Any commercial vehicle conforming to the accepted standard of construction may be divided in two parts, the chassis and the body. The chassis or running gear as it is sometimes called, consists of the frame, power plant, springs, axles, wheels, brakes and in fact all units which enter into the propulsion and control of the vehicle.
There are three general types of chassis when classified according to the type of power plant, the gasoline, the steam and the electric. The gasoline propelled vehicle is by far the most popular and will be considered in this work.
The general layout of the chassis covers such points as the location of the driver's seat or cab in relation to the location of the power plant, which controls the distribution of the useful or pay load of the vehicle. This affects the overall length and turning radius of the vehicle.
The principal problem confronting the commercial vehicle designer is how to make use of the overall length of the chassis to the best advantage, considering accessibility and all other factors which enter into this problem.
Most designers have placed the driver's seat back of the motor. This necessitates the making of the total length of the machine somewhat greater than it would be for the same capacity when the drivers seat is placed above the motor. In some cases a compromise is effected between these two by placing the driver's seat and steering to the right or left side of hood which encloses the motor, thus saving about half the space used in the design which has the seat placed in back of the motor.
Advocates of each type have a number of arguments in favor of their design, all of which have merit. The one who places the seat over the motor, claims that by this arrangement, the load is shifted somewhat forward and the center of gravity is brought somewhat nearer the center of the machine. This is claimed to create more even tire wear all around and a reduction of the total overall length of the chassis. The last is an advantage as it permits a shorter wheel base and turning radius.
Those who place the seat back of the motor claim that in the above construction too much weight is placed on the front axle, that the motor is more accessible and that it is placed in a higher position in the frame. The front springs can be made somewhat lighter, since they are not required to carry as large a percentage of the total load. And being lighter, they are less stiff and take the shocks of the road more readily, which tends to increase the life of the motor. These widely varying views of the makers are echoed in many different lengths of commercial vehicles which have been placed on the market.
Probably no single feature of commercial car design merits more attention than does that of arranging the power plant in such a manner as to offer the user motor accessibility in the greatest degree consistent with reasonable compact design. The importance of the first desideratum will be admitted by anyone having experience in the operation of an internal combustion engine. The day is still far distant when a gasoline engine may be locked in a box and with a supply of essence, be expected to mote satisfactorily until like the justly famous Shay, its multitudinous parts give out simultaneously as a result of legitimate wear. The desirability of compact arrangement will be endorsed as a purely academic proposition and will be heartily subscribed to as a thoroughly practical feature by truck operators, who have had to deal with metropolitan street and garage conditions.
As far as the power plant arrangement is concerned American makers have formed themselves into three distinct classes. First, the class comprising those who place accessibility above all other considerations. Second, the class comprising those whose greatest satisfaction arises from the contemplation of a design in which the compact arrangements of parts is accentuated. Third, the class comprising those who have attempted to effect a compromise between the above two types, to secure both accessibility and compactness.
A layout of the trend of the first class of commercial car design is depicted in Fig. 1. It will be noted that the motor is carried under a removable hood in front of the driver's seat and control elements, being typical of current passenger car chassis design. The loading platform is divided into approximately equal parts fore and aft of the rear axle. This design permits of easy access to the motor and accessory parts from both sides and top, making it equal in point of accessibility to passenger ears. The driver's seat is also accessible while the clutch and change gear can be easily reached through the floor board opening when they form a unit with the motor. The greater percentage of the paying load is concentrated upon the rear or driving wheels, resulting in good traction and easy steering. This construction also results in a pleasing appearance, which is of some advantage in smaller vehicles, which are used to some extent as an advertising feature.
Fig. 3. Three Prominent Types of Chassis.