DAMPING Its Importance
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in Suspension Systems
By J. PICKLES, The Function of Shock Absorbers in the A.M.I.A.E. Control of Leaf-spring Action is Here
Fully Discussed
SHOCK absorbers are of many types and are intended to modify the characteristics cf the ordinary leaf spring. In fact, "shock absorber" is a misnomer, because, for some obscure reason, this device cannot function without causing shock, and any suggestion of its absorbing it is misleading.
In a hypothetical vehicle, the springs will be frictionless, and will be deflected to an extent which is dependent on the applied load, comprising a proportion of the weight of the sprung mass and the passengers or goods. If the body be subjected to some external exciting force, such as that resulting from passing over a road bump, it will vibrate with a frequency, the rate of which will depend primarily on the original compression, and will continue with decreasing amplitude until the applied energy be dissipated in the form of heat, air resistance, and material hysteresis.
No suspension system is, however, frictionless. Link bearings and leaf friction resist motion, so that a reduction of speed of vibration is obtained, and the energy retained in the springs is dissipated to an extent relative to the value of the friction.
Damping by Interleaf Friction
The application of some friction mechanism in the suspension, therefore, has the advantage that relative motion between axle and body is impaired, and, in consequence, the time required for the oscillations caused by the external exciting force to "die,' is reduced. Such a result is obtained by the interleaf friction in laminated springs, and, to a greater degree, with friction shock absorbers, Advantageous as this form of damping may be, it suffers from overriding disadvantages, in that the extent of the friction is governed by the materials in sliding contact, on the finish of the surfaces, and on their lubrication. In the laminated spring, the conditions existing between the leaves of two otherwise identical springs may vary between ample lubrication on smooth surfaces, to mud between rusty faces. No scientific suspension control could possibly be designed to function equally well under each condition.
Still further to complicate matters, although the extent of the friction damping is dependent on the pressure and coefficient of friction between the respective surfaces, the breakaway friction or resistance. to the commencement of sliding will rise to a value much above that required to maintain motion In consequence, when a vehicle passes over a road. bump that causes compression of the spring, the friction of the leaves and/or the damper resists such motion, so that the road shock will be transmitted directly to the body, and thence to the occupants, until it has reached a value suffi .0 6 cient to overcome the friction and compress the spring. When the spring has compressed to its maximum extent and begins to unload by returning the axle towards its normal running position, it is once more resisted by the friction, although the influence on riding comfort is by no means so important.
To attain optimum efficiency, a shock absorber should not resist the primary shock, but should damp out the subsequent vibrations. The attainment of this ideal is, of course, impossible, as road shocks may follow one another in rapid succession, with the result that a further shock may be applied before the spring has become stationary. In consequence, the damping must be a continuous process. To attain the best result, and maintain designed performance, damping by hydraulic means is undoubtedly the answer. • The nature of the damping obtainable by hydraulic means is almost unlimited in variety, but the systems available fall into a few distinct classes. In the simplest case the necessary resistance is obtained by means of a calibrated hole bored in the damper cylinder, so that motion of the piston causes a flow through the hole. In this type of damper the resistance becomes progressively greater as the piston speed is increased. (Fig. f.)
Controlled Fluid Flow
In a damper of somewhat more advanced form, the port area is closed by means of a spring-loaded valve, which is opened when the flow begins. The spring is calculated to increase in strength ai the opening under the valve becomes greater, thus permitting a more rapid flow to maintain a constant increase in pressure. (Fig. 2.) A further type may have the spring pressure so proportioned as to give a much smaller increase in pressure buildup, so that the damper resistance does not greatly vary with increased piston speed. (Fig. 3.)
Another method of obtaining_the desired result is to incorporate a relief valve in the cylinder, in addition to the port of fixed dimensions. Thus, pressure will build up with increased piston speed, as in the first case, but, at some predetermined figure, the relief valve will be opened and the rate of increase of resistance will then be reduced. Alta.natively, the relief valve may be so designed that the resistance remains constant at any piston speed above that required to " blow " the valve. (Fig. 4.)
Iii addition to these basic types, it is, of course, possible to use any required combination of fixed port aperture, spring-valve-controlled aperture, and relief valve.
The ideal design, as already stated, is that which gives maximum damping and minimum shock, and encourages the road wheels to follow most nearly the contour of the road. Two major disturbing conditions are encountered. and each is followed by a secondary disturbance.
Most important is the rapid rise of a road wheel when passing over a bump. This will occur with great rapidity and is the cause of greatest shock. This is followed by event No. 2, when the axle returns to normal at a speed dependent only on spring strength.
The passage of the wheels over a depression in the road surface must next be considerecl, when the wheel will fall under the influence of the spring. On meeting the rim of the pot-hole, the wheel will again be hurled upwards towards the normal position, when, as in the first case, shock will occur because of the high rising speed.
In the past, single-acting shock absorbers, coupled to work on the compression stroke, have been used. (Fig. 5.) They operate against the bump force, but, obviously, this is bad practice, in that the shock transmitted to the body is increased, while on the less vital rebound no damping is obtained. This type has, however, proved satisfactory when restricting rebound, in that, for both pot-hole and bump shocks, the spring is left to absorb the shock without further restriction, whilst full damping is obtained when the axle is being returned to its normal position. (Fig. 6.)
Tyres Part of Suspension System
The tyres fitted to the road wheels are, in themselves. springs, and the unsprung weight, in the sense of not being suspended by the main road springs, oscillates by deflecting the tyres directly as a result of road impact and, indirectly, from gyroscopic disturbances caused by such impacts. In this case there is no damping of upward axle disturbance arising from this cause.
Double-acting shock absorbers, with equal resistance on rise and fall (Fig. 7) adequately damp main vibrations and unwanted wheel oscillations. To improve riding characteristics, it is customary to reduce the damping effect on the compression stroke, thereby obtaining some assistance and maintaining axle control, whilst obtaining the greater proportion of damping on rebound. (Fig. 8.) In its simplest form, the hydraulic damper consists of a plunger working in a. cylinder, and actuated by a lever connected to the axle. Movement of the lever in one direction causes motion of the piston, with consequent displacement of fluid through an aperture in communication with the reservoir. On the return stroke a spring, situated behind the piston, causes it to follow the actuating lever, but as the rate of oil return is dependent on atmospheric pressure, it is possible for the piston to lag behind and eventually close the gap with an audible impact.
Double-acting types, using the principles of a springreturned plunger, are employed on several modern vehicles. These do not suffer from this trouble so long as a relief valve is not used or when one is incorporated in the circuit between the pistons. Noise is largely overcome because, on both compression and rebound, the cylinder behind the rising piston is filled by the transfer from the exhausting cylinder. Filling is thus positive and no longer dependent on atmospheric pressure.
To eliminate the danger of spring breakage and still obtain positiveness of operation, the opposed-piston type was introduced. .This, in one form, consists of a long cylinder closed at. each end. In it operates a double-ended piston having a slot in the centre to engage the actuating lever. A port at each end of the cylinder communicates with a passage running the length of the cylinder, so that movement of the piston displaces oil, which passes to the other side. In this design the long communicating passage introduces parasitic friction akin to that inherent in leaf springs, but a good modern design has the valves incorporated in the pistons with the object of reducing the oil-transfer distance and eliminating the narrow _passage.
Yet another design, based on the same principle, but having the cylinders arranged in a parallel manner, is the well-known Armstrong damper. In this component the operating spindle has splined to it a built-up crank to which connecting rods are articulated, and these communicate motion to a piston in each cylinder—one rising as the other falls. Thus, oil is passed from one cylinder to the other via a short connecting port in which is interposed a springloaded compound valve.
Viscosity Change Control
This is arranged with a bleed, so that slow axle-motions pass oil from cylinder to cylinder with the characteristics of the constant-orifice type. More rapid movements lift the valves to permit of a more rapid oil flow, the resistance imposed being readily proportioned on compression and rebound to give the desired effect.
The forcing of oil back and forth through a restricted orifice raises the temperature of the fluid, and hence reduces its viscosity. In consequence, the damping effect is reduced with prolonged driving, and is modified by the prevailing atmospheric temperature. A recent introduction by the Armstrong company is a thermostatic control, arranged to ensure constant results over a wide range, of temperature variation.
The method of phasing front and rear suspension charac teristics, to minimize pitching, is generally known. To obtain satisfactory results is a matter of finding the best theoretical combination of springs, and then, by diligent experiment, correcting the practical result until the ideal is reached. Satisfaction is, however, hard to achieve when a correct balance of frontand rear-roll stiffness and optimum riding comfort, in the sense of softass, are sought simultaneously.
There is a temptation, in this case, to obtain the required frequency combination by modifications to the dampers. although this practice is ene which may be resorted to only in small measure without danger of losing correct damping characteristics.
With modern suspensions of orthodox design, the spring softness is limited by, among other things, the room available under the frame for axle rise, and, on small cars, by the rear-seat position. Many post-war motorcycles are, however, fitted with front forks embodying coil springs and dampers, and a development of this scheme may well lead to softer suspension on passenger vehicles, In this case the fluid flow, under normal running conditions, would be limited only by the usual valves, and would be via a large aperture in the cylinder end.
Towards the end of the piston stroke a conical extension would enter the cylinder-end aperture. which would be gradually closed until, eventually, minion would be arrested by a cushion of oil formed by the closing off of the transfer port. Thus, soft springing would be possible for normal riding without danger of occasional heavy impacts from spring bottoming.
In my opinion, the ideal suspension unit is possible only with the widespread employment of coil and torsion springs. It is likely that the future will see a gradual tendency for a complete suspension unit to be supplied by specialists, just as, to-day, we have proprietary brake assemblies. carburetters, and electrical equipment.