The Analysis And Improving Of The Steering Returnability Of A Car Using Simulation Methods

In the driving processes, steering is a basic motion of the car. The driver changes the car's driving direction by controlling the steering wheel. Steering system is a necessary part in modern cars which consists of several transmission components and connects to the chassis, suspension and wheels. Toward the whole vehicle's dynamics system,the steering system is the input part of the car's turning motion and it's characteristic has a direct influence to the automotive handling stability and driving security.With the development of the freeway and the increasing of the vehicle's speed, the turning road feel driving car gets more and more attentions.if the front wheels of a car's don't have steering returnability moments or steering returnability moments are too small, the ability of the car's beeline running must be harmed, as long as a small force could make it turn,and the steering wheel must be turned back after running, it also returns its'beeline driving without returnability moments, and the driver doesn't have the feeling of the turning speed and driving performance too, and there is still a danger the car deviating the road because the driver couldn't turn steering wheel after finishing turning. Because it is concerned about the car's handling sense-"road feel"especially in high-speed car, and it also affects the safety of the accident at the same time, hence, studying on the steering returnability in handling stabilities is a very important.This thesis emphasizes the process of using virtual test method to improve the steering returnability of cars. Focusing on the topic, the following work is done. 1 The theoretical analysis about vehicle parameters affecting steering returnabilityHere the software Matlab/Simulink was used to build a vehicle model of force input with two Degree of Freedom , through simulation analysis, we could acquire the effect of the whole vehicle parameters, which could help improvement analysis for structure-based vehicle dynamic model using ADAMS. we did the theoretical analysis about wheel alignment parameters, steering system and tires affecting steering returnability through examples,graphics or formulas, getted these factors which are caster angle, kingpin inclination angle, friction, damping, stiffness, inertia, assistance characteristic, and tire affecting steering returnability most. 2 Hydraulic power steering systems and vehicle modelingIn Adams/Car built a hydraulic power steering system dynamics model, modified the steering system template of the Adams/Car and added the hydraulic power steering system characteristics that we got from the experiments into the template. After that carried out some simulations, such as Pylon course slalom test, Steering wheel angle step input, Steering Returnability test and Steering efforts test procedure to validate the model. The results showed that the model we have built is correct.3 The improvement and analysis for steering returnability performanceHave briefly introduced the objective evaluation indices and evaluation method of the steering returnability performances in national standards .Have analysed the effects of caster angle, kingpin inclination angle, friction, damping, stiffness, inertia, assistance characteristic, and tire to the whole vehicle steering returnability performances through the ADAMS dynamic model which we have built. The conclusions we had gotten were as following:1) Reducing the friction of the steering system's gemel could advance the vehicle's steering returnability performance, meanwhile too small friction transferred would increase the impact of different road surfaces to steering wheel.2) Reducing the backpressure of hydraulic power steering system appropriately, reducing the cycle pressure of redirector and reducing oil pressure loss, could reduce fluid damping, which could advance the vehicle's steering returnability performance. Meanwhile the fluid damping could reduce the system vibration energy and improve vehicle's smoothness, hence, it should not be too small.3) Allocating steering system's the moment of inertia rationally, especially increasing the upper moment of inertia of redirector, could advance the vehicle's steering returnability performance.4) Increasing the torsional stiffness of steering system appropriately could advance the vehicle's steering returnability performance, meanwhile too excessive torsional stiffness brought risk of excessive turn. Besides, increasing the stiffness between suspension and vehicle body could advance the returnability of suspension flexibility.5) Adopting hydraulic electronic power steering system (HEPS) or electric power steering system (EPS) whose assistance characteristic are changed based on the vehicle speed could advance the vehicle's steering returnability performance.6) Increasing caster angle and caster moment arm, could advance the vehicle's steering returnability performance, meanwhile consider steering ponderosity because of too big caster angle; Increasing kingpin inclination angle and scrub radius especially steering returnability in high speed could advance the vehicle's steering returnability performance, but it is not too large, too large scrub radius could lead that the aligning torque longitudinal force generated are too sensitive which affects the beeline driving ability and make against the vehicle's understeer. 7) Replacing the tire whose lateral stiffness is smaller could advance the vehicle's steering returnability performance, meanwhile should assure vehicle's handling stabilities performance.4 Confirm and validate the improvement projectConsidering the manufacturers'sample car and the upper methods about improving steering returnability, we confirm the project on caster angle, kingpin inclination angle, tire and returnability of suspension flexibility. Introduce the difference depression between left wheel and right wheel of understeer degree's objecttive evaluation method in steady static circular test, and do the simulation analysis of steady static circular characteristic for our dynamic model. Separately validate the steering returnability and steady static circular characteristic of the three confirmed projects, the three improvement projects improve the steering returnability effectively through the contrast analysis, and understeer degree changes a little ,ensure the vehicle's handling performance.