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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.
SHOCK ABSORBERS A BRIEF HISTORY
In the early 1900's, cars still rode on carriage springs. After all,
early drivers had bigger things to worry about than the quality of their
ride - like keeping their cars rolling over the rocks and ruts that
often passed for roads.
Pioneering vehicle manufacturers were
faced early on with the challenges of enhancing driver control and
passenger comfort. These early suspension designs found the front wheels
attached to the axle using steering spindles and kingpins. This allowed
the wheels to pivot while the axle remained stationary. Additionally,
the up and down oscillation of the leaf spring was damped by device
called a shock absorber.
These first shock absorbers were simply two arms connected by a bolt
with a friction disk between them. Resistance was adjusted by tightening
or loosening the bolt.
As might be expected, the shocks were
not very durable, and the performance left much to be desired. Over the
years, shock absorbers have evolved into more sophisticated designs.
WHAT SHOCKS DO
Let's start our discussion of shock absorbers with one of very
important point: despite what many people think, conventional shock
absorbers do not support vehicle weight. Instead, the primary purpose of
the shock absorber is to control spring and suspension movement. This
is accomplished by turning the kinetic energy of suspension movement
into thermal energy, or heat energy, to be dissipated through the
hydraulic fluid.
Shock absorbers are basically oil pumps. A
piston is attached to the end of the piston rod and works against
hydraulic fluid in the pressure tube. As the suspension travels up and
down, the hydraulic fluid is forced through tiny holes, called orifices,
inside the piston. However, these orifices let only a small amount of
fluid through the piston. This slows down the piston, which in turn
slows down spring and suspension movement.
The amount of
resistance a shock absorber develops depends on the speed of the
suspension and the number and size of the orifices in the piston. All
modern shock absorbers are velocity sensitive hydraulic damping devices -
meaning the faster the suspension moves, the more resistance the shock
absorber provides. Because of this feature, shock absorbers adjust to
road conditions. As a result, shock absorbers reduce the rate of:
- Bounce
- Roll or sway
- Brake dive and Acceleration squat
Shock absorbers work on the principle of fluid displacement on both the
compression and extension cycle. A typical car or light truck will have
more resistance during its extension cycle then its compression cycle.
The compression cycle controls the motion of a vehicle's unsprung
weight, while extension controls the heavier sprung weight.
Compression cycle
During the compression stroke or downward movement, some fluid flows
through the piston from chamber B to chamber A and some through the
compression valve into the reserve tube. To control the flow, there are
three valving stages each in the piston and in the compression valve.
At the piston, oil flows through the oil ports, and at slow piston
speeds, the first stage bleeds come into play and restrict the amount of
oil flow. This allows a controlled flow of fluid from chamber B to
chamber A.
At faster piston speeds, the increase in fluid
pressure below the piston in chamber B causes the discs to open up away
from the valve seat.
At high speeds, the limit of the second
stage discs phases into the third stage orifice restrictions.
Compression control, then, is the force that results from a higher
pressure present in chamber B, which acts on the bottom of the piston
and the piston rod area.
Extension cycle
As the piston and rod move upward toward the top of the pressure tube,
the volume of chamber A is reduced and thus is at a higher pressure than
chamber B. Because of this higher pressure, fluid flows down through
the piston's 3-stage extension valve into chamber B.
However,
the piston rod volume has been withdrawn from chamber B greatly
increasing its volume. Thus the volume of fluid from chamber A is
insufficient to fill chamber B. The pressure in the reserve tube is now
greater than that in chamber B, forcing the compression intake valve to
unseat. Fluid then flows from the reserve tube into chamber B, keeping
the pressure tube full.
Extension control is a force present as a result of the higher pressure in chamber A, acting on the topside of the piston area
SHOCK ABSORBER DESIGN There are several shock absorber designs in use today:
- Twin Tube Designs
- Gas Charged
- PSD
- ASD
- Mono-Tube
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
valving
values for a particular vehicle to achieve optimal ride characteristics
of balance and stability under a wide variety of driving conditions.
Their selection of valve springs and orifices control fluid flow within
the unit, which determines the feel and handling of the vehicle.
Twin Tube - Gas Charged Design
The development of gas charged shock absorbers was a major advance in
ride control technology. This advance solved many ride control problems
which occurred due to an increasing number of vehicles using uni-body
construction, shorter wheelbases and increased use of higher tire
pressures.
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 100 to 150 psi, depending on the amount
of fluid in the reserve tube. The gas serves several important
functions to improve the ride control characteristics of a shock.
The prime 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 and helping
keep the tire firmly planted on the road surface.
An additional
benefit of gas charging is that it creates a mild boost in spring rate
to the vehicle. This does not mean that a gas charged shock would raise
the vehicle up to correct ride height if the springs were sagging. It
does help reduce body roll, sway, brake dive, and acceleration squat.
This mild boost in spring rate is also caused by the difference in the
surface area above and below the piston. With greater surface area below
the piston than above, more pressurized fluid is in contact with this
surface. This is why a gas charged shock absorber will extend on its
own.
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.
Advantages: - Improves handling by reducing roll, sway and dive
- Reduces aeration offering a greater range of control over a wider variety of road conditions as compared to non-gas units
- Reduced
fade - shocks can lose damping capability as they heat up during use.
Gas charged shocks could cut this loss of performance, called fade
Disadvantages: - Can only be mounted in one direction
Current Uses: - Original equipment on many domestic passenger car, SUV and light truck applications
Twin Tube - 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).
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.
Advantages: - Allows
ride engineers to move beyond simple velocity sensitive valving and use
the position of the piston to fine tune the ride characteristic
- Adjusts more rapidly to changing road and weight conditions than standard shock absorbers
- Two shocks into one - comfort and control
Disadvantages: - If vehicle ride height is not within manufacturer's specified range, piston travel may be limited to the control zone
Current Uses: - Primarily aftermarket under the Sensa-Trac brand name
Twin Tube -ASD Design
We have discussed the compromises made by ride engineers to bring
comfort and control together into one shock absorber. This compromise
has been significantly reduced by the advent of gas charging and
position sensitive damping technology.
A new twist on the comfort/ control compromise is an innovative
technology which provides greater control for handling while improving
ride comfort called
Acceleration Sensitive Damping (ASD).
This technology moves beyond traditional velocity sensitive damping to
focus and address impact. This focus on impact is achieved by utilizing a
new compression valve design. This compression valve is a mechanical
closed loop system, which opens a bypass to fluid flow around the
compression valve.
This new application specific design allows
minute changes inside the pressure tube based on inputs received from
the road. The compression valve will sense a bump in the road and
automatically adjust the shock to absorb the impact, leaving the shock
with greater control when it is needed.
Due to the nearly
instantaneous adjustment to changes in the road's condition, vehicle
weight transfer is better managed during braking and turning. This
technology enhances driver control by reducing pitch during braking and
roll during turns.
Advantages: - Control is enhanced without sacrificing driver comfort
- Valve automatically adjusts to changes in the road condition
- Reduces ride harshness
Disadvantages: Current Uses: - Primarily aftermarket applications under the Reflex brand name
Mono-tube design These are high-pressure gas shocks with only one tube, the
pressure tube. Inside the pressure tube there are two pistons: a
dividing piston and a
working piston.
The working piston and rod are very similar to the twin tube shock
design. The difference in actual application is that a mono-tube shock
absorber can be mounted upside down or right side up and will work
either way. In addition to its mounting flexibility, mono-tube shocks
are a significant component, along with the spring, in supporting
vehicle weight.
Another difference you may notice is that the mono-tube shock absorber
does not have a base valve. Instead, all of the control during
compression and extension takes place at the piston.
The
pressure tube of the mono-tube design is larger than a twin tube design
to accommodate for dead length. This however makes it difficult to apply
this design to passenger cars designed OE with a twin tube design. A
free-floating dividing piston travels in the lower end of the pressure
tube, separating the gas charge and the oil.
The area below the
dividing piston is pressurized to about 360 psi with nitrogen gas. This
high gas pressure helps support some of the vehicle's weight. The oil
is located in the area above the dividing piston.
During
operation, the dividing piston moves up and down as the piston rod moves
in and out of the shock absorber, keeping the pressure tube full all
times.
Advantages: - Can be mounted upside down, reducing the unsprung weight
- May run cooler since the working tube is exposed to the
Disadvantages: - Difficult to apply to passenger cars designed OE with twin tube designs
- A dent in the pressure tube will destroy the unit
Current Uses: - Original equipment many import and performance domestic passenger cars, SUV and light truck applications
- Available for many Aftermarket applications
