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Transient Response Analysis and Synthesis of an FSAE Vehicle using Cornering Compliance
ISSN: 0148-7191, e-ISSN: 2688-3627
To be published on November 21, 2019 by SAE International in United States
Event: NuGen Summit
OBJECTIVE Race vehicles are designed to achieve higher lateral acceleration arising at cornering conditions. A focused study on the steady state handling of the car is essential for the analysis of such conditions. The transient response analysis of the car is also equally important to achieve best driver-car relationship and to quantify handling in the range suitable for a racing car. This research aims to investigate the design parameters responsible for the transient characteristics and optimize those design parameters. This research work examines the time-based analysis of the problem to truly capture the non-linear dynamics. Apart from tires, chassis can be tuned to optimize vehicle handling and hence the response times. METHODOLOGY To start with, the system is modelled with governing parameters and simulation is carried out to set baseline configurations. Steady state and transient handling simulations run independent of each other with independent logic, coded on MATLAB. The static testing of the chassis is carried over using a Kinematic & Compliance (K & C) testing rig to get Compliance Budget and hence the calculated Understeer Gradient. After the simulations results and static tests, the chassis is set up for dynamic testing. On track dynamic open loop step response test is carried out, with data acquisition system to capture vehicle motion metrics. Post-processing and filtering the test data is carried over to find the response times of yaw rate, lateral acceleration, sideslip and roll rate against steering wheel angle. Dominant chassis parameters were altered to study the new transient characteristics. RESULTS From the steady state simulation carried over for ISO 4138, the characteristics speed was computed to be 49 km/h. This necessitated the step response test to be run at less than characteristic speed. From the K&C testing rig, the front axle Aligning Moment Steer Compliance was found to be 2 deg/g and rear Axle Compliance was 1.5 deg/g. This result is less than the simulated target values. The Lateral Acceleration Response Time was simulated to be 0.186 secs for 2.3 deg/g and 2.0 deg/g of Front and rear compliance respectively. This was validated by dynamics step response test ISO 7401 test of the car which gave a response time of 0.21 secs. The deviation can be attributed to road surface properties, compliance and design departure in manufacturing of the prototype. LIMITATIONS The steady state simulation runs on a time stepping approach without considering the longitudinal forces of the tires and hence the understeer-oversteer due to differential properties. The transient response simulation incorporates modelling of transfer functions for respective responses, and giving a step steering input. The dynamic testing was carried out in limited space, on a concrete surface, whereas tire data was modelled for tarmac. INNOVATION This study will help to set a benchmark for the Formula Student category of vehicles, since this work hasn't been performed on this scale yet. This research will open floor to further optimization techniques for vehicle handling design and development of improved and more complex models for handling characterization. CONCLUSION The handling of the FSAE vehicle was studied through simulations. The simulation results were later verified by on-track testing, albeit with limitations. A lower axle compliance overall, with front compliance being greater than the rear will ensure a higher characteristic speed also minimizing the Lateral Acceleration Response Time. The LART is established as one of the most paramount factors for determining the transient characteristics of a car. Furthermore, a benchmark for FSAE vehicle can be set using this research work.