Singularly Exceptional: Shock Absorbers and the Advantages of a Monotube Design

All vehicles – from the smallest passenger car to the largest Class 8 truck – feature damping control technology designed to enhance on-road performance and help protect critical components against premature wear and failure. Without it, driver comfort and safety would suffer, as would the durability of other parts and vehicle systems. An effective damping control system comprises multiple components, one of the most important of which is the shock absorber.

Shock absorbers are devices positioned between the frame of a vehicle and the wheels to soften the movements generated by the vehicle’s springs during the ride. They work in concert with other suspension components to control and reduce vehicle bounce, lean, sway, brake dive and acceleration squat. By mitigating such motions, shock absorbers help play a critical role in providing consistent handling and braking performance, maintaining dynamic wheel alignment and preventing premature degradation of components like brakes and tires.

All modern vehicles are fitted with shock absorbers, but not all shock absorbers are created the same. Today’s shocks feature a broad range of technology and material differences that impact performance. In addition, their design can be based on two very distinct design types: traditional twin-tube or monotube. Each type of shock absorber offers its own advantages, but monotube shock absorbers are particularly beneficial in many modern applications. To appreciate these benefits, it is important to understand how a shock absorber works, and that begins with understanding that energy can neither be created nor destroyed – only transferred and transformed. 

The Science of Damping

Vehicle suspension systems are constructed utilizing different types of springs designed to support the weight of the vehicle and absorb shock from bumps and uneven road surfaces. The most common of these, the coil spring, absorbs vehicle’s vertical wheel motions. In a suspension system using only springs, the absorbed energy would be continuously released back into the vehicle, creating uncontrollable bounce. A vehicle operating under such conditions would be both uncomfortable and unsafe to drive.

At its core, a shock absorber is an oil pump with a piston, attached to a piston rod, that pushes against hydraulic fluid in a pressure tube. It dampens spring motion by transforming kinetic energy stored during suspension movement into thermal energy (heat), which is then dissipated through the hydraulic fluid. As a vehicle travels down the road, wheel motion impacts on the suspension system and the springs. Suspension movement forces hydraulic fluid to travel back and forth through tiny holes inside the piston. Because these holes let only a small amount of fluid through at a time, the piston slows down, which in turn slows spring and suspension movement.

Shock absorbers work in two cycles – compression and rebound. Compression occurs as the piston moves downward, pushing down on the hydraulic fluid, forcing it to pass through the compression valve. During the rebound cycle, the piston moves upward, compressing fluid in the chamber above and forcing it back through the holes in the piston. The level of resistance a shock absorber offers in each cycle depends on multiple factors, including the number and size of the orifices in the piston; the quantity and thickness of the valve disks; and the speed of the suspension. The faster the suspension moves, the more resistance is provided.

The Dichotomy of Dual Tubes

Most commonly found on light vehicles like passenger cars, SUVs and pickups, twin-tube shock absorbers are named as such because they feature two cylinders – an oil chamber set inside the shell case and a second, internal cylinder that contains the piston valve. These shock absorbers come in one of two varieties: hydraulic and low-pressure gas.

When a twin-tube hydraulic shock absorber goes through a compression stroke, some oil in the lower working chamber transfers across the piston via a lightly loaded inlet valve. The remaining oil ­– which corresponds to the volume of the piston rod entering the inner tube – is forced through a valve system and into an outer oil reservoir called the equalization chamber. When the shock absorber rebounds, the inlet valve closes and oil in the upper working chamber is forced through a valve system in the piston. Oil passes from the equalization chamber into the lower working chamber via a lightly loaded inlet valve, ensuring the inner tube always stays full of oil.

Twin-tube low-pressure gas shock absorbers function similarly, but with two notable changes. First, the air in the upper part of the reserve tube is replaced by pressurized gas – usually nitrogen or impressed atmospheric air– at a pressure ranging from 2.5 to 8 bars. Second, the oil seal surrounding the piston rod in the upper portion of the housing features unique design elements, including a lip to prevent dirt from entering the system and two sealing lips to prevent oil or gas from escaping.

Twin-tube shock absorbers of both types are cost-effective and offer superior comfort and steering response in many driving conditions, but they suffer from a few drawbacks. In a twin-tube design, there is no physical barrier separate oil from gas. During compression and rebound strokes, oil is forced to flow from a high- to low-pressure area, creating a sudden pressure drop that can cause aeration or cavitation.

Aeration occurs when air is circulated with a hydraulic fluid, which can temporarily render damping inefficient. Cavitation occurs when air bubbles – which have mass but are compressible – form in the oil. When this happens, the initial piston rod travel of each stroke will compress the air bubbles, creating a damping control lag and reducing shock absorber efficiency. Though less likely to happen in a low-pressure gas shock, it is still possible and either scenario can negatively affect shock absorber function and efficiency.

The Monotube Advantage

Contrasting with the twin-tube design, a monotube shock absorber is a high-pressure gas shock comprising a single cylinder that is divided into two chambers – one filled with pressurized nitrogen gas and another above it with hydraulic fluid. The gas and oil are separated by a floating piston – preventing any aeration or cavitation from occurring – and a piston valve assembly connected to the piston rod is seated in the fluid portion of the tube. This type of design offers numerous advantages.

When the piston rod displaces oil on compression, the nitrogen in the chamber below also compresses. Because the nitrogen is subject to variations in volume, it functions much like a spring. The continuous pressure exerted on the oil in the tube by the gas ensures instantaneous response to changing road conditions, offering faster damping reaction and, as a result, improved steering control and road-holding capabilities. While these characteristics may lead to the perception of a stiffer ride in some passenger cars, monotube shock absorbers are an ideal solution for larger sedans, sports car applications and SUV rear axles.

Though the exterior dimensions of a twin-tube and monotube shock absorber may be the same, the size of the piston in a twin-tube design is limited by the outer cylinder, reducing oil capacity and piston size. Generally, this results in lower possible pressure. By comparison, monotube shocks feature more oil and a larger piston, which creates a wider area of pressure. Not only does this improve damping precision, but it also allows for more efficient heat dissipation.

One of the few ​ limitation in terms of usage of the monotube shocks absorbers is related to its use at off-road applications. Where the piston stroke is a key factor. In off-road vehicles the stroke of the shock used to be an important technical requirement of the suspension in order to be able to absorb the movements of the tire while it passes the irregularities and obstacles of the road. In the case of mono tube shocks due to the internal design, the piston rod stroke is limited to the part of the working chamber that is placed over the floating piston, while at the twin-tubes, this limitation is not present because the piston can move until the lower end of the working chamber where the compression valve is located. Therefore twin tube technology is the preferred one for off-road vehicles.

Tenneco offers monotube shocks to meet a range of needs and price points, from the Monroe Original high-value replacement line to the new premium Monroe OESpectrum® range. Featuring Tenneco's innovative M-RTECH®2 Rebound Valving Technology™, OESpectrum gas-charged shock absorbers leverage Tenneco's global OE leadership for a superior driving experience. Tenneco confidently backs each OESpectrum product with its first-ever 5-year limited warranty, offering consumers the peace of mind of a lasting investment. Complete warranty terms and conditions can be found online at www.Monroe-OESpectrum.com.

Monroe aftermarket products are supported through comprehensive training and vehicle diagnostic information available on Technicians Advanced Digital Information System (TADIS), a technical support platform from Tenneco that includes thousands of helpful files designed for technicians, installers and other aftermarket professionals. To access TADIS and other resources, visit www.training.tenneco.com . Independent aftermarket professionals also have access to Monroe advertising and point-of-sale promotional materials, as well as fully illustrated catalogues with 360-degree product images that can be found online at www.monroecatalogue.eu. To learn more about Monroe monotube shock absorbers and other Monroe ride control technologies, please contact your Monroe supplier or Tenneco sales representative. Additional information is available online at www.monroe.com.