Experimental evidence shows that road racing motorcycles are often subject to peculiar stability issues, which commonly arise due to the high stresses that all mechanical parts and systems suffer in this kind of vehicles. These events generally consist of self-excited vibrations, which involve one or more elements of the motorcycle. Several studies can be found in the literature on this subject, dealing with different instability phenomena, e.g. chatter, weave, wobble, front wheel patter. Each of them can be identified with a mode of vibration of the motorcycle, with its own typical frequency and modal shape, which can be reproduced by suitable multibody models. The aim of this work is to identify the switching mechanisms of these self-excited vibrations, focusing on the in-plane modes of the motorcycle. To this purpose, novel tools for stability analysis are employed, along with root loci and time domain simulations, which have a wider documented use in the literature. In particular, a numerical algorithm is developed for computing stability maps descending from symbolic algebra multibody models. In this thesis the two main instability mechanisms that affect the motorcycle longitudinal dynamics are studied, the so-called “chatter” of rear wheel and “patter” of front wheel, both of them consisting of self-excited vibrations which can arise during heavy braking in straight motion. In both kinds of motion, the out-of-plane components are negligible, or they are not a necessary condition to the onset of instability, hence planar multibody models are employed, with rigid bodies. The full motorcycle multibody models employed feature all the relevant moving parts of the vehicle, therefore they are suitable to carry on transient manoeuvres in the time domain. The stability of these models is studied next to modal analysis of reduced or simplified models with the lowest possible number of degrees of freedom. These minimal models represent sub-systems of the vehicle, since they are designed in order to capture the essential characteristics of the specific instability mechanism under study, taking into account only the elements which are necessary to the onset of the self-excited vibrations. Taking advantage of the simplicity of these models, the equations of motion are linearised in steady-state conditions, and the system matrices are analysed to highlight the crucial parameters that can bring the actual vehicle to instability. Experience of race engineers and riders confirms that in motorcycles all kinds of instability are always to prevent, since they weaken the vehicle overall performance, due to the severe oscillations of the tyre-ground vertical loads that can arise, causing a loss of grip and making the vehicle harder to control. The results of the present work give a comprehensive description of the in-plane instability phenomena of a racing motorcycle, highlighting the parameters that play a key role in the onset of chatter and patter vibrations. In particular, the geometry of the rear (swing-arm) and front (telescopic fork) suspensions are demonstrated to have major role in the unstable phenomena, together with their damping and stiffness. The roles of the chain transmission and of the structural compliance of the frame and the forks are also studied. These results can be useful to prevent instability, by careful design of the motorcycle elements, by the correct choice of stiffness and damping parameters or by introduction of new devices on the vehicle.

L’evidenza sperimentale mostra che le motociclette stradali da corsa sono spesso soggette a particolari problemi di stabilità, che comunemente insorgono a causa delle forti sollecitazioni a cui tutte le parti meccaniche sono sottoposte in questi veicoli. Questi eventi generalmente consistono di vibrazioni auto eccitate, che coinvolgono uno o più elementi della motocicletta. Vari studi su questo tema sono reperibili in letteratura, inerenti diversi fenomeni di instabilià: chatter, weave, wobble, patter. Ognuno di questi fenomeni può essere identificato con un modo di vibrare della motocicletta, con le proprie frequenze tipiche e forme modali, e può essere riprodotto da modelli multicorpo adeguati. Lo scopo del presente lavoro è di identificare il meccanismo scatenante queste vibrazioni auto eccitate, concentrandosi sui modi entro piano del veicolo. A questo scopo, innovativi strumenti per l’analisi di stabilità sono impiegati, insieme a luoghi delle radici e simulazioni nel dominio del tempo, che hanno un utilizzo più largamente documentato in letteratura. In particolare, viene sviluppato un algoritmo numerico per calcolare mappe di stabilità a partire dalla modellazione multicorpo simbolica. In questa tesi vengono studiati i due principali meccanismi di instabilità che affliggono la dinamica longitudinale della motocicletta, i cosiddetti “chatter” della ruota posteriore e “patter” dell’anteriore; entrambi consistono di vibrazioni auto eccitate che possono scatenarsi durante forti frenate in rettilineo. In entrambi i moti, le componenti fuori piano sono trascurabili, o non sono una condizione necessaria per l’insorgenza dell’instabilità, per cui vengono utilizzati modelli multicorpo piani a corpi rigidi. I modelli multicorpo dell’intera motocicletta qui impiegati presentano tutte le parti mobili rilevanti nel moto del veicolo, quindi sono adatti ad eseguire simulazioni di manovre transitorie nel dominio del tempo. La stabilità di questi modelli viene studiata affiancadola all’analisi modale di modelli ridotti o semplificati, con il minor numero possibile di gradi di liberà. Questi modelli minimali rappresentano sottosistemi del veicolo, e sono progettati per catturare le caratteristiche essenziali dei fenomeni instabili in studio, tenendo conto solo degli elementi necessari per l’insorgenza delle vibrazioni auto eccitate. Approfittando della semplicità di questi modelli, le equazioni del moto vengono linearizzate in condizioni stazionare, e le matrici di sistema vengono analizzate per evidenziare i parametri chiave che possono condurre il veicolo reale all’instabilità. L’esperienza degli ingegneri di pista e dei piloti conferma che nelle motociclette tutti i fenomeni di instabilità sono da prevenrie, poichè riducono le prestazioni del veicolo, a causa delle intense oscillazioni del carico vericale tra ruota e suolo che possono avvenire, causando perdita di aderenza e rendendo il veicolo più difficile da controllare. I risultati di questo lavoro danno una descrizione completa dei fenomeni instabili entro piano di una morocicletta da corsa, evidenziando i parametri che giocano un ruolo chiave nello scatenarsi delle vibrazioni di chatter e di patter. In particolare, si dimostra che le geometrie delle sospensioni posteriori (a forcellone oscillante) e anteriori (a forcella telescopica) hanno un ruolo fondamentale in questi fenomeni, insieme ai loro smorzamenti e rigidezze. Inoltre vengono studiati i ruoli della trasmissione a catena e della cedevolezza strutturale del telaio e della forcella. Questi risultati possono essere utili per prevenire l’instabilità tramite un attento progetto degli elementi della motocicletta, la corretta scelta dei parametri di rigidezza e smorzamento, o con l’introduzione di nuovi dispositivi sul veicolo.

Analisi di stabilità dei modi entro piano di motociclette da competizione mediante modellazione multicorpo simbolica / Stefano Cattabriga , 2020 Sep 15. 32. ciclo, Anno Accademico 2018/2019.

Analisi di stabilità dei modi entro piano di motociclette da competizione mediante modellazione multicorpo simbolica

CATTABRIGA, STEFANO
2020

Abstract

Experimental evidence shows that road racing motorcycles are often subject to peculiar stability issues, which commonly arise due to the high stresses that all mechanical parts and systems suffer in this kind of vehicles. These events generally consist of self-excited vibrations, which involve one or more elements of the motorcycle. Several studies can be found in the literature on this subject, dealing with different instability phenomena, e.g. chatter, weave, wobble, front wheel patter. Each of them can be identified with a mode of vibration of the motorcycle, with its own typical frequency and modal shape, which can be reproduced by suitable multibody models. The aim of this work is to identify the switching mechanisms of these self-excited vibrations, focusing on the in-plane modes of the motorcycle. To this purpose, novel tools for stability analysis are employed, along with root loci and time domain simulations, which have a wider documented use in the literature. In particular, a numerical algorithm is developed for computing stability maps descending from symbolic algebra multibody models. In this thesis the two main instability mechanisms that affect the motorcycle longitudinal dynamics are studied, the so-called “chatter” of rear wheel and “patter” of front wheel, both of them consisting of self-excited vibrations which can arise during heavy braking in straight motion. In both kinds of motion, the out-of-plane components are negligible, or they are not a necessary condition to the onset of instability, hence planar multibody models are employed, with rigid bodies. The full motorcycle multibody models employed feature all the relevant moving parts of the vehicle, therefore they are suitable to carry on transient manoeuvres in the time domain. The stability of these models is studied next to modal analysis of reduced or simplified models with the lowest possible number of degrees of freedom. These minimal models represent sub-systems of the vehicle, since they are designed in order to capture the essential characteristics of the specific instability mechanism under study, taking into account only the elements which are necessary to the onset of the self-excited vibrations. Taking advantage of the simplicity of these models, the equations of motion are linearised in steady-state conditions, and the system matrices are analysed to highlight the crucial parameters that can bring the actual vehicle to instability. Experience of race engineers and riders confirms that in motorcycles all kinds of instability are always to prevent, since they weaken the vehicle overall performance, due to the severe oscillations of the tyre-ground vertical loads that can arise, causing a loss of grip and making the vehicle harder to control. The results of the present work give a comprehensive description of the in-plane instability phenomena of a racing motorcycle, highlighting the parameters that play a key role in the onset of chatter and patter vibrations. In particular, the geometry of the rear (swing-arm) and front (telescopic fork) suspensions are demonstrated to have major role in the unstable phenomena, together with their damping and stiffness. The roles of the chain transmission and of the structural compliance of the frame and the forks are also studied. These results can be useful to prevent instability, by careful design of the motorcycle elements, by the correct choice of stiffness and damping parameters or by introduction of new devices on the vehicle.
Stability analysis of in-plane modes of racing motorcycles employing symbolic algebra multibody modeling
15-set-2020
SORRENTINO, Silvio
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Utilizza questo identificativo per citare o creare un link a questo documento: https://hdl.handle.net/11380/1210575
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