Design and Analysis of a Tractor-Trailer Cabin Suspension


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Design and Analysis of a Tractor-Trailer Cabin Suspension

  1. 1. 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
  2. 2. Symposium on International Automotive Technology 2007 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
  3. 3. Symposium on International Automotive Technology 2007 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
  4. 4. Symposium on International Automotive Technology 2007 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
  5. 5. Symposium on International Automotive Technology 2007 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
  6. 6. Symposium on International Automotive Technology 2007 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
  7. 7. Symposium on International Automotive Technology 2007 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.
  8. 8. Symposium on International Automotive Technology 2007 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
  9. 9. Symposium on International Automotive Technology 2007 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 : Figure 28 : Subjective Rating of Suspended Cab Figure 29 : Measured Acceleration at Driver Seat