Shock Absorbers

2.5.3. Shock Absorbers

"Shock absorber" refers specifically to the hydraulic piston that absorbs and dissipates vibration. A shock absorber (in reality, a shock "damper") is a mechanical or hydraulic device designed to absorb and damp shock impulses. It does this by converting the kinetic energy of the shock into another form of energy (typically heat) which is then dissipated.

Fig: Shock absorber

In a vehicle, shock absorbers reduce the effect of traveling over rough ground, leading to improved ride quality and vehicle handling. While shock absorbers serve the purpose of limiting excessive suspension movement, their intended sole purpose is to damp spring oscillations. Shock absorbers use valving of oil and gasses to absorb excess energy from the springs.

Shock absorbers bolt between the vehicle's frame and suspension members near each wheel. The mounting points are typical described as either "eye" (round bushing which bolt passes through) or "pin" (threaded shaft which receives nut). Inside the shock absorber is a metal rod attached to a piston that moves up and down inside a cylinder filled with hydraulic oil. Most modern shocks also contain nitrogen gas to reduce shock absorber fade that happens if oil becomes aerated and bubbly due to heat build-up.

Necessity of damper (shock-absorbers):

Hydraulic dampers are prevalent. During bumps or compression, rod & piston move into the Shock Absorber. In rebound, or extension, they move out.

For dampening to be effective, resistance is needed in both directions – provided by oil and valves.

Without a damper structure a car spring will extend and release the energy it absorbs from a bump at an uncontrolled rate.

The spring will continue to bounce at its natural frequency until all of the energy originally put into it is used up.

A suspension built on springs alone would make for an extremely bouncy ride and, depending on the terrain, an uncontrollable car.

In a vehicle, shock absorber reduces the effect of traveling over rough ground, leading to improved ride quality. Shock absorber is use of soft (lower rate) springs while controlling the rate of suspension movement in response to bumps.

Types of shock absorber:

According to construction: Telescopic types

1. Mono tube / Single tube

2. Twin tube

According to medium:

1. Hydraulic shock absorber

2. Gas charged / Pneumatic shock absorber

Gas charged shocks provide a firmer, sportier ride because they contain gas (typically nitrogen) which is injected under high pressure (260-360 Psi) into the shock. This pressure serves to compress air and oil in the shock absorber faster, leading to a more responsive feel.

With hydraulic shocks, fluid compression happens at a slower pace because air and fluid inside the shock are not under such high pressures. This gives hydraulic shocks a softer and smoother ride. Note that if you’ve got a classic car, it was probably designed with hydraulic shocks.

While all shock absorbers contain hydraulic fluid, “gas charged” and “hydraulic” shock absorbers are laid out differently.

Fig: Twin & Mono Shock absorber

Two main types of internal designs exist for shock absorbers: mono-tube and twin-tube layouts. As it sounds, these designs differ primarily in the number of metal tubes found within each shock absorber. Twin-tube shock absorbers are fitted primarily as OEM equipment because they produce a more compliant, comfortable ride and cost less to produce. Conversely, mono-tube shock absorbers are oriented toward performance driving, and their sophisticated design is more expensive to manufacture.

In a twin-tube shock absorber, the piston is contained in a small tube that sits within a larger outer one. The inner tube (known as the "pressure tube") and the outer tube (known as the "reserve tube") are separated into two chambers by a pass-through valve located in between. This serves to protect the shock absorber from damage because if the outer tube should become dented, there are no ill effects on the operation of the piston itself. Nitrogen gas and hydraulic oil are not separated, which can add to fluid aeration and bubbling under heavier-use conditions such as off-roading or racing.

The valve which allows fluid and gas to pass through as needed serves to keep gas pressures low and produces a more comfortable ride. Low gas pressure avoids stress on seals, ensuring they generally have a longer lifespan. However, that piston valve is usually smaller than mono-tube ones because of limited space. Other disadvantages of the twin-tube layout include limited hydraulic oil capacity compared to mono-tubes, and limited angles at which twin-tube shocks can be installed.

In mono-tube shock absorber, the outer housing doubles as the sole cylinder tube for the piston, hydraulic oil, gas, valve, and other bits. Unlike twin-tube designs, nitrogen gas is usually housed within the main cylinder, and is kept completely separated from the oil chamber by a floating piston. The main benefits of mono-tube shocks are less oil aeration because it's kept separate from gas, and lower operating temperature because the greater quantity of oil dissipates more heat. Both of these factors allow a more stable damping force to be generated consistently without fade.

Unlike twin-tube designs, these shocks can also be custom mounted at extreme angles. Because the gas needs to be under higher pressure in this layout, ride quality mono-tube shocks produce is stiffer - making them ideal for aggressive driving situations. This higher pressure can be harder on seals over the long term. Any physical damage to the outer shell case makes mono-tube shocks virtually unusable, because there's no squish room between the casing and piston cylinder walls.

Double Acting Telescopic Shock Absorber

The ‘Double Acting Hydraulic Telescopic Shock Absorber’ is the commonly used shock absorber which is described as shown in the figures. It offers resistance both during compression and rebound of the spring.

In telescopic dampers two main tubes are coincide to each other say outer and inner tube with inserting piston rod through it. It develops resistance to the spring by forcing a fluid through check valves and small holes.

                 Fig: Double Acting Telescopic Shock absorber

Construction:

The telescopic shock absorber consists of a cylinder to which a head is welded to screwed to the outer tube. The space between outer and inner tube is called reservoir. A pressed steel cap and axle eye by means of which cylinder is screwed to the axle are welded to the outer tube. A piston slides inside the cylinder and screwed to the piston rod at which its upper end of chassis eye, it is attached to the frame of the vehicle. The part of the piston rod that is outside of the cylinder is protected by a cover which is welded to the chassis eye. A piston rod gland packing prevent the leakage, when the piston passes through the head and any fluid is trapped by it is supplied to the reservoir through drain hole.

Its upper eye is connected to the axle and the lower eye to the chassis frame. A two way valve ‘A’ is attached to as rod ‘G’. Another two way valve B is attached to the lower and of the cylinder C. The fluid is in the space above and below the cylinder C and tube D, which is connected to the space below the valve B. The J has glad H. Any fluid scrapped off the rod G is brought down into the annular space through the inclined passage.

Fig: Valves at compression & tension 

Working:

When the vehicle comes across a bump the lower eye E moves up. Therefore the fluid passes from the lower side of the vehicle A to its top side. But since the volume of the space above valve A is less than the volume B. This pressure of the fluid through the valve opening provides the damping force. Similarly when the lower eye E moves down, the fluid passes from the upper side of the valve A to the lower side and also from the lower of the valve B to the upper side.