THE TRACTIVE ability of a motor-vehicle, as stated by the author, is the measure of its power to overcome outside resistances to its translation, based on the tangential force exerted by the driving wheels at their points of contact with the road. The propelling force is derived from the engine. To compute the “tangential force” of the foregoing definition it is engine torque that interests us rather than the horsepower, he states. If the horsepower is given, it can be converted into torque.
After analyzing this point mathematically, the author discusses typical tractive-factors of modern motor-trucks so that he is enabled to develop an economic factor mathematically and thus be prepared to discuss tractive resistance as opposed to tractive effort. Air resistance is considered in detail as a particularly important factor concerning motorcoaches, and the author's points are backed up by diagrams and charts as well as by numerous tables of statistical and computed data.
In an Appendix, other tabulated data are presented which show the results obtained with a testing machine in which the conditions of a tire rolling on a hard smooth surface are reproduced and rolling resistance is measured correctly.