KYB技報第58号

2019―４

製品紹介

Double Hydraulic Stop（DHS）Suspension
Javier Lizarraga Senar

1

Introduction

2

Double Hydraulic Stop suspension concept

From a physical point of view, vehicle
suspensions are characterized by their stiffness

The Double Hydraulic Stop（DHS）suspension
is based on providing the right blend of stiﬀnessand damping. Since the beginning of theautomobile industry, these two parameters havebeen adapted to provide the best driving
experience according to the market standards oneach moment. In general, their optimal design isdefined as a compromise between ride comfort
and handling. As they are involved in the car
performance not only for vertical movement, butalso for roll, yaw and pitch behavior, the best
setting changes from one driving condition toanother.
Many developments have been carried out toﬁnd and improved compromise between stiﬀness
and damping such as air suspension,hydropneumatic suspension or semi-active

and damping throughout the full suspension
travel.
As shown in Fig. 1, the standard passive
suspension is composed of elastic and damping
components. The suspension spring, the stabilizer
bar, the bumper, the rebound stop and the rubberbushings could be classiﬁed as elastic parts. Apartfrom the limited rubber viscous damping, theshock absorber is the only damping component in
car suspension. Whenever the vehicle rides, thekinetic energy induced by the road or driveractions is stored by the elastic components anddissipated, mainly, by the shock absorber.

dampers. All these technologies are focused on
improving the standard passive suspension by
better adapting suspension stiffness or/and
damping to the vehicle instantaneous demands. To
do so, they use a common architecture where acertain combination of sensors, ECUs andactuators is essential, adding cost and complexity
to vehicle suspension. As a result, these systems
are frequently being relegated to the higher
vehicle trim levels, or even disappearing from theMs
kscsmarket.
In the last years, KYB has faced this subject
from a diﬀerent perspective. KYBʼs proposal is anenhanced passive suspension that respects thesimplicity and effectiveness of standard
suspension, boosting its performance at anaﬀordable cost.

Fig. 1 Quarter-car suspension model（lumped parameters）. Elastic and damping components in astandard passive suspension and their function― 19 ―

Double Hydraulic Stop（DHS）Suspension

The spring is the main elastic component insuspension, being responsible for supporting thesprung mass（vehicle body, frame, passengers. . .）and defining the ride frequency as a relation
between its stiﬀness（ks in the quarter-car model,
and the slope of red line in Fig. 2）and the sprungmass（m s in the quarter-car model）. The ride
frequency is a decisive parameter that establishes
the frequency of the sprung mass verticalmovements, that is directly related to thehandling/comfort compromise.
The shock absorber, as it is the main energy
dissipating element, will control the sprung massoscillations decay after each disturbance. The
right amount of damping（Cs in the quarter-carmodel）to be added is a function of the sprungweight, the spring rate and the desired
performance for each suspension velocity range. A
dimensionless parameter, named damping ratio, isdeﬁned to describe the oscillations decay pattern.
As shown in Fig. 2, a damping ratio of 1 meansthat the suspension is critically damped and its
response will be the fastest without overshoot.
Values close to this level are chosen for low
velocity oscillations, which are associated withhandling. Low damping ratios will provide a fast
and underdamped response, well adapted toprovide comfort and usually chosen for medium
and high suspension velocities.

impacts and noise.
The standard solution for this issue is toincorporate two new elastic components, therebound stop for extension and the bumper forcompression. They will reduce suspension travel
velocity as it reaches its stroke end, but theenergy in excess will be fully stored andsubsequently released leading to discomfort andtoo many oscillations before settling. In addition,
as these components are not velocity sensitive,
they cannot guarantee an optimal performance athigh suspension travel speeds, still allowing
undesired noise and acceleration peaks.
In Fig. 3, this is analyzed in terms of damping
ratio. The spring elastic curve（red graph）has
been reinforced in the surroundings of each strokeend. However, the response type of the shock
absorber subjected to a sinusoidal oscillation（bluegraph）shows a decrease in damping capability,
linked to travel speed reduction in the stroke ends
surroundings. The stiﬀness increment along with
damping decline, conducts to an undesired
damping ratio decrease that will degrade
suspension performance.

Fig. 3 Damping ratio change in the surroundings ofthe stroke ends after bumper and reboundstop incorporation

The damping ratio approach is very useful toMs
ks

understand DHS philosophy. DHS function has
been added to a standard damping cycle in Fig. 4（blue graph）. The DHS shock absorber includes
two subsystems, one focused on smoothening thecsextension stroke ends（Hydraulic Rebound Stop,HRS）and the other acting for compression stroke
Fig. 2

Roll of suspension spring and shock absorberin damping ratio deﬁnition

The suspension is now perfectly balanced
whenever it works around its reference position.

ends（Hydraulic Compression Stop, HCS）
. Both ofthem are able to progressively reinforce damping
capability as the stroke ends are reached,accommodating their response to the movement

However, neither the spring nor the shock
absorber are capable to adapt their response invelocity. The elastic curve（red graph）has also
been modiﬁed in order to lower the bumper andthe surroundings of the suspension stroke ends,
where and additional effort is required to avoid

rebound stop contributions. As a result, thedesired damping ratio is kept along the entire― 20 ―

KYB技報第58号

stroke length, minimizing suspension oscillations
with an improved ride comfort. As DHS is mainly
based on damping, it is fully capable to manage
high velocity stroke ends.

2019―４bumper-like function. The simpliﬁed bumper will
still be responsible for managing the last
millimeters of compression stroke, to avoid therisk of a damping lack when the stroke end isreached slowly. HCS has been conceived to workin parallel with the main shock absorbercomponents and ﬂows, without altering them andrespecting the main damping law.
As shown in Fig. 6, HCS is partially installed on

Fig. 43

Damping ratio improvement after including
DHS concept and the adjustment of bumperand rebound stop

DHS architecture: HCS & HRS

As mentioned before, DHS is integrated by theHydraulic Compression Stop （HCS）, a firstthe piston rod and mainly placed in the bottom of
the inner tube. Once the shock absorber iscompressed enough to overlap the HCS plunger
with the HCS tube, the shock absorbercompression chamber is divided in two portions.
The outer one, delimited by the plunger, the HCS
tube, the shock absorber inner tube and the HCS
base valve, is named HCS chamber.

subsystem devoted to control compression stroke
ends, and the Hydraulic Rebound Stop（HRS）, a
second subsystem dedicated to improve extension
stroke ends. Both are passive systems that areinstalled inside the shock absorber body, asdisplayed in Fig. 5. A standard shock absorberequipped with DHS will generate the forcedisplacement curve shown below, characterized
by an additional energy dissipation capability thatbuilds progressively as both stroke ends arereached. HCS and HRS will be analyzed separately
in the following paragraphs.
Fig. 6 HCS: installation, construction and components
description

As the shock absorber is further compressed,
the pressure in HCS chamber rises. The trappedoil has two possible paths to reach the originalcompression chamber and join the main shock
absorberʼs oil ﬂow: a group of oriﬁces practiced in
the plunger and through the HCS valve（see Fig.
7）, which is preloaded to operate as a pressure
Fig. 5 HCS and HRS installation in the shock absorber and sample of function

4

DHS subsystems: HCS description

relief valve. In consequence, while the pressure in
HCS chamber doesnʼt exceed the relief pressure,the oil will be forced to ﬂow through the plunger
orifices. These passages are gradually closed by

HCS is a damping-based fully dissipative device
that is intended to work together with a simpliﬁed

the HCS tube as the shock absorber iscompressed, bringing an additional pressure
increment. The HCS pressure raise is limited tobumper version. It oﬀers a tunable progressivity
with suspension stroke to provide a dissipative

the preset relief pressure of HCS valve, which will
open to allow the oil quitting the HCS chamber― 21 ―

Double Hydraulic Stop（DHS）Suspension

easily. This condition is normally reached at theend of the compression stroke, allowing theplunger oriﬁces to control the HCS force increase
with stroke, while the HCS valve is responsible forregulating the maximum force at eachcompression velocity.

5DHS subsystems: HRS description

HRS is a combination of elastic and hydraulicdevice focused on replacing the conventional
mechanic rebound stop. The HRS elastic content
guarantees the function at low extension velocities,
while the hydraulic one reinforces its performanceat higher velocities, by making the systemsensitive to movement speed. Short stroke
versions are focused on noise and acceleration
peaks reduction, while long versions are also
suitable for vehicle dynamics improvement. In
that case, the dissipative content of HRS is a great
tool to improve roll and pitch control. HRS has
been conceived to work in parallel with the main
shock absorber components and flows, without

Fig. 7

HCS working principle and ﬂow distribution

The working principle described above has theadvantage of limiting the maximum HCS force
even for the high compression velocities registeredin suspension during normal driving. Moreover,the high preload/stiﬀness ratio used for HCS valve
provides high damping force at low velocity while
keeps it moderate at high velocity.

interfering with them and respecting the main
damping law.
HRS is essentially implanted in the upper areaof an inner tube that has been radially expandedwith a conical shape（see Fig. 9）, comprising arubber stop, a spring with plastic washers and an
open segment. The system is completed with ametallic washer assembled on the piston rod.
When the shock absorber stroke is enough tomake the rebound stop washer contact thesegment, the shock absorber rebound chamber isdivided in two sections. The upper one, that isdefined by the guide, the rod, the deformed
portion of the inner tube, the segment and therebound washer, is named HRS chamber.

Fig. 8 HCS function: tunability options

As summarized in Fig. 8, HCS is a highly
tunable technology. Apart from system stroke andmaximum force, HCS progressivity, preload anddependence on velocity can be tuned.
Progressivity is deﬁned by the chosen sequence ofplunger oriﬁces. HCS preload and force gain withvelocity increase are adjusted with the assembly
deformation of HCS valve and its stiffness
respectively.

Fig. 9HRS: installation, construction and components
description

As the shock absorber is further extended, theoil in HRS chamber is compressed and its pressure
rises. The open segment counts with two kinds of― 22 ―

KYB技報第58号

2019―４

passages to regulate this pressure build-up: thegap between ends constitutes the main control

car body, thus being able to impose really highcompression velocities. On the contrary, extension

path, while a number of reliefs are responsible forthe force increment regulation.

strokes are regulated by the elastic energy
released to the low inertia unsprung mass bysuspension spring, deploying predictable travels at
no so high velocity. In such a scenario, it is notnecessary to use a pressure relief device to avoid
excessive load at high velocity. As shown in Fig.
11, adjusting the stroke, the maximum force and,eventually, the damping force build-up is enough.

6Conclusion

A new passive suspension concept has beendeveloped. The treatment of suspension stroke
Fig. 10

HRS working principle

At the HRS entry point, the open segment restsin the maximum diameter of the inner tube
deformation. At this position, the separation
between both segment ends is maximum and thereliefs are fully open, allowing the oil to escapeeasily from the HRS chamber. As shown in Fig.
10, the more the segment is dragged through theconical shape of the tube expansion, the smaller
becomes the gap between segment ends and thereliefs passage, resulting in an increased dampingforce. Once the stroke end is reached, the springis blocked and a rubber stop deforms to ﬁlter any
eventual acceleration peak transferred to thechassis.

ends provided by DHS brings a new comfort
standard to automobile industry. It is an aﬀordable
passive solution easy to implement in no matterswhich suspension architecture, and highly tunable
to match customer expectations. In fact, this is a
key factor to explain the system success, because
DHS allows to car makers offering a high classsuspension starting from the most basic trim levelof a vehicle.
A second key point is the system versatility:
DHS can be conﬁgured to create an outstanding
comfortable vehicle, or to achieve a better dynamicperformance by combining a good, well-tuned
passive shock absorber with DHS to, for instance,compete with adaptive suspensions.
Last but not least, DHS is easily perceived byanyone who drives the vehicle, due to the granted
smoothness and the absence of rebounds whendriving through obstacles and bad roads.
As a result, DHS is receiving a very good
acceptance from the market both, in sales andwith positive reviews from the specialized press.

7

Further development

DHS, as a passive hydro-mechanical device, isalso well positioned to contribute to improve semi-Fig. 11 HRS function: tunability options

Due to the more deterministic character of thesuspension extension movements compared tocompression ones, HRS needs not be as tunable as
HCS. Compression movements are induced byroad irregularities working against a high inertia

active suspensions. Properly managing thesuspension stroke ends demands shock absorbers
adapting their response in few milliseconds. Thisis a hard task for an electronic suspension,
because not only a fast response actuator isrequired, but also the sensors and control strategy― 23 ―

Double Hydraulic Stop（DHS）Suspension

have to make the right decision within such ashort time interval. On the contrary, it is an easytask for a passive system, which instantaneously
acts in a predictable and repeatable way
whenever a certain input is applied. Merging thegreat potential of semi-active suspensions with thefast passive response of DHS would boost thesystem performance.

8Acknowledgements

I would like to express my deep gratitude andsincere appreciation to all my colleagues,
engineers and technicians, who have contributeddecisively to the ﬁnal success of this challenging
development.

AuthorJavier Lizarraga Senar
Joined the company in 2003.
Currently at KYB Europe GmbHas R&D engineer.

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