Abstract
The chassis stiffness of a vehicle influences road dynamic behaviour. In a standard vehicle elastic behaviour is demanded to suspension elements; hence chassis has only the task to be tough and stiff enough to avoid elastic interaction with flexible parts. To satisfy the philosophy of simplicity, competition go-kart regulations require the absence of suspension systems and differential gear. Thus elastic frame characteristics are highlighted by the absence of suspension elements and the global dynamic behaviour is influenced by chassis shape and stiffness and by tires characteristics. Hence frame stiffness must be carefully evaluated in order to compensate the absence of differential gear by rising up the rear internal wheel during a turn. This avoids tire slip in favour of stability and acceleration and simultaneously assures a rigid connection between mechanical members. Moreover the development of tires performance has increased chassis importance whose shapes and materials have not substantial evolved. The reason is that tires development is practically the same of other formulas. Therefore kart tires can take advantage of a research results obtained elsewhere, on the contrary kart frames were developed only with knowledge of many years of experimentation and track tests. Nevertheless the great diffusion and popularity of karting sport, means great investments from corporations, privates and amateurs and karting is going to include a such high technological content to need design and analysis criterions peculiar to modern engineering.
The aim of this study is to develop and validate a numerical model which integrates FEM analysis with Multi-Body technique, in order to evaluate dynamic behaviour of a racing kart. The result is a useful tool in kart frame design. The FEM model was developed on the geometric model to evaluate the chassis stiffness. This model was validated by means of a static and dynamic loading tests. The static test consists of measuring displacements due to an application of a known load. This test is also useful to estimate a gross torsional stiffness value. Instead the dynamic response of the chassis, in order to check the FEM model dynamic capability, was measured by means of a load cell hammer/accelerometer system. Being the tires radial stiffness a very important parameter, because it ranges in values similar to the chassis stiffness and deeply influences load transfer in a kart, a set of experimental investigation was dedicated to properly evaluate this parameter.
The FEM analysis permits to estimate the kart frame displacements related to different load conditions (lateral and vertical force, flexural moment) applied to wheel hubs. The estimated stiffness is represented with an equivalent springs in an home-made Multi-Body model. The handling model was validated by results comparison with a commercial software.
The proposed procedure, based on the use of commercial software for FEM analysis and an home-developed code for Multi-Body, permits the optimisation of an existing kart frame and the new concept design.
A possible improvement of this procedure is the introduction of a shape optimisation algorithm that converge in a semiautomatic way to an optimal solution: structural parameters are varied to obtain a goal in dynamic simulation results, minimising, for instance, the lap time.