Motor-car principles; the gasoline automobile

by Roger B. Whitman

In case of the failure of the brakes to operate, which may result from poor adjustment or worn bands and shoes, the speed may be checked by throwing out the clutch, switching off the ignition, engaging the intermediate speed gears, and letting in the clutch very slowly. Great care must be taken that the clutch is not permitted to bind suddenly, for that would probably result in the stripping of the gears. If the low-speed gears are engaged, the checking would be so sudden, no matter how slowly the clutch might be engaged, that the shock would probably throw the passengers from their seats.

The failure of the brakes when descending a hill produces a condition that requires skill and coolness, and danger can only be averted by a steady hand and a clear head.

Brakes applied to the rear wheels must have an equal grip on each, for if one binds more tightly than the other, the car will have a tendency to skid, or slide sideways. In the best cars this is taken care of by an equalizer, in which the pull of the lever or pedal is not applied directly to the brakes, but to the center of a bar, each end of which is connected to one of the bands or shoes. The lever action of this bar distributes the pull equally between the two brakes, and unless there is a great difference in the grip on the two drums, as might be the case if one were oily and the other dry, the effect will be the same on both sides.

TIRES

For low speeds, solid tires give good results in traction and the absorption of jolts from small obstacles, but for anything above six or eight miles an hour, pneumatic tires are a necessity in preventing the rapid shaking to pieces of the mechanism. A hard tire touches the ground at but one point, and its grip on the road will be much less than that of a pneumatic tire which, being slightly flattened by the weight it bears, presents an oval or elliptical surface to the road. While a pebble will force a solid tire to roll over it, it will sink into a pneumatic tire, and the jolt that it might cause will be entirely absorbed. Pneumatic tires are formed of alternate layers of heavy canvaslike fabric and soft rubber, and the processes through which they are put in manufacture are supposed to effect their perfect combination; but as it is not the nature of rubber to be absorbed by the fabric, the layers are only bound together by its tenacity. The bending of the sides of the tire under the weight of the car tends to separate these layers, and water or dirt entering between them through cuts quickly brings ruin; it is obvious that the less the sides bend, the smaller will be the opportunity for the layers to work apart. A pneumatic tire should always be pumped as hard as possible, so that it stands up practically round under a loaded car. While the car will ride a little harder under these conditions than when the tires are soft, there will be greater resistance to punctures, and the life of the tires will be increased. The normal wear to a tire should give it a smooth surface, but if it is noticed that the tread is rough and uneven, it may be taken for granted that the wheels do not run true. Rear wheels will be thrown out of true by the springing or bending of the axle, and front wheels also from this cause, but more probably from faulty adjustment of the steering mechanism or the bending of the drag link or steering arms.

The grip of the tire on the road is much affected by the nature of the surface, the traction on dry macadam being much greater than on wet asphalt. When the pull of the engine on the wheel exceeds the grip of the tire on the road, there will be a slip, and the wheel will revolve without moving the car. This will wear the tread of the tire far more rapidly than will ordinary running. The better the traction of the tires on the road surface, the less will be the tendency of the car to skid or slide sideways, and less power will be lost through the slipping of the wheels. To reduce the chance of slipping, because of wet asphalt or muddy roads, various devices are in use, all of which encircle the tread of the tire, and present a rough surface. The form in most general use consists of chains that fit across the tread, these being detachable and used only in case of necessity. While it is often done, it is nevertheless bad practice to apply chains or other antiskid devices to only one of the rear wheels instead of to both, for it increases the diameter of the wheel and makes a difference in the resistance against the wheel, causing the differential to operate at all times. The differential is not constructed to operate steadily, and will wear rapidly if forced to do so.

SPRINGS

Fig. 48.—A, Full elliptic spring; B, half elliptic spring.

In addition to tires, an automobile is fitted with springs, which are necessary to absorb the shocks and jolts that are too great to be taken up by the tires. These are usually full or half elliptic (Fig. 48), and made of flat plates, or leaves, of different lengths, the small being placed on the large, and all bound together at the center. The combined action of the springs and tires permits the frame and body of the car to move in a nearly straight line, while the wheels and axles follow the inequalities of the road. When springs break, as is frequently the case, it is from the rebound of the body that results when the wheels drop into a deep hole, the upward movement separating the leaves, and the entire strain coming on the long leaves alone. To prevent this, shock absorbers are recommended, which permit the springs to have a certain amount of action, but check them if they tend to expand or compress to too great an extent. They act either by the friction between metal plates and washers, or by air or oil in a cylinder that permits a piston to move freely to a certain degree, but presents resistance to a greater motion. Shock absorbers are placed between the axles and frame, and there should be four, two to each axle.