Development of a New Software for Racecar Suspension Kinematics

Abstract

In the racecar design process the definition of suspension type and geometry is an important stage. A good design of the main parameters, in terms of static and dynamic angles in bump and steer, is the basis of a successful racecar quite often. This paper aims at introducing the development of a new software tool, called MLKrace, that can analyse the suspensions kinematics for a wide range of different layouts. There are today a lot of commercial tools that analyse or simulate suspension behaviour. In general their application is restricted to the classic double wishbone suspension and in this case the calculation of the kinematics is not so difficult. This layout is used on both ends of the majority of formula and sports race cars but it isn’t the only possible one. Figure 1 – Example of multilink suspension The MLKrace software is instead more suitable to analyse innovative suspension geometries, the mathematics being based on the resolution of the socalled Stewart platform, a method originally devised for parallel robot kinematics. Therefore full multilink or hybrid systems can be designed as easily as a double wishbone. In addition push rod, pull rod or outboard spring systems can be added and the driveshaft required float is computed. Multilink-McPherson hybrid suspensions can be designed as well. The flexibility is the great advantage of this new software. Apart from the mathematic model, a massive effort was devoted to the design of the Windows interface with the aim of making the designer’s job particularly easy and effective compared to outdated MS-Dos interfaces of most of the existing software. The output is shown in the form of 2D and 3D animations, a large number of predefined diagrams, numeric tables and regression functions. INTRODUCTION The suspension design process is an important point in the development of a new car or in the tuning of an existing car. As a matter of fact tyres can produce forces to generate high lateral and longitudinal acceleration only if they work in the proper way, where this means with a good contact area and angles. These angles are camber and toe. Figure 2 shows the effect of camber angle in cornering force. A general rule is that negative angles give more lateral force. But the optimum angle depends on the type of tyre and its application. The designer tries to keep it constant to have the best performance. In the design phase the suspension is conceived in a static configuration that defines the main parameters of the geometry. On the track, road bumps, load transfer and aerodynamic downforce produce a relative movement of the wheel. It is also important to study the variation of these parameters when the wheel is moving in bump and steer. A typical problem is to avoid “bump steer” or the toe change along the bump motion; this is a negative characteristic as it can upset the vehicle behaviour. Figure 2 – Camber effects on tyres The developed software “MLKrace” allows the designer to analyse the suspension system and to modify its characteristics for improvement. The choice of the suspension type is the first step in suspension design and MLKrace gives the opportunity to analyse different layouts to compare parameters, performance and overall dimensions. When the main layout is defined the design phase begins where the designer can move links and rods to obtain the best compromise for the specific car and application (road car, sportscar or formula). MLKrace aims to simplify the design process with the following features: • Modelling of the most common configurations • High quality and user-friendly interface • Fast and accurate calculation • Graphic representation of the suspension • Wide selection of displayed results Compared to other commercial suspension design software MLKrace is easy to use and complete and results are particularly accurate. SUSPENSION DESCRIPTION In the description of a new suspension the designer needs to input a lot of information. MLKrace’s main screen presents various options to define the suspension type and layout. The first parameter to set in the suspension is the layout. There are 5 possibilities: 1. Multilink: the most general type of suspension that gives the possibility to have virtual steering axis and a good control of angles. 2. Bottom multilink Top wishbone: a multilink where top arms are connected to form a wishbone. 3. Top Multilink – Bottom wishbone: a multilink where bottom arms are connected to form a wishbone. 4. Double wishbone: top and bottom arms are connected to form a wishbone. 5. McPherson: A strut geometry where the bottom links can be separated or connected to form a wishbone. 1 – Multilink 2 – Bottom Multilink 3 – Top Multilink 4 – Double wishbone