# Baja and Formula SAE Frame Design

The purpose of this post is to give an idea of how to design a tubular space frame for the Baja or Formula SAE competitions. This is the procedure that I have come up with after being involved in the design, construction and testing of frames for Formula SAE vehicles.

First, what is a frame? What is its role in the vehicle? The frame is a bracket that holds many systems of the car together. The frame also transmits the loads of the suspension! These two are the two most important general roles of the frame.

Where do you start? I have experienced myself through the years all the possible combinations: define suspension points and engine first, then design the frame and adapt systems to the frame design, to the other end where you let all your systems floating in space and design a frame around the systems. My conclusion so far is that you should try to design everything at once and iterate as much as possible. This is because the frame is another system of the car!

Where to start? Pencil and paper, with a sketch, many sketches. The idea at this stage is to generate as many designs as possible. In your sketch of the frame try to also incorporate other systems (e.g. engine).  When sketching first just draw the required rules members and then add the rest.  Also have in mind the manufacturability of the design (angles of notches and diameter of tube bends).  Once the sketches are generated look at them and start to combine the good parts of the sketches and leave the parts you don’t like. At the end choose at least 4 designs but no more than 8. Then decide what are going to be the metrics by which you will judge the design (e.g. weight, cg, torsional stiffness).

This leaves us with the task to model the frame in CAD software. It does not matter what software you are using these steps are generalized:

1. Make a hand drawn sketch with front, side and top view.

2. Identify all the nodes of the sketch and number them.

3. Make a table with the coordinates of all the nodes (at this point these will be rough numbers but the idea is to start, they can be changed later).

5. Create all the points from the table in step 3.

6. Draw lines between points (for curve sections a center point of the arc is needed most of the time).

7. Then almost all software packages have a piping, frame or beam toolbox where you can select the beam cross section and apply it to the line.  This step can vary greatly between different CAD software, but the idea underneath is the same.

8. Most likely the beams are crossing each other at the nodes, thus usually the same toolbar where the cross sections were applied to the lines will have a mating or coupling section where you can specify the connectivity between them (which tube goes first and which one is notched).

9. Save.

Once you have the model go into assembly mode and start adding all the components even if they are not completely designed.  At this point the integration between systems starts an iteration process.  At the same time, the metrics by which the design of the chassis was going to be chosen now can be calculated.

Steps 4 through 8 are shown using PTC CREO 2.0

These post will always be evolving and if you have any suggestions to improve it feel free to comment below or send me an email JLugo{at}ND.edu. Thanks to Bob Kobayashi and Oliver Chmell for their suggestions.