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An automobile shock absorber is basically a hydraulic piston that absorbs and dissipates vibration using spring-loaded check valves and orifices to control the flow of oil. The first production hydraulic dampers, to act on the main leaf spring movement, were probably those based on an original 1908 concept by Maurice Houdaille.

One design consideration, when designing or choosing a shock absorber, is where that energy will go. In most shock absorbers, energy is converted to heat inside the viscous fluid. In hydraulic cylinders, the hydraulic fluid heats up, while inair cylinders, the hot air is usually exhausted to the atmosphere. In other types of shock absorbers, such as electromagnetictypes, the dissipated energy can be stored and used later. In general terms, shock absorbers help cushion vehicles on uneven roads.

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. Spring rates are chosen by the manufacturer based on the weight of the vehicle, loaded and unloaded. Some people use shocks to modify spring rates but this is not the correct use. Along with the tires, they damp the energy stored in the motion of the unsprung weight up and down. Effective wheel bounce damping may require tuning shocks to an optimal resistance.

Spring-based shock absorbers commonly use coil springs or leaf springs, though torsion bars are used in torsional shocks, as well. Springs alone, however, are not shock absorbers – they only store and do not dissipate or absorb energy. Vehicles typically employ both hydraulic shock absorbers and springs or torsion bars.

Types of Shock Absorbers

Most vehicular shock absorbers are either mono-tube or twin-tube types, with some variations on these designs.

Diagram of the main components of a twin-tube and mono-tube shock absorber


The mono-tube shock absorber was considered a revolutionary advancement when it appeared in the 1950s. As its name implies, the mono-tube shock consists of only one tube- the pressure tube- though it has two pistons. These pistons are called the working piston and the dividing or floating piston, and they move in relative synchrony inside the pressure tube in response to changes in road smoothness. The two pistons also completely separate the shock's fluid and gas components. Mercedes became the first auto manufacturer to install mono-tube shocks as standard equipment on some of their cars starting in 1958. They were manufactured by Bilstein and first appeared in 1954. Because the design was patented, no other manufacturer could use it until 1971, when the patent expired.

Bilstein shocks/struts first appeared on the Challenger in 2008, as part of the SRT’s handling package.

Magnetic Ride Control

Magnetic Ride Control does not use mechanical valves or small moving parts that are prone to wear. Instead, a Magnetic Ride Control shock absorber uses the following components:

  • A mono-tube damper filled with magnetorheological fluid located at each wheel of a vehicle.
  • A set of sensors.
  • An electronic control unit (ECU) responsible for coordinating the entire system.
This unique system was first used on the 2003 Cadillac Seville STS and Corvette C5. Ford first employed this system, in 2015, in its Mustang Shelby GT350.


Basic Twin-Tube

Also known as a "two-tube" shock absorber, this device consists of two nested cylindrical tubes, an inner tube that is called the "working tube" or the "pressure tube", and an outer tube called the "reserve tube". At the bottom of the device on the inside is a compression valve or base valve. When the piston is forced up or down by bumps in the road, hydraulic fluid moves between different chambers via small holes or "orifices" in the piston and via the valve, converting the "shock" energy into heat which must then be dissipated.

Twin-Tube Gas Charged

Variously known as a "gas cell two-tube" or similarly-named design, this variation represented a significant advancement over the basic twin-tube form. Its overall structure is very similar to the twin-tube, but a low-pressure charge of nitrogen gas is added to the reserve tube. The result of this alteration is a dramatic reduction in "foaming" or "aeration"- the undesirable outcome of a twin-tube overheating and failing which presents as foaming hydraulic fluid dripping out of the assembly. Twin-tube gas charged shock absorbers represent the vast majority of original modern vehicle suspensions installations.

Position Sensitive Damping

Often abbreviated simply as "PSD," this design is another evolution of the twin-tube shock. In a PSD shock absorber, which still consists of two nested tubes and still contains nitrogen gas, a set of grooves has been added to the pressure tube. These grooves allow the piston to move relatively freely in the middle range of travel (i.e., the most common street or highway use), and to move with significantly less freedom in response to shifts to more irregular surfaces when upward and downward movement of the piston starts to occur with greater intensity (i.e., on bumpy sections of roads). This advance allowed car designers to make a shock absorber tailored to specific makes and models of vehicles and to take into account a given vehicle's size and weight, its maneuverability, its horsepower, etc. in creating a correspondingly effective shock.

Acceleration Sensitive Damping

The next phase in shock absorber evolution was the development of a shock absorber that could sense and respond to not just situational changes from "bumpy" to "smooth" but to individual bumps in the road in a near instantaneous reaction. This was achieved through a change in the design of the compression valve, and has been termed "acceleration sensitive damping" or "ASD". Not only does this result in a complete disappearance of the "comfort vs. control" tradeoff, it also reduced pitch during vehicle braking and roll during turns. However, ASD shocks are usually only available as aftermarket changes to a vehicle and are only available from a limited number of manufacturers.


Coil-over shock absorbers are usually a kind of twin-tube gas charged shock absorber inside the helical road spring. They are common on motorcycle and scooter rear suspensions, and widely used on front and rear suspensions in cars.

Spool Valve

Spool valve dampers are characterized by the use of hollow cylindrical sleeves with machined-in oil passages as opposed to traditional conventional flexible discs or shims. Spool valving can be applied with mono-tube, twin-tube, and/or position-sensitive packaging, and is compatible with electronic control.

Adjustable Shocks

After market upgraded adjustable rear shocks are available, from for 2015+ Challenger Hellcats. (Note- The 2018 Demon came standard with adjustable dampers). Viking Performance shocks have special valving for street, drag race, handling or pro-touring requirements. Its Warrior shocks are great for a direct replacement and offer better than stock ride quality through improved valving and adjustability. The Crusader shocks are even better and engineered for maximum traction. All shocks offer external adjustment to control compression and rebound.


Air-adjustable shocks, which were popular with hot-rodders in the early 1970s to provide rear tire clearance, provide customizable performance and support that can be adjusted and readjusted to handle temporary towing or hauling needs and keep a vehicle from bottoming out. Air is added, using a valve, to raise the suspension and air is bled off to return the vehicle to its normal height. Two popular brands are Monroe Max-Air and Gabriel Hi-Jackers.

  • Unlike the shock absorber, the strut has a reinforced body and stem.
  • Strut already plays the role of shocks absorbers, so the shock absorber may well act as part of the strut, not the other way around.
  • Struts is subjected to multidirectional loads, while the shock absorber only damps vibrations and receive an impact along its axis.
  • The strut gives to the car greater reliability.
  • Strut and shock absorber have a different way of attachment. Shock absorbers are mounted through silent blocks without a swivel device and are equipped with a small diameter rod. Strut replaces the upper ball and rotator.

Premium Member
The Bacon Hauler (‘12 Cop Charger)
9,760 Posts
Interesting, I always thought of a strut as the shock absorber hootus and a coil spring. The two together equal one strut. But you’re saying the shock absorber part of what I called a strut is actually the strut. Okay, if that’s the case, what is the coil spring plus the strut (plus strut mount, bellows, jouncer, etc) called?

I’m thinking “strut assembly” is the proper term for that...
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