The Power of Oil – Influence of Shock Absorber Oil on Vehicle Ride and Handling Performance

技術解説

The Power of Oil – Influence of Shock Absorber Oil on
Vehicle Ride and Handling Performance
Dr. Shinji Kato

Abstract
When the suspension deforms, damping force isof an own developed device that can measure thespeed dependence of friction force in reciprocating

generated as the piston of the shock absorbermoves, but a time difference occurs before thelarge hydraulic damping force rises. At the same

motion with high accuracy.
The result of our analysis shows that friction
dissipates energy at a speed level of 0.002m/s

time the frictional force in the shock absorber isplaying an important role in generating effective
damping force against deformation of thesuspension, the generated force itself is small but
its response is high. The frictional force in theshock absorber occurs in reciprocating motion invarious speed ranges from amplitude of less than
one millimeter to larger amplitudes, its behavior
changes dynamically and is non-linear. Recent
studies have revealed that controlling the friction
is more important than reducing it for improvementof the performance. Because of this we haveinvestigated the dynamic friction characteristics ofshock absorber oil and sliding parts, by the means

where hydraulic dissipation is not working yet. By
knowing this phenomenon, we have developed
different kind of oil types in order to generatevarious dynamic friction characteristics.
The influence of our developed oil types on theride and handling performance was tested andanalyzed in different vehicles. Subjective
assessments and objective measurement show thevehicle performance improvement, which can begenerated by using more advanced oil types.

１

Keywords: shock absorber, damping force,
dynamic friction, riding comfort, stability, vertical
dynamics

responsiveness is affected by elastic deformation
1）of oil（oil column rigidity）
especially at the point

Introduction

The damping force of the shock absorbers（SAs） is one of the elements making up thecharacteristics of the suspension system, and it
mainly affects vehicle stability and ride comfort. In
recent years, vehicles experience smallerfluctuations in the vertical direction even at highspeed as road conditions have improved. Therefore,
SAs are often operated at high frequencies withsmall amplitude, where the direction of SA motion
is frequently reversed and the SAs are mainly
operated at the low speed.
SA generates damping force by hydraulic oil
passing through valves attached to a piston, SA

where the direction of SAs is reversed. There are
new technologies that adjust hydraulic force, forexample, a hydraulic valve structure 2） and anelectronically controlled SA3） aiming for generating
appropriate damping force according to the roadsurface conditions by changing hydraulic force.
However, these technologies are limited in termsof their ability to improve the damping force when
the direction of motion is reversed because of deformation of oil. The current situation calls for SAs
that improve ride comfort and safety（stability,
steering response）when driving on good roads as
moved very low speed of SAs.

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KYB技報第62号

2021―４

Experience with on-vehicle subjective evaluationof SA performance indicates that ride feeling on
good roads can vary greatly with hydraulic oil
difference. In this paper, we focus on effect ofhydraulic oil on vehicle ride and handling. And then,we developed our own hydraulic oil to adjust thedamping force in the very low speed range where
direction of motion reverses, in order to clarify thefactors that influence the change in feeling.

２
2. 1

Fig. 1

Shock Absorber Design
 onventional damping force measurementCmethod and its problems

A conventional indicator of the performance of SA
is shown in Fig. 1. The right figure is a magnified
one of the surrouded area by the red square in theleft figure. Here, the performance of the SA isevaluated by a plotting the peak damping force
generated by the SA versus stroke speed, which iscalled “damping force - speed characteristic line
diagram”. Generally, it does not show speeds of
10mm/s or below. It is a quasi-static characteristics
and not suitable for the evaluation of dynamic
performance. Hence, in order to evaluate the SA
performance on good roads, we believed that it wasnecessary to evaluate the dynamic characteristics
from the time of reversing the moving direction to
the very low speed region, which has not been
noticed in the conventional performance evaluation.
2. 2 F luid resistance and friction force indamping force
Fig. 2 shows the structure and damping force

 amping force - velocity characteristics lineDdiagramcomponents of a twin-tube-type SA. As shown in
the figure, hydraulic force is generated by thepressure change as hydraulic oil passes throughthe valves4）, and friction force is generated fromthe oil seal, the rod guide, and the piston band.
The damping force of SAs is the sum of thehydraulic force and the frictional force. We believethat in the very low speed region, a region which
has not yet been extensively studied, the hydraulicforce would have a smaller impact on the damping
force, while the friction force, which has nonlinear
characteristics, would have a greater impact［Fig. 3］.
In order to understand the relative contributionof hydraulic force and frictional force to the total
damping force, we investigated the damping forcespeed characteristics. The measurement experiments
were conducted as follows.
First, we prepared a valve-less SA by removing
valves in order to generate no hydraulic force, andwe measured the damping force which now
consisted only the friction force. This enables us

Fig. 2 Twin-tube-type shock absorber structure and damping force component― 35 ―

The Power of Oil – Influence of Shock Absorber Oil on Vehicle Ride and Handling Performancefrom the neutral position of the SA. We found thatfor at least 50％ of the time on a typical good road,the displacement of the SA would be less than orequal to ± 1.5mm from the stroke center. We also
found that for 50％ or more of the time, the piston
speed was less than or equal to ± 30mm/s, andthe most frequent speed was less than or equal to± 10mm/s.

Fig. 3 R elationship between piston speed and dampingforce

to determine the speed characteristic of thehydraulic force from the difference between the

In these low-speed and low-amplitude regions,little hydraulic force is generated, and thedamping force consists mostly of the friction force.
In other words, on good roads, the damping forceof SAs is largely dominated by the friction force.

damping force - speed characteristic of a normal
SA and that of the valve-less SA. Fig. 4 shows thechanges in the proportion of hydraulic force andthe frictional force in the damping force as afunction of the piston speed. The figure showsthat the proportion of friction force is the highest
in the very low speed region where the piston
speed is 30mm/s or less.

Fig. 4

Fig. 5 xternal view of suspension displacementEbehavior measurement during actual vehicle
running

Fig. 6

 peed dependency of load ratio of hydraulicSpressure and friction force in damping force

Frequency distribution of piston speed

Next, we analyzed the behavior of the shock
absorber on the test course in order to determine
their functionality on a good road. In theexperiment, we drove a passenger car with adisplacement of 1.8L at 60km/h on a test courset h a t r e p r o d u c e d t h e u n e v e n r o a d s u r f a ceequivalent to a good road on a national highway.
During the experiment, a displacement sensor was
mounted in parallel to the SA, as shown in Fig. 5,
and the displacement was measured at 2-kHz
intervals, to calculate the frequency distribution of
the stroke speed and the displacement. Fig. 6
shows the frequency distribution of the SA speed
and Fig. 7 shows the displacement distribution

Fig. 7

 requency distribution of displacement fromF
neutral position

2. 3

E valuation methods and problems in the

low-speed operation region
In order to evaluate the dynamic characteristicsof the damping force, we use a plotting the force
generated by SA versus stroke speed when the― 36 ―

KYB技報第62号

SA is operated at a constant frequency andamplitude, as shown in Fig. 8. The low-speed
region around the zero point（surrounded by the
red square）in Fig. 8 is magnified in Fig. 9.
Damping force has hysteresis due to thecompressibility of the hydraulic oil1）, that works inthe form of oil column rigidity as shown in Fig. 10,2021―４

Friction adjustment using hydraulic oil has beenstudied in order to improve SA responsiveness,but the response which was observed as thegradient around zero point and the magnitude of
the friction force are simultaneously changed asshown in Fig. 11 in order to avoid the occurrenceof stick-slip. However, when this method is used,

and in the very-low-speed zone starting at thezero line where the direction of motion reverses,
the damping force of SAs is in the direction of

the responsiveness is improved, but at the sametime, the ride comfort is deteriorated, and thistrade-off characteristics have not been improved.

acceleration. We believe that the SA response canbe improved by reducing this zone.

Therefore, we thought that we can improve theresponsiveness of SAs without deteriorating theride comfort by adjusting only the gradient of thefriction without changing its magnitude as shownin Fig. 12.

Fig. 8F-V characteristics

Damping force100

Decelerationarea

Fig. 11

Accelerationarea

Decelerationarea

 echnology for changing the magnitude ofT
friction

0Damping force
response angle
Deceleration area

-100

Acceleration area
-50

Fig. 90Stroke Velocity, mm/s

50

F-V characteristic around the zero point

Fig. 12

2. 4

Technology for adjusting responsiveness

Friction influence and related components

In order to evaluate friction characteristics
while separating friction response and friction
force and develop our own hydraulic fluid
Fig. 10 Influence of oil column stiffness characteristics

technology, we developed our own device as― 37 ―

The Power of Oil – Influence of Shock Absorber Oil on Vehicle Ride and Handling Performance

shown in Fig. 13 and use it to examine theevaluation index5）.

direction of motion is reversed without changing
the average friction force. Fig. 15 compares theactual characteristics of the oils that we developedwith the conventional hydraulic oil characteristics.

Fig. 13

2. 5

Schematic of reciprocating force-tester

Response index

The Response index （RI） was defined as aquantitative index to evaluate the response offriction when the direction of movement was
reversed. Fig. 14 shows the range where we
extracted the RI in the time waveforms ofdisplacement and friction. F sa is the maximum
friction in the range from the start of motion up to
the phase of π/4 radians（that is, top and bottom
dead centers, π/2 to 3π/4）, and normalizes thedifference by the average friction. We divided thedifference by the average in order to obtain theacceleration-side RI using formula（1）.
Response index（RI）/Fave，＝（Fsa－Fave）
π/2≦θ≦3π/4

⑴

Fig. 15 C haracteristics comparison between conventionaloils and oils we developed

Hydraulic oil A is a high-response, and thehydraulic oil B is low-response, although themagnitude of friction is almost the same in eachcase. By changing the blending ratio of thesehydraulic oils, it is possible to adjust only the
response without changing the magnitude of thefrictional force. This technique enables us toinvestigate the effect of the responsiveness on thebasic performance in on-vehicle and bench tests.

４

Oil types Influence on Ride and Handling
Performance（Vehicle Assessment Result）

The new KYB oil types and their performanceFig. 14 Calculation concept of the Response index（RI）

variation on component level were analysed. Inorder to prove the impact of the oil types on
vehicle ride and handling performance, an objective
and subjective vehicle assessment is carried out by
an engineering service company specialized in
vehicle dynamics measurement and assessment.

We used our specially made friction test device
to develop our own hydraulic fluid technology

For this analysis, three sets of SAs with threedifferent oil types, Base（reference, not modified
from the original equipment）, KYB A/B 100％and KYB A/B 75％ are set up. The viscosity ofwhich can adjust only the response when thethe two KYB oils were adjusted so that it would

３

New KYB OIL types and their characteristics― 38 ―

KYB技報

第62号

Table 12021―４

Oil properties
Base

A/B75

A/B100Density（15℃）
Viscosity index
Kinematic viscosity（40℃）

3

0.83
167

0.85
168

0.86
152

mm2/s

Kinematic viscosity（100℃）

mm2/s

11.7
3.3

12.9
3.9

12.9
3.4

g/cm

be the same as the Base specification and theinfluence of kinematic viscosity and the magnitude

negative）. The vehicle speed was varied between
30-50km/h in steps of 5km/h and from 60-100km/h

of friction would not appear［Table 1］.
Apart from the oil, the SA hardware and tuning
are not changed. It should be mentioned that by notin steps of 10km/h. Measurements were conducted
3times on every lane and at every speed step, andall of the measurements were averaged for thechanging the tuning of the SA while changing theoil, the performance might not be optimally balanced
at vehicle level, since the tuning is made usually

evaluation. All accelerations, body modes andmovements were assessed for each lane, for low,high, and full speed range.

after choosing the oil. However, since the target of
this analysis is to isolate the oil impact on vehicle
performance, the tuning is not changed. Fig. 16
shows the damping force-velocity characteristics ofthe three different SA sets. It can be confirmed that
there is almost no difference in damping force
characteristics, and one would not expect that thevehicle ride and handling performance will besignificantly differing.

Fig. 17

4. 2

Vehicle measurement setup

Objective assessment results

The results of the analysis are shown in Fig. 1819. Compared the base configuration shown on thedotted line, the A/B 100％ oil shown on thedashed line exhibits a smaller body acceleration in
Fig. 16

the low-frequency region of 1-2Hz on and larger
body acceleration in medium-frequency region ofDamping force-velocity characteristics

4. 1 Objective assessment analysis conditions
The vehicle used for this analysis is a MercedesBenz A-Class（W177）which was equipped with

4-15Hz. The A/B 75％ oil shown on the solid line
exhibits a smaller body acceleration than the baseconfiguration in both the low- and mediumfrequency region.

various acceleration sensors on knuckle/wheel
hubs, top mount at all 4 corners and the seat rail.
The full sensor setup is shown in Fig. 17.

In general, there is a trade-off to be made whenusing valves to control the hydraulic force. If the
body vibration near sprung mass resonance

The test track on which the measurements havebeen carried out consist of different road surface
types: Nevada（smooth tarmac）, South Africa（harshfrequency of 1-2Hz is reduced to emphasize
safety, the body vibration of 4-15Hz increases. To
emphasize ride comfort, the opposite approach is
required6）. The characteristics are contradictory,
making it difficult to achieve both goals. Thesetarmac）and Country-Road（bad local highway）anda special lane with 30mm steps（3 positive and 3― 39 ―

The Power of Oil – Influence of Shock Absorber Oil on Vehicle Ride and Handling Performance

results show that by simply adjusting the friction
responsiveness while keeping the characteristics

adjusted to improve the riding comfort, it is at theexpense of safety（manoeuvrability and stability）,

of the damping force unchanged, it is possible toproduce a difference in the vibration levels toachieve both ride comfort and handling. The effectand vice-versa. We were able to enhance both theriding comfort and safety performance bychanging the hydraulic oil to adjust the dampingof friction response on the body vibration in theregion can be summarized as follows.
Compared to the base configuration, our A/B

force after the reversal of the direction of motion
in a region where it has been difficult to achieveboth goals. The results verified that this is an

100％, which has the highest responsiveness,changes the vibration in a way that emphasizes
safety. The A/B 75％ oil, which has an intermediate
responsiveness between the A/B 100％ and theimportant technique for meeting the requirements
for ride comfort and safety depending on the typeof vehicle and the customer’s preference.
4. 3 Subjective assessment

base configuration, does not change the vibration of
the 7-15Hz region, and yet decreases vibration in

The subjective evaluation of ride comfort andhandling/safety was made on different roads andthe 1-2Hz region. This oil can be said to improve
the stability of the vehicle body without impairing
the riding comfort. This improvement, which is
generally difficult to achieve with hydraulic valves,various conditions such as German motorway（with speed up to 180km/h）, well-used roads,
potholes, asphalt patches and state roads withconditions between brand new and well-used.

appears to be the result of the oil’s improved
responsiveness, which reduces the acceleration
zone in the very low speed range.
We investigated the effect of a SA equippedwith this more responsive hydraulic oil on an
actual vehicle, and we confirmed that it has asignificant impact on handling and ride comfort
performance.
Traditionally, when the hydraulic force isFurther the assessment was made on the test
course containing comfort tracks, simulated state
road, single obstacles, handling track, long straight.
As shown in Fig. 20, varying the responsivenessof the oil resulted in significant changes in ride
comfort and safety. In addition, we confirmed that
it is possible to modify characteristics such as“safety” and “comfort” depending on the purpose
and taste of the vehicle.
The subjective assessment of our A/B 100％ andFig. 18

Vertical Vehicle Body Acceleration for Nevada

Fig. 19 ertical Vehicle Body Acceleration for SouthV
Africa― 40 ―

Fig. 20

 esponse adjustment hydraulic fluidRperformance

KYB技報第62号

A/B 75％ oils compared to the base configuration
was also commented by the test driver as if avalve setting such as the bypass or bleed area of
the SA were modified. Driving the vehicle with thenew oils changes the vehicle dynamics like it has adifferent positioning e.g. a sport type derivate, or adifferent manufacturer tuning philosophy.

５

Conclusion

2021―４compromising ride comfort.

６

OutlookIn this paper, we proposed a technique for
improving the performance using ordinary
conventional SAs, but we anticipate that thistechnique will improve performance even further
if applied to a SA with the latest sated-of-the-art

1. W hen driving on good roads, SAs typicallyoperates in a very-low-speed region between

valve structure. In the future, in addition to
pursuing the practical application of these results,
we plan to further study on the relationshipzero（where the direction of motion is reversed）
and 30mm/s.
2. O
 n good roads, the very-low-speed region of the

between the friction response and vehicle
performance, and will continue to develop the
potential of this approach as a new performance

SA has a greater effect on the friction force
than the hydraulic force, which is very
important role of the ride comfort and handling.
3. T he conventional friction adjustment technique ina very-low-speed region of the SA, which involves
varying the magnitude of friction, has drawbackin that it negatively impacts the ride comfort.
4. The friction response was confirmed to be an
important factor suitable for safety and ride
comfort depending on the type of vehicle andcustomer preferences.
5. T he mechanism of our results is considered from
improvement in performance produced bycompensating for the damping force that is
generated by compressibility of the hydraulic oilin the acceleration zone part of the very-low-speed

enhancement technique.

region in which the direction of motion is reversed.
6. Using the KYB developed oils can open a new5）Kato, Shinji, and Shinya Sasaki. “Effects of hydraulic oil anddimension of creating variations and optimizing
the vehicle drive feel. As the result, we developed
a technique that adjusts only the friction response
and confirmed that it can improve safety without

various reciprocating sliding conditions.” Friction（2019）.

References
1）Ernst, Walter. “Oil hydraulic power and its industrial
applications.”（1960）.
2）K AMAKURA, Ryosuke, et al. “Development ofExternally-Mounted Shock Absorber with Adjustable
Solenoid Damping Force.”（2017）.
3）K linger, Florian, Johannes Edelmann, and Manfred
Plöchl. “Characterization and potential analysis ofpassive and（semi-）active suspension systems bymeans of equivalent suspension parameters.” 10th
International Munich Chassis Symposium 2019.
Springer Vieweg, Wiesbaden,（2020）.
4）Dixon, John C. The shock absorber handbook. John
Wiley & Sons,（2008）.
lubricant additives on dynamic friction properties under
6）Kaldas, Mina MS, et al. “Triple-control-mode for semi-

Author 

Dr. Shinji Kato
Senior Manager Suspension
Engineering Headquarters,
KYB Corporation, 2548, Dota,
Kani-shi, Gifu 509-0298, Japan,
E-Mail: katou-shin@kyb.co.jp― 41 ―

active suspension system.” SAE International Journal ofCommercial Vehicles 8.2015-01-0621（2015）.