Motor Trucks.— The principal differences in the construction of chassis for motor trucks and those for motor cars are the following, speaking in general and with reference to the preceding account of common motor car con struction: The engine is heavier, slower and larger, the maximum engine speed rarely ex ceeding 1,500 revolutions per minute and fre quently not reaching half of that figure. The general type of engine is shown in Fig. 29. Self-starting and electric lighting equipments are usually omitted, though provision is often made for fitting them afterward at the pur chaser's option. The frame is usually made of industrial iron shapes but especially designed pressed-steel frames are gradually displacing the cheaper construction. The frame projects from one to three feet in front of the front axle and as far to the rear of the rear axle as necessary for providing load space. In heavy trucks from three-fourths to eight-ninths of the load is carried over the rear axle. (By this system the cost of can be some what reduced, as the front portions of trucks of different load capacities can be made nearly alike, but the better manufacturing practice tends toward providing- a complete individual design for each size of truck). As a heavily loaded platform demands a broad support on the vehicle springs, these are generally of the semi-elliptic or platform type at the rear and are often supplemented by overload springs (see VEHICLE SPRINGS) which come into action only when full load is carried or severe shocks are received on the road. Rebound checks and shock absorbers are rarely used. Instead of ball bearings or roller bearings in the wheel hubs, parallel bearings are quite generally used, especially in Europe; .in England often with floating bushings and special provisions for keeping lubricant from leaking out. Each rear wheel is usually shod with two solid rubber tires, side by side, known as twin tires; each front wheel with a single solid rubber tire. Pneumatic tires in front or on all four wheels are used only for delicate loads, such as pianos, dynamite, glass, crockery.
The transmission system for motor trucks is sometimes the same as that described as typi cal for motor cars, but recently worm gear sets are taking the place of bevel gear sets, allow ing more gear reduction for a given road clear ance under the middle of the rear axle and being credited also with silent running and, for heavy work, with greater durability. Fig.
30 shows the nature of a worm gear as used in motor trucks. The worm may be either above or below the worm wheel. Further de tails are given under GEARS and WORM GEAR ING. Reliable lubrication, accurate alignment and suitable end-thrust bearings are conditions for the successful use of worm gears in motor vehicle driving systems. A general view of chassis and body arrangements of a motor truck with worm drive is given in Fig. 31.
Another variation of the ordinary shaft drive transmission which is commonly em ployed in motor trucks is known as the internal gear drive, of which an example is shown in Fig. 32. A variation less commonly yet not infrequently used is the compound rear axle gear by which two pairs of spur wheels trans mit the rotary movement from bevel gear and differential to the wheel shafts within the rear axle housing, in order to effect a reduction of speed. But chain drive, without casing for the chain, is still the type of transmission most numerously represented in this class of vehicles, the general construction of it being the same as for motor wagons. Radius rods for transmit ting the driving thrust to the truck frame are always used with this drive and are arranged to be lengthened or shortened by a screwthread adjustment in order to adjust the distance be tween sprocket wheels and thereby compensate for wear of the chain. Roller chains are gen erally used. See SPROCKET CHAINS.
The remaining classes of automobile vehicles, which are dealt with in other articles, are the following: •Electric trucks and wagons (sec ELECTRIC VEH ICLE$) ; electric carriages (see to ignorance of suitable methods for varying the fuel feed and, in lesser degree to ignition, by hot tube or defective electric apparatus. Unreliability at any speed, due to poor cast