SHAFT DRIVE.--The shaft of the engine, being on the frame, has a vertical movement, and the axle, to which power is to be transmitted, is below. The engine must be mounted so the shaft inclines, or, be placed low enough, so that it will be on a direct line with the rear shaft. In either case some flexible means must be provided between the engine shaft and axle on account of the relative vertical motion between engine and rear axle. The _straight line drive_ is most desirable, in every way, as the full power of the engine is available, and this is usually arranged for by lowering the engine bed sufficiently so that the shaft will point straight to the axle when the car is loaded. [Illustration: Fig. 38. Shaft Drive.] [Illustration: Fig. 39. Straight Line Drive.] TRAIN OF SHAFTING.--Several lengths of shafting are often interposed between the engine shaft and axle, and some cars have two universal joints in the shaft line, one mounted forward of the transmission case and the other to the rear of it. Or, more frequently, one in the rear of the transmission and one in front of the rear axle. It seems, however, to be the most general practice to have a single universal joint directly behind the gear case, and the shaft forward of the case only slightly inclined. Figs. 38 and 39, show the two types, the former being the straight line drive, and the latter a form of construction where the two universal joints make the drive through a line which minimizes the angles as much as possible between the shafts. Figs. 38 and 39 are not intended to show all the elements in the train of shafting, such as joints and connections, but is merely designed to illustrate the disposition of the drive shafting relative to the engine and rear axle. CHAPTER VII CLUTCHES Clutches are essential in all gasoline cars, for the reason that the driving power of the motor must be frequently disconnected from the running gear. These devices are designed to transmit motion from the engine to the transmission shaft, so that when the clutch is engaged the transmission shaft will turn with the engine shaft. CLUTCH REQUIREMENTS.--The first requisite of a clutch is its ability to firmly hold the two shafts together; the second is, that it may be engaged gradually, and not suddenly; third, that it must disconnect instantaneously; and, fourth, that the force required to hold the two parts of the clutch together must not produce an end thrust on either shaft. These requirements must be met by a condition that the act of engaging the clutch will not necessitate a long movement of the foot pedal which sets the clutch. Other considerations must be taken into account, also, and that is facility for examining and repairing, easy removal of worn or broken parts, and capability of adjustment as the contact surfaces wear. [Illustration: Fig. 40. Cone Clutch.]