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 on air-springs and the measurement of automobile riding-quality.
| Identifier | ExFiles\Box 43\3\ Scan014 | |
| Date | 1st September 1924 | |
| Reprinted from The JOURNAL of the SOCIETY of AUTOMOTIVE ENGINEERS for September, 1924. Air-Springs and the Measurement of Automobile Riding-Quality By JOHN J.{Mr Johnson W.M.} McELROY¹ DETROIT SECTION PAPER Illustrated with PHOTOGRAPHS and DRAWINGS [C 54 The Society shall not be responsible for statements or opinions advanced in papers or in discussions at its meetings] ABSTRACT AN air-spring and a steel-spring combination has a characteristic load-curve that allows maximum flexibility in the general working-range of the axle yet has an increasing resistance to dissipate heavy shock-loads. By varying the compression volume in the air-spring, the load curve of the combination can be made more flexible or stiffer as occasion demands. Tests show that the steel-spring vibration alone had a duration of 5 1/2 sec. with a period of 87.2 vibrations per min.; the combination, a 3-sec. duration with 60 vibrations per min. Field tests of front-axle movement were made, the test apparatus for these and other tests being illustrated and explained. The maximum axle-movement either above or below the normal line is increased when using air-springs, and the subsequent rebound shows more action on the underside of the normal line, the general tendency of the air-springs being to float the chassis on a slightly higher plane at the time of rebound. A reduction in the vibration period and its duration with increased up-and-down axle-travel was indicated also. An axle-movement recorder was then developed and used in tests made over an average rough country-road test-course. Total up-and-down axle-amplitude is increased about 28 per cent by the use of air-springs and the amplitude above the normal line is reduced about 29 per cent; but the total amplitude below the normal line is increased 106 per cent, thus pushing the theoretical normal line of axle of the axle vibration away from the chassis under running conditions. This tendency to float allows the air-spring combination to have slightly less axle-clearance at rest than would be provided for the steel spring alone. The net reduction of axle-vibration duration over a period of time, is approximately 13 per cent or say a reduction in axle movement of 5 vibrations per sec. without air-springs to 4.12 vibrations per sec. with them. This reduction is a natural function, as it still leaves the axle free and flexible in both directions, yet capable of very fast action in absorbing shocks and making contact with the ground in depressions. Hence, the general effect is to smooth out the body line of travel and reduce the objectionable vibrations in the chassis and in the body. Testing for riding quality involves two constant factors, the course and the testing instrument. The speed and the ability of the driver to retrace his course at identically the same path at identically the same speed are the variables. The instrument used for measuring riding quality is illustrated, and the results obtained are discussed. Air-springs have been in common use on automobiles for the past 10 years, but very little technical information concerning them has been published. This paper treats the subject of air-springs from an engineering standpoint. THE present air-spring suspension is a typical counter-spring, and produces better riding quality due to the inherent characteristics of its combined spring-action. The general principles of the counter-spring are fairly well known, and a number of supplementary coil-spring shock-absorbers are on the market; they are used as auxiliary members of the spring-suspensions. Some manufacturers of foreign cars have gone so far as to interpose as standard equipment an auxiliary coil-spring between the frame and each end of the steel spring. Unquestionably, the auxiliary spring so mounted absorbs the smaller vibrations and reduces the acceleration of the free end of the steel spring, and its value is generally a plus quantity in the direction of more comfortable riding. Ethelbert Favary has stated that the ordinary coil-spring used in practice has a very much higher vibration period than have leaf springs that carry the same amount of load and that, in a poor combination, the efficiency of a counter-spring suspension is negligible. I agree with him. The ordinary coil-spring must have considerable free length to form a satisfactory combination with the leaf spring but, if care is taken in the design throughout, improvement in riding quality can be obtained. LOAD CURVES In addition to the typical counter-spring action, the air-spring suspension has a characteristic load-curve that is entirely different from that of the steel spring and coil suspension. This is due to the fact that the compression or load curve of the air-spring is practically isothermal and this characteristic is felt in the load curve of the combination. Fig. 1, at the left, is a diagrammatic drawing showing the combined action of a typical air-spring suspension. FIG. 1—DIAGRAMMATIC DRAWING SHOWING THE COMBINED ACTION OF A TYPICAL AIR-STEEL SPRING-SUSPENSION The Views from Left to Right Illustrate the Air-Spring at Rest in Its Mid Position, the Compression Position when Passing over an Obstruction and the Position Assumed by the Spring when the Wheel Drops into a Rut. The Following Table Gives the Data Relating to the Suspension Position of Air-Spring: At Rest, Compression 2 in., Extension 2 in. Effect at Axle, in.: 0, 1.40, 1.58 Steel-Spring Camber, in.: 2.00, 0.60, 3.58 Total Load, lb.: 400, 640, 170 Combined Axle Movement, in.: 0, 1.10 Above Normal, 1.58 Below Normal Spring Scale, lb.: 190, 240, 145 ¹M.S.A.E.—Chief engineer, Westinghouse Air Spring Co., New Haven, Conn. 3 Handwritten text at bottom: 47. 42.5 25mm 46 3 49 | ||