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Published in Nonfiction/Automotive
The current world-wide production of vehicle dampers, or so-called shock absorbers, is difficult to estimate with accuracy, but is probably around 50-100 million units per annum with a retail value well in excess of one billion dollars per annum. A typical European country has a demand for over 5 million units per year on new cars and over 1 million replacement units, The US market is several times that. If all is well, these suspension dampers do their work quietly and without fuss. Like punctuation or acting, dampers are at their best when they are not noticed - drivers and passengers simply want the dampers to be trouble free. In contrast, for the designer they are a constant interest and challenge. For the suspension engineer there is some satisfaction in creating a good new damper for a racing car or rally car and perhaps making some contribution to competition success. Less exciting, but economically more important, there is also satisfaction in seeing everyday vehicles travelling safely with comfortable occupants at speeds that would, even on good roads, be quite impractical without an effective suspension system.
The need for dampers arises because of the roll and pitch associated with vehicle manoeuvring, and from the roughness of roads. In the mid nineteenth century, road quality was generally very poor. The better horse-drawn carriages of the period therefore had soft suspension, achieved by using long bent leaf springs called semi-elliptics, or even by using a pair of such curved leaf springs set back-to-back on each side, forming full-elliptic suspension. No special devices were fitted to provide damping; rather this depended upon inherent friction, mainly between the leaves of the beam springs. Such a set-up was appropriate to the period, being easy to manufacture, and probably worked tolerably well at moderate speed, although running at high speed must have been at least exciting, and probably dangerous, because of the lack of damping control.
The arrival of the so-called horseless carriage, i.e. the carriage driven by an internal combustion engine, at the end of the nineteenth century, provided a new stimulus for suspension development which continues to this day. The rapidly increasing power available from the internal combustion engine made higher speeds routine; this, plus the technical aptitude of the vehicle and component designers, coupled with a general commercial mood favouring development and change, provided an environment that led to invention and innovation.
The fitting of damping devices to vehicle suspensions followed rapidly on the heels of the arrival of the motor car itself. Since those early days the damper has passed through a century of evolution, the basic stages of which may perhaps be considered as:
(1) dry friction (snubbers);
(2) blow-off hydraulics;
(3) progressive hydraulics;
(4) adjustables (manual alteration);
(5) slow adaptives (automatic alteration);
(6) fast adaptives ('semi-active');
(7) electrofluidic, e.g. magnetorheological.
Historically, the zeitgeist regarding dampers has changed considerably over the years, in roughly the following periods:
(1) Up to 1910 dampers were hardly used at all. In 1913, Rolls Royce actually discontinued rear dampers on the Silver Ghost, illustrating just how different the situation was in the early years.
(2) From 1910 to 1925 mostly dry snubbers were used.
(3) From 1925 to 1980 there was a long period of dominance by simple hydraulics, initially simply constant-force blow-off, then through progressive development to a more proportional characteristic, then adjustables, leading to a mature modern product.
(4) From 1980 to 1985 there was excitement about the possibilities for active suspension, which could effectively eliminate the ordinary damper, but little has come of this commercially in practice so far because of the cost.
(5) From 1985 it became increasingly apparent that a good deal of the benefit of active suspension could be obtained much more cheaply by fast auto-adjusting dampers, and the damper suddenly became an interesting, developing, component again.
(6) From about 2000, the introduction, on high-price vehicles at least, of controllable magnetorheological dampers.
Development of the adaptive damper has occurred rapidly. Although there will continue to be differences between commercial units, such systems are now effective and can be considered to be mature products. Fully active suspension offers some performance advantages, but is not very cost effective for passenger cars. Further developments can then be expected to be restricted to rather slow detail refinement of design, control strategies and production costs. Fast acting control, requiring extra sensors and controls, will continue to be more expensive, so simple fixed dampers, adjustables and slow adaptive types will probably continue to dominate the market numerically for the foreseeable future.
The basic suspension using the simple spring and damper is not ideal, but it is good enough for most purposes. For low-cost vehicles, it is the most cost-effective system. Therefore much emphasis remains on improvement of operating life, reliability and low-cost production rather than on refinement of performance by technical development. The variable damper, in several forms, has now found quite wide application on mid-range and expensive vehicles. On the most expensive passenger and sports cars, magetorheologically controlled dampers are now a popular fitment, at significant expense.
The damper is commonly known as the shock absorber, although the implication that shocks are absorbed is misleading. Arguably, the shocks are 'absorbed' by the deflection of the tires and springs. The purpose of dampers is to dissipate any energy in the vertical motion of body or wheels, such motion having arisen from control inputs, or from disturbance by rough roads or wind. Here 'vertical' motion includes body heave, pitch and roll, and wheel hop. As an agglomeration of masses and springs, the car with its wheels constitutes a vibrating system that needs dampers to optimise control behaviour, by preventing response overshoots, and to minimise the influence of some unavoidable resonances. The mathematical theory of vibrating systems largely uses the concept of a linear damper, with force proportional to extension speed, mainly because it gives equations for which the solutions are well understood and documented, and usually tolerably realistic. There is no obligation on a damper to exhibit such a characteristic; nevertheless the typical modern hydraulic damper does so approximately. This is because the vehicle and damper manufacturers consider this to be desirable for good physical behaviour, not for the convenience of the theorist. The desired characteristics are achieved only by some effort from the manufacturer in the detail design of the valves.
Damper types, which are explained fully later, can be initially classified as
(a) dry friction with solid elements;
(b) hydraulic with fluid elements;
Only the hydraulic type is in use in modern times. The friction type came originally as sliding discs operated by two arms, with a scissor action, and later as a belt wrapped around blocks, the 'snubber'. The basic hydraulic varieties are lever-arm and telescopic. The lever-arm type uses a lever to operate a vane, now extinct, or a pair of pistons. Telescopics, now most common, are either double-tube or gas-pressurised single-tube.
The early days of car suspension gave real opportunities for technical improvement, and financial reward. The earliest suspensions used leaf springs with inherent interleaf friction. Efforts had been made to control this to desirable levels by the free curvature of the leaves. Further developments of the leaf spring intrinsic damping included controlled adjustment of the interleaf normal forces, Figure 1.1.1, and the use of inserts of various materials to control the friction coefficients, Figure 1.1.2.
Truffault invented the scissor-action friction disc system before 1900, using bronze discs alternating with oiled leather, pressed together by conical disc springs and operated by two arms, with a floating body. The amount of friction could be adjusted by a compression hand-screw, pressing the discs together more or less firmly, varying the normal force at approximately constant friction coefficient. Between 1900 and 1903, Truffault went on to develop a version for cars, at the instigation of Hartford in the US, who began quantity production in 1904, as in Figures 1.1.3-1.1.5. Truffault, well aware of the commercial potential, also licensed several other manufacturers in Europe, including Mors and Peugeot in France, who also had them in production and use by 1904. A similar type of damper was also pressed into service on the steering, Figure 1.1.6, to reduce steering fight on rough roads and to reduce steering vibrations then emerging at higher speeds and not yet adequately understood.
Figure 1.1.7 shows an exploded diagram of a more recent (1950s) implementation from a motorcycle. This is also adjustable by the hand-screw. Subsequent to the Truffault-Hartford type, The Hartford Telecontrol (the prefix tele means remote) developed the theme, Figure 1.1.8, with a convenient Bowden cable adjustment usable by the driver in situ. A later alternative version, the Andre Telecontrol, had dry friction scissor dampers, but used hydraulic control of the compression force and hence of the damper friction moment.
In 1915, Claud Foster invented the dry friction block-and-belt snubber, Figure 1.1.9, manufactured in very large quantities by his Gabriel company, and hence usually known as the Gabriel Snubber. In view of the modern preference for hydraulics, the great success of the belt snubber was presumably based on low cost, ease of retrofitment and reliability rather than exceptional performance.
The spring-loaded blocks are mounted on the body, in particular on the chassis rails in those days, with the leather belt being fixed to the wheel upright or axle. In upward motion of the suspension, the snubber has no effect, but the spring-loaded blocks take up any slack. Any attempt by the suspension to extend will be opposed by the belt which has considerable friction where it wraps over itself and around the blocks. Hence the action is fully asymmetrical. The actual performance parameters do not seem to have been published. Some theoretical analysis may be possible, derived from the standard theory of wrapped circular members, with friction force growing exponentially with wrapping angle, for prediction of the force in relation to block shape, spring force and stiffness and belt-on-belt and belt-on-block coefficients of friction. The overall characteristic, however, seems to be an essentially velocity-independent force in extension, i.e. fully asymmetrical Coulomb damping. The characteristics could have been affected in service conditions by the friction-breaking effect of engine vibrations.
An early form of hydraulic contribution to damping was the Andrex oil-bath damper, Figure 1.1.10. This had metal and leather discs as in the dry damper, but was immersed in a sealed oil bath. There may also have been a version with separated metal discs relying on oil in shear. Another version, Figure 1.1.11, was adjustable from the dashboard, with oil pressure transmitted to the dampers to control the normal force on the discs, or perhaps in some cases to adjust the level of oil in the case. The pressure gauge in Figure 1.1.11 suggests that this type was controlling the normal force. The early development timetable of dampers thus ran roughly as follows:
1901: Horock patents a telescopic hydraulic unit, laying the foundations of the modern type.
1902: Mors actually builds a vehicle with simple hydraulic pot dampers.
1905: Renault patents an opposed piston hydraulic type, and also patents improvements to Horock's telescopic, establishing substantially the design used today.
1906: Renault uses the piston type on his Grand Prix racing cars, but not on his production cars. Houdaille starts to develop his vane-type.
1907: Caille proposes the single-lever parallel-piston variety.
1909: A single-acting Houdaille vane type is fitted as original equipment, but this is an isolated success for the hydraulic type, the friction disc type remaining dominant.
1910: Oil damped undercarriages come into use on aircraft.
1915: Foster invents the belt 'snubber' which had great commercial success in the USA.
1919: Lovejoy lever-arm hydraulic produced in the USA.
1924: Lancia introduces the double-acting hydraulic unit, incorporated in the front independent pillar suspension of the Lambda. The Grand Prix Bugatti uses preloaded nonadjustable drum-brake type.
1928: Hydraulic dampers are first supplied as standard equipment in the USA.
1930: Armstrong patents the telescopic type.
1933: Cadillac 'Ride Regulator' driver-adjustable five-position on dashboard.
1934: Monroe begins manufacture of telescopics.
1947: Koning introduces the adjustable telescopic.
1950: Gas-pressurised single-tube telescopic is invented and manufactured by de Carbon.
2001: Magnetorheological high-speed adjustables introduced (Bentley, Cadillac).
The modern success of hydraulics over dry friction is due to a combination of factors, including:
(1) Superior performance of hydraulics, due to the detrimental effect of dry Coulomb friction which is especially noticeable on modern smooth roads.
(2) Damper life has been improved by better seals and higher quality finish on wearing surfaces.
(3) Performance is now generally more consistent because of better quality control.
(4) Cost is less critical than of old, and is in any case controlled by mass production on modern machine tools.
During the 1950s, telescopic dampers gradually became more and more widely used on passenger cars, the transition being essentially complete by the late 1950s. In racing, at Indianapolis the hydraulic vane type arrived in the late 1920s, and was considered a great step forward; the adjustable piston hydraulic appeared in the early 1930s, but the telescopic was not used there until 1950. Racing cars in Europe were quite slow to change, although the very successful Mercedes Benz racers of 1954-55 used telescopics. Although other types are occasionally used, the telescopic hydraulic type of damper is now the widely accepted norm for cars and motorcycles.
It was far from obvious in early days that the hydraulic type of damper would ultimately triumph, especially in competition with the very cost-effective Gabriel snubber of 1915. The first large commercial successes for the hydraulic types came with the vane-type, developed from 1906 onwards by Maurice Houdaille. The early type used two arms with a floating body, a little like the dry friction scissor damper. The later type still used vanes, but had a body mounted on the vehicle sprung mass, operated by an arm with a drop link to the leaf spring suspension, Figures 1.1.12-1.1.14.
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