Stating the desirability of reducing unsprung weight in motor vehicles as a recognized fact and that 75 of 100 engineers interviewed favor such reduction, the particular advantages resulting are given as improved riding qualities, economy in tire wear and better acceleration. Mathematical deductions to establish the most desirable ratio of sprung to unsprung weight are not attempted, the intention being rather to state the reasons favoring lighter wheels and axles.
Unsprung weight effects depend primarily upon the ratio of sprung to unsprung weight. No data determining the most desirable ratio are available, but an investigation of the proportional weight of the unsprung and sprung parts of good-riding-quality automobiles showed it to be about 1 to 3. By constructing the wheels and the axles of light metal it is possible to maintain such a ratio, assure good riding qualities and reduce the total weight.
Laboratory and road tests prove that more mileage can be obtained from a light than from a heavy wheel. A comparative test was made between an aluminum and a steel wheel mounted at opposite ends of the same rear axle of a truck, each equipped with a 36 x 6-in. tire. Running on the surface of a drum having cleats to cause shock, the tires and wheels were subjected to a total of 6,009,300 impacts. The steel-wheel tire showed deep cracks, but that on the aluminum wheel only small fractures; there was no trace of any metal fatigue.
By decreasing the wheel and axle weight, the inertia that must be overcome in accelerating is reduced proportionately. In the test described, the moment of inertia of the aluminum wheel was 53 and that of the steel wheel 123. The force needed to produce a given angular velocity of the wheels in a given time is directly proportional to the moments of inertia of the wheels. It follows that an aluminum wheel consumes less power in attaining its angular velocity than a wheel of the same mass distribution but of greater weight.