Motorbike Suspensions - Modern Design and Optimisation

This book addresses the fundamental aspects of the structural design and optimisation of a motorbike front suspension. Starting from diverse loading conditions that the normal use of the vehicle implies, it has been found that the emergency braking is the most severe loading case. A novel analytical model that, given a few geometric parameters of the motorbike and of the fork, allows calculating the stress field on the legs during the brake is presented. The analytical model was designed to account for the unequal stress distribution on the two legs that arises from the geometrical asymmetry of single brake disc architectures. The model accuracy was tested by comparison with FEA and experimental stress analyses. The possibility of balancing the loads on the legs of single disc architectures was examined and some structural optimization strategies were proposed. The design of fork–pin compression-fit couplings in motorbike front suspensions owes much to conjecture because of the poor knowledge of the starting friction coefficient and the mean coupling pressure. In these mechanical applications, the friction coefficient is a variable parameter because it depends on coupling assembly conditions, while the mean coupling pressure is uncertain because the geometry of the fork is not axisymmetric, so it is impossible to define the pressure by means of the thick walled cylinders theory, that requires a polar axial symmetry. The axial releasing force, which is the fundamental design parameter for such couplings, indeed depends on the aferomentional factors, usually unknown, and on the coupling area A, usually known: the book addresses this important issue. The standard procedure to assembly the steering shaft and the fork is realized by press fitting the components using a standing press. In order to guarantee the axial releasing force, high interferences are usually adopted. This assembly condition could produce a dangerous tensile state on the components, in particular referring to their fatigue behavior. For these reasons the possibility of realizing hybrid joints (interference fit and adhesively bonded), by applying anaerobic adhesive before the assembly operation, has been deeply investigated. This allows to reduce the amount of interference, improving the stress field on the components, by taking advantage of the adhesive strength. The design and optimization of clamped joints usually applied to front forks, is difficult to carry out with theoretical formulas mainly because of the shape of the hub, which does forbid to define the maximum bending stress easily. The fundamental idea is to define the structural behaviour of the clamp during the tightening phase. The aim is to find the relationship between the bolt pretension load and the maximum bending stress by means of the definition of a theoretical stress concentration factor developed by the presence of the bolts holes and spot facings. Investigations focused into various axle clamps first. Secondly, the methodology was extended to the joint realised between the fork and the stanchions. Non-linear FEA were performed, to take into account the contact between the external surface of the shaft and the internal surface of the clamp. Experimental tests were done in order to assess the validity of the numerical analyses. Brake calipers can be subdivided into standard and radial ones, where the latter usually equip high performances motorbikes. These components are crucial to safety: therefore they must withstand a number of requirements established by motorbike manufacturers, in terms of structural strength and stiffness. In this book were presented numerical and experimental investigations carryed out to understand the behaviour of these components under the loads generated by the normal use of the vehicle. In recent years front fork’s manufacturers have spent a valuable effort in improving powered two wheelers safety conditions. In fact, suspensions are a crucial component in defining the vehicle dynamic performances, which are tightly related to the rider's safety. As new electronic control technologies allow suspensions manufacturers to provide real time adjustments of the suspensions behaviour in the case of critical events, different configurations of active and semi active forks have been designed and tested.