SAE Baja Frame Design Project Final Report

Luke Blocker and Brandt Lamonte

May 3, 2017

ET493 Spring 2017

Advisor: Ho-Hoon Lee

Abstract

The SAE Baja frame team is charged with the responsibility of designing and developing a frame to be used for an off-road racing vehicle. The frame team’s primary duty is to account for the safety and comfort of the driver while maintaining the efficient usability and optimization of the design. The second-most important purpose for the frame team to serve as the facilitators for the Baja buggy as a whole. This facilitating role is in the form of ensuring that each of the other design teams’ components are accommodated for final assembly.

Introduction

The challenge of designing a frame was given to two separate groups of two people. Each of these subgroups act entirely independent of one another which should yield two different methods of approach and two very different final designs. Frame Team 1 cannot speak on behalf of Frame Team 2 as the following is our own methodology and developments.

Method

Frame Team 1’s method has changed notably since the proposal of the project. As more experience with the project begin to reveal the specific requirements that we were required to fulfill, the method was developed in proportion to the development of the frame itself. Frame team one first chose a desirable OD of tubing to construct the frame. With this pre-decided, an initial shape was formed.

initial frame design

The initial construction above was our first go at a design that we believed would conform to the needs of the other design teams while also providing safety measures set forth by the SAE. Using the first iteration of the frame, we set up an equation for solving for the weight distribution of the frame. We calculated weight distribution with respect to motor, driver, and steering components’ weights.

We conducted several stress analyses to find the weaknesses of the frame design. These stress analyses were numerous and repetitive, but resulted in several iterations. These developments were on-going even as we awaited input from the other design teams. The first developments were to create “strong points” for the suspension shocks to mount to. After several iterations we had a frame that was sufficiently rigid under the full-load forces of the baja at rest.

early frame design

With the frame proving to be satisfactorily efficient at bearing a load for it’s total weight, we began introducing new forces that would simulate several different load situations including impact and excessive static loading. These tests not only showed how stress would travel through the members, but also shed light on what members were necessary and what members were frivolous and not worth the additional weight to keep.

In addition to constant improvement of the frame design, we kept vigilant watch for the requirements of the other teams.

After many stress analyses and accounting for the requirements of the other groups, we went through several more iterations of the frame. Each iteration was intended to meet the needs of the final assembly as well as simplify the future fabrication of the design.

mid-semester frame design

With final inputs from the suspension, steering, and power transmission teams, the final iteration of the frame was created. The last modifications were the results of several loading cases that included, most importantly, ten-foot drops onto various parts of the frame.

Forces Used in Analyses

The forces we used in our analyses were calculated using our weight distribution formula in conjunction with simple physics to find forces equivalent to the impact associated with a drop of from a specified height. A large variety of heights were used to gain an understanding of how stress translated across the frame when force was exerted at various locations. Though many heights were tested for this reason, our benchmark of quantitative comprehension of design requirements was set to be the forces caused by a drop from ten feet.

Assumptions Made in Analyses

Because there is no way to generate numerical results from all the variables of the Baja’s design (i.e. angle of suspension, completed build weight, etc…), we had to make best-guess style assumptions in order to test the frame. These assumptions were:

Suspension

· Angle of spring-dampers equal to 30°

· Angle of spring-dampers held constant on impact

· Spring-dampers remain rigid on impact

Weight

· Total weight of frame and mounted components equal to 225lb

· Total curb weight equal to 560lb

Fixed Geometry in Analyses

When conducting a stress analysis in SolidWorks, it is required to have some fixed joints that will remain unchanged. As the test is running, fixed joints do not allow force to translate through them. Because of this, the pictures of the stress analyses have areas of stress concentration around these points. This is unavoidable in these tests thus requiring each test to be repeated as the fixed points are moved away from the exerted forces in stages. Below is an example of how we used this procedure to evaluate the frame. This example is from our “100-ft drop onto the front of frame” test.

100 ft drop onto front end

It should be noted that as the force is given more members to travel through, the stress in each member reduces and the stress concentration areas move. The stress concentration will continue to move with the fixed points until a weak member is uncovered—at which point the stress concentration will show in the same place with each additional test. This is how we were able to determine where our actual stress concentrations were.

Understanding Stress Concentrations

In SolidWorks, the color-coded result produced after a study is ran is given by a relative, case-specific scale. This means that even with a negligible inputted force, there will be some modeled deformation and some red indication of an area afflicted with a concentrated stress. Without understanding the color scale for each test, it may seem as if SolidWorks is always giving a critical reading as result of a test. When a stress analysis is complete, the first thing that should be understood is the case-specific scale. Secondly, the maximum value on the scale should be compared to the design stress that was decided. And lastly, if necessary, the frame should be modified and retested to ensure the stress concentration was alleviated.

Cross Member Placement

Along the way, it became evident that our initial geometry would not be strong enough to endure the harshness of the course. In order to make our cart stronger, careful analysis of stress concentration was conducted in order to find the best places to relieve stress via additional members.

Seen below is the gradual addition of cross members on our roll cage after stress concentration was found. eventual addition of roll cage cross members and their results

Max Load

As previously mentioned, our frame was tested for a max possible load that was set as the equivalent of the Baja with full curb weight being dropped from ten feet. As covered before, in order to ensure the forces in the test are far greater than any forces that are expected during normal operation of the Baja, we assumed the suspension to be perfectly rigid and the impact surface to be completely immovable. The results are below—note the max stress is given as the yield strength. During the operation of the Baja, we anticipate normal forces acting on the frame to be around 3 times the static load of the Baja with all weights considered.

10 ft drop onto rigid suspension

10 ft drop onto roll cage

Material Selection

With so many factors unknown to us, we had to make some assumptions based on our drop-test findings to assume what forces would be acting on the frame under normal circumstance. Using these assumptions, we were able to select 4130 Alloy Steel tubing as our design material. This steel alloy provides high impact resistance as well as ease of welding. With a tensile stress rating of 460 MPa, this alloy is strong enough to survive our extreme circumstance drop tests, much less the frame’s normal load.

While our initial designs used a schedule 40 tubing (1.315” OD 1.049” ID), this material size is not readily available. Instead, we have moved to using 1.375” OD 1.135” ID tubing, which provides a similar weight and overall area for stress distribution.

4130 Alloy Steel is not available locally. Despite contacting numerous Louisiana vendors, no one can supply this material. Online however, we were able to find the exact material and size we needed.

Design Factor

We computed both maximum stress from the extreme circumstances (crash test), the maximum stress to be expected from race conditions (35mph “speed bump”), and the maximum stress that our frame can handle when applied constantly in a repeated-reversed loading. Our frame held up well in each of the tests that used these constraints as the maximum stress allowed. We used the following:

Sn’ = Sn*Cm*Cs*Cr*Cst

where:

Sn = Endurance Limit = 27.5 ksi

Cm = Material Factor = 1.0

Cst = Stress-Type Factor = .8

Cr = Reliability Factor = .75

Cs = Size Factor = .85

With this, Sn’ was equal to 14.025 ksi (or 96.7 MPa). We used Sn’ to ensure that our design would hold up to the loads to be expected during operation of the Baja with 99.99% reliability.

We then divided Sn’ by a safety factor of “N”. We decided to go with N = 2, which is reserved by dynamic loading that hosts relatively few unknowns. We feel this is a justifiable decision because of the amount of tests regarding various circumstances that we conducted as well as other safety factors that are built into our test (such as assuming no reduction in impact force due to suspension or the tires).

Frame static with weight of driver, motor, and total curb weight reaction forces on suspension according to weight distribution.

3x static load at rear suspension mounts and 8x static load at front suspension mounts with the design stress set as allowable (red)

Overall Dimensions

83” x 46.5” x 28”

Cost Analysis

Item

Amount

Price

1.325 “ OD x 1.209 “ ID 4130 Alloy Steel Tube

119.2’ (15 * 8’ pieces)

20.1” (1 * 2’ piece)

$1319.50

$162.25 Shipping

1/8” ER80S-D2 TIG Filler Rods

15 lbs.

$142.50

$41.33 Shipping

Waste & Error Factor

15%

$249.84

Total Cost

$1915.42

Price sources: Onlinemetals.comArc-zone.com

Deliverables

In the beginning of the semester, we promised the following:

· Solidworks 3D model (Brandt & Luke)

· Full stress analysis for 3D model (Luke)

· Interface for component mounting (Brandt)

· Material Choice (Luke)

· Cost analysis (Brandt)

Along the way, we added more to the list:

· Add findings-based cross members

· Crash-force testing

initial frame design

final frame design

Conclusion

The assignment to design and develop a frame for the Baja buggy has been completed following strict and, to the best of our knowledge, the most correct procedures. Each member was added only after testing found it was necessary to add it to the frame. Using this method of approach, we feel confident that our deliverables are pertinent to the task we were charged with. In addition to our deliverables, the long hours spent working with SolidWorks and developing a frame with such careful consideration has done a lot to enhance our understanding of the design process. The constant need for modification and design adaptations made it clear to see that the design’s component variables should be first made into variable equations to allow for quick changes in resultant forces and modified distances--in other words, assumptions should be kept to a minimum. Though before this may have seemed like a rule of thumb, we now understand it through experience. This is sure to be monumental in future design projects.