. They help engineers ensure they are providing adequate packaging space for all components that will occupy a specific area of a vehicle.

The typical section at the rocker:

What is the side of a car called?

Body sides can be broken down into two assemblies:

1. Body Side Outer (BSO) – all the class A (customer-facing components). Most BSOs are stamped out of one giant die. Due to the complex geometry, the type of steel that this panel is made out of must have good ductility while proving good dent resistance such as bake-hardening steel.
2. Body Side Inner (BSI) – The load-bearing elements behind the class A surface

What is automotive platform sharing?

the underbody subassembly can also be referred to as the vehicle platform. An example is the Bronco Sport and the Ford Escape, which are on a shared platform, Ford’s C2 platform. The underbody interfaces with a vehicle’s suspension system so by keeping the underbody similar you can carry over many of the same components in the chassis and thus drive down the cost of the vehicle development.

Underbody Members:

What are the front parts of a car called?

The front part of a car is commonly referred to as the front motor compartment. On a traditional gas or diesel-powered vehicle, this is where the internal combustion engine would be packaged. A breakdown of the front motor compartment can be seen below.

What is the bottom of the car called?

The bottom of a car is known as the floor pan assembly. However, the dirty side (facing the road) is commonly known as the undercarriage. Several automakers are currently experimenting with casting large parts of the floor pan and rear compartment out of one mold thus reducing the number of parts and complexity of the underbody subassembly.

What is the rear panel of a car?

The rear compartment (rear of a car) is made up of the rear BIW structure, the rear bumper, and the quarter panel (part of the body side outer). The rear bumper on most vehicles is just a plastic part that mounts the rear-end panel of a BIW. The purpose of the rear bumper is just to act as a beauty cover and provide a location to mount the license plate.

The structural bumper is behind the plastic rear bumper. The quarter panel is a class A body panel rear of the rear doors. The breakdown of the parts of the rear compartment can be seen below: