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 large 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 5½ sec. with a period of 87.2 vibrations per min.; the combination, a 3-sec. duration with 60.0 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 105 per cent, thus pushing the theoretical normal line or axis 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 springs alone. The net reduction of axle-vibration duration over a period of time is approximately 16 per cent or say a reduction in axle movement of 5.00 vibrations per sec. without air-springs to 4.12 vibrations per sec. with them. This reduction is a natural function and 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 in 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.


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