Development of recirculating ball and nut and rack and pinion type steering systems to be used with double axis Macpherson front suspension for a passenger car

With developing vehicle technologies in nowadays, work on electric vehicles is increasing, and major automotive manufacturers are investing more in electric vehicles. Energy consumption, safety precautions and customer-oriented designs are also important in conjunction with electric vehicles. Steering systems tend to be different designs with developing technology, systems with less energy consumption and more customer focus are being developed by the manufacturers. In this study, two different steering systems with right-hand and left-hand steering were used for a front wheel drive passenger car with a double axis Macpherson type front axle suspension. Two different type of steering system concepts were designed for use in different markets and each type of them is suitable to use with same front axle system for the common use of the vehicle. A steering system that has a capability to reach U-turn diameter of 8,535 meter was designed for the right-hand drive vehicle. It is demanded in the tender request of London taxi that the driver is able to start his movement in parallel from one of the walls at a distance of 8,535m between the two walls and to return from the opposite wall without going back and forward. Some changes have been made in the dimensions and layout of the vehicle in consideration of the turning radius target. In the benchmark researches carried out, only front axle steering is aimed to provide the desired turning radius of the vehicle. For this reason, a double axis Macpherson type front suspension system has been selected in order to allowe the wheel to turn to provide the desired turning radius. It is generally deemed necessary to use recirculating ball and nut (RCB) steering gears on heavy commercial vehicles (trucks, buses etc.) and also off-road vehicles. Recirculating ball and nut steering gear must be accompanied by an additional set of parts and brackets. A steering system design has been carried out that includes the steering box and other steering linkage components, as the vehicle dimensions, load capacity, engine location and type (front wheel drive engine, electric motor), collision test requirements and the other regulations are considered. A further version of the vehicle with a left-hand drive rack and pinion steering system has been worked on for the same front axle. A common front axle has been developed on this side to enable the vehicle to be used both as a right-hand drive version with respect on the London city requirements and as a passenger car in other markets with left-hand drive. During thesis studies, a double axis Macpherson type front suspension system, which allows for high turn angles, is mentioned first. The front suspension geometry, which affects steering system parameters and geometrically limits the steering system components such as kingpin axis, caster angle, caster trail, camber angle, scrub radius, is worked and decided. And after that, we talked about the design of a recirculating ball and nut steering system for both right hand drive cars and a rack and pinion steering system for left hand drive vehicles. In this case, two different steering systems performance were examined mutually. A method has been developed that includes calculating the self aligning wheel torque in parking condition in order to determine maximum output steering box torque in the design of a recirculating ball and nut type steering system. It is examined on Mitchke's and bore torque formulas regarding on the self aligning torque. It is observed that Mitchke's formula is more likely considered on the front suspension geometry and not considered the tire effects. That's why the self alignin moment which is calculated from it is not sufficient enough and not simulate the real life exactly. On the other side bore torque method is developed in order to find out the self aligning torque, but in this method you have to be known all tire parameters such as contact patch and elastokinematic behaviours. This method is not so easy to reach. In the thesis, in order to find the self aligning torque without measuring the tire parameters, some measurements had been done on the another vehicle. With respect on the Mitchke's formula and measurements on the another vehicle, the self aligning torque is calculated. In the following sections the steering kinematics to provide the demanded turning radius and the design of the steering system components. Steering gear box geometrically packaged into the vehicle and different gear ratios are investigated both vehicle dynamics and steering performance. Steering linkage parts such as steering column, I-shaft, pitman arm, draglink, intermediate rod, tie rod and relay lever arms are also designed with considering the limits of packaging area, hardpoints which is coming from kinematic analysis and the durability of the parts. Steering column, I-shaft and pinion shaft location of the RCB gear is designed and packaged according to the target of the non-uniformity which is directly related on the angular velocity and steering wheel torque. Basically, non-uniformity of the angular velocities of the joints are packaged with considering the all position of the steering column. Non-uniformity of the joints is calculated with a formula in basically. Designer and engineers started to design their own system with some initial acceptance criteria. Total weight and weight distribution is some of these acceptance criteria. It is a common issue with changings the weights in the middle of the vehicle design project. In particular, the effects of varying weights on steering turn characteristics especially understeer gradient and the work done to correct these effects have been addressed in order to change the accepted weight at the beginning of the project, which emerged in vehicle designs, towards the end of the project. Without redesign the suspension geometry, just making small modification on the steering system parameters, bad effect of the changing weight distribution is eliminated. Variations in the turning characteristics of the vehicle with different steering ratios are also examined. Using the same methodology, the rack and pinion steering system was also investigated and designed. Rack force is calculated with same formula and method in the RCB gear section. Rack and pinion dimensions such as inner and outer tie rod locations, pinion tower angle, length and subframe connections of the rack and pinion is designed with the limits of the other parts in vehicle and steering kinematics. Rack gear ratios are also investigated with regarding on the turning characteristics and steering catch performance. Thus, the ball and nut type and rack type steering systems are designed with the same metric criteria. Finally, the issue of determining the optimum steering ratio for both recirculating ball and nut type and rack and pinion type steering system. It has been examined from different steering gear ratios which are also examined in terms of vehicle dynamics. In the selection of the steering gear ratio, steering catch performance is examined. In the electro-hydraulic steering drive mechanism, the capacities of the off the shelf pump units in different models and different type of steering gear ratios for both rack and pinion and RCB gear were also examined for two different steering systems and the optimum steering gear ratio was selected.