Wind turbines normally have a long operational lifetime and experience a wide range of operating conditions. A representative set of these conditions is considered as part of a design process, as codified in standards. However, operational experience shows that failures occur more frequently than expected, the more costly of these including failures in the main bearings and gearbox. As modern turbines are equipped with sophisticated online systems, an important task is to evaluate the drive train dynamics from online measurement data. In particular, internal forces leading to fatigue can only be determined indirectly from other locations' sensors. In this contribution, a direct wind turbine drive train is modelled using the floating frame of reference formulation for a flexible multibody dynamics system. The purpose is to evaluate drive train response based on blade root forces and bedplate motions. The dynamic response is evaluated in terms of main shaft deformation and main bearing forces under different wind conditions. The model was found to correspond well to a commercial wind turbine system simulation software (ViDyn).1 data in terms of drive train dynamics and its effect on drive train performance and components' fatigue life, based on blade root forces and moments, as well as bedplate motion. This analysis is crucial in evaluating different operation conditions. It can also be used as a priori to evaluate different proposed sensor positions. Multibody dynamics techniques are commonly used to analyze the component loads in mechanical systems [2,3]. Wind turbine drive trains could be considered to constitute a multibody system with flexible and rigid components interconnected with each other by kinematic constraints, and which experience force during different operating conditions. For instance, in [4], a flexible multibody dynamic system modelling is employed to study the dynamic response of a horizontal wind turbine gearbox composed of planetary and conventional parallel axis gear sets. In [5], the dynamic stability analysis of a horizontal axis wind turbine (HAWT) is presented. The analysis framework is established based on separation of the complete HAWT system model into rigid and flexible body subsystems. The most popular methods within flexible multibody dynamics modelling are floating frame of reference (FFR) formulation [2,6,7,8,9], absolute nodal coordinates formulation (ANCF) [10,11], and corotational frame of reference (CFR) [12,13].The FFR formulation is based on separating the overall motion of the elastic body into a large rigid body motion (or reference motion), and superimposing small deformations by introducing a body fixed coordinate system, which translate and rotate with the body and experience small deformations (measured relative to this coordinate system). In [10], the traditional FFR approach combined with linear elastic finite element (FE) models is illustrated. In contrast, the ANCF does not separate the total elastic body motion into a reference motion and elastic deformations re...