Chassis/Body


Heidts 

 

This document is designed to give the reader a feel for the techniques used by the HEV chassis team at SDSU. If desired it can also be used as a template for interested people with similar chassis design options to perform some analysis of their own. The accompanying Excel files work for a square tubular ladder frame. In our particular design, the most crucial length of frame is the section that runs underneath the doors of the vehicle. This section has the shortest length in the vertical direction to supply bending resistance as well as being the section that has the greatest distance from the supports, which causes a large moment-arm.  Throughout this span of frame, our design contains a rectilinear box which serves three purposes:

    1. Supplying additional bending resistance.
    2. The composites that make up the top and the bottom of the box are graphite-reinforced laminates with the majority of their stiffness in the 45-degree direction, which aids in torsional stiffness.
    3. This box supplies the needed volume for mounting the 28 batteries chosen to power the vehicle.

We realize that most people designing a ladder frame for a vehicle will be doing so without the addition of this "Battery Box".  Only slight modifications of the linked files will need to be made.  These modifications are mainly to the Area Moments link.  By excluding the portion of the total area moments that are contributed by the battery box, the user will bypass any problems.  To do this, simply adjust the equation in question to use the Cross Sectional properties that are contributing to the users design.

In order to design a tubular chassis, much information needs to be obtained.  The main concerns in design of a chassis are; Bending Stiffness, Torsion Stiffness and Weight.  Bending stiffness is a measure of an individual chassis to resist deflection due to a vertical load; it is measured in pounds per inch of deflection.  Equations for deflections are well published and available in most Mechanics of Materials textbooks.  The controlling factor involved in most of these calculations is the cross sectional area moment.

1) To solve for area moments click here

Using the area moment we can approximate total deflection for a certain point load introduced at the center of the wheelbase.  This is the worst case possible, if we distribute the load over an area or assume more point loads the deflection value goes down rapidly.

2) To solve for deflection click here

With the introduction of a load on any cross section of the chassis, a stress is induced as well.  The value of the induced stress changes throughout the cross section according to the distance from the neutral axis and the Bending Moment at that point. (The stress value is zero along the neutral axis.) The maximum value of this stress is at the points located farthest from the neutral axis. In our case the upper surface of the laminated composite is the surface in question.

3) For a look at these stresses click here

 

 

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HEV Team
Department of Mechanical Engineering
San Diego State University
5500 Campanile Dr.
San Diego, CA 92182-1323
Fax: (619) 594-3599
E-mail: hev@kahuna.sdsu.edu