Intro to Automotive Sheet Metal Design

29 Oct.,2022


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In this article, we see how the automotive design system evolves and how different parts of an automobile are developed from scratch.  

The CAD process alone will not fulfill the Original Equipment Manufacturer's (OEM) requirements. The engineering that goes into designing the automotive part is the most vital aspect. Hence, in this article, you will also be able to recognize the fine line between designing and engineering. 


Basics of Car Bodies (Passenger Cars) 

A lot of engineering goes into the sheet metal panels of a passenger car, which is why they are often deemed to be the most critical component in the automotive sector.  



Depending on the construction of the body panels, they are divided into two types: 

  1. Body-on-frame
  2. Monocoque or Unibody


Let's throw some light on their key differences:



The body-on-frame type has a rigid reinforced frame at the bottom over which the body panels are built. 

On the other hand, in the monocoque type construction, the floor panel itself acts as a frame and is integrated with the body built above it.  



The body-on-frame type of construction significantly increases weight. It is mostly used in larger vehicles like heavy-duty trucks or vans. 

In comparison, the unibody construction is favored by regular passenger cars because of the substantial reduction in weight as well as improved performance.


Main Feature 

The body-on-frame emphasizes the rigidity and structural integrity of the vehicle by having the walls of the components thicker and heavier. 

In contrast, the monocoque structure emphasizes on performance and comfort. It can dissipate energy across the entire body and away from the passengers in the event of a crash. 


Classification of Car Bodies 

Depending upon the outer shape of a car, they can be divided into various categories like: 

  • SUV
  • Sedan
  • Truck
  • Minivan
  • Convertible
  • Coupe
  • Wagon
  • Luxury
  • Crossover
  • Hatchback


What Is Body in White (BIW)? 

In layman terms, the Body in White (BIW) is a monocoque body that refers to the integration of the upper body, closure (the center part of a vehicle), and the floor of a passenger car. It is the stage in the design and manufacturing process where all the sheet metal parts and components are joined together.  

A BIW structure of a passenger car typically consists of the following: 

  • High-pressure cast aluminum shock tower
  • Aluminum struts
  • Other struts
  • Torsion frame


Vehicle Development Process 

The complete development of a passenger car from scratch to mass production takes about three to five years on average—a tremendous improvement when compared to the early days of automotive development.

Today, a significant part of the simulations, designing, modeling, and prototyping is done using highly capable and advanced computers with stellar processing speeds.  

The entire vehicle development process can be split into four phases. The first three phases are very critical in terms of engineering, and as a design engineer, you will have complete control over them. The final stages will be affected by various factors, such as the consumer mindset, marketing, and more. 


1. Predesign and Development 

This phase is subdivided into four parts: 

  • Benchmarking 1: Comparing two or more products and processes of the competitors to adopt them into your business.
  • Concept Preparation: This is the process of preparing and verifying a concept obtained from a stylist or an artist.
  • Carryover Study: This goes hand-in-hand with concept preparation. It involves how and which parts of an existing product can be translated into the newer version. An example will be OEMs do not have separate wheel rims for different cars. Instead, they have a different standard wheel rim for their platforms.
  • Benchmarking 2: This process revisits the entire predesign and development phase with newer information and tries to increase optimization with the data available.

At the end of the predesign phase, automakers get a rough idea about the budget needed to manufacture the vehicle. They also understand the budget that should be appropriated to each team working on the project. 


2. Design Engineering 

The design engineering phase has three parts: 

  • Level 1 CAD Construction: This includes all the necessary data and design information.
  • Level 2 CAD construction with CAE feedback: This is a revision of the first CAD after the feedback from the engineering or simulation team.
  • Final Data for Blank Release: The final approved design is set out for prototyping, and its manufacturability is explored.


3. Development Engineering 

This further has four different parts: 

  • Coordination with suppliers and tooling confirmation with GD&T
  • First level physical par receivable
  • Solving concerns raised in the plant and assembly
  • Final vehicle validation before the SOP


4. Start of Production (SOP) and Vehicle Market Survey 

This is the final stage in the vehicle development process. It includes: 

  • Start of sales and customer feedback
  • JDP test and score reduction to improve the quality of the vehicle



The hood or bonnet is the frontal part of a car that protects the engine. The interior parts of a hood include: 

  • Reinforced assy-hood engine
  • STF NR-hood
  • Base assy-hood lock striker


Hoods are of two types: 

  1. Active Hoods: Additional parts to improve safety and ratings
  2. Passive Hoods: Only compensate for strengths



Fenders define the overall look of the car. They determine the radius of the arc and the gap between the body and tire. They also have anti-chipping commodities to prevent the car's exterior body from external damages. Hence, they radically improve the life cycle of the vehicle. 


There are two types of fenders: 

  1. Front Fenders: Just beside the hood
  2. Rear Fenders: Integrated with the body




The creation of the roof of a car typically starts from a concept that is inspired by preexisting automobiles. This is then optimized to suit the specific needs of newer products and the regions where the car would be used. For this reason, design engineers must consider factors like heat transfer. 



Tailgate Back Door 

This commodity is largely steel-based. It includes: 

  • Left and right hinges
  • Hinge reinforcement
  • Wiper motor reinforcement
  • Gas stay reinforcement
  • Outer upper panel
  • Outer lower panel
  • Latch reinforcement
  • Inner panel



Since the inner panel of a tailgate back door is a rather complex structure, a casting operation is required for it. The outer panel can be made and assembled by simpler methods. 


Side Doors 

Side doors are highly complex structures separated into three features: 

  1. Header reinforcement (used for high-end luxury cars)
  2. Separate header
  3. Integrated header


When you design the inner parts of the side doors, you should keep in mind the commodities that would be placed inside like power window switches, window rollers, speakers, cup holders, bottle holders, handles, and more.

Additionally, some vehicles need to be viewed with a different approach when it comes to side body design. Some examples are:

  • Convertibles
  • Sashless cars
  • Coupe
  • Center pillar-less
  • 4-door coupe


Flange Details of Entire Body Side

Below is a picture that accurately explains the considerations of various portions of the side body flange.




After all of that is done, what comes next are the proportions and tolerances of joining all of them. This is known as 'gap and flushness.'



Field Concern and Quality Improvement of a Vehicle

After production, the car goes to the field where manufacturers receive additional feedback from the customers. The design is further tweaked as per consumer demand.

Below is an example of in-depth feedback where water enters into the headlamp of the car.




You have now successfully gained knowledge on the basics of car bodies, Body in White (BIW), and the different car components. Design engineers must have a strong background in these topics to make it big in the industry. 

If you are a design engineering student looking to upskill and build a core foundation in BIW and automotive sheet metal design, check our Post Graduate program in Body-in-White


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