SAE BAJA Offroad Vehicle - Rear Suspension
Above photo is our BAJA vehicle Derecho. This is during a race at UW - Stout in 2022.
Placed 13th overall and 9th in Suspension and Maneuverability out of over 100 teams
Introduction:
Objective:
To engineer a robust and efficient rear suspension system for the Baja SAE vehicle, optimizing for stability, traction, and durability, while ensuring ease of maintenance and compliance with competition rules and regulations.
Suspension goals for Derecho:
Retain handling and performance characteristics, iterate on the previous car, identify key failure points of prior iterations to improve component reliability and develop a greater understanding of suspension geometry to improve vehicle dynamics.
Summary of Rear Suspension:
Our Baja vehicle's rear suspension features a resilient design incorporating dual shock coilovers and a three-link suspension system. Custom-made in-house shocks ensure precise control over damping. Each season, tire selection is meticulously tailored to meet performance goals, with ATV tires providing traction on various terrains. Constructed from durable steel, the suspension components ensure reliability under demanding off-road conditions. Adjustable camber links, equipped with opposite-threaded rod ends, offer flexibility for fine-tuning alignment settings, contributing to optimal handling and performance.
Design Concept:
Initial Geometry Design:
Figure 1: 2D Rear Suspension Geometry Figure 2: 3D Rear Suspension Geometry
Component Design Considerations:
What was considered when designing: vehicle dynamics, terrain handling, load distribution, suspension travel, kinematics, durability, adjustability, compliance with rules, cost-effectiveness, and overall system integration.
Some aspects of the rear suspension was kept the same but a lot was changed. This vehicle, Derecho, was the first 4WD we had ever made. Much of the car had to be built from scratch. The rear suspension had some crucial changes. Below are some improvements and changes I planned out during the designing phase. (issues with 2021 Baja car)
The mounting location on the frame for the dual link coilover shocks, trailing arm, and camber links have to be moved due to frame changes.
The trailing arm will be seeing more forces than usual due to increased weight.
Lower camber link were getting banged up by rocks.
Both camber links were sheering bolts.
Needed heavier spring selection due to increased weight from 4WD.
Brake and shock mounts were overbuilt.
Function, Failure, and Analysis:
Rear Suspension Data Collection for Analysis
Design Studies:
Shock travel and ground clearance: Rear suspension travel: 9.44 inches. Ground clearance: 3.5 - 14.5 inches.
Spring Selection:
Due to increased weight, new springs needed to be selected. This requires calculations to determine best set up. Each dual coilover suspension setup has 2 springs; a main spring, and a tender spring.
Research:
Inboard camber link bolts needs to shear before the outboard bolts. Bolts need to shear before other components of the suspension like the links and shocks.
Individual Component Selection:
Trailing Arm:
The trailing arm required a look into many different aspects of design. The geometry and kinematics were first to determine the length, angle, and attachment points of the trailing arm.
The material selection and thickness was next. This trailing arm experiences forces from impacts and dynamic loads. The material selected was steel pipes. This was instead of a more robust trailing arm.
Camber links:
The camber links are what keep the tire oriented and set the camber angle for the suspension. The main considerations for the camber links after deciding geometry were the material and thickness. The lower camber link experiences impacts from rocks so we chose a thicker OD. The upper was a good location to reduce weight. Aluminum camber links were looked into but any failure of the camber links would be dangerous. We went with steel.
Frame Mounts:
The frame mounts were changed in result of the previous years vehicle. They were too thick and beefy. As 4WD was a big change, we needed to save as much weight as possible.
One aspect of the frame mounts is that they need to be welded to the frame exactly where the geometry calls them out. To do this I created 3D printed mount locaters that used the frame as reference.
The bolts were a huge factor in the success of failure of our vehicle, specifically the camber link bolts. The goal is for the the failure point of the suspension to be the outboard camber link bolts. This is so that the camber link will sheer off and not damage any more of the vehicle. To ensure this, the outboard bolts were increased from 5/16 to 3/8 inch. This new bolt would fail at 10,539 in lbs compared to 7000 in lbs for in inboard bolt.
Shock Absorber:
Above it was mentioned that our shocks were built in house. This is due to a few reasons. First, it gives us so much customizability and control over the shock. We are able to tweak it as we see fit due to real testing of the vehicle and how it performs. Second, if anything goes wrong with our shocks, we are able to take it apart and figure out the problem. This saves a lot of money. The drawbacks are that we are not professionals and can make errors in the manufacturing of the shock. This could lead to failure and even injury.
Main and Tender Spring:
The main spring sits below the tender spring. Between them is a crossover bump stop. This is what holds the two springs together and how the load is transferred from the main spring to the tender spring for when there are extreme loads on the suspension.
Eibach coilover springs were selected as the brand. The main spring was a 10 inch 130 lb while the tender spring was 6 inch 105 lb. This provided the best ride frequency and support for our vehicle.
Manufacturing:
A majority of the rear suspension components were manufactured in house. This was done by utilizing the Boyd Lab at ISU and the skills learned through manufacturing classes. The only parts that were not manufactured in house were the shock components, springs, and wheel and tire. Many spares had to be made for competitions for when components broke or needed replacing.
Conclusion:
The redesign of the rear suspension system for the Baja vehicle "Derecho" shows a meticulous approach to enhancing performance, durability, and safety in off-road racing conditions. By addressing a large amount of design considerations, from vehicle dynamics and terrain handling to material selection and compliance with regulations, the team successfully optimized the suspension system to meet the demands of 4WD operation.
Through strategic adjustments to mounting locations, material choices, and bolt sizes, the rear suspension was fortified to withstand increased weight and forces while minimizing the risk of failure and damage to the vehicle. Additionally, the utilization of custom-built shock absorbers and carefully selected spring configurations provided a tailored solution that prioritized ride quality and support.
Furthermore, the implementation of innovative design solutions, such as 3D printed mount locators and carefully engineered bolt shear points, underscored the team's commitment to efficiency, precision, and safety.
Ultimately, the result of these efforts resulted in a rear suspension system that not only met the performance goals for "Derecho" but also laid a foundation for continued innovation and success in Baja SAE competition.
