Designing Your Own Car

Designing Your Own Race Car, Sports Car or Off-Road Truck

Designing your own vehicle, even if you don’t build it, can be a very rewarding experience. You’ll learn a lot about how cars work and if you do build it, you’ll be part of a select group of people who’ve taken on the challenge of building their dream.

Once you choose the type of vehicle you’re interested in, it’s time to begin thinking about how your vehicle is going to look, perform and cost.

Vision/Concept

Car design is a process that begins with a vision or concept of the vehicle to be created. Much like production vehicle design, DIY car design seeks to define goals and objectives for the design. In passenger vehicles, the objectives are usually around price, acceptable performance and safety and satisfying the transportation needs of the target market.

Common Race or Performance Vehicle Objectives

For racing cars, the objectives are much more oriented toward pure performance. The primary objectives of most race cars or performance cars are:

  • Maximum acceleration and deceleration
  • Maximum cornering speed
  • Driver/occupant safety
  • Cost of the final product

Almost every objective you can dream up will relate to performance, cost, safety or aesthetics. Regardless of how important you believe an objective to be, it should be included for your consideration as it is in the details that a good design becomes great.

Sanctioning Body Regulations

If you are planning to participate in a sanctioned race series, the regulations published by the sanctioning body will also form part of the vision for the race vehicle. Although regulations are often seen as limitations, they should also be seen as opportunities, assuming there is leeway in the interpretation of them.

Example Objectives/Vision Statements

Let’s consider some example objectives and vision statements:

  • Total Parts Cost to remain below $10,000 (Vision/objective)
  • Track width front not to exceed 72 inches (Regulations)
  • Tires restricted to Greatrich XZ100 series (Regulations)
  • Total vehicle weight not to exceed 1300 lbs (Regulations)
  • Center of Gravity reduced 2″ from last year’s car (Vision/Objective)
  • Side impact protection sustain 80 mph impact (Vision/Objective)
  • Sleek, Flowing, Curving bodywork (Vision/Objective)

Once you have defined and documented the vision for the car then you will have a baseline of concepts and specifications to design from.

To assist you in documenting your vision and objectives we have prepared a Conceptual Drawing Template and a Component Worksheet document which are available as free downloads. The conceptual drawing can help you define the look/layout of your vehicle, while the Component Worksheet can help record the objectives/vision/regulations for each component in your car design.

Design Process

The Design of a vehicle is an iterative process where you attempt to meet all objectives you have defined. In practice, you usually won’t meet all of them, but hopefully most.

If you are designing a car from a donor car chassis, your process will involve determining how the vehicle components can be changed to meet your vision/objectives. If you are designing from scratch, your process will involve the design and selection of components from the rubber on the road on up.

Iterations

Car design is iterative (that is, requiring repeated re-designs) because with the exception of a few parts on a car, all parts attach to another in some way.

To illustrate, let’s picture a wheel on a race car. The wheel attaches to a tire and it also attaches to a wheel hub. If I decide to change the wheel from a 4 bolt by 4″ bolt circle to a 5 bolt by 5″ bolt circle, I’ll need to change the wheel hub as well. Now let’s say that wheel is no longer available in the size we need to fit the tire. I’ll need to change the tire as well. Further, what happens if the new wheel hub doesn’t fit the spindle I have? I’ll need to change it too and so on…

As you can see, changing one part can affect many parts “upstream” and “downstream” from that one part which is why we need to “Iteratively” design by going back and re-designing around a changed part.

The process itself is really not that complicated, but it does require some patience if you are scratch-building. Donor car builders will find their job a little easier in that the components the car comes with usually limit the design options and therefore the number of iterations necessary to get the best design possible.

Design Process Variations

Depending on whether you are starting from scratch or a donor car (The “Design Start point”) or are designing with sanctioning body regulations or your own vision (The “Design Basis”), there are four possible variations in the car design process.

In the diagram below is a matrix of the four variations. Click on the “View Design Process” to view the design process variation that matches your chosen vehicle type.

View Donor Vehicle With Regulations Design ProcessDonor Vehicle With Vision Design ProcessScratch built with VisionView Scratch-Built With Regulations Design Process

Engineering Considerations

As you design, it is important that you understand the demands placed upon your vehicle in the environment it is intended to operate in. If you consider that driving at 100 mph on a paved race circuit places far different demands on a vehicle than traveling at the same speed over a desert trail, then the ability to design or select components that match your needs becomes important.

How do you design or select the right components? This is where two types of knowledge come in handy: Book knowledge and Practical/Research Knowledge.

Engineering Knowledge

Book knowledge lays the foundation for our understanding of the engineering used in cars and trucks. The basics of metallurgy, structural design, and aerodynamics (among other disciplines), enable us to understand “How” stuff works. This knowledge also enables us to formulate a rough estimation of how a particular component might perform. Every amateur designer should have this knowledge, but scratch-builders even more so. (If you are just getting started learning about cars, check out our free Car Basics, How-To & Design Tips .)

Practical Knowledge

Practical observation and research ties our book knowledge to real-world examples. For instance, if you are designing a rally car, then researching the material, tube diameter and structure of a typical rally roll cage will enable you to tie your basic engineering knowledge to a practical example. Given even more practical examples of roll cages, you could use your knowledge to build a roll cage for your car and potentially even for other types of cars. The best practical examples are from vehicles that closely match the weight, speed and operating environment of your car.

Testing/Verification

Finally, while both engineering knowledge and practical observations can provide reasonable confidence in component designs or selections, physically or virtually testing the component can be the final way of assessing if your component works. Mechanical testing has been around for a very long time, and modern software has evolved over the years to enable amateurs to run basic simulations of structures (FEA or Finite Element Analysis) and airflows (CFD or Computational Fluid Dynamics or the “virtual wind tunnel”).

Check out our Design Tools section for some of the available software packages for design and simulation.

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