within selected limits. Thus any transmission ratio most suitable for a particular driving condition may be selected, giving best car performance for a predetermined engine size. Moreover, noise, time loss and effort experienced when shifting gears in an ordinary transmission are eliminated and adaptability for automatic control is increased. These advantages were recognized early, but com plexity, weight, cost and low efficiency have limited the use of such transmissions to a small field where performance and smooth ness are paramount and the objectionable features are of sec ondary importance, as for instance in American motor-bus and rail-car operation. Friction drive was among those first utilized. Early types used a belt and two conical pulleys. Friction discs using wood, leather or fabrics on the driving surfaces, or metal to metal contact, have been tried on small cars with limited success. Variable throw mechanisms oscillating intermittent members which in turn drive through devices simulating ratchets are used in the De Lavaud transmission. The intermittent action of the several individual members combine in a continuous driving effort. Other transmissions use the inertia reaction of weights oscillated by a constant throw mechanism, the inertia reaction driving through ratchets or similar devices.
Hydraulic transmissions use an engine-driven pump, deliver ing an arbitrarily variable amount of fluid to an hydraulic motor operating the driving wheels. In electric transmissions an engine driven generator supplies current to electric driving motors. In some arrangements the structural parts of generator and motor have been combined to reduce weight and cost. Electric trans missions are widely used in American motor-buses and on a few trucks. By combining an infinitely variable transmission with an epicyclic gear, part of the torque may be transmitted through gears, thus reducing weight and costs of the variable mechanism and increasing efficiency. Besides this, two ratios may be obtained at which the variable mechanism does not perform any relative motion. In general, the control of transmission ratio may be in herently automatic, governor operated, manual, or a combination of these. Automatic control may be based on engine or vehicle speed, road torque requirements or combinations thereof. Only a few passenger cars at present use any of these forms of infinitely variable transmission, because when cost, weight, size, complica tion and general reliability are considered, the manually operated sliding gear transmission is more practical than any infinitely variable type so far developed. At the present time the electric transmission used on American motor-buses is generally not con sidered suitable for passenger cars.
general types of propeller shafts, depending upon the manner in which the rear axle is attached to the car. In the torque tube drive, the rear axle housing is extended in the form of a tube surrounding the propeller shaft, and this tube is flexibly attached to the rear of the gear-box or to some point on the frame adjacent thereto, so that the rear axle, in moving up and down, rotates about this joint. In this case only one universal joint placed coaxially with the point of attachment of the torque tube is required. In the so-called "Hotchkiss" drive, the rear axle is fastened rigidly to the springs, and moves up and down without rotating about the wheel axis, in which case it is necessary to have two universal joints, one at each end of the shaft, and a sliding connection to allow the shaft to change its length to allow for movement of the rear axle about a centre not coaxial with either joint. Some cars have a beam or "radius rod" which per forms the function of locating the rear axle, in which case two joints are invariably employed.
Universal joints are used to transmit torque from one shaft to another through an angle. Where loads and speeds are high, this angle must be small, not over 6 to 8°, although for light work angles of up to 3o° are practical. Many types of universal joints are used, but the most common are the mechanical "Cardan" joint and the flexible type. The former consists essentially of two forked members, one on each of the two intersecting shafts, so disposed that a pin between the two "prongs" of each fork will pass through the point of intersection of the shafts. The two pins form a rigid cross which transmits the torque from one fork to the other regardless of their relative angle. The flexible type of universal joint consists essen tially of two similar forks with a ring of flexible material, usually a rubberized fabric between them, and the relative movement of the two forks is taken up in the flexing of the ring.
Rear Axle and Final Drive. —When the axis of the power plant is disposed longitudinally, the final transmission has to provide a right-angle drive com bined with the necessary reduc tion of rotational speed; this may be effected in various ways, of which at the present time the spiral bevel and the hypoid gear (an offset type of spiral bevel) are principally used. The straight bevel gear has about disappeared from passenger-car practice. The worm is used occasionally in Europe, but passed from American design after having some vogue a few years ago. Advantages of the worm drive are quietness of operation, permanence of adjust ment, and freedom from vibration. Underslung worm construction permitted considerable lowering of the chassis, a feature primarily responsible for widespread adoption of hypoid rear-axle gears in recent years.