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).
Vehicle roadability, test procedures, and the performance of air-springs and shock absorbers, with comparative data tables and diagrams.
| Identifier | ExFiles\Box 43\3\ Scan020 | |
| Date | 28th September 1923 guessed | |
| 14 AIR-SPRINGS AND RIDING-QUALITY gether for a short space that a magnifying glass is required to count them, it will be appreciated why we feel the instrument is, for all practical purposes, dead-beat. "ROADABILITY" Another factor exists that we call “roadability”; it is closely related to riding-comfort. The instrument is of no help in measuring this factor, as it is purely one that is felt in the control or stability of the car. As this is a difficult subject to discuss, we will dismiss it with the comment that the present low-center-of-gravity car with its refinement in balance has automatically improved this factor. The improvement in this direction removes the main objection that has existed in the past to a more flexible spring-suspension. TEST PROCEDURE Our procedure in testing is to place the instrument on the floor of the car and to have the test operator hold it between his knees to prevent it from sliding or moving sideways. It can be placed anywhere in the car. We test principally to check-up the results obtained with air-springs. If the improvement is under the average figure expected for a particular make of car, further inspection is made to find the trouble and remove the cause before the car is released. Naturally, various makes and models of car in the same weight-class ride differently, and the improvement on some is not so great as on others, but the instrument reading generally will check closely on two different cars of the same make and model, provided they are both in the same condition. Fig. 20 is a diagrammatic drawing showing a comparison of two tape records. The tapes were carefully traced and the perforations located as accurately as possible. The difference in vibration is shown graphically by the intensity charts under each tape. The only similar feature that can be recognized in both tapes is a bump that occurs in section No. 9 and section No. 10 on both; these vibrations are marked S on the intensity charts. This bumpy spot in the road is toward the end of the course, and is an unusually large chuck-hole that no one would drive through under ordinary conditions. The upper tape shows three large vibrations at this point while the lower one shows only two, although the intensity of the initial shock appears to be the same in both cases. Fig. 21 gives more clearly the story of the ride on the tape. Table 5 is presented to show the results obtained with the instrument in an endeavor to bring out the fact that the error is divided somewhere between the instrument and the driver. The largest variations in readings are shown in tests Nos. 29 to 33. As I remember, the airing equipment had just been put on and the springs were somewhat stiff. This might easily account for the difference in readings. We find, generally, that two cars of the same make and type, tested under the same conditions, will ride in a very similar manner if they are in good condition. All our tests are made by skilled men who try to produce the same conditions in testing cars. As our course is little used and well marked with wheel grooves, this is not a difficult matter. We find that any average driver will turn in approximately the same readings if a given speed is maintained. Our practice is to test each car that comes into our local service-station with and without the various devices that come with it. All TABLE 6—AVERAGE RESULTS OF ROAD TESTS ON THE 1-MILE FACTORY COURSE AT 15 M.P.H. Test No. | Make of Car | Type | Model of | Auxiliary Device | A Factor - Number of Vibrations | A Factor - Per Cent | B Factor - Accumulated Riding-Factor, In. | B Factor - Per Cent 1,2 | Overland | Touring | 1920 | None/Air-Springs | 495/... | ... | 106.87/... | ... 3,4 | Oakland | Roadster | 1919 | None/Air-Springs | 428/384 | -10 | 124.00/60.31 | -51 5,6 | Maxwell | Sedan | 1924 | None/Air-Springs | 360/345 | -4 | 84.06/68.93 | -18 7,8 | Dodge | Ambulance | 1923 | None/Air-Springs | 464/349 | -25 | 123.31/84.71 | -31 9,10 | Essex | Coach | 1924 | None/Air-Springs | 310/233 | -25 | 99.87/61.87 | -38 11,12 | Lincoln | (Chassis l.) 640 Lb./Sedan | 1924 | None/Air-Springs | 333/199/203 | -40/-39 | 94.62/39.34/36.03 | -58/-62 13,14,15 | Pierce-Arrow | Sedan | 1923 | None/Hydraulic Snubbers/Air-Springs | 287/282/197 | -2/-31 | 46.43/43.93/28.45 | -5/-38 16,17,18 | Dorris | Sedan | 1924 | None/Hydraulic Snubbers on Rear Alone/Air-Springs | 366/390/217 | +6/-41 | 101.06/82.18/43.96 | -19/-54 19,20,21 | Studebaker | Coupe-Sedan | 1924 | None/Snubbers/Air-Springs | 363/305/165 | -16/-55 | 96.65/82.80/29.50 | -14/-69 22,23,24 | Paige | Coupe-Sedan | 1922 | None/Snubbers/Air-Springs | 356/393/321 | +10/-10 | 68.40/61.31/43.87 | -10/-36 AIR-SPRINGS AND RIDING-QUALITY 15 TABLE 7—COMPARATIVE TESTS WITH FOUR-WHEEL-BRAKE EQUIPMENT¹ Test No. | Make of Car | Type | Model of | Auxiliary Device | A Factor - Number of Vibrations | A Factor - Per Cent | B Factor - Accumulated Riding-Factor, In. | B Factor - Per Cent 1,2 | Cadillac | Touring | 1921 | None/Air-Springs | 141/121 | -14 | 25.87/13.81 | -47 3,4 | Cadillac (Four-Wheel Brake) | Coupe | 1924 | None/Air-Springs | 287/157 | -45 | 29.00/13.89 | -52 ¹This test was run over the 1-mile factory course at a speed of 15 m.p.h. For this test, the instrument was adjusted to lift at a 70-vibration period with a 2½-in. movement. tests are duplicated, and in some cases triplicated, so as to avoid error and get a fair average-reading. RESULTS OF TESTS Table 6 covers the results obtained with a few different cars; it shows the difference in riding-quality. Often a particular car of a given model is either much better or worse than the average of that model. This is due, principally, to its general condition, as the engine vibration and the spring efficiency are reflected in our readings. A study of Table 6 brings out the interesting fact that all the devices we have tested so far do improve the riding-quality and that the improvement is generally proportionate to the condition of the car. A startling example of this is the Studebaker sedan, which shows a 14-per cent improvement in riding-quality with snubbers, and a 69-per cent improvement with air-springs. Both percentages are above the average improvement gained with either device on the average car. The general average improvement shown by snubbers, either of spring or of hydraulic type, is 10 per cent; and that of the air-spring, 35 per cent. The results of our tests so far seem to indicate that a good spring-type snubber is equally as efficient as an hydraulic type. It is again apparent that a snubber of either design does not decrease the number of vibrations materially. Generally, it shows a 2-per cent decrease, and under certain conditions will increase the vibrations to such an extent that the improvement in riding-quality is lost. The air-spring, on the other hand, usually will show a 30-per cent decrease in the number of vibrations. Snubbers that act like air-springs, some are better than others. It is not my intention to give an opinion on the performance of any certain snubber or air-spring. These comparisons are made only to show the general difference in performance between the two types of auxiliary suspension. A comparison of the performance of a four-wheel-brake car is interesting. We made some tests recently on two cars of the same make. The rear-brake car was an open model, 2 years old; the four-wheel-brake car was a coupe, almost new. Table 7 shows the results. It indicates that an auxiliary device will show a greater improvement on a four-wheel-brake car than on the regular rear-wheel-brake type. AIR-SPRING DESIGN AND CONSTRUCTION The design of an air-spring is built up around the elements as shown in Fig. 22. This elementary spring is little more than a refinement of a veteran spring of 1841, and is adapted to installation on the modern automobile. The cup-washer packing around the piston-head serves to retain the oil above the piston; and the oil, in turn, is the medium for retaining the air under pressure within the cylinder. Air under pressure is introduced into the upper chamber through an air-valve from an external source. The pressure in the lower chamber is normally atmospheric, but a sudden movement of the piston serves to build-up momentarily a pressure that FIG. 22—AN ELEMENTARY DIRECT-ACTING AIR-SPRING SHOWING ITS GENERAL CONSTRUCTION AND PRINCIPAL ELEMENTS Diagram Labels: Air-Chamber—The Cushion, Oil To Prevent Leakage, Moving Member Connected with the Steel Spring, Outer-Shell-or-Cylinder Rigidly Attached to Frame of Car, FRAME, STEEL SPRING. The Cup-Washer Packing around the Piston-Head serves to Retain the Oil Above the Piston and the Oil, in Turn, Is the Medium for Retaining the Air under Pressure within the Cylinder. Air under Pressure Is Introduced into the Upper Chamber through an Air-Valve from an External Source. The Pressure in the Lower Chamber Is Normally Atmospheric, But a Sudden Movement of the Piston Serves to Build-up Momentarily a Pressure that Acts as a Rebound Check. The Outer Shell of the Air-Spring Is Bolted to the Frame of the Car and the Lower End of the Piston-Rod Is Connected to the Existing Steel Spring | ||