Monroe® - The right choice for hard-working trucks
from class 3 to 8.
Monroe® Commercial Vehicle ride control products are
application-engineered to match your driving needs.
Monroe® Commercial Vehicle shock absorbers are built for durability, even with the demands of high mileage and severe use.
Your single source supplier – featuring a complete line of ride control products for today’s medium, heavy-duty and industrial markets.
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MONROE® COMMERCIAL VEHICLE: Support
Monroe's Technical Support will help you find answers to the most frequently asked product and installation questions and provide invaluable technical training information. If you need additional information or assistance, Monroe's Ride Control Technical Assistance Team can assist you.
MONROE® COMMERCIAL VEHICLE: What Do Shock Absorbers Do?
Commercial vehicle shock absorbers control spring movement to help keep your tires in contact with the road. Shock absorbers are critical safety components that can affect tire wear, stability, braking, vibration, driver comfort, and the life of other steering and suspension parts.



MONROE® COMMERCIAL VEHICLE: The Vital Functions That Shocks Perform
  • Control spring movement
    Shocks work with a commercial truck’s suspension system to maintain tire-to-road contact by controlling spring movement.
  • Protects springs and air bags
    Shocks work with a commercial truck’s springs - if one is weak, it will wear the other out quickly.
  • Help keep tires in contact with the road surface
    Maintaining firm tire-to-road contact is critical for safe steering, handling and load control.
  • Provides extension stop for air suspensions
    If extension limits are exceeded, damage to the air spring - and the truck - may result.
  • Transform movement to heat
    These velocity-sensitive dampers transform the kinetic energy produced by suspension movement into thermal energy, which is dissipated via hydraulic fluid.
  • Reduced cost per mile
    Properly functioning shocks can help reduce operating expenses by extending tire life, reducing wear and tear to other components and protecting your truck investment.
  • When replacing worn air springs, remember to replace worn shocks.
MONROE® COMMERCIAL VEHICLE: The Vital Functions That Shocks Perform



MONROE® COMMERCIAL VEHICLE: Commercial Vehicle Shock Absorber Design
There are several hydraulic shock absorber designs in use today for commercial vehicles:

BASIC TWIN TUBE DESIGN

MONROE® COMMERCIAL VEHICLE: BASIC TWIN TUBE DESIGN
The twin tube design has an inner tube known as the working or pressure tube and an outer tube known as the reserve tube. The outer tube is used to store excess hydraulic fluid.

There are many types of shock absorber mounts used today. Most of these use rubber bushings between the shock absorber and the frame or suspension to reduce transmitted road noise and suspension vibration. The rubber bushings are flexible to allow movement during suspension travel. The upper mount of the shock absorber connects to the vehicle frame.

Notice that the piston rod passes through a rod guide and a seal at the upper end of the pressure tube. The rod guide keeps the rod in line with the pressure tube and allows the piston to move freely inside. The seal keeps the hydraulic oil inside and contamination out.

The base valve located at the bottom of the pressure tube is called a compression valve. It controls fluid movement during the compression cycle.

Bore size is the diameter of the piston and the inside of the pressure tube. Generally, the larger the unit, the higher the potential control levels because of the larger piston displacement and pressure areas. The larger the piston area, the lower the internal operating pressure and temperatures. This provides higher damping capabilities.

Ride engineers select valve codes or damping force values for a particular vehicle to achieve optimal ride characteristics of balance and stability under a variety of driving conditions. Their selection of bleeds, deflective valve discs, springs and orifices control fluid flow within the unit, which ultimately determines the feel and handling of the vehicle.

TWIN TUBE - NON GAS CHARGED DESIGN

MONROE® COMMERCIAL VEHICLE: TWIN TUBE - NON GAS CHARGED DESIGN
In the past, ride comfort and vehicle control were compromised by the design limits of conventional hydraulics. A shock absorber can provide either a more comfortable ride or greater vehicle control, but not the optimum of both in the same unit. A shock damps excessive vehicle spring motion by the control of movement of fluid under pressure. Fluid provides the resistance to movement. Valving controls the amount of resistance.

Previously, shock valve orifices could not be enlarged to increase riding comfort without losing damping effectiveness. So valving was compromised in one of two directions - soft or hard valving. With soft valving, the fluid flows more easily. The result is a smoother ride, but with poor handling and a lot of roll and sway. When valving is hard, fluid flows less easily. Handling is improved, but the ride can become harsh.

To operate correctly, twin tube shock absorbers must be mounted in a somewhat vertical position. If you mount the shock upside down, air, not oil, flows through the pressure tube during the extension cycle, causing control loss.

In these non gas charged designs, a condition called aeration can occur, and is primarily caused two ways. The first condition occurs under severe suspension movement, or high velocity piston travel. In this situation, the fluid passes rapidly through holes in the piston, the fluid pressure drops so low, so quickly, that the fluid creates a void. The pressure drop can also occur in the oil when the shock fluid is forced violently and repeatedly through the piston. This violent agitation cause the oil to mix with the air bubbles and creates foam.

Because foam compresses, the amount of resistance the fluid will produce will be hard to predict.

In any non-pressurized shock absorber, an aeration zone builds up around the piston. When this happens, the shock heats up and the shock’s damping ability is reduced. This loss of damping capability is called fade.

TWIN TUBE - GAS CELL OR CELLULAR GAS DESIGN

MONROE® COMMERCIAL VEHICLE:TWIN TUBE - GAS CELL OR CELLULAR GAS DESIGN
There have been many shock absorbers designed to reduce aeration. The gas cell is one solution.

The gas cell is either a synthetic, air tight “envelope” of hexasulphafloride gas or a blanket of closed cell foam. This “envelope” is installed between the reserve tube and the pressure tube of a twin tube shock absorber. The gas cell, or cellular gas replaces the air in the reserve tube. The cell is compressed as the piston rod displaces hydraulic fluid from the pressure tube to the reserve tube. This cell then exerts pressure on the hydraulic fluid.

This design keeps the air and oil separated and in the appropriate places, and ultimately reduces aeration. Additionally, this design will allow the shock to operate in a horizontal position (steering stabilizers).

TWIN TUBE - GAS CHARGED DESIGN

MONROE® COMMERCIAL VEHICLE: TWIN TUBE - GAS CHARGED DESIGN
The development of gas charged shock absorbers was a major advance in ride control technology. The design of twin tube gas charged shock absorbers solves many of today's ride control problems by adding a low pressure charge of nitrogen gas in the reserve tube. The pressure of the nitrogen in the reserve tube varies from 60 to 150 psi, depending on the application and construction. The gas serves several important functions to improve the ride control characteristics of a shock.

The primary function of gas charging is to minimize aeration of the hydraulic fluid. The pressure of the nitrogen gas compresses air bubbles in the hydraulic fluid. This prevents the oil and air from mixing and creating foam. Foam affects performance because it can be compressed - fluid can not. With aeration reduced, the shock is able to react faster and more predictably, allowing for quicker response time.

The final important function of the gas charge is to allow engineers greater flexibility in valving design. In the past such factors as damping and aeration forced compromises in design.

GAS CHARGING BENEFITS
Reduce Fade: Shocks can lose damping capabilities as they heat up during use. Gas charged shocks can cut this loss of performance, called fade.

Reduce Excessive Vibration: As tires bounce up and down, road roughness is transmitted to the vehicle's body and cargo, causing it to vibrate. Excessive vibration may increase a truck's cost per mile by increasing downtime, reducing tire mileage, reducing vehicle life and lowering resale value. Gas charged shocks control tire motion better than non-gas units, so vibration is reduced.

Provide a Wider Range of Control: Because gas charged shocks reduce aeration and dumping, they can provide improved performance levels over a wider variety of road conditions.

Reduce Aeration: Reduced aeration means greater valving range for improved control and a reduction in excessive vibration.

Extend Shock Life: By virtually eliminating the effects of aeration, the shock will operate at a much cooler temperature. This extends the life of the shock and decreases the cost of maintaining your fleet.

Improve Handling: When turning a corner or a sharp curve, a vehicle's body tends to lean away from the direction of the turn and then rebounds. This motion is called roll. And, excessive roll may cause a loss of control. With gas charged shocks, roll is reduced.

Gas charged shocks are available for air spring and taper leaf suspensions. The nitrogen gas charge provides a quick response essential to today's heavy truck suspension systems.The extra large piston bore can handle the demand of high mileage and severe use. When combined, the gas charge and large bore provide the consistent control and efficiency necessary for demanding truck suspensions.

MONROE® COMMERCIAL VEHICLE: TWIN TUBE - GAS CHARGED DESIGN
UNIQUE HYDRAULIC LOCKOUT
On commercial vehicles with air suspensions, the shock absorber limits suspension travel. They also protect the air spring from over travel, which will result in premature failure.

The Unique Hydraulic Lockout feature is a secondary piston above the valving piston. This unique feature helps to transfer the forces generated as the piston travels, throughout the entire diameter of the piston rod. Its primary function is to reduce the velocity at full extension. The hydraulic pressure created as the lockout piston travels into the extended rod guide collar reduces the velocity and minimizes metal to metal contact, at full extension.

The hydraulic pressure created as the lockout piston travels into the extended rod guide collar reduces the velocity and minimizes metal to metal contact, at full extension.

TWIN TUBE - POSITIVE SENSITIVE DAMPENING (PSD) DESIGN

MONROE® COMMERCIAL VEHICLE: TWIN TUBE - POSITIVE SENSITIVE DAMPENING (PSD) DESIGN
In our earlier discussion of hydraulic shock absorbers we discussed that in the past, ride engineers had to compromise between soft valving and firm valving. With soft valving, the fluid flows more easily. The result is a smoother ride, but with poor handling and a lot of roll/sway. When valving is firm, fluid flows less easily. Handling is improved, but the ride can become harsh. With the advent of gas charging, ride engineers were able to open up the orifice controls of these valves and improve the balance between comfort and control capabilities available in traditional velocity sensitive dampers.

A leap beyond fluid velocity control is an advanced technology that takes into account the position of the valve within the pressure tube. This is called Position Sensitive Damping (PSD) a feature found in the Gas Magnum cab shock.

The key to this innovation is precision tapered grooves in the pressure tube. Every application is individually tuned, tailoring the length, depth, and taper of these grooves to ensure optimal ride comfort and added control. This in essence creates two zones within the pressure tube.

The first zone, the comfort zone, is where normal driving takes place. In this zone the piston travel remains within the limits of the pressure tube's mid range. The tapered grooves allow hydraulic fluid to pass freely around and through the piston during its midrange travel. This action reduces resistance on the piston, assuring a smooth, comfortable ride.

The second zone, the control zone, is utilized during demanding driving situations. In this zone the piston travels out of the mid range area of the pressure tube and beyond the grooves. The entire fluid flow is directed through the piston valving for more control of the vehicle’s suspension. The result is improved vehicle handling and better control without sacrificing ride comfort. It’s like having two shocks in one – comfort and control.

PSD technology allows ride engineers to move beyond simple velocity sensitive valving and use the position of the piston to fine tune the ride characteristics. This technology adjusts more rapidlyto changing road and weight conditions than standard shock absorbers.

The only use of PSD in commercial vehicles is Monroe heavy duty cab shocks.