Shear stress and first normal stress difference relaxation data were measured for a range of shear strains using three polymers: polystyrene ( PS) with narrow molecular weight distribution, PS with large polydispersity index ( PI), and a commercial polyethylene, HDPE, with large PI. From these experiments a damping function, h(gamma), for each polymer was determined and compared with theoretical predictions of the recent force balanced network (FBN) model proposed by Marrucci et al. [ 1, 2]. For comparison purposes the data were also compared with Doi and Edward [ 3] theory. Both models were found to overestimate the shear softening of h( c). Nevertheless, a somewhat better description of the observed behavior was obtained with the FBN model without using any adjustable parameter.