Shock absorber
170829
218705482
2008-06-11T20:30:54Z
170.64.0.35
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{{Citations missing|date=January 2008}}
[[Image:Gas damper mov.gif|thumb|Shock absorber with internal reservoir. The components are: the rod (A), the piston with seals (B), the cylinder (C), the oil reservoir (D), the floating piston (E), and the air chamber (F).]]
A '''shock absorber''' in common parlance (or '''damper''' in technical use) is a mechanical device designed to smooth out or [[damping|dampen]] [[shock (mechanics)|shock]] impulse, and dissipate [[kinetic energy]]. It is analogous to a [[resistor]] in an electric [[RLC circuit]].
== Explanation ==
Shock absorbers must absorb or dissipate energy. One design consideration, when designing or choosing a shock absorber is where that energy will go. In most [[dashpot]]s, energy is converted to heat inside the viscous fluid. In [[hydraulic cylinder]]s, the hydraulic [[fluid]] will heat up, while in air cylinders, the hot air is usually exhausted to the atmosphere. In other types of dashpots, such as [[Electromagnetism|electromagnetic]] ones, the dissipated energy can be stored and used later.
== Description ==
[[Pneumatic]] and hydraulic shock absorbers commonly take the form of a cylinder with a sliding [[piston]] inside. The cylinder is filled with a fluid (such as hydraulic fluid) or air. This fluid filled piston/cylinder combination is a [[dashpot]].
== Applications ==
Shock absorbers are an important part of [[automobile]] and [[motorcycle]] [[suspension (vehicle)|suspension]]s, [[aircraft]] [[landing gear]], and the supports for many industrial [[machine]]s. Large shock absorbers have also been used in [[structural engineering]] to reduce the susceptibility of structures to [[earthquake]] damage and [[resonance]].
[[Image:R75-rear-shock.jpg|thumb|150px|left|Rear shock absorber and spring of a BMW R75/5 motorcycle]]
=== Vehicles suspension ===
In a vehicle, it reduces the effect of traveling over rough ground, leading to improved [[ride quality]]. Without shock absorbers, the vehicle would have a bouncing ride, as energy is stored in the spring and then released to the vehicle, possibly exceeding the allowed range of [[Suspension (vehicle)|suspension]] movement. Control of excessive suspension movement without shock absorption requires stiffer (higher rate) springs, which would in turn give a harsh ride. Shock absorbers allow the use of soft (lower rate) springs while controlling the rate of suspension movement in response to bumps. They also, along with [[hysteresis]] in the tire itself, damp the motion of the [[unsprung weight]] up and down on the springiness of the tire. Since the tire is not as soft as the springs, effective wheel bounce damping may require stiffer shocks than would be ideal for the vehicle motion alone.
[[Spring (device)|Spring]]-based shock absorbers commonly use [[coil spring]]s or [[leaf spring]]s, though [[torsion bar]]s can be used in [[torsion (mechanics)|torsional]] shocks as well. Ideal springs alone, however, are not shock absorbers as springs only store and do not dissipate or absorb energy. Vehicles typically employ both springs or torsion bars as well as hydraulic shock absorbers. In this combination, "shock absorber" is reserved specifically for the hydraulic piston that absorbs and dissipates vibration.
=== Structures ===
Applied to a structure such as a [[building]] or [[bridge]] it may be part of a [[seismic retrofit]] or as part of new, [[earthquake]] resistant construction. In this application it allows yet restrains motion and absorbs [[Resonance|resonant energy]], which can cause excessive motion and eventual [[structural failure]].
== Types of shock absorbers ==
There are several commonly-used approaches to shock absorption:
*[[Hysteresis]] (hysteresis is like a "memory" of the material, if you press down rubber disks, they tend to back to its normal uncompressed state as the pression of fingers is relieved) of structural material, for example the [[physical compression|compression]] of [[rubber]] disks, [[stretching]] of rubber bands and cords, [[bending]] of [[steel]] [[spring (device)|spring]]s, or twisting of [[torsion spring|torsion bar]]s. Hysteresis is the tendency for otherwise [[Elasticity (physics)|elastic]] materials to rebound with less force than was required to deform them. Simple vehicles with no separate shock absorbers are damped, to some extent, by the hysteresis of their springs and frames.
*Dry [[friction]] as used in wheel [[brake]]s, by using disks (classically made of [[leather]]) at the pivot of a lever, with friction forced by springs. Used in early automobiles such as the [[Ford Model T]], up through some British cars of the 1940s. Although now considered obsolete, an advantage of this system is its mechanical simplicity; the degree of damping can be easily adjusted by tightening or loosening the screw clamping the disks, and it can be easily rebuilt with simple hand tools. A disadvantage is that the damping force tends not to increase with the speed of the vertical motion.
*Solid state, tapered chain shock absorbers, using one or more tapered, axial alignment(s) of [[granular]] spheres, typically made of metals such as [[nitinol]], in a casing. [http://physicsweb.org/articles/news/5/10/15/1],[http://unisci.com/stories/20014/1022011.htm]
*[[Fluid]] friction, for example the flow of fluid through a narrow orifice ([[hydraulic]]s), constitute the vast majority of automotive shock absorbers. An advantage of this type is that using special internal valving the absorber may be made relatively soft to compression (allowing a soft response to a bump) and relatively stiff to extension, controlling "jounce", which is the vehicle response to energy stored in the springs; similarly, a series of valves controlled by springs can change the degree of stiffness according to the velocity of the impact or rebound. Specialized shock absorbers for racing purposes may allow the front end of a [[dragster]] to rise with minimal resistance under acceleration, then strongly resist letting it settle, thereby maintaining a desirable rearward weight distribution for enhanced traction. Some shock absorbers allow tuning of the ride via control of the valve by a manual adjustment provided at the shock absorber. In more expensive vehicles the valves may be remotely adjustable, offering the driver control of the ride at will while the vehicle is operated. The ultimate control is provided by dynamic valve control via computer in response to sensors, giving both a smooth ride and a firm suspension when needed. Many shock absorbers contain compressed [[nitrogen]], to reduce the tendency for the oil to [[foam]] under heavy use. Foaming temporarily reduces the damping ability of the unit. In very heavy duty units used for racing and/or off-road use, there may even be a secondary cylinder connected to the shock absorber to act as a reservoir for the oil and pressurized gas. Another variation is the [[Magneto rheological damper]] which changes its fluid characteristics through an [[electromagnet]].
*Compression of a gas, for example [[pneumatic]] shock absorbers, which can act like springs as the air pressure is building to resist the force on it. Once the air pressure reaches the necessary maximum, air dashpots will act like hydraulic dashpots. In [[aircraft]] landing gear air dashpots may be combined with hydraulic damping to reduce bounce. Such struts are called ''oleo struts'' (combining oil and air) [http://www.hangar9aeroworks.com/Aeroncastrut/Aeroncastrut.html].
*[[Magnet]]ic effects. [[Eddy current]] dampers are [[dashpots]] that are constructed out of a large magnet inside of a non-magnetic, electrically conductive tube.
*[[Inertia]]l resistance to acceleration, for example prior to 1966 [http://www.citroenet.org.uk/passenger-cars/michelin/2cv/2cv-01.html] the [[Citroën 2CV]] had shock absorbers that damp wheel bounce with no external moving parts. These consisted of a spring-mounted 3.5 kg (7.75 lb) iron weight inside a vertical cylinder [http://www.canadiandriver.com/articles/bv/2cv.htm] and are similar to, yet much smaller than versions of the [[tuned mass damper]]s used on tall buildings
*Composite [[hydropneumatic]] devices which combine in a single device spring action, shock absorption, and often also ride-height control, as in some models of the [[Citroën]] automobile.
*Conventional shock absorbers combined with composite pneumatic springs with which allow ride height adjustment or even ride height control, seen in some large trucks and luxury sedans such as certain [[Lincoln (automobile)|Lincoln]] and most Land Rover automobiles. Ride height control is especially desirable in highway vehicles intended for occasional rough road use, as a means of improving [[car handling|handling]] and reducing aerodynamic drag by lowering the vehicle when operating on improved high speed roads.
*The effect of a shock absorber at high (sound) frequencies is usually limited by using a compressible gas as the working fluid and/or mounting it with rubber bushings.
==See also==
*[[Strut]]
*[[Strut_bar|Strut Bar]]
[[Category:Automotive suspension technologies]]
[[Category:Motorcycle technology]]
==External links==
* [http://www.240edge.com/performance/tuning-shocks.html Shocks and Race Car Suspension Tuning]
* [http://www.monroe.com/tech_support/tec_shockabsorbers.asp Monroe Technical Support - What is a Shock Absorber?]
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