Design and Analysis of a Tractor-Trailer Cabin Suspension

Symposium on International Automotive Technology 2007

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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 rideproblem of a Tractor-trailer vehicle. Ride of any vehiclecan be improved by maintaining the low frequency of itssuspension. The typical target frequencies for a car are1-1.5 Hz whereas for a truck, it is 2-2.5 Hz. In heavycommercial vehicles, load carrying capacity is animportant selling parameter, which does not allowsoftening the suspension beyond a limit. A unique four-point suspension has been designed to achieve the lowride frequencies of the cabin to improve the ride comfort.This paper describes some of the insight and knowledgegained from the effort to lead cabin suspension designfor a heavy commercial vehicle.

INTRODUCTION

Drivers of heavy trucks, especially tractor-trailers spend12-14 h a day on wheels. A good ride is primerequirement for these long haulage trucks. Few yearsback, in India truck ride comfort was seldom consideredas a vehicle selling feature because these vehiclesnever driven by truck owners and also there is nolegislative requirement for the ride comfort. But todaythe driver’s environment, especially cabin ride hasbecome an important factor in the vehicle marketability.A better ride, including both reduced jounce and pitchbut particularly the latter, was sought by more operatorsthan any other single improvement. A cab suspensionnot only provides the good ride but also improves thecab life by isolating the cab from the frame and enginevibrations, reduces the interior noise and make ridemuch less sensitive for the 5th wheel locations.

GENERAL LAYOUT OF CAB MOUNTINGS : The idealfully suspended cabin system would be based on athree-point layout with the singular point at the front ofthe chassis [1]. However, with a tilt cab, this is

SAE Paper No. 2007-26-047

generally unpractical due to the load bearing requirementsat the front end during tilting the cab. So the four-pointsuspension is the best solution. A four-point suspensioncan be subcategorized into two groups:

FRONT SUSPENSION: The basic considerations of frontmounting 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 abush type mounting having two torsion bars to providethe cab tilting. The vertical deflections depend upon thestiffness of the bushes. The new cabin suspensionreplaced this arrangement by a vertical coil spring witha damper on the each side of the cabin and a hydrauliccylinder and pump replaced the torsion bar. The objectiveof this replacement was to increase and control thevertical motion of the cabin to improve the comfort.

Figure 1 : Existing Front Cab Mounting

Copyright © 2007 The Automotive Research Association of India, Pune, India


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REAR SUSPENSION : The basic considerations of rearsuspension design are to provide:

� General Cab Mounting Configuration

� Cab Locking at Rear

Existing fixed type rear mounting shown in Fig. 2 rigidlyconnects the cabin with the frame and has manual cablocking arrangement. This arrangement was replaced bya vertical coil spring with a damper on each side andtwo lateral dampers to provide the roll and lateralstiffness to cabin.

Design Process : Following steps was followed duringthe cabin suspension design:

� Subjective and Objective ride evaluation of existingvehicle

� Comparative ride study with benchmarked vehicles

� Adams modeling of existing vehicle (withoutsuspension)

� Adams model validation with test data

� Target setting for ride comfort

� Optimization of vehicle suspensions for ride comfort

� Layout of cabin suspension, considering designspace

� Adams Analysis

� Development and validation

Ride Comfort Evaluation : There are two ways toaccess the ride characteristics of any automobile vehicle.

� Subjective evaluation, and

� Objective evaluation

A panel of juries generally, performs subjectiveassessment of ride comfort experienced during rideevaluation. For assessing the ride, 10 various rideparameters [2] considered and these parameters ratedby the panel on the scale of 10. An overall vehicle riderating was derived from these parameters. Threecompetitive vehicles have been identified and the samepanel has performed subjective ride evaluation. Fig. 4shows the subjective ride rating of first concept vehicleagainst three benchmarked vehicles.

Figure 2 : Existing Rear Cab Mounting

Figure 3 : Typical Analog Computer Model

Fig. 3 shows an analog model of the newly designedcab suspension. It has two vertical coil springs withdampers at front along with a front tilting arrangementto provide the cabin tilting, two vertical coil springs withdampers at rear to control rear vertical motion and twolateral dampers to control the cabin roll and lateralmotion.

Figure 4 : Subjective Rating of First Concept VehicleAgainst Benchmark


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Further to verify this subjective rating an objectiveevaluation has been performed on all four vehicles.

Objective Evaluation : An objective evaluation for ridecomfort basically involves the measurement ofaccelerations (g’s level) at various locations of cabinespecially driver seat, driver seat back and at driver feet.For a commercial vehicle, these vibrations are measuredin vertical (Z-direction) and in fore-aft (x-direction)directions. Lateral direction is optional as these vibrationsare not significant for ride assessment in commercialvehicles.

MEASUREMENT INSTRUMENTATION

ACCELERATION AND SIGNAL CONDITIONING : Ridevibrations measured using SEIKO accelerometers with aninput range of +/-10 m/s2. SOMAT e-DAQ was used forsignal conditioning with 32 channels. Sixteen uni-axialand two tri-axial accelerometers were used for datacollection.

Data Analysis : As ride vibrations are limited to the 1Hzto 25 Hz frequencies range, all the data has beenanalyzed in this band only. The acquired vibration datain time domain has converted into the frequency domainusing Fast-Fourier Transform (FFT) with suitable windowand percentage overlapping. Frequency domain record isuseful for both to verify the data and help to determinethe nature of the ride phenomenon.

ISO-2631guidelines [3] have been referred for definingthe ride comfort level of the vehicle. It was found thatthese guidelines show better representation of subjectiveevaluation than any other method.

ISO-2631 [3] defines the tolerable acceleration level fordifferent exposure time at different frequency anddirection. Fig. 5 and Fig. 6 show the RMS accelerationlevel at different frequencies and exposure duration forvertical and fore and aft directions respectively.

Sensitivity of human body is dissimilar at differentfrequency of acceleration, e.g. for vertical vibrationshuman body is most sensitive for frequency range of4-8 Hz where as in fore-aft 1-2 Hz frequency range.Thus it is required to normalize acceleration data fordifferent frequencies. Fig. 7 shows the weighing curvesfor vertical and fore-aft directions used to normalize theacceleration 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 ofcabin for objective evaluation and these data plotted asper ISO-2631 [3] guidelines. Fig. 8 and Fig. 9 showsthe driver seat accelerations in vertical and in fore-aftdirections respectively, for first concept vehicle againstthree benchmarked vehicles on ISO-2631 [3] guidelines.The benchmarked vehicles selected for study wereequipped with cabin suspension whereas Eicher vehiclewas without cabin suspension. The first natural frequencyof the existing cabin was around 5 Hz for the pitchingmode.

It has been noticed that acceleration level at driver seatof BM-1 and BM-2 were below the 8 h tolerable line

Figure 7 : Frequency Weighting Curve


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whereas BM-3 and Eicher vehicle were crossing the4 h tolerance line. These objective evaluation showsgood level of correlation with subjective rating shown inFig. 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 withmeasured mass and inertia properties has been used forinitial quick prediction of work. A flex body representationhas also been used for final run. Fig. 10 shows theAdams model of existing cabin used for validation.

Chassis : The dynamic behavior of a heavy truck isvery much affected by the frame’s fundamental mode.To capture the affect of chassis stiffness a flex modelof the chassis has been used. Fig. 11 shows the MNFmodel of the frame used for the analysis.

Figure 8 : Driver Seat Accelerations in VerticalDirection

Figure 9 : Driver Seat Accelerations in Fore-aftDirection

These ride evaluations demand to develop a uniquesystem to cater the vehicle’s ride comfort requirement.A well-designed four-point cabin suspension along withoptimized vehicle suspensions can provide the goodcabin ride comfort. For optimizing the various parametersof the cabin and the vehicle suspensions, it required todevelop 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 ofvehicle with cab suspension. Aggregate based modelingapproach has been used to develop the vehicle model.Both aggregate model verification and vehicle levelverification with test data approach has been used. Thefollowing is the brief key list of aggregates used fordefining the vehicle:

Figure 10 : Quick Prediction Model of the Cabin

Figure 11 : MNF Model of the Frame


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Vehicle Suspension: Front and Rear suspension of thevehicle was modeled using SAE 3-Link method andparameterized on hard points. Adams Leaf tool modulewas used for calculating the stiffness of the bushes inthe suspension. The suspension systems consist of axle,leaf springs, and dampers. Fig. 12 shows the Adamsmodel of tractor’s front and rear suspension with trailersuspension. Front and rear suspensions modeled forvariable stiffness. Leaf springs were validated separatelywith test data, and the force-velocity data was fitted toa curve and used to define an Adams spline.

Cargo Body : Cargo body and payload modeled asrigid block with calculated mass and inertial properties.Fig. 14 below shows the Adams model of cargo bodywith payload.

Engine and Cargo body Engine and transmissionmodeled as two rigid bodies and attached to the frameby flexible joints. Calculated mass and inertia propertieswere used to define the Adams model (Fig. 13).

Figure 12 : Adams Model of Vehicle SuspensionSystems

Figure 13 : Engine and Transmission

Figure 14 : Cargo Body and Payload

Actuators : An eight poster virtual actuators havemodeled for wheel input motion of different road condition.Three different road profiles were acquired and used asactuators input.

Vehicle : Virtual vehicle was prepared by merging allaggregate models and validated with test data. Fig. 15shows the Adams model of the vehicle on an 8-posterused for the validation of numerical simulation.

Figure 15 : Adams Model of Tractor-trailer

Following data were measured and used for building themodel.

� 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 cabsuspension

Validation of the Model: In order to validate the model,extensive test data was acquired. The truck wasinstrumented with 22 accelerometers and then driven onthree different road profiles and at two different speeds.The vehicle modeled as per the existing configurations


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and validated with test data. Vehicle axle’s load andaccelerations level at driver seat have been measuredand compared with calculated values.

Fig. 16 shows the various axle loads calculated fromAdams model. These axle weight calculated at contactpatch of the tire after the model got static equilibrium.The total calculated GVW was 42.28 ton againstmeasured 43.3 ton. Table- I below shows the percentagedifference between calculated and measured axle loads.The differences of axle loads are within 5 %, thiscorrelation confirms the mass and inertia properties ofthe Adams model as per actual vehicle.

Further to verify the model, driver seat accelerationscalculated from Adams and measured in the field byusing accelerometers. Fig. 17 shows the calculated andmeasured accelerations at driver seat. These plots showa good level of correlation and confirm the validation ofthe model.

Target Setting for Ride Comfort : Any analysis requiresthe target values to achieve but there is no simple valueor a procedure by which the ride comfort can bedefined. ISO-2631 [3] guidelines were used for targetsetting in numerical simulation.

We targeted the cabin acceleration level should be belowthe 8 h tolerable line of ISO-2631 [3] in vertical andhorizontal direction.

Optimization of vehicle suspensions for ride comfort:Design sensitivity analyses were performed on variousvehicle parameters for minimum driver seat verticalaccelerations. Following parameters were considered forride sensitivity analysis.

� Tractor front suspension Stiffness

� Tractor front suspension damper

� Fifth wheel location

� Tractor Rear Suspension Stiffness

Tractor’s Front Suspension : Fig. 18 shows theacceleration level at driver’s seat for different value offront suspension stiffness. Analysis was performed for+/-10 % values of existing suspension. Stiffness againstminimum acceleration level has been used for furtheroptimization of other parameters.

Tractor Front Damper : Fig. 19 shows the driver seatacceleration for existing damper and new optimizeddampers. Force vs. velocity spline was used for definingthe damper’s characteristics.

Table- I : Difference in Measured and CalculatedLoad

Figure 16 : Calculated Axle Load

Figure 17 : Driver Seat Acceleration Level for Test andSimulation Data

Figure 18 : Driver Seat Accelerations for DifferentFront Suspension Stiffness


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Adams Analysis : After doing successful optimizationof vehicle front suspension, the fixed type cab wasreplaced with suspended cab. Fig. 21 shows the Adamsmodel of tractor-trailer vehicle modeled with suspendedcab. Basic calculated spring stiffness and dampingvalues used in first iteration. DOE has been performedfor cab suspension springs for targeting the naturalfrequencies between 1.8 Hz to 2.5 Hz keeping lineardamping values. Analysis carried out for three differentroad 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, lineardamping values replaced with non-linear damping (Force-velocity). No of iterations performed for reductionof acceleration level for the frequencies range between1-25 Hz.

Fifth Wheel Location : It has been observed that fifthwheel location plays an important role in vehicle ridecomfort but due to design space constraint it has notbeen optimized further.

Tractor Rear Suspension : Effect of tractor rearsuspension stiffness on cabin ride was not significant.Proto Vehicle has been modified with optimizedsuspension and subjective ride performance evaluationhas been performed. It has been observed that vehicleride rating improved by 2 points on subjective scale. Toimprove it further fixed type cabin mounting replaced withcabin suspension arrangement. Front bush type mountinghas been replaced with two coil springs with dampersand new cab titling system. The front suspension hasbeen designed to control the vertical motion and toprovide the required cabin tilting. The existing torsion bararrangement has been replaced by hydraulic cylinder andpump. Rear fixed mounting has been replaced with 2vertical coil springs with dampers and 2 lateral dampersfor controlling the roll and lateral motion of the cabin.Fig. 20 shows the Adams model of the suspendedcabin.

Figure 19 : Vehicle Front Damper Optimization

Figure 20 : Fully Suspended Cabin

Figure 21 : Vehicle with Suspended Cab

Ride Comfort Study : In the final analysis, rideimprovement system judged on the basis of comfortimprovement it provides. A good indication of cabinsuspension effectiveness can be judged by comparingthe vertical acceleration level measured with and withoutcabin suspension.

With suspension, the cab natural frequencies changedto around 1.8 Hz to 2.2 Hz corresponding to tractorbounce and pitch mode.

Fig. 22 shows the calculated PSD of driver seat verticalacceleration plotted on ISO-2631 [3] scale for the ladencabin 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 thatgeneral shape of the response spectra was same.

Fig. 23 shows the driver seat acceleration for averagecabin loading i.e. four people in cabin and for the cementroad. In both loading condition the vertical accelerationswere well below the 8h tolerance line.

Fig. 24 contains the vertical acceleration calculated atdriver seat for three different road input and with fourpersons in cabin. Only for pave road input theaccelerations level are marginally crossing the eight-hourtolerance curve.


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Fig. 25 shows the calculated driver seat acceleration forladen cab and half loaded cabin against measured dataof the BM-1 which have the best ride comfort amongfour vehicles. Significant improvement was noticed withsuspended cabin.

Similar acceleration reduction has also been observed atother location i.e. co-driver seats, lower berth, upperberth, driver feet, cab front and rear mountings.

DEVELOPMENT AND VALIDATION

After optimizing the suspension parameters for minimumacceleration level it is required to verify the same byphysical testing. Based on the analysis feed back,physical model of the suspended cabin has beendeveloped and it has been evaluated on subjective andquantitative bases against existing cabin. Fig. 26 andFig. 27 shows the front and rear suspension of protovehicle respectively.

Figure 22 : Driver Seat Vertical Acceleration on ISOScale

Figure 23 : Driver Sear Vertical Motion on ISO Scalefor Four People in Cabin on CementedRoad

Figure 24 : Driver Sear Vertical Motion on ISO Guidefor Laden in Cabin on Three Different Road

Figure 25 : Vertical Vibration Level Comparison levelfor Modified Vehicle with Suspended Cab

Figure 26 : Front Cabin Suspension

Figure 27 : Rear Cabin Suspension


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Fig. 28 shows the subjective comfort rating of thesuspended cabin against benchmarked vehicles.Significant improvement in cabin ride comfort has beennoticed.

Fig. 29 shows the objective evaluation results of ridecomfort. It can be seen that acceleration level are wellbelow the 8 h ISO curve. This testing result is alsohaving fair degree of correlation with numericalcalculations.

CONCLUSION

Initial ride evaluation and driver’s feedback has indicatedthe universal desire for great ride comfort. A significantimprovement in ride can be achieved through theapplication of low frequency system at cabin mountingareas. The discussed methodology is very effective forimproving the ride comfort of any commercial vehicle.This methodology can also be used for other passengervehicles. Numerical simulation methodology discussedhere can be used for predicting the dynamic behaviorof the any vehicle in different conditions. ISO-2631guidelines along with subjective evaluation give fairrepresentation of ride in quality and quantitative manner.

ACKNOWLEDGMENTS

The author would like to thank Eicher Motors forauthorization of publication of this paper.

The author is thankful to Mr. Rakesh Grover (DeputyGeneral Manager, Eicher Motors) for his support andguidance during this work.

REFERENCES

1. Sternberg, E. R., “Heavy-Duty Truck Suspension”SAE Technical Paper SP-402

2. Measurement and Presentation of Truck RideVibration- SAE J1490, Sept. 1999

3. ISO-2631 guidelines4. MSC-Adams 2005 R1 Help Manual5. Prashant S. Rao, et al., “Developing an ADAMS

Model of an Automobile Using Test Data”, SAEPaper No. 2002-01-1567

6. Davis Anderson and Gregory Schade, “Tractor/SemiTrailer Ride Quality Prediction Using a TemplateBased Approach”, ADAMS Users Conference, 2001

CONTACT

Paras JainEicher Motors PithampurE-mail : [email protected]

Figure 28 : Subjective Rating of Suspended Cab

Figure 29 : Measured Acceleration at Driver Seat

Documents

Design and Analysis of a Tractor-Trailer Cabin Suspension