Msc User Conference, Pune, India 2006

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Cabin Ride Comfort Improvement of a Tractor Trailer Vehicle
Paras Jain
Eicher Motors Ltd. (CV Unit)
ABSTRACT
This paper presents the work done to overcome the
ride problem of Tractor Trailer vehicle. Ride of any
vehicle can be improved by maintaining the low
frequency of its suspension. The typical target
frequencies for a car are 1-1.5 Hz where as for a
truck it is 2-2.5Hz. In heavy commercial vehicles
load carrying capacity is an important selling
parameter which doesn’t allow softening the
primary suspension beyond a limit. A cabin rear
suspension has been designed to achieve the low
ride acceleration of cabin to improve the driver ride
comfort in the cabin. This paper has been written to
share some of the insight and knowledge gained
from the effort to lead cabin suspension design for a
heavy commercial vehicle at Eicher Motors Ltd.
INTRODUCTION
Heavy trucks especially tractor trailer vehicle
drivers spends 12-14 hr a day on wheels. A good
ride is prime requirement for these long haulage
trucks. Few years back, in India truck ride comfort
was seldom considered as a vehicle selling point
because these vehicles never driven by truck owner
and also there is no legislative requirement for the
ride comfort. But now a day’s driver’s environment
especially cabin ride is an important factor in
vehicle feature list. A better ride, including both
reduced jounce and pitch but particularly the latter,
was sought by more operators than any other single
improvement. A cabin suspension not only provides
the good ride but also improves the cab life by
isolating the cab with frame and engine vibration
and also reduces the interior noise.
MAIN SECTION
EXISTING LAYOUT OF FIXED CABIN
MOUNTINGS
FRONT MOUNTING
The front mounting of any tilting cab is required to
design considering:
• Method of Tilting
• Front Cab Mounting
The present cabin is using a pair of torsion bar to
achieve the cabin tilting. As the name suggest the
system basically consist of 2 spring steel bar
Figure 1: Present Torsion Bar Arrangement
which, when the cab is seated on its mounting, are
twisted inside and form a torsional reaction between
the fixed frame bracket and the lower cab structure
members. When the cab lock released the reaction
cause a vertical upward movement on the cabin and
thus with a minimum of manual effort the whole
can be easily lifted. The configuration is relatively
cheap, simplest, and light weighted. Figure 1 shows
the present torsion bar arrangement provides cabin
tilting along with the rubber bush type mounting.
The dynamic vertical movement of the cabin is
equivalent to the bush deflection.
Rear Mounting
A basic rear suspension design consideration is to
provide the arrangement for:

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• General Cab Mounting Configuration
• Cab Locking at Rear
Cabin Mounting Configuration
Figure 2 shows the conventional cabin rear
mountings consist of fixed rear mounting poster
assembly (RMPA) which got bolted rigidly on
chassis, provides two rubbers isolated seat on which
cabin got rested.
Locking Arrangement
The AIS-029 which is equivalent to E.C.E. R29
requirements state that two independent cabin
locking arrangements are incorporated on any
tilting, cabin over truck application. Considering
front torsion bar arrangement it is required to
provide the secondary cab locking. The present cab
locking shown in figure 3 is a mechanical type cab
locking. The arrangement provides the cabin
locking in two stages. First it locked automatically
with the cabin down position. In second stage one
has to rotate the lever downward to provide the
complete locking. ECE R29 also states that during
the test the component by which the cabin is
secured to the chassis frame may be distorted but
should not be detached from the chassis.
Figure 2: Existing Cab Rear Mounting
Figure 3: Existing Cab Rear Locking Arrangement
NEW REAR SUSPENDED CABIN LAYOUT
Front Mounting
The torsional bar arrangement of front mounting
has been replaced with hydraulic tilting
arrangement. This is a very common method of
cabin tilting for the heavier type cab consist a
hydraulic pump and a ram although in some
application two rams are used. This system
eliminate the constant loading on the cabin floor
structure occurs due to torsion bar reactions. This
also eliminated the constant upward movement of
cabin which causes the poor cabin ride. We will
discuss the effect of torsion bar on ride comfort in
more detail in analysis section.
Due to the mechanical advantage of this system
there are no operational restraints as allied to the
torsion bar but is more expensive and by the use of
hydraulics could increase the maintenance
requirements.
Rear Mounting
The fixed type cabin rear mounting has been
replaced by a cabin rear suspension system. The
suspension consists of 2 vertical coils over shocks
to control the vertical movement of cabin and lateral
bar has been provided to control the cabin roll and

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lateral movement. Figure 4 shows the designed rear
cabin suspension.
Figure 4: Cabin rear suspension
The manual cab locking arrangement has been
replaced with hydraulic locking arrangement which
operate through the discussed hydraulic pump.
OPTIMIZATION OF SUSPENSION
PARAMETER
MSC: Adams/View 2005 r1 has been used for the
dynamic analysis of vehicle with cab suspension.
Aggregate based modeling approach has been used
to develop the vehicle model. Both aggregate model
verification and vehicle level verification with test
data approach has been used. A virtual 8 poster
actuator has been modeled for displacement input at
tire contact patches for correspond to various road
conditions. Three different road profiles have been
considered for motion input at tire. The following is
the brief key list of aggregates used for defining the
vehicle:
1. Cabin with Suspension
2. Tractor Frame
3. Front Suspension
4. Rear Suspension
5. Trailer Chassis
6. Trailer Suspension
7. Cargo body with payload
8. Engine and transmission
9. Actuators
Cab Sub System
A rigid representation of cabin with cab suspension
with measured mass and inertia properties has been
used for initial quick prediction of work. A flex
body representation has also been used for final run.
Figure 5: Suspended Cabin
• Chassis - The dynamic behavior of a heavy
truck is very much affected by frame’s
fundamental mode and should be used a flex
model for the analysis. A MNF model of the
frame has been used for assembly.
Figure 6: Chassis
• Vehicle Suspension

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Front and Rear suspension of the vehicle has
been modeled using SAE 3-Link method and
has been parameterized on hard points. Adams
Leaf tool module has been used for finding the
bushes stiffness of suspension. The suspension
systems consist of axle, leaf springs, and
dampers. Figure 7 shows the Adams model of
tractor’s front and rear suspension with trailer
suspension. Front and rear suspension modeled
for variable stiffness. Leaf springs were
validated separately with test data, and the
force-velocity data has fitted to a curve and used
to define an Adams spline.
Figure 7: Vehicle Suspension Systems
• Engine and Cargo body
Engine and transmission modeled as a sub-
system and model as two rigid bodies and
attached with flexible joints and model has
updated as per calculated mass and inertia
properties.
Figure 8: Engine
Figure 9: Cargo Body
• Actuator:
8 poster virtual actuators have been modeled for
simulating the different road condition.
Different road profiles has acquired and used as
actuators inputs.
• Vehicle:
Virtual vehicle has prepared by merging all
these aggregate models and validated with test
data. Figure 10 below shows the Adams model
of the vehicle with existing fixed type
mountings used for the validation of numerical
simulation.

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Figure 10: ADAMS Vehicle
Following data were measured and used for
building the model.
• Hard Points of the for suspension and
mountings
• Geometry of Chassis
• Force vs. Deflection data for bushes
• Suspension hard points and deflection curves
• Mass and Inertia properties
• Damper force vs. velocity data
• Basic calculated stiffness and damping data of
cab suspension
VALIDATION OF THE MODEL
In order to validate the model extensive test data
was acquired to understand the vehicle behavior.
The truck was instrumented by 22 accelerometers
and then driven on three different roads and for two
different speeds.
Vehicle has been modeled as per the existing
configuration and verified with test data. Vehicle
Axle’s load and Acceleration level at driver seats
have been measured and same data has been
generated from simulation. Fine tuning has been
done by changing the mass and inertia properties of
systems.
Figure 11 below shows the various axle loads
calculated and verified with measured data. This
correlation confirms the mass and inertia properties
of the vehicle as per actual vehicle.
Figure 12 shows the driver seat acceleration
calculated from Adams and measured on field.
These plots show a good level of correlation and
this confirm the validation of the model.
Figure 11: Axle load from Simulation and Measured
Figure 12: Driver Seat Acceleration Level for Test and
Simulation Data
TARGET SETTING FOR RIDE COMFORT
Any analysis required the target values to achieve.
Unfortunately there is no single value or procedure
which can define the ride comfort for all vehicles in
all running condition. There are 2 ways to assess
the characteristics of ride vibration of a vehicle
during its operation
Subjective Evaluation:
Subjective assessment of ride vibration experienced
by drivers during ride evaluations is generally
performed by a panel of drivers and passengers.
Objective Evaluation:
Objective assessment used to define the ride in
quantitative manner. This generally contains the
acceleration data at various locations. Ride
vibration for any commercial vehicle is measured in
vertical and fore-aft direction at the locations of:
• Driver Seat
• Co-Driver Seat
• Passenger Seat or Berth (if there is any)

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The measurements in lateral direction (Y-axis) are
optional as these vibrations from ride assessment
standpoint are seldom significant in commercial
vehicles. For defining the ride quality in complete
manner it is recommended to perform both
objective and subjective testing with good level of
correlations.
MEASUREMENT INSTRUMENTATION
Data Analysis
As ride vibrations are limited to the 1 to 25 Hz
frequency range, all data has been analyzed in this
band only. The acquired ride data in time domain
has converted into the frequency domain using Fast-
Fourier transform (FFT) with suitable window and
percentage overlapping. Frequency domain record
is useful for both to verify the validity of the data
and help to determine the nature of the ride
phenomenon.
ISO-2631 standard has been referred for defining
the ride level of the vehicle. We found that this
method is most suitable for defining the ride quality
of vehicle and having the best correlation with
subjective rating.
ISO 2631 define the human comfort and discomfort
level at different frequency, direction and exposure
time.
Figure 12 and 13 below shows the vibration level of
(RMS) at different frequency and exposure duration
for vertical and fore and aft direction respectively.
Figure 13: ISO-2631 Guide for vertical accelerations
Figure 14: ISO-2631 Guide for fore-aft accelerations
We targeted the cabin acceleration level should be
below the 8 Hr. comfort line in vertical and
horizontal direction.
ADAMS ANALYSIS
After doing successful correlation of the Adams
model with test data, fixed type cab has been
replaced with rear suspended cab. Basic calculated
spring stiffness and damping values used for first
iteration. Sensitivity analyses have been performed
on various vehicle parameters for minimum driver
seat vertical acceleration. Following parameter has
been considered for ride sensitivity analysis.
• Tractor front suspension Stiffness
• Tractor front suspension damper
• Fifth wheel location
• Cabin Front mounting bushes
• Tractor Rear Suspension Stiffness
• Cabin Suspension’s Damper and Spring
Stiffness
Tractor’s Front Suspension: Figure 15 shows the
acceleration level at driver’s seat for different value
of front suspension stiffness. Analysis had been
performed for +/-10 % values of existing
suspension. Stiffness against minimum value has
been used for other DOE.

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Figure 15: Driver Seat Acceleration for Different Front
Suspension Stiffness
Tractor Front Damper: Figure 16 shows the driver
seat acceleration for existing damper and new
optimized dampers.
Figure 16: Vehicle Front Damper Effect
Fifth Wheel Location: It has been observed that
fifth wheel location plays an important role in
vehicle ride comfort but due to design space
constraint it has not been optimized further.
Cabin Front Mounting Bushes: Driver seat
acceleration marginally by stiffening the front
mounting bushes.
Rear Cabin Suspension: After optimizing the all
existing vehicle parameter the rigid mounted cabin
has been replaced with the suspended cabin with
hydraulic tilting arrangement instead of torsion bar.
First DOE has been performed on coil spring
stiffness for linear damping value and optimized the
stiffness for cabin natural frequencies. The Targeted
natural frequencies were 2.5 to 3 Hz for cabin and
1.5Hz to 2 Hz for coil spring.
After achieving the desired natural frequencies
linear damping values replaced with non-linear
damping (Force-velocity) forces. No of iteration
performed for reduction of acceleration level for the
frequencies range between 1-25 Hz.Three Different
roads Tar Cemented and Rough profile have been
considered for wheel input motion. Model has been
optimized for rated payload and for 2 different
cabin loads. 4 people inside the cabin and 8 people
inside the cabin.
Figure 17 and 18 below shows the measured
acceleration level of existing vehicle in comparison
with benchmarked vehicle on ISO 2631 scale in
vertical and in fore-aft direction respectively. Three
vehicles have been identified for the benchmarking.
Acceleration data have acquired on these vehicles
under same test conditions. Although all
benchmarked vehicles shown higher accelerations
level as per ISO 2631 standard but EML vehicle
had highest among them in both vertical and fore aft
direction.
Figure 17: Existing Vertical Vibration Level
Figure 18: Existing Fore-Aft Vibration Level

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Figure 19 below shows the Adams model of tractor
trailer vehicle developed for suspended cab. Consist
of 2 vertical coils over dampers and a Panhard rod
for lateral control. Existing manual locking has been
replaced with hydraulic locking and torsional bar
system replaced with hydraulic tilting ram.
Figure 19: Vehicle with Suspended Cab
RIDE COMFORT STUDY
In the final analysis, ride improvement system
judged on the basis of comfort improvement it
provides. A good indication of cab suspension
effectiveness has judged by comparing the vertical
acceleration level with measured with cab
suspension and without cab suspension.
Figure 20 shows the PSD of acceleration data on
driver seat in vertical direction plotted on ISO 2631
standard for the fully loaded cabin running on
cemented road at 40 kmph. Similar plot has also
been generated for other 2 roads. Comparing the
acceleration for different road spectra it has been
noticed that general shape of the response spectra
reserved.
Figure 21 shows the driver seat acceleration for
average cab loading i.e. 4 people in cabin running
on cemented road. In both loading condition the
vertical acceleration are well below the 8-hr line.
Figure 22 contain the vertical acceleration plot
calculated at driver seat for three different road
input and with 4 person load in cabin. Only for
Belgian Pave road profile the acceleration levels are
marginally crossing the 8 hr ISO-2631 level.
Figure 20: Driver Seat Vertical Acceleration on ISO Guide
for Laden Cabin
Figure 21: Driver Sear Vertical Motion on ISO Guide for
4person in Cabin on Cemented Road
Figure 22: Driver Sear Vertical Motion on ISO Guide for
4person in Cabin on 3 different Road

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Similar acceleration reduction has also been
observed at other location i.e. co-driver seats, lower
berth, upper berth, driver feet, cab front and rear
mountings.
RESULTS
After optimizing the suspension parameters for
minimum acceleration level it is required to verify
the same by physical testing. Both subjective and
quantitative testing has been performed on new
suspended cabin as well as on existing cabin. Ride
vibrations measured using SEIKO accelerometer
capable of taking acceleration +/-10 m/sec^2 with 5
% transitivity. A 32-channel SOMAT e-DAQ used
for signal conditioning. 16 Uni-axial and 2 tri-axial
locations identified and acceleration measured in
both vertical (z-direction) and fore-aft (x-direction).
Figure 23 and 24 indicate the comparative overall
subjective rating of EML tractor vehicle with
benchmarked vehicle for fixed cab and suspended
cabin respectively. Significant improvement has
been observed in cabin ride comfort. The overall
rating calculated based on the subjective rating on
the scale of 1 to 10 given by 4 different people for
10 different parameters e.g. impact damping,
pitching, roll, lateral shack and vertical vibration
etc.

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Figure 23: Subjective Rating for Existing Vehicle
Figure 24: Subjective Rating for Modified Vehicle
It is necessary that every subjective rating should be
backed by objective testing. Acceleration data has
been acquired at various locations and compare
study has done. Figure 25 and 26 shows the vertical
acceleration at driver seat for existing vehicle and
for modified vehicle respectively. A good
correlation has been noticed in subjective and
objective evaluation of ride comfort.
Figure 25: Measured Driver Seat Acceleration for Existing
Vehicle
Figure 25: Measured Driver Seat Acceleration for
Modified Vehicle
CONCLUSION
The above quantitative data substantiate that a
significant ride comfort improvement has been
made. It can be seen that cab suspension isolates the
cabin and driver much better than the fixed
mountings when compared under identical
conditions. cab suspension not only improved the
driver comfort but also increase the cab life as it
also filtered out the forces coming from chassis and
engine vibrations. After successful optimization of
cab suspension for ride comfort it was necessary to
do the analysis for handling also. A Handling
analysis has done for considering ISO single lane
change and ISO- double lane change.

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ACKNOWLEDGMENTS
The author would like to thank Eicher Motors for
authorization of publication of this paper.
The author is thankful to Mr. Rakesh Grover
(Deputy General Manager, Eicher Motors) for his
support and guidance during this work.
REFERENCES
1. J. I.Ribartis, J. Aurell, and E. Andersers -Ride
Comfort Aspects of Heavy- SAE Technical
Paper No. 781067
2. E. R. Sternberg- Heavy-Duty Truck Suspension-
SAE Technical Paper no. SP-402
3. Prashant S. Rao, Davis Roccaforte, Ron
Campbell, and Hao Zhou- Developing an
ADAMS Model of an Automobile using Test
Data, SAE Technical Paper Series 2002-01-
1567.
4. Davis Anderson, Gregory Schade- Tractor/Semi
Trailer Ride Quality Prediction Using a
Template Based Approach-2001 ADAMS Users
Conference.
5. Measurement and Presentation of Truck Ride
Vibration- SAE J1490 Sep 1999
6. MSC-Adams 2005 R1 Help Manual

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