Another type of four-wheel drive construction is that employing a live front and rear axle: that is. an axle which bub propels the car and carries the weight of the vehicle and load. The F.W.D. and Nevada trucks are of this type.
In the F.W.D. chassis. the transmission is centrally located with a very broad, silent chain, which transmits the power to a shaft parallel to and far enough to one side of the transmission to permit proper clearance between the engine and propeller shaft. Each of the two propeller shafts carries a brake drum, so that, the braking force is applied to all four wheels. Steering is by the front wheels only, so that the rear axle is of conventional full-floating design with bevel-gear drive, excepting that the differential housing is located to the left of the axle center.
Fig. 146. F.W.D. Front Axle.
The steering-knuckle pivot of the front axle (Fig. 146) is in the form of a spherical joint, which has two trunnions, one on the upper side and one on the lower side, so that only a pivoting action, instead of a universal action is obtained. A divided housing surrounds these joints, to which is bolted the spindle, upon which the wheel revolves. This pivot and wheel spindle are hollow and contain the drive shaft and universal joint. The center of this universal is in line with the center of the wheel pivot and the shaft from one end extends into the differential, while the other end has a square, which fits into the flange that drives the wheel.
In the Nevada, the steering is also by front wheels only. The general application of the power to the front wheels differs somewhat from the above, although this design also employs full-floating axles. In this vehicle, the differential in the axles is located to the left of the center and the front axle has a somewhat different power-transmitting device in the steering knuckle.
Fig. 147. Nevada Front Wheel Drive.
The steering knuckles (Fig. 147) are similar to the usual type, consisting of a yoke at the end of the axle tube, through which passes a king bolt, on which revolves a solid wheel spindle. On the knuckle, which surrounds the king bolt, is revolvably disposed a double bevel pinion. The lower teeth of this bevel pinion are engaged by a bevel at the end of the axle shaft, which is disposed within the axle tube. The upper teeth mesh with a larger bevel gear, this being fastened to a stub shaft extending through the wheel to the outer edge of the hub. The end is squared and engages the driving flange, which drives the wheel.
In the chain-driven vehicles, the thrust and torque are taken by the conventional radius rods, while the remainder are divided between spring and radius rod construction. This is also true of differentials, as some use three and others two, in compensating for the division of power between the wheels.
Fig. 148. Couple Gear Electric Drive.
The fourth type, or electric, is particularly adaptable to one, two or four-wheel drives, and affords a very simple construction mechanically. The Couple Gear Freight Wheel Company makes a type of road wheel, which is constructed from two steel discs, between which is mounted an electric motor. This wheel (Fig. 148) can be attached to an axle, regardless of whether or not it is pivoted. This company builds two types of trucks, one straight electric and the other gasoline electric. In the former, batteries are used to supply the current to the motors in the wheels, while in the latter type, a gasoline engine drives an electric generator, which furnishes the current consumed by the motors, in driving the wheels. The motor is carried by the steering knuckle, the armature having a pinion at each end, one pinion pulling up on one side and the other pulling down on the opposite side and both mesh with large ring gears attached to the two discs of the wheel This affords a single reduction of twenty-five to one. A device, which is termed an "evener," permits of compensating movements and divides the force equally between the two pinions, regardless of any unequal wear or adjustments.
Both front-wheel drives and four-wheel drives are capable of further developments, and, if demands for these types of vehicles continue, we may expect to see considerable improvement in details.