The process of designing a solar car can seem daunting at first, but like any endeavor, once some initial groundwork is laid, the rest of the trip falls into place with some persistence. It is my hope that this page will help you with the groundwork, and that some simple principles can be learned in order to apply them to the more formidable task at hand. You may find that many of the items discussed below are applicable over a wide range of design challenges, not necessarily revolving around the design of a solar car, persay. Good luck with your endeavors, and Have Fun!

Overview:
The Solar-powered car has taken many different shapes over the years. The general shape of the car has been refined and modified over the years, primarily to reflect a few major aspects:

     A) Changes in Technology
     B) Changes in Funding
     C) Previous Knowledge/Experience
     D) Race Regulations

It seems that the last item has had the most visible effect in the design process, rather than the particular outcome. Each and every design, if the builders intend to race it, must pass some general specifications and requirements. It seems prudent, therefore, to give the inquisitive mind a sense of what these regulations entail. Click here to see a copy of the American Solar Challenge (ASC)’s regulations for the 2005 race.

Like any project, the solar car is composed up of many subsystems. Within the Mechanical subsections are the following:

     A) Chassis
     B) Body
     C) Suspension
     D) Brakes
     E) Steering
     F) Human Factor

Below you will find each of these subsystems in more detail. It should be noted, however, that each of these divisions are completely interrelated. Ideally, they will all be designed together so that they work in harmony, and compliment each other. However, in practice, this is rarely done. The end product, however, is often much better off when the integration between the subsystems is thoroughly thought out, and incorporated.

It can be thought about as if you were designing a house: Each room is connected, yet separate. Each room has it’s own function, but functions overlap (one can sit down in a bedroom, dining room, living room, etc.). It would not be functional to have one walk through a bathroom to enter the kitchen, just as it would not be reasonable to have the main door in the floor. Also, does it make sense to have a separate hallway that connects each room to each other, or could one hallway have entrances to all the rooms? These questions are similar to the questions faced in designing a car’s systems, only in a different, somewhat more familiar, context.

Simple Mechanics:

In order to continue with some of the later design sections, one must become familiar with some simple physics principles. These have been adequately simplified here, for ease of understanding. Note that the information as mentioned, is close to what is used in the design of our car. However, it should be noted, that the issues have been simplified for ease of calculations, and the science is ever-expanding, and can be researched into more depth as necessary.

Stress:
In Mechanical Engineering, the term “stress” can often be associated with the more familiar term often stemming from high expectations and lots of work, but mainly deals with a more physical concept. In the simplest terms, stress can be defined as:

     Stress:The intensity of internal force¹

A more precise way to think about stress is that it is an internal pressure within a material. The general equation for stress is:

     [1]        

The units of stress are the same as that of Pressure: N/m² (Pa), and lb/in² (psi). Also, a commonly used unit is the ksi (1000 psi).

There are two “types” of stress. The most applicable to us is the tensile stress (the other is shear stress). It is easy for one to think of the tensile stress, denoted s (sigma), if one considers uniaxial tension. Pretend you have a solid wire, with diameter 1 inch, and you tie one end to a huge rock, and you pull on the other end with a force of 500 lbs (yeah, you’re pretty strong). Now, by Newton’s law, we know that the rock is also pulling back and equal and opposite 500 lbs, just for completeness. Well, to find the stress in the solid wire, we can take the force applied (500 lbs), and divide by the area (area of a circular cross section is p r². Which means, from equation 1, the stress is:

     

Knowing the possible stresses in a body can be very useful; it can help us predict if a part is going to fail, before it is actually built and tested. This is extremely beneficial in time, money, and safety. We can do this due to the property that, in general, a material fails at a given stress. People have been testing this for many years, and have compiled tables of common “yield stresses.”

For calculating stress, there are certain physical parameters which must be known, depending on the type of equation. This can include: Moment of Inertia, Young’s Modulus, Slenderness Ratio, and so forth. Some of these will now be discussed.

Young’s Modulus:
Also known as the Elastic Modulus or Modulus of Elasticity. The Young’s Modulus of a material is a useful characteristic in calculating various values in a system. It’s physical meaning comes from the relationship of the force to the elongation (how much some portion of an object “stretches” when it is given some load). This is a very simplified explanation, but should suffice for now. The Elastic Modulus is often denoted by the letter E, and has units of stress (Pa or psi). A common value for E for steel is approximately 29 million psi.

Moment of Inertia and Bending:
In order to understand the concept of Moment of Inertia ( I ), it may be easier to visualize given the context of bending. Bending is a relatively simple concept to visualize, as everyone has encountered it many times. From the bending of a diving board to the bending (and breaking) of a pencil, we are in some way or another familiar with bending, and even Moment of Inertia!

Take a ruler, or some similar-shaped object. If you were to try to bend it (don’t break it), you would inherently try to bend it by trying to bring the two thick parts together. You know, instinctively, that this is much easier than bending it by turning it ninety degrees (trying to bring the thin parts together). This is an example of moment of inertia. It is easier to bend the ruler in the first orientation because of the fact that the moment of inertia is less. That said, here are two simple equations for calculating the moment of inertia around the center of the shape ( w = width, h = height, and r = radius ):

     [2]        

     [3]        



Bibliography:
    1.      Riley, W., Sturges, L.D., Morris, D.H., Statics and Mechanics of Materials: An Integrated Approach, John Wiley & Sons, Inc, New York, 1996.