The author has selected fourteen automobiles on which to make a study of the factors of safety used in their design. He considers specifically the front axles, front-wheel spindles, propeller-shafts, clutch-shafts, transmission drive-shafts and rear-axle drive-shafts. The method of calculating the stresses is outlined; compositions of the steels used are given; and complete data are presented showing the factors of safety of the various parts, together with the intermediate figures used in obtaining the factors.