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 solutions for harshness in automobiles, focusing on suspension and structural rigidity.
| Identifier | ExFiles\Box 154a\2\ scan0003 | |
| Date | 1st January 1939 | |
| January, 1939 HARSHNESS IN THE AUTOMOBILE 3 who say that this condition is as it should be since a harshness which is not noticeable to a car passenger is not important. However, in the day-to-day study of the problem, suitable instrumentation would be of inestimable value in enabling the detection of minor differences which would indicate trends or effects of certain changes made in the car. General Method of Harshness Reduction Since harshness refers to one of the more disturbing aspects of car ride, an important purpose in car ride improvement should be the elimination of harshness or reduction of accelerations (both meaning the same thing). Even if this change results in increasing the amplitude of the accompanying motion, a greater softness and better feel will be the consequence. This statement does not mean that amplitude of motion is not important, but only that acceleration is more important. In reducing these accelerations, two possible methods are at hand. One is to make all roads smooth. For obvious reasons, this can be dismissed as a possible solution – at least for the next-few years. The second method is indicated by a couple of well-known equations or relations. Force = mass times acceleration. (F = M X A). Work = Energy = Force times the distance through which it acts. (Energy = F average X S). From the first equation, it can be seen that, if we are to reduce the accelerations A of a given mass M, we must reduce the force. The manner in which this force can be reduced is indicated by the second equation. Assuming the energy to remain constant, if the space S is very small, the force F must be correspondingly large whereas, if the space S can be made much greater, the force will then be reduced in direct proportion. These principles will be referred to in the subsequent discussion of certain parts to which they apply. Suspension Vs.{J. Vickers} Harshness Since this is a discussion of car harshness in its relation to structural rigidity, only a brief mention of the suspension will be included. The principle of force reduction through increase in cushioning deflection, as mentioned previously, has been applied to car springs throughout the past few years with the general result that springs have a much lower rate and wheel travel is decidedly greater. Little car harshness results from the normal free action in a vertical direction of present-day springs. The so-called conventional type of suspension, using leaf springs all around, afforded a small measure of fore-and-aft shock dissipation. With the adoption of that independent suspension which is in most general use today, fore-and-aft shocks became more pronounced, since this suspension would deflect very little in a fore-and-aft direction. The presence of the fore-and-aft shocks just mentioned usually is attributed to the rearward component of a radial load through the tire contact and passing through the wheel axis or spindle. It is possible that the extreme harshness noticeable at times may be due even more to longitudinal forces of another sort than to the component just mentioned. When a wheel strikes an obstacle, some deflection of the tire will result. The rolling radius of the wheel is thereby decreased and, since the car speed remains substantially constant, the wheel must suffer a very sudden angular acceleration. To impart this acceleration, a rearward force at tire tread is required and this must be reacted at the wheel spindle. With the car traveling at a moderately high rate of speed, the time interval during which this acceleration takes place is very short, and the force is surprisingly high. Its value may be calculated as follows: F = 310S²dIp/[R(R-d)²√(N²-(R-h)²)] (1) where F = force at tire tread, lb. S = car speed, m.p.h. R = normal rolling radius, in. N = undeflected tire radius, in. h = height of obstacle, in. d = tire deflection due to obstacle, in. Ip = polar moment of wheel and tire, lb-in. sec.² (See Appendix for development of formula) Assuming a tire of 13.5-in. rolling radius and a polar moment of 11.5 lb-in. sec.² (which is for a wheel with a 6.00-16 tire), a car speed of 50 m.p.h. and a tire deflection of 1.5 in. caused by an obstacle 2 in. high, the force is calculated to be 860 lb. This value is very considerable, and it appears that it can be much greater than the rearward component before mentioned. The two are in the same direction, however, and probably act simultaneously. If both wheels strike the obstacle at the same instant, the forces, of course, are doubled. Their effect is transmitted through the car structure to all the passengers, and the result is something like that produced by a sledge-hammer blow on the end of the car frame. The solution to this type of harshness requires some fore-and-aft resilience and, for a structure as rigid in a fore-and-aft direction as the car frame and body, only a little cushioning will give very great improvement. Car Harshness Vs.{J. Vickers} Structural Rigidity The trend in car structure is toward greater rigidity. In the early days of the automobile, the open-section frame was used. This frame provided ease of attachment for cross-members, brackets, and units. The open-type bodies of that period contributed but little to the rigidity of the car. This condition was because the door frames did not give side-panel strength and because of the absence of the rigid top. The major assistance offered the frame by the body was through the wooden sills bolted to the top of the frame which increased the effective depth of the frame over the body length. At this time the engine generally was bolted firmly to the front portion of the frame and greatly assisted the frame in torsional rigidity in that section. With the coming of the closed body considerable gain in overall rigidity resulted even though a wooden structure was used and the body had a fabric-covered top deck. The mounting of the engine on resilient supports decreased the torsional rigidity in this section, but the introduction of the present X-member frame more than compensated for this loss. The “all-steel” body gave better joining of members and provided a continuity of structure which considerably improved rigidity. Welding of both frame and body further increased the stiffness. The steel roof, by providing efficient shear bracing in the roof panel, also increased the torsional rigidity of the entire car to a great extent. Fig. 1 illustrates this trend. Let it be understood that some of the developments may have been made for other reasons than to increase rigidity of the car. Some may have been done to improve passenger safety, to reduce cost both in material and fabricating labor, or to provide adequate strength for the increasing severity of service. However, the effect has been a definite trend – step-by-step – to give greater rigidity. You may ask the question, “why is all this rigidity necessary?” The answer is that the performance of the automobile has increased since the early days. Speeds have increased – not only top speeds but average driving speeds. The car is capable of greater accelerations and shorter braking distances at given speeds. In addition, rigidity provides greater passenger comfort and car control. In early days, cars had considerable shake because of the | ||