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).
Article detailing various chassis, brake, and component endurance testing procedures, including cold-weather trials and measuring brake efficiency.
| Identifier | ExFiles\Box 160\3\ scan0239 | |
| Date | 3rd February 1939 | |
| 182 The Autocar FEBRUARY 3rd, 1939. "Third Degree" The front wheels flip violently from side to side under gyroscopic forces induced by the sudden rise and fall. The test is so severe that tyres and shock absorbers have to be cooled by water sprays. If a part will stand up without fracture to 50 hours of this intensive maltreatment, it will run indefinitely on the road. Another important feature of this test is that everything going on can be measured with precision as to frequency, amplitude, and so forth, so that reliable data are established. In the same building another process is carried out which is equivalent to vivisection, for the chassis may be dismantled piecemeal whilst it is running. This is done to track down and analyse vibrations of all kinds. Drums similar to those used in the bump test are driven by an engine of 150 b.h.p. up to speeds equivalent to 80 m.p.h. or so. By placing the rear wheels of the chassis on the drums, its whole working mechanism can be driven, and every component can be dissected. It may happen that there is an irritating little vibration in evidence on a new design, incited by some unexpected cause. It may be in the valve gear, or the crankshaft, or some lesser component. So the engine is stripped piecemeal until the offender is traced. Actually, vibrations due to crankshaft dampers, flywheels, and clutch parts have thus been determined. Shimmy and Tramp Conversely the front wheels may be placed on the drums, and information with regard to “shimmy” and high-speed wheel wobble obtained. “Tramp” can be aggravated by fastening cams and irregularities to the drums. Synchronous movements of the frame, radiator, wings, road springs, or wheels can be examined with exactness by the aid of the Stroboscope, which is an optical instrument designed for producing a slow-motion picture effect. There is something else in the daily work of a car which imposes stresses of a high order upon the suspension and frame, namely, braking. So the chassis is subjected to a brake test rig which is uncommonly like the medieval rack. In this test the wheels are locked to the axles by plates and bolts so that they cannot revolve. Then a coupling rod is fastened to the front of the chassis, and is passed through an enormous coil-spring. The latter is suitably connected up to a heavy framework which is caused to rock back and forth over an adjustable distance. Adjustment of the spring enables a draw-bar pull to be applied to the front of the chassis equivalent to the maximum torque of which the brakes of the car would be capable. This pull is repeated and released at intervals of a few seconds for hour after hour. The test is responsible for analysing a host of problems relating to the brakes themselves, and also to their reactions upon the chassis. When the lining of a stationary brake shoe is forced into contact with the rotating brake drum, friction is produced which tries to arrest the movement of the drum. Omitting for the moment such factors as loads and speeds, the degree of friction also varies with the nature of the two materials causing it, and with the changes in the temperature which result. The degree of friction is normally known as the “coefficient of friction,” and is usually designated by the Greek letter µ. The co-efficient of friction of brake linings, and the ability of the latter to maintain their standard, is of paramount importance to the efficiency of brakes in actual use. A falling off in µ as the effect of continuous brake use down a really long hill, such as is common in the Alps, is the cause of brake “fade out.” On cars of the Bentley and the Rolls-Royce calibre such things just must not happen; hence the extremely careful watch kept on brakes and linings. Keeping a Check on “Mu” One piece of apparatus used for the purpose consists broadly of a single wheel, tyre, and brake mounted on a framework which can be moved into position with the tyre in contact with the jacked-up rear wheel of a car. The car engine then drives the testing apparatus, which is provided with means of adjustment of brake application and with reaction-measuring devices, and is fitted with chronograph pens which trace on a strip of paper a record of what is happening to the co-efficient of friction during the test. This is a brief description of what is actually a somewhat complicated piece of scientific apparatus. What is brake horse power to the owner of a car? Does the ability of one cart horse to raise a weight of 33,000 lb. through a height of one foot in one minute mean much to the average lady driver? When we buy a car with an engine rated at 20 h.p. and are informed that this engine can develop 70 brake horse power, are we much wiser? What does really matter to the owner is the amount of useful tractive effort exerted on the road by the back wheels of his car as the result of putting his hand in his pocket to buy gallons of petrol. In short, brake horse power delivered at the road wheels is the real criterion of the efficiency of a car. That is the common-sense attitude towards power, and although exhaustive bench tests of engines are carried out for comparison, when it comes to hard facts the tests are applied at the road-wheel end of the car. Such tests are carried out once again upon a pair of sunken 4ft. diameter drums, which are coupled to a large Heenan and Froude water brake, this constituting the road-wheel dynamometer. A car of any make can be run on to this dynamometer and tested extensively on all gears, with provision made to cool those parts which normally would be cooled by moving through the air. To run a car on a low-gear under full torque conditions for an indefinite period is an extremely severe test. One of the difficulties in ( 2 [Image Caption] Twenty degrees below zero, “Observers, who go into the cold chamber through a ‘sealing’ chamber have to wear arctic clothing whilst carrying out their work.” | ||