From the Rolls-Royce experimental archive: a quarter of a million communications from Rolls-Royce, 1906 to 1960's. Documents from the Sir Henry Royce Memorial Foundation (SHRMF).
Paper discussing the causes of and methods for reducing harshness and noise in automobiles, including mathematical analysis.
| Identifier | ExFiles\Box 154a\2\ scan0007 | |
| Date | 1st January 1939 | |
| January, 1939 HARSHNESS IN THE AUTOMOBILE 7 possible that the frequency has become such that the structure responds to blows from the road with a vibration that is more harsh than in the more flexible structure. Obviously, the most practical method of attacking this problem is to soften the blow. 3. Our tests have shown that bodies carry load. As rigidity is improved by extensions of the structure forward, toward or over the front axle, body loads are increased. If such an automobile in which the body provides great rigidity strikes a bump, the loads which are introduced into the body are greater than in the less-rigid car. Stressed bodies which transmit more load will transmit shocks to the passengers. 4. Every effort has been made in the past to reduce unsprung weight. The important criterion, however, is not so much the magnitude of the unsprung weight as it is the ratio of the sprung to the unsprung weights. To increase arbitrarily the sprung weight in order to augment this ratio would be very poor engineering. To fail in utilizing the full effect of the sprung masses that are a necessary part of the car is equally poor engineering. A rigid structure will hold all the items of weight, which are distributed throughout its length, to constant relationship with one another. They can then resist, as one mass, every attempt from an outside source to excite vibration, and the result will be an increase in effective sprung weight with no addition to the actual car weight. This discussion has been confined to vertical and torsional rigidity of our structures. Rigidity in a fore-and-aft direction always has been very large compared to stiffness vertically. The frame rails function as relatively flexible beams under vertical loads and as straight rigid columns under any fore-and-aft loads. Regardless of the type of construction, there is no appreciable flexibility available in this direction for force dissipation. Therefore, it is to be expected that fore-and-aft blows transmitted from the road through the suspension to the car must, by the nature of the structure, be shocks that are harsh. Fig. 8 - Harshness can be reduced by means of harshness and noise insulation applied to the spring ends Method of Reducing Harshness Attempts have been made to alleviate the harshness condition by use of cushioning pads. In the type of construction where the frame is of the conventional X-braced type, rubber-spool-type insulators have been placed between the body and frame at the attaching points. Their movement has been limited so that, for any serious structural distortion, they bottom and the body functions with the frame. For small movements, they give a softening of the car. Any loss in stiffness, however, small though it be, is begrudged since structural rigidity is a primary aim. Another type of harshness and noise insulation has been developed for use at the spring ends. The advantages of insulating at this location are quite real. There is no reduction in structural rigidity. It can be applied equally well to both the conventional and the unit type of construction. The insulation is placed closer to the source of vibration. (Fig. 8.) These insulators, or shock and vibration cushioners, consist of rubber bonded to small metal discs and so mounted at the spring ends that the rubber is loaded in shear. They are free to deflect in all directions in a vertical and longitudinal plane but are definitely limited laterally. For the front independent suspension, the shear rubber may be built into the knuckle support. Since deflections up to 3/16 in. each way from the normal position are obtainable easily, the formula, “energy equals force x space,” shows that, if the deflection under a given load were even as much as 1/32 in. for the normal spring connections, the force transmitted would be reduced to one-sixth its former value by the insulators. Although but few tests have been made thus far, this method of insulation has shown promising results in harshness reduction. There are probably many ways of solving the problem of harshness. Automotive engineers still have countless unsolved problems. They always will have as long as their aim is a continuing progress toward the ideal set forth in the beginning of this paper. An energetic attack on and solution of these problems is our surest guarantee of the continued retention by the automobile of its predominant position in the field of transportation. The suspension and structural engineers must continue to work hand-in-hand for progress toward that ideal ride which will be characterized by a complete absence of harshness. Appendix Development of Equation (1) Fig. 9 Torque required for acceleration = F(R-d) = I_p α̈ α̇₁ = (88S × 12) / 60R = 17.6 S/R α̇₂ = 17.6 S/(R-d) α̈ = (α̇₂ - α̇₁)/Time To find time: x = distance = √N² - (R-h)² Time = √N² - (R-h)² / ((88S × 12) / 60) = √N² - (R-h)² / 17.6S then α̈ = 17.6S(1/(R-d) - 1/R) / (√N² - (R-h)² / 17.6S) α̈ = 310S²d / R(R-d)√N² - (R-h)² from the foregoing: F = I_p α̈ / (R-d) = 310S²dI_p / R(R-d)²√N² - (R-h)² α̇ = Angular velocity (radians per sec.) α̈ = Angular acceleration (radians per sec.²) | ||