Bellows flexible joints are included in automobile exhaust systems to allow for engine movements and thermal expansion and to reduce vibration transmission. Generally the joint consists of a flexible bellows, an inside liner and an outside braid. In this work the bellows is considered. A straightforward way to model the bellows is to use shell finite elements. Due to the convoluted geometry of the bellows that procedure requires however a high number of elements, meaning that the bellows model would constitute a large part of the model of the exhaust system. For more effective dynamics simulations a beam finite element representation of the bellows has been presented in a prior work. This modelling procedure was implemented in the commercial software I-DEAS and was verified against experimental results available in the literature for single-ply bellows of constant mean radius. This paper suggests adjustments by which this procedure can be extended to model also multi-ply bellows of variable mean radius. Experimental investigations of a double-ply bellows having decreasing mean radius towards its ends are included for verification. The agreement between theoretical and experimental results is very good, implying that the suggested extension of the modelling procedure is valid. It is also shown that the procedure can easily be implemented into other commercial software (in this case ABAQUS). The experimental investigation reveals an intriguing resonance frequency shift at small excitation force levels. Although considered to be of minor significance for the present application of the bellows, a hypothetic qualitative explanation to the observed phenomenon is given.