Design and Analysis of a Tractor-Trailer Cabin Suspension

Symposium on International Automotive Technology 2007
549
Paras Jain
Eicher Motors Ltd. Indore, India
Design and Analysis of
a Tractor-Trailer Cabin Suspension
ABSTRACT
This paper presents the work done to overcome the ride
problem of a 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 whereas for a truck, it is 2-2.5 Hz. In heavy
commercial vehicles, load carrying capacity is an
important selling parameter, which does not allow
softening the suspension beyond a limit. A unique four-
point suspension has been designed to achieve the low
ride frequencies of the cabin to improve the ride comfort.
This paper describes some of the insight and knowledge
gained from the effort to lead cabin suspension design
for a heavy commercial vehicle.
INTRODUCTION
Drivers of heavy trucks, especially tractor-trailers spend
12-14 h 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 feature because these vehicles
never driven by truck owners and also there is no
legislative requirement for the ride comfort. But today
the driver’s environment, especially cabin ride has
become an important factor in the vehicle marketability.
A better ride, including both reduced jounce and pitch
but particularly the latter, was sought by more operators
than any other single improvement. A cab suspension
not only provides the good ride but also improves the
cab life by isolating the cab from the frame and engine
vibrations, reduces the interior noise and make ride
much less sensitive for the 5th
wheel locations.
GENERAL LAYOUT OF CAB MOUNTINGS : The ideal
fully suspended cabin system would be based on a
three-point layout with the singular point at the front of
the chassis [1]. However, with a tilt cab, this is
SAE Paper No. 2007-26-047
generally unpractical due to the load bearing requirements
at the front end during tilting the cab. So the four-point
suspension is the best solution. A four-point suspension
can be subcategorized into two groups:
FRONT SUSPENSION: The basic considerations of front
mounting design of a titling cabin are to provide:
Method of Tilting
Front Cab Mounting, and
Front Vertical Travel
The existing front cab mounting, shown in Fig. 1, is a
bush type mounting having two torsion bars to provide
the cab tilting. The vertical deflections depend upon the
stiffness of the bushes. The new cabin suspension
replaced this arrangement by a vertical coil spring with
a damper on the each side of the cabin and a hydraulic
cylinder and pump replaced the torsion bar. The objective
of this replacement was to increase and control the
vertical motion of the cabin to improve the comfort.
Figure 1 : Existing Front Cab Mounting
Copyright © 2007 The Automotive Research Association of India, Pune, India

550
REAR SUSPENSION : The basic considerations of rear
suspension design are to provide:
General Cab Mounting Configuration
Cab Locking at Rear
Existing fixed type rear mounting shown in Fig. 2 rigidly
connects the cabin with the frame and has manual cab
locking arrangement. This arrangement was replaced by
a vertical coil spring with a damper on each side and
two lateral dampers to provide the roll and lateral
stiffness to cabin.
Design Process : Following steps was followed during
the cabin suspension design:
Subjective and Objective ride evaluation of existing
vehicle
Comparative ride study with benchmarked vehicles
Adams modeling of existing vehicle (without
suspension)
Adams model validation with test data
Target setting for ride comfort
Optimization of vehicle suspensions for ride comfort
Layout of cabin suspension, considering design
space
Adams Analysis
Development and validation
Ride Comfort Evaluation : There are two ways to
access the ride characteristics of any automobile vehicle.
Subjective evaluation, and
Objective evaluation
A panel of juries generally, performs subjective
assessment of ride comfort experienced during ride
evaluation. For assessing the ride, 10 various ride
parameters [2] considered and these parameters rated
by the panel on the scale of 10. An overall vehicle ride
rating was derived from these parameters. Three
competitive vehicles have been identified and the same
panel has performed subjective ride evaluation. Fig. 4
shows the subjective ride rating of first concept vehicle
against three benchmarked vehicles.
Figure 2 : Existing Rear Cab Mounting
Figure 3 : Typical Analog Computer Model
Fig. 3 shows an analog model of the newly designed
cab suspension. It has two vertical coil springs with
dampers at front along with a front tilting arrangement
to provide the cabin tilting, two vertical coil springs with
dampers at rear to control rear vertical motion and two
lateral dampers to control the cabin roll and lateral
motion.
Figure 4 : Subjective Rating of First Concept Vehicle
Against Benchmark

551
Further to verify this subjective rating an objective
evaluation has been performed on all four vehicles.
Objective Evaluation : An objective evaluation for ride
comfort basically involves the measurement of
accelerations (g’s level) at various locations of cabin
especially driver seat, driver seat back and at driver feet.
For a commercial vehicle, these vibrations are measured
in vertical (Z-direction) and in fore-aft (x-direction)
directions. Lateral direction is optional as these vibrations
are not significant for ride assessment in commercial
vehicles.
MEASUREMENT INSTRUMENTATION
ACCELERATION AND SIGNAL CONDITIONING : Ride
vibrations measured using SEIKO accelerometers with an
input range of +/-10 m/s2
. SOMAT e-DAQ was used for
signal conditioning with 32 channels. Sixteen uni-axial
and two tri-axial accelerometers were used for data
collection.
Data Analysis : As ride vibrations are limited to the 1Hz
to 25 Hz frequencies range, all the data has been
analyzed in this band only. The acquired vibration 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 data and help to determine
the nature of the ride phenomenon.
ISO-2631guidelines [3] have been referred for defining
the ride comfort level of the vehicle. It was found that
these guidelines show better representation of subjective
evaluation than any other method.
ISO-2631 [3] defines the tolerable acceleration level for
different exposure time at different frequency and
direction. Fig. 5 and Fig. 6 show the RMS acceleration
level at different frequencies and exposure duration for
vertical and fore and aft directions respectively.
Sensitivity of human body is dissimilar at different
frequency of acceleration, e.g. for vertical vibrations
human body is most sensitive for frequency range of
4-8 Hz where as in fore-aft 1-2 Hz frequency range.
Thus it is required to normalize acceleration data for
different frequencies. Fig. 7 shows the weighing curves
for vertical and fore-aft directions used to normalize the
acceleration data in the frequency band of 1-25 Hz.
Figure 5 : ISO-2631 Guide for Vertical Accelerations
Figure 6 : ISO-2631 Guide for Fore-aft Accelerations
Acceleration data was acquired at various locations of
cabin for objective evaluation and these data plotted as
per ISO-2631 [3] guidelines. Fig. 8 and Fig. 9 shows
the driver seat accelerations in vertical and in fore-aft
directions respectively, for first concept vehicle against
three benchmarked vehicles on ISO-2631 [3] guidelines.
The benchmarked vehicles selected for study were
equipped with cabin suspension whereas Eicher vehicle
was without cabin suspension. The first natural frequency
of the existing cabin was around 5 Hz for the pitching
mode.
It has been noticed that acceleration level at driver seat
of BM-1 and BM-2 were below the 8 h tolerable line
Figure 7 : Frequency Weighting Curve

552
whereas BM-3 and Eicher vehicle were crossing the
4 h tolerance line. These objective evaluation shows
good level of correlation with subjective rating shown in
Fig. 4.
Cabin
Tractor Frame
Front Suspension
Rear Suspension
Trailer Suspension
Engine and transmission
Cargo body with payload
Actuators
Cabin Sub System : A rigid representation of cabin 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. Fig. 10 shows the
Adams model of existing cabin used for validation.
Chassis : The dynamic behavior of a heavy truck is
very much affected by the frame’s fundamental mode.
To capture the affect of chassis stiffness a flex model
of the chassis has been used. Fig. 11 shows the MNF
model of the frame used for the analysis.
Figure 8 : Driver Seat Accelerations in Vertical
Direction
Figure 9 : Driver Seat Accelerations in Fore-aft
Direction
These ride evaluations demand to develop a unique
system to cater the vehicle’s ride comfort requirement.
A well-designed four-point cabin suspension along with
optimized vehicle suspensions can provide the good
cabin ride comfort. For optimizing the various parameters
of the cabin and the vehicle suspensions, it required to
develop a numerical simulation model of the Tractor-
trailer.
Building the Adams Model : MSC: Adams view 2005
[4, 5, 6] 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. The
following is the brief key list of aggregates used for
defining the vehicle:
Figure 10 : Quick Prediction Model of the Cabin
Figure 11 : MNF Model of the Frame

553
Vehicle Suspension: Front and Rear suspension of the
vehicle was modeled using SAE 3-Link method and
parameterized on hard points. Adams Leaf tool module
was used for calculating the stiffness of the bushes in
the suspension. The suspension systems consist of axle,
leaf springs, and dampers. Fig. 12 shows the Adams
model of tractor’s front and rear suspension with trailer
suspension. Front and rear suspensions modeled for
variable stiffness. Leaf springs were validated separately
with test data, and the force-velocity data was fitted to
a curve and used to define an Adams spline.
Cargo Body : Cargo body and payload modeled as
rigid block with calculated mass and inertial properties.
Fig. 14 below shows the Adams model of cargo body
with payload.
Engine and Cargo body Engine and transmission
modeled as two rigid bodies and attached to the frame
by flexible joints. Calculated mass and inertia properties
were used to define the Adams model (Fig. 13).
Figure 12 : Adams Model of Vehicle Suspension
Systems
Figure 13 : Engine and Transmission
Figure 14 : Cargo Body and Payload
Actuators : An eight poster virtual actuators have
modeled for wheel input motion of different road condition.
Three different road profiles were acquired and used as
actuators input.
Vehicle : Virtual vehicle was prepared by merging all
aggregate models and validated with test data. Fig. 15
shows the Adams model of the vehicle on an 8-poster
used for the validation of numerical simulation.
Figure 15 : Adams Model of Tractor-trailer
Following data were measured and used for building the
model.
Vehicle suspension hard points and stiffness curves
Geometry of Chassis and cargo body
Force vs. Deflection data for bushes
Mass and Inertia properties of cabin
Force vs. velocity data for damper’s spline
Basic calculated stiffness and damping data for cab
suspension
Validation of the Model: In order to validate the model,
extensive test data was acquired. The truck was
instrumented with 22 accelerometers and then driven on
three different road profiles and at two different speeds.
The vehicle modeled as per the existing configurations

554
and validated with test data. Vehicle axle’s load and
accelerations level at driver seat have been measured
and compared with calculated values.
Fig. 16 shows the various axle loads calculated from
Adams model. These axle weight calculated at contact
patch of the tire after the model got static equilibrium.
The total calculated GVW was 42.28 ton against
measured 43.3 ton. Table- I below shows the percentage
difference between calculated and measured axle loads.
The differences of axle loads are within 5 %, this
correlation confirms the mass and inertia properties of
the Adams model as per actual vehicle.
Further to verify the model, driver seat accelerations
calculated from Adams and measured in the field by
using accelerometers. Fig. 17 shows the calculated and
measured accelerations at driver seat. These plots show
a good level of correlation and confirm the validation of
the model.
Target Setting for Ride Comfort : Any analysis requires
the target values to achieve but there is no simple value
or a procedure by which the ride comfort can be
defined. ISO-2631 [3] guidelines were used for target
setting in numerical simulation.
We targeted the cabin acceleration level should be below
the 8 h tolerable line of ISO-2631 [3] in vertical and
horizontal direction.
Optimization of vehicle suspensions for ride comfort:
Design sensitivity analyses were performed on various
vehicle parameters for minimum driver seat vertical
accelerations. Following parameters were considered for
ride sensitivity analysis.
Tractor front suspension Stiffness
Tractor front suspension damper
Fifth wheel location
Tractor Rear Suspension Stiffness
Tractor’s Front Suspension : Fig. 18 shows the
acceleration level at driver’s seat for different value of
front suspension stiffness. Analysis was performed for
+/-10 % values of existing suspension. Stiffness against
minimum acceleration level has been used for further
optimization of other parameters.
Tractor Front Damper : Fig. 19 shows the driver seat
acceleration for existing damper and new optimized
dampers. Force vs. velocity spline was used for defining
the damper’s characteristics.
Table- I : Difference in Measured and Calculated
Load
Figure 16 : Calculated Axle Load
Figure 17 : Driver Seat Acceleration Level for Test and
Simulation Data
Figure 18 : Driver Seat Accelerations for Different
Front Suspension Stiffness

555
Adams Analysis : After doing successful optimization
of vehicle front suspension, the fixed type cab was
replaced with suspended cab. Fig. 21 shows the Adams
model of tractor-trailer vehicle modeled with suspended
cab. Basic calculated spring stiffness and damping
values used in first iteration. DOE has been performed
for cab suspension springs for targeting the natural
frequencies between 1.8 Hz to 2.5 Hz keeping linear
damping values. Analysis carried out for three different
road input motion acquired on Tar road, Cemented road,
and rough/pave road on different vehicles speed i.e.
40 km/h and 60 km/h.
After achieving the desired natural frequencies, linear
damping values replaced with non-linear damping (Force-
velocity). No of iterations performed for reduction
of acceleration level for the frequencies range between
1-25 Hz.
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.
Tractor Rear Suspension : Effect of tractor rear
suspension stiffness on cabin ride was not significant.
Proto Vehicle has been modified with optimized
suspension and subjective ride performance evaluation
has been performed. It has been observed that vehicle
ride rating improved by 2 points on subjective scale. To
improve it further fixed type cabin mounting replaced with
cabin suspension arrangement. Front bush type mounting
has been replaced with two coil springs with dampers
and new cab titling system. The front suspension has
been designed to control the vertical motion and to
provide the required cabin tilting. The existing torsion bar
arrangement has been replaced by hydraulic cylinder and
pump. Rear fixed mounting has been replaced with 2
vertical coil springs with dampers and 2 lateral dampers
for controlling the roll and lateral motion of the cabin.
Fig. 20 shows the Adams model of the suspended
cabin.
Figure 19 : Vehicle Front Damper Optimization
Figure 20 : Fully Suspended Cabin
Figure 21 : 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 cabin
suspension effectiveness can be judged by comparing
the vertical acceleration level measured with and without
cabin suspension.
With suspension, the cab natural frequencies changed
to around 1.8 Hz to 2.2 Hz corresponding to tractor
bounce and pitch mode.
Fig. 22 shows the calculated PSD of driver seat vertical
acceleration plotted on ISO-2631 [3] scale for the laden
cabin and the cement road input motion at 40 km/h.
Similar plot was generated for other two roads.
Comparing the different road spectra, it was noticed that
general shape of the response spectra was same.
Fig. 23 shows the driver seat acceleration for average
cabin loading i.e. four people in cabin and for the cement
road. In both loading condition the vertical accelerations
were well below the 8h tolerance line.
Fig. 24 contains the vertical acceleration calculated at
driver seat for three different road input and with four
persons in cabin. Only for pave road input the
accelerations level are marginally crossing the eight-hour
tolerance curve.

556
Fig. 25 shows the calculated driver seat acceleration for
laden cab and half loaded cabin against measured data
of the BM-1 which have the best ride comfort among
four vehicles. Significant improvement was noticed with
suspended cabin.
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.
DEVELOPMENT AND VALIDATION
After optimizing the suspension parameters for minimum
acceleration level it is required to verify the same by
physical testing. Based on the analysis feed back,
physical model of the suspended cabin has been
developed and it has been evaluated on subjective and
quantitative bases against existing cabin. Fig. 26 and
Fig. 27 shows the front and rear suspension of proto
vehicle respectively.
Figure 22 : Driver Seat Vertical Acceleration on ISO
Scale
Figure 23 : Driver Sear Vertical Motion on ISO Scale
for Four People in Cabin on Cemented
Road
Figure 24 : Driver Sear Vertical Motion on ISO Guide
for Laden in Cabin on Three Different Road
Figure 25 : Vertical Vibration Level Comparison level
for Modified Vehicle with Suspended Cab
Figure 26 : Front Cabin Suspension
Figure 27 : Rear Cabin Suspension

557
Fig. 28 shows the subjective comfort rating of the
suspended cabin against benchmarked vehicles.
Significant improvement in cabin ride comfort has been
noticed.
Fig. 29 shows the objective evaluation results of ride
comfort. It can be seen that acceleration level are well
below the 8 h ISO curve. This testing result is also
having fair degree of correlation with numerical
calculations.
CONCLUSION
Initial ride evaluation and driver’s feedback has indicated
the universal desire for great ride comfort. A significant
improvement in ride can be achieved through the
application of low frequency system at cabin mounting
areas. The discussed methodology is very effective for
improving the ride comfort of any commercial vehicle.
This methodology can also be used for other passenger
vehicles. Numerical simulation methodology discussed
here can be used for predicting the dynamic behavior
of the any vehicle in different conditions. ISO-2631
guidelines along with subjective evaluation give fair
representation of ride in quality and quantitative manner.
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. Sternberg, E. R., “Heavy-Duty Truck Suspension”
SAE Technical Paper SP-402
2. Measurement and Presentation of Truck Ride
Vibration- SAE J1490, Sept. 1999
3. ISO-2631 guidelines
4. MSC-Adams 2005 R1 Help Manual
5. Prashant S. Rao, et al., “Developing an ADAMS
Model of an Automobile Using Test Data”, SAE
Paper No. 2002-01-1567
6. Davis Anderson and Gregory Schade, “Tractor/Semi
Trailer Ride Quality Prediction Using a Template
Based Approach”, ADAMS Users Conference, 2001
CONTACT
Paras Jain
Eicher Motors Pithampur
E-mail : Pjain1@eicher.co.in
Figure 28 : Subjective Rating of Suspended Cab
Figure 29 : Measured Acceleration at Driver Seat

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