Wheel loaders are fundamental construction equipment to assist handling of bulk material e.g. gravel and stones. During digging operations, it withstands forces that are both large and very complicated to predict. Moreover, it is very expensive to develop prototypes of wheel loader for verification. Consequently, the Discrete Element Method (DEM) was introduced for gravel modeling a couple of years ago to enable prediction of these forces. The gravel model is connected with a Multibody System (MBS) model of the wheel loader, in this thesis a Volvo L180G. The co-simulation of these two systems is a very computer intensive operation and hence, it is important to investigate which parameters that have the largest influence on the simulation results. The aim of this thesis is to investigate the simulation sensitivity with respect to co-simulation communication interval, collision detection interval and gravel normal stiffness.The simulation results are verified by comparison with measurement data from previous tests performed by Volvo CE. The simulations are compared to investigate the relevant parameters. The conclusion of this thesis is that DEM is a method that in a very good way can predict the draft forces during digging operations.