The physical properties of rubber crumb modified technical elastomers from NR-, CR- and NBR (with 18 and 38% ACN) are demonstrated and interpreted by elementar models for multi component Systems. Starting from a chemical and morphological characterization of the rubber crumb from truck tire treads, the influence of composition and particle size distribution on technical relevant properties like tensile strength, static and dynamic tear strength, compression set, swelling, thermal conductivity and dynamical resistance in flexometer tests is shown. Negative and positive effects of rubber crumb are found, dependent on the specific properties of the crumb and the rubber matrix.