1942 - 1947 Chevrolet Shop Manual



	10-4

	useful life of his tires by taking the turns a little slower or a little wider. The tires and wheels should be interchanged in accordance with instructions given under the heading "Correction of Irregular Tire Wear." No changes can be made to the car or tires to improve cornering wear, and it is advisable to check alignment only if abnormal camber wear is encountered and changes should only be made when the camber is found to be beyond factory specifications. TIRE BALANCE Original equipment tires and tubes are marked at source, so that the light portion of the casing is counterbalanced by the heavy portion of the tube. Due to irregularities in tread wear, caused by sudden brake applications, misalignment, low inflation pressure, or tube and casing repairs, a casing and tube assembly can lose its original balance. If a disturbance is felt in the steering wheel due to the action of the front wheels, the first items to check are pressures and the balance of the tire and wheel assembly. BALANCING WHEELS AND TIRES Wheel balance is the equal distribution of the weight of the wheel and tire assembly around the axis of rotation. There are two ways in which every wheel must be balanced-statically and dynamically. STATIC BALANCE Static balance (sometimes called still balance) is the equal distribution of the weight of the wheel and tire assembly about the axis of rotation in such a manner that the assembly has no tendency to rotate by itself, regardless of its position. For example: A wheel with a chunk of dirt on the rim will always rotate by itself until the heavy side is at the bottom. Any wheel with a heavy side like this is statically out of balance. Static unbalance of a wheel causes a hopping or pounding action (up and down) which frequently leads to wheel flutter" and quite often to wheel "tramp." DYNAMIC BALANCE Dynamic balance (sometimes called running balance) means that the wheel must be in static balance and also run smoothly at all speeds on an axis which runs through the center line of the wheel and tire, and is perpendicular to the axis of rotation. To explain the principle of dynamic balance, let us first consider what happens when we swing a weight attached to a string. If we start to swing this weight slowly, it is apparent that the weight swings in a sharp angle with reference to	the axis of rotation (the hand). If the speed is increased, the weight climbs until the weight mass is at right angles to the axis of rotation. Now, let us apply this principle to a spinning wheel.

		Fig. 9-Wheel in Static Balance By referring to Fig. 9-A, it can be seen that when a wheel and tire assembly is in static balance, the sum of the weights of sections 1 and 2 is equal to the sum of the weights of sections 3 and 4; or, in other words, the weight is equally distributed about the axis of rotation. Fig. 9-B is a drawing of a wheel that is in static balance because the shaded heavy point 2 is balanced by the shaded heavy point 3. However, it can be seen that with reference to the center line, section 1 is lighter than section 2, and that section 4 is lighter than section 3. When we start to spin this wheel (as in Fig. 10-A) the center line of the weight masses 2 and 3 tries to get at right angles to the axis of rotation, just as the weight on the string tried to get at right angles to its axis of rotation (the hand). This tendency to get at right angles exerts a force on the wheel (as shown by the arrows). This force, in turn, tends to move the center line of the wheel, and, in so doing, distorts the axis of rotation. When the wheel has turned 180°, the forces exerted by the heavy sections 2 and 3 now tend to move the center line of the wheel in the opposite direction. In other words, the wheel tries to rock first in one direction, then in the other. The result of the movement of these unbalanced forces causes the wheel to wobble or shimmy, and the condition becomes more violent with increased speeds.