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.