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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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