Experimental and numerical investigation on the motorcycle front frame flexibility and its effect on stability

• Two modern motorcycle forks (one enduro and one sport) are modally tested.
• At low frequencies the response is dominated by one lateral and one longitudinal mode.
• Wheel twist axis characterizes static and dynamic fork deformation.
• A single mode approach is enough to derive front fork models for stability analysis.
• A lumped fork model simulating both static and dynamic tests results is developed.

It is well known that front fork flexibility may have a significant effect on motorcycle stability. This work addresses the problem of developing lumped element models of the front fork from experimental results. The front forks of an enduro motorcycle and of a super sport motorcycle are characterized performing static, dynamic and modal tests by means of specific testing equipment. The concept of wheel twisting axis is proposed to characterize static and dynamic deformability of the front fork. Modal analysis results show the presence of two important modes of vibration of the front assembly in the low frequency range: the lateral mode and the longitudinal mode. Different lumped models are discussed and a new model that takes into account information obtained from static and dynamic tests is proposed. Simulations are carried out by means of a multibody code and show the effect of the front assembly deformability on the weave and wobble vibration modes.