Design of a suspension system for an off-road vehicle as well as failure mode and effective analysis for the vehicle.
The goal of competition was to successfully design, build and test an offroad vehicle. Goal for suspension was to safeguard vehicle’s members, driver from off-terrain shocks, provide suitable vehicle handling conditions. For the same, suspension kinetics, kinematics, shocks, dampers were studied, a suitable system was designed, simulated and then built. The system was then tested on the endurance track and suspension track. As a result, the vehicle and suspension endured 4 hours of off roading event successfully.
Research into existing suspension systems showed that the most suitable suspension system for an offroading vehicle is double wishbone suspension system. The double wishbone system is good for its flexibility (to control camber, castor, toe), for its negative camber and independent working nature. By adjusting the lengths of arms of wishbones, it becomes easier to adjust the suspension parameters such as camber and toe.
The suspension system working can be roughly divided into 2 parts- kinematics which studies motion of wheel and wishbones when the vehicle goes over a bump, and kinetics which studies motion of bodies under forces. For studies of kinematics, msc Adams software was used to simulate the movement of wishbones which also gave values of camber, toe change. For studies of kinetics, finite elemet analysis was carried out using Ansys workbench, where expected forces were simulated to know critical stress points.
The components that were designed through the kinetics study were springs, wishbones, upright (knuckle), steering arms to know if the design is able to withstand forces of off-terrain. The forces of off-terrain were researched and approximated by taking into consideration factor of safety. The material selected should be lightweight, high strength and easily machinable, so high grade aluminium was selected for manufacturing upright, steering arm, whereas high strength steel was used for manufacturing wishbones. The finite element analysis was carried out using Ansys to know maximum stress on a component.
Study of kinematics was highly iterative, where multiple combinations of wishbone,upright designs were simulated to find out the most suitable one for the vehicle. Roll cage department, steering department, brakes department and transmission department provided the necessary constraints for identifying the mounting points, and thus, specifications of wishbones. As a result, the simulator gives values of caster change, camber change, toe change with wheel displacement. The mounting points suggested by simulator, were then visualized and combined with roll cage using Catia software.
There were components that were specific to the vehicle and components that can be sourced directly from the market. The components directly sourced from market were bearings (cylindrical bearing, uniball ball joint), dampers for the shock absorber. Components that were manufactured were springs for shock absorber, wishbones, upright, steering arms and brackets for mounting.
Design Failure Mode and Effect Analysis is a systemic way to analyse design to identify where and how it is most prone to fail and to assess relative impact of different failures. The analysis was carried out for the whole vehicle and respective S-O-D (Severity-Occurance-Detection) and RPN (Risk Priority Number) were calculated. Corrective action plan was also suggested to reduce the RPN.
The suspension and overall vehicle was tested at the testing facility of Natrax in Madhya Pradesh, India, where vehicle was subjected to harsh conditions. In the end, the buggy ran 4 hours of endurance race on the off road track at the facility. The vehicle that we built sustained those conditions but also showed points of improvement for next year.
- For the design, inside out plan was devised, which led to changes in design of subsystems to accommodate already existing parts in the design. A more outside-in approach is suggested.
- In some cases, the factor of safety was more than necessary, leading to over-designing of subsystems which decreased the performance of buggy. An optimisation is necessary which will come with more experience of off-roading.
- Design tasks should have a deadline to stop design efforts at correct time and complete the project within the timeline.
- Components should be designed not only by considering the requirements of conditions it will face, but also by considering machineability and ease of manufacturing.