In the foregoing chapters on the final drive, we considered all types of final rear wheel drives. There are also a number of commercial cars, in which the final drive is through the front wheels, and others in which it is through all four wheels. Both of these types have gained considerably in popularity of late. This is possibly due, to some extent, to the demands of the various war departments, for motor-driven vehicles, which can propel themselves to any point where mules can pull a wagon.
This same ability is also of great importance in many other classes of service, where departure from the road surface is sometimes necessary. It is especially of importance to coal dealers, who are generally called on to make deliveries in narrow alleys: excavating contractors, who must haul material in and out of excavations, building supply concerns, handling such as lumber, gravel, brick, etc., over unpaved roads and sometimes across swampy land to deliver the material to the building site. There are also other cases to which the four-wheel drive is adaptable.
There is also a variety of cases in which extremely low bodies are necessary, as in hauling long structural girders, timbers and heavy stone. On the other hand, there are such problems as handling light paper, boxed paper and tin cans and tubes and other articles which present a very bulky load. For these purposes, the front-wheel drive lends itself to best advantage, while it is also of considerable advantage for fire apparatus, etc., where it is desired to retain the vehicles used with the horse equipment.
Foreign manufacturers were the first to experiment with these types of vehicles and have perhaps led the way: however, there are a number of companies in America, which are prepared to manufacture these types of vehicles. As the front-wheel and four-wheel drives are closely related, and practically involve the same mechanical problem in driving and steering, they will be considered together.
In comparing the various designs we find four methods of driving and two methods of steering. In the first construction, the whole axle pivots about its center and the wheels are driven, as in the ordinary dead or live rear axle. This construction is mostly used with front-drive units, that is, in vehicles propelled through the front wheels only. In the second type, the axles have steering knuckles, as in the conventional type, but the flexibility is obtained through universal joints or chains driving the wheels through internal gearing. In the third type, the wheels are driven through bevel gears mounted inside the wheel and on the steering knuckles, another pair of bevel gears mounted on the steering knuckles driving the wheels, through shafts extending from the central pair of gears in the axle housing. These three are termed mechanical drives; while the fourth construction is a non-mechanical drive, electric motors being mounted in the wheels and driving direct through reduction gearing.
The front-wheel drives, in some cases, are so constructed that this forms the power unit, which may be attached to any type of vehicle the user may desire, while a rear frame construction, upon which any type of body can be mounted, may also be provided. With the latter, the rear wheels may be shod with steel or rubber tires as desired. The Devon, Pull-More, and Meyers may be mentioned as examples of this type.
Referring to Fig. 140, illustrating the Devon Tractor Trailer, the engine is mounted in front of the axle and placed lengthwise. It is located toward the left side and enclosed by a metal hood. At the rear of this hood is the vertical steering column, and back of this, a metal seat. The power from the engine is transmitted to the clutch and transmission at the rear, the transmission being a unit with a jack-shaft, as in the ordinary chain-driven truck. The front axle is similar to the ordinary dead rear axle, and carries a set of brakes and sprockets. From the jackshaft, the power is transmitted to the front wheels through chains, which are enclosed. In steering, the entire unit is turned bodily by means of the steering wheel, through bevel gears, rotates a worm, engaging the circumferece of a large worm wheel attached to the forward part of the main frame and concentric with the pivot. This worm is mounted on a splined shaft between heavy coil springs to absorb the road shocks.
Fig. 140. Plan View of the Devon Tractor.
Fig. 141. The Meyers Tractor.
The Pull-More front drive is similar to the above: but has a power steering device operated by the engine, this steering device working through a sprocket on the crankshaft, which, by means of a chain, drives a countershaft, located in the lower half of the crank case. This countershaft operates the arm of the steering reach, which is a part of the king bolt. This king bolt is located midway between the front wheels, and forms; the axis upon which the entire front unit revolves. Differentials are provided to permit right-and-left-hand steering. An ordinary steering post and wheel is used to operate the power steering device.