Chassis Design for Baja SAE

University of Colorado

ABSTRACT

This paper acts as a mock design report describing the overall chassis design that I developed for the Baja SAE competition. It is a basic design, acting primarily as an artifact to help further my academic writing skills.

INTRODUCTION

Baja SAE is an intercollegiate competition sponsored by the Society of Automotive Engineers (SAE). The over-arching goal is to simulate the real world engineering and design conditions by having each team competing for their design to be manufactured by a fake firm. Thus, each team must design, build, test and promote their design to be not only the best but also operate within the given rule specifications. The vehicle itself is a single seat, all-terrain buggy that is powered by a 10-hp Briggs & Stratton single cylinder engine. In order to compete in the dynamic events, the vehicle must first pass the technical and safety inspection. This is where the car is inspected to insure that it is safe to operate.

After the vehicle has passed the initial inspection, it can be scored. The scoring is broken up into two parts, static and dynamic. The static events consist of the design report, the cost

analysis and the sales presentation. The Dynamic events are acceleration, traction, land maneuverability, rock crawl, suspension and an endurance race. Teams are scored with respect to the standard guidelines, and the team with the most points wins. The incorporation of the static events, not only prevent the team with the most money and therefore material quality from winning outright, but they also encourage good engineering practice which is also a goal of this competition.

This paper covers the ideas behind my design and its over-all methodology.

1 Chassis - Overview

A chassis at its most basic, acts as the skeleton for the entire vehicle. It provides a rigid connection between the front and rear suspension, creates structural support for the other necessary systems and provides protection for the driver. There are a variety of different frame designs to choose from; however a tubular space frame is most commonly used due to its inherent structural properties, and ease of fabrication and modification. This style of chassis consists of a series of tubes connected together in different ways to form a coherent

structure. An example of this style of frame can be seen in figure 1.

2 Design Considerations

Before an actual design can be put down on paper, the design parameters necessary to the design must first be addressed. Several parameters for instance could be total weight, material, mounting points, etc. Typically these design parameters are selected after the suspension, engine, drive train and the other major components of the vehicle have already been laid out. Then, the parameters can be chosen such that the frame is designed exactly to the specifications needed, which saves time, money and weight. For my project, since I did not have the entire vehicle designed, I chose frame stiffness as my primary design consideration since stiffness incorporates many other design considerations into it and allows me to showcase the design process, which is a primary goal of this paper.

Frame Stiffness

The suspension is designed to keep all of the tires on the ground when subjected to the dynamic loads of the vehicle. To do this, the frame is usually assumed to be a rigid structure[2]. Because this assumption is made, if the fame is in actuality, not a rigid structure, un-anticipated forces and degraded suspension performance can occur. This can lead to either lost vehicle performance in the best case and total frame failure in the worst. That being said, most frame designs walk the fine line between stiff enough to handle the loads yet light enough to deliver good performance[3]. This is where most of the time spent on a design goes. Furthermore, due to the frame requirements set down by the SAE rules committee, most teams agree that if the frame is stiff enough to withstand design loads then it will not fail if crashed[3].

There are two main types of stiffness when it comes to frames: torsional and bending. Bending stiffness, which corresponds to the frame tubes between the front and rear suspension bending, is not as critical within this vehicle design as torsional is. This is because midpoint bending of the tubes does not affect suspension performance as critically as torsional loads do[2].

Torsional stiffness – Torsional stiffness is the resistance of the frame to “twisting” loads. These torsional loads develop primarily due to the fact that there are four suspension attachment points, two on each side of the vehicle. Since the suspension loads react the loads on the opposite sides of the vehicle, moments are created that twist the frame resulting in

Figure 1. Tubular Space Frame

torsional loads. When the frame twists, it creates misalignments within the suspension which can cause the problems mentioned earlier.

There are several ways to increase torsional stiffness. One method involves adding copious amounts of material to the frame structure. This strategy is usually detrimental to the over-all performance because it also increases the weight. Another method is called triangulation which incorporates the polar moment of inertia [3].

Triangulation – It is well known that the triangle is one of the strongest principle design structures and because of this, it is beneficial create a frame of interconnecting triangular sections. A good example of this triangulation can be seen in figure 2.

Notice how most of the frame members connect and combine to form triangles. This significantly strengthens the frame without adding lots of weight. This strength to weight ratio is key when designing a race chassis. Furthermore, when using triangulation, the nodes where members connect become extremely strong and stiff, making them excellent mounting points for large force producing components such as the suspension and engine[3]. Thus, the frame should be designed in such a way that the main triangulation

nodes for the chassis also be the attachment points for the suspension.

Performing triangulation in this way also creates a coherent load path within the chassis. The load path corresponds to the path that the force and subsequent stress follows through a structure. If there is not a coherent load path within a structure and forces are being reacted in middle of long tubes then the structure will fail because the load is only being reacted by one element, which by itself if very weak. By having a coherent load path, then the force and stress can be spread out over many interconnected members which recduces the force and stress felt by any individual member. Thus the possiblity of a member failure goes down and lighter materials can be used to react those loads.

Since frame members consist of thin walled tubes, triangulation is very important. This is because tubes of this type perfrom very well in compression and tension but not in bending [3]. This means inorder to avoid bending in the main structural members, those members must be kept short. And the best way to create a coherent structure from short members is through triangulation.

Polar Moment of Inertia – Torsional stiffness of a frame is also heavily influenced by the concept of polar moment of inertia. This moment of inertia corresponds to an object’s ability to resist torsion. It is commonly used to determine shear stress acting in beams under torsion and in power calculations for turbines and the like. In the case of the chassis, only the

Figure 2: UM-Rolla 1996 FSAE Chassis[3]

basic concept is used to enhance torsional stiffness.

From the definition of polar moment of inertia, it is understood that the farther structural material is away from the axis of twist, the stiffer will be in bending and torsion [3].

3 My Design

Figure 3: Isometric view

Figure 4: Front View

Figure 5: Side View

Figure 6: Birds-eye View

Figures 3-6 are different views of the chassis that I designed. To create this chassis, I used the 3D-modelling software called SolidWorks. It belongs to the class of programs known as computer aided design (CAD) suites and is commonly used across all disciplines in industry. As can be clearly seen in the figures, my chassis is a tubular space frame consisting of a mix between 1.25” and 1” tubular steel members. I debated between what alloy of steel (4340 or 4130)to use, which becomes important later in

the analyisis process. I reached my decision by weighing the strength, cost and machinability for each material against one another. After that, I decided to go with the 4130 alloy as it is cheaper and easier to machine than 4340. It is a little weaker but that being said, with an ultimate strength of 97.2 ksi, it is more that adaquate for this design.

Other aspects that drove my design were the required structural members as dictated by by the rules, weight, and over-all frame dimensions.

The required members can be seen in figure 7 and consist of the main support hoop (MSH), the roll over bars (ROB), side support bars (SSB) and rear support bars (RSB)[ref].

The rules require only the members mentioned above and then stipulate that they can be connected in any way to from a coherent structure with adequate strength.

Inorder to keep the weight down, I tried to minimize the total

number of members in the design. SolidWorks actually has a tool in it that will calculate the combined mass of all of the solid bodies in the model, but I was unable to use it. This is because I had to create the model using solid tubes instead of hollow ones due to computational and time constraints. However, most teams aim for a chassis weight of around 50 to 60 pounds [1].

The over-all dimensions of my design were also a driving factor. The rules stipulate that the courses for the dynamic competition are designed for a maximum vehichle size of 64 in. at the widest point and 108 in. at the longest[1]. The longer and wider the car is, the more stable it is. However, it is more a function of aspect ratio versus outright dimensions. As in, how long the car is with respect to how wide it is. If the car is too long, then it will tend to “push” through corners, causing a poor turning radius and loss of speed. However, too short of a car will be really “twitchy” in steering at high speeds which can result in a roll over. Furthermore, since this is an off road competion, a longer wheel base would be beneficial in the rock-crawling competion for an extened reach, but during the hill-climb portion, a longer wheel base can cause the front end to lift off the ground earlier than a shorter one would. It is all a matter of compromises and suspension design. Due to the lack of suspension in my project, I chose a wheel base of 97 in. and a width of 62 in. This is one parameter that would need to be investigate in much more detail if the chassis were to be built.

Figure 7: Required Members

I chose to keep the design wide in order to maximize my polar moment of inertia. As seen in figure 4 (reproduced below),

the

chassis members extend farther out past the central axis of the design. This helps increase the torsional stiffness of my design. Having the chassis widest where the driver is sitting also makes driver ingress and egress easier and safer, which is one part of the technical inspection.

I tried to triangulate as much as possible too. As can be seen in figures 3-6, most of the structural members form triangles with the surrounding elements. Also, at the extreme four corners of the chassis, there are points where more than 3 members attach into a node. These points are very strong and have a good load path to the rest of the structure. I designed these nodes as the suspension attachment points, where most of the force will be transmitted to the chassis.

4 Analysis

When I use the term analysis in this context, I mean to analyze the

stress and deflection within the members of the chassis, under reponse to structural loads. To do this, I used a computer program called ANSYS. This is a finite element analysis (FEA) program that allows the user to analyze virtually any complex geometry.

FEA is a type of analysis where the solid geometry created in a CAD program is “chopped” up into a bunch (typically on the order of 100,000) pieces and each of those pieces is analyzed with respect to the type of analysis being done. This is a very powerful tool, however for large models like the chassis, this analyis mode takes a lot of computational power, which equates to lots of times. In order for me to be able to solve a model in under 20 minutes, I had to use soild bars in my model. If I were to use hollow tubes with a thickness of .045 in, ANSYS would have to represent the model with elements no bigger than .045 in. Thus in such a big model, there would be a very large number of pieces, probably close to 106, which would take almost 2 hours to run.

My goals for the analyis I performed were to get a rough sense of the primary load path through the chassis and to see if there where any stress “hot-spots”. For this analysis, solids bars will generate similar results as what one could expect from hollow bars. This is because the applied load will travel along the same path whether the bars are solid or not. Furthermore, if there are any stress concentrations in the solid bar model then they will still be there and

Figure 4: Front view

exagerated in the hollow model and must be remedied any ways. Because of this and the time constraint I determined that a solid bar model would perform as needed.

The Setups

I decided to analyze my frame based upon several different loading schemes, vertical loading, side-impact and front impact. I decided not to perform a torsional stiffness analysis because the use of solid bars does have a large impact on this type of analysis. The solid bars have too much inertial stiffness which leads to errouneous results when subjected to torsional loads. This would be an analysis that needs to be completed farther down the design path, once it becomes prudent to invest the time needed to perform the analysis.

Vertical Loading - For this analysis setup, I rigidly fixed the model in ANSYS at the four suspension points and loaded the chassis at the center of gravity (CG) with a large 700 lbf.

This setup is fairly realistic of a dynamic loading situation during vehicle operation, as the force at the

CG would be reacted equally by the four suspension points. The results can be seen in figure 9.

Figure 9 shows the deformation of the chassis as well but keep in mind that the scale is on the order of 10-3, which is not even visible to the naked eye. What is seen from this stress distribution is that the vertical loading is reacted by the majority of the structural members within the chassis. This is indicative of a good load path and transfer within the members, which is what I wanted to see. Also, there are no bright red spots in the model which indicates that there are no large stress concentrations which is also good to see.

Side Impact – For this analysis, I kept the chassis rigidly connected at the four corners again and applied a force to the side support member. This can be seen in figure 10.

Figure 8: Vertical Loading

The stress distribution can be seen in figure 11.

Once again, there are no stress concentrations and although the impacted side of the chassis distributes the load pretty well, I would like to see the load transmited to the opposite side of the chassis a little better. Also, this style of analyis with the chassis rigidly fixed, represents a much harsher impact than what would ever be possible in a real impact. That is because in a real impact, the suspension would give and the car would slide over the ground, lessing the impulse force of the collision, but this analysis gives a good idea of what load transmission to expect in the event of a side impact.

Front Impact – The chassis is still rigidly fixed in this analysis and the load is applied to the front of the chassis. See figure 12.

As seen from the stress distrubtion in figure 13, there is almost no load transmitted to the rear of the chassis.

Figure 9: Vertical Loading Stress Distribution

Figure 10: Side Impact

Figure 11: Side Impact Stress Distribution

Figure 12: Front Impact

Figure 13: Front Impact Stress Distribution

Upon review, I think this is due to the fact that there is only a comined loading of 600 lbf, which is probably not enough for a solid bar model. Also, the front rigid supports should be removed if the analysis were to be run again as well since they are holding the chassis in one place, which would not be the case in a real front impact. The results are still useful and it can be seen that all the structural members attached to the node are being loaded almost equally. This is very good to see and indicates that there is adaquate structural support for a frontal impact.

Conclusion

Frame design, as most design problems are, comes down to a series of tradeoffs between various competing aspects. With a chassis, the main aspects are stiffness, weight and cost. There are several ways to maximize these trade-offs as discussed earlier and this is essentially what the design process consists of.

I feel that my design incorperated the concepts of triangulation and polar moment of inertia into a coherent chassis design that is representative of a first cut. The actual design process, is an iterative effort and once the analysis results are in, the design can be tweaked and updated to accommodate the discovered weaknesses. This is where my design currently sits. It has been created and analyzed and the next step would be to update the design to address the issues found in the analysis.

However, within the scope of this project, I feel I have suceeded in designing a basic chassis that satifies the design requirements of the Baja SAE competion and allowed me to explore and write about the design challenges and process.

References

[1] Formula SAE Rules. Warrendale, Pa: SAE International.

[2] Smith, Carroll. Tune to Win. Fallbrock, CA: Aero Publishers 1978.

[3] Gaffney III, Edmund, and Anthony Salinas. "Introduction to Formula SAE Suspension and Frame Design”.