Fig. 28 shows the principle on which all differential automobile gearing is based, that is, that both wheels receive half of the driving power even if one wheel should turn faster, as shown at Fig. 29, which is the case when turning a corner. This is what causes the power to drive both wheels at all times, whether going straight or on a turn. [Illustration: Fig. 34a. Differential Gears.] If, however, one wheel gets on slippery ground, then A, Fig. 29, will move forward, without pulling on the lower end. As the lever A has the same action as the pinion in a differential, shown in Fig. 34a, it will be seen that if the pinion center is moved in the direction of the arrow, and if the wheel W^1 slips, the pinion will simply roll on the bevel gear G^2 without driving it on the wheel W^2. This is the disagreeable characteristic of a differential, that makes one wheel spin when it touches a slippery spot on the road, and stalls the car, because the other wheels cannot get any driving power. CHAPTER VI THE DRIVE The term used to designate the transmission of power from the engine to the wheels, is called the _drive_. In nearly all cars the engine shaft runs fore and aft, and consequently is at right angles to the axles. This, of course, necessitates some sort of gearing between the engine shaft and axle. This change is made in the bevel gear drive hereafter explained. As the engine is mounted on the frame of the car, which rests on springs, and the axle is below the springs, it is obvious that the drive must be transmitted between two parts which have a relative up and down movement. This necessitates several things, structurally, which should be considered. First. A flexible joint must be interposed in the system, where a shaft is used to transmit the power. Second. Torsion rods are necessary to prevent the housing or casing of the rear axle from turning, due to reaction of the driving bevel gear. Third. A rod, or rods, are required to prevent a fore and aft movement of the rear axle. The rods run from the ends of the rear axle housing to some convenient point on the frame. ILLUSTRATING POWER TRANSMISSION.--For convenience, these mechanical elements are illustrated on a frame. [Illustration: Fig. 35. Radius Rods.] Fig. 35 shows a frame which has its rear axle provided with a pair of radius rods A A. These have their rear ends attached, in any suitable manner, to the axle housing, near the springs and the forward ends are brought forward and pivoted to the cross beam B. TORSION ROD.--These rods thus take care of any undue strain which takes place by the wheel striking obstructions. C represents the torsion rod which has its rear end firmly secured to the housing D, and its forward end to the cross piece E. This prevents the housing from turning, and also serves to provide against any undue thrust of the driving bevel.