When an all-terrain vehicle (ATV) is running on a rough surface, the frame is subject to strong forces transmitted from the underbody and other parts, so it must have sufficient strength resistance to such stresses.
One of the methods for checking the strength of motorcycle and ATV parts is to measure the applied stresses by using strain gauges. By evaluating the stresses measured under various running patterns, it is possible to determine if the part has sufficient strength. However, measuring the stresses requires an actual machine, and solving problems that have been identified and checking their effects is costly and time-consuming.
Jump-landing, which is an indispensable test for checking the strength of an ATV frame, is a particularly severe condition, where heavy loads hit the body and high stresses act on the frame. If the stresses generated upon jump-landing could be calculated in the early stage of design review by computer simulation, the problems encountered while checking the strength on prototype vehicles could be reduced.
Forces acting on the ATV frame are more complex than those acting on the frame of a motorcycle because of the difference in the number of wheels, the link system of the suspension arms, etc. Jump-landing radically changes the body position within a matter of seconds and drives the ATV forward, complicating the calculation of forces transmitted to the frame from underbody parts and the inertia of heavy parts. This was why it had been difficult to determine the boundary conditions by simple static analysis and to check the frame strength by computer simulation in the design review stage.
Therefore, engineers from Suzuki Motor Corp. developed a new method of analysis for ATV jump-landing that can be implemented at the early stage of design review. The stresses acting on the frame were calculated by combining mechanism analysis and structure analysis and repeating the calibrations to make the results of analyses correspond to the results of actual experiments.
To determine the conditions of the structure analysis, calibrate the results of analysis with the actual behavior of the ATV body, and calibrate the results of the structure analysis with actually generated stresses, ATV jump-landing experiments were conducted using a jump ramp on the test course and measuring the displacement of the axles, loads placed on the body, and stresses acting on the frame.
By calibrating the results of the analysis with the results of the experiments, a mechanism analysis model was developed and conducted. The stresses generated were calculated by inputting the results of calculating the loads generated on joints into the structure analysis of the frame. For this purpose, mechanism and structure analysis models were developed.
As link data for the mechanism analysis, underbody parts such as the suspension arms and knuckles were used and set up so that they were aligned in the same way as on the actual ATV. The suspension was reproduced with spring damping constraints added between the suspension arms and the frame. As for the engine, a simplified model was developed as the link fixed to the frame, in which the mass and center of gravity were the same as those of the actual engine.
Heavy parts, such as the fuel tank and muffler, were fixed to the ATV frame of the mechanism analysis model, with weights placed on their centers of gravity. Since weights were not created for all of the parts, the above addition alone left the frame lighter than the actual vehicle. Therefore, dummy weights were placed at the front and rear of the body and the mass of the dummy weights was adjusted so that the weight and the center of gravity of the body became the same as those of the actual vehicle.
A simplified model of the tires was developed with spring damping constraints added on the wheels and contact constraints on the ground. The spring coefficients were based on the values measured on actual tires, and the damping coefficient, coefficient of restitution to the ground, and friction coefficient were determined by calibrating the results of the analysis with the actual behavior.
Stresses generated on the frame upon jump-landing were calculated by doing a mechanism analysis based on the data measured in the jump-landing experiments, calculating the reaction on the frame from the other parts, and inputting the results into the structure analysis.
The 3-D CAD data creation, mechanism analysis, structure analysis, and pre- and post-treatment were carried out using NX CAD software from Siemens PLM. Suzuki engineers made it easy for designers to do the analysis themselves in the design review stage by developing a software program that allows them to easily input the results of the mechanism analysis into the data for structure analysis.
The results of comparison between the experimental values and the analytical values of the stresses generated at the stress concentration points and the stress contour figures at the maximum pressure on the rear suspension are shown in the bar graph.
A comparison of the analytical values and experimental values showed that all the symbols matched and that the magnitude relations at the respective points also matched, except at point 5. Furthermore, the values of stress at points of high tensile stress (points 7 and 8), which are important in the evaluation of strength, were close between the experimental and analytical values.
This article is based on SAE Tech Paper 2012-32-0101 by Daisuke Iwata and Yoshinobu Matsumoto of Suzuki Motor Corp.