14 A Proposal of Dynamic Behaviour Design Based on Mode Shape Tracing: Numerical Application to a Motorbike Frame 153 2000 4000 6000 8000 10000 12000 14000 Engine Speed [rpm] 0 100 200 300 400 500 600 700 800 Motorbike Frequencies [Hz] 1 EO 2 EO 3 EO 4 EO Fig. 14.1 Motorbike Campbell diagram 0 Chassis-Engine Mode [-] 0 5 10 15 20 100 200 300 400 500 600 700 800 Motorbike frequency [Hz] 0 10 20 30 40 50 60 70 80 90 Fig. 14.2 Front sub-assembly vs motorbike The four horizontal lines correspond to the natural frequencies of the motorbike modes that intersect the engine orders in the working frequency range analysed. In Fig. 14.2 only the first three engine harmonics find out a correspondence in the front sub-assemblies, while the last driving frequency is not influenced by the front sub-assembly. Modes 8, 12 and 13 of the front sub-assembly have a high influence on the motorbike global behaviour at those frequencies, respectively 225 Hz, 455 Hz, and 655 Hz. The influence of the rear sub-assembly is very important for the second and third driving frequency, with high participation of modes 14 (455 Hz), 22 (655 Hz), and a lower influence of mode 16 at 695 Hz as illustrated in Fig. 14.3. The mode shape tracing should continue with the comparison between sub-assemblies and components, as reported in Figs. 14.4, 14.5, and 14.6. The front sub-assembly can be compared only with respect to the chassis. No comparison with the engine is performed because it is treated as a rigid mass linked to the chassis.
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