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1TheEffectofaViscousCouplingUsedasaFront-WheelDriveLimited-SlipDifferentialonVehicleTractionandHandling1ABCTRACTTheviscouscouplingisknownmainlyasadrivelinecomponentinfourwheeldrivevehicles.Developmentsinrecentyears,however,pointtowardtheprobabilitythatthisdevicewillbecomeamajorplayerinmainstreamfront-wheeldriveapplication.ProductionapplicationinEuropeanandJapanesefront-wheeldrivecarshavedemonstratedthatviscouscouplingsprovidesubstantialimprovementsnotonlyintractiononslipperysurfacesbutalsoinhandingandstabilityevenundernormaldrivingconditions.Thispaperpresentsaseriousofprovinggroundtestswhichinvestigatetheeffectsofaviscouscouplinginafront-wheeldrivevehicleontractionandhanding.Testingdemonstratessubstantialtractionimprovementswhileonlyslightlyinfluencingsteeringtorque.Factorsaffectingthissteeringtorqueinfront-wheeldrivevehiclesduringstraightlinedrivingaredescribed.Keyvehicledesignparametersareidentifiedwhichgreatlyinfluencethecompatibilityoflimited-slipdifferentialsinfront-wheeldrivevehicles.Corneringtestsshowtheinfluenceoftheviscouscouplingontheselfsteeringbehaviorofafront-wheeldrivevehicle.Furthertestingdemonstratesthatavehiclewithaviscouslimited-slipdifferentialexhibitsanimprovedstabilityunderaccelerationandthrottle-offmaneuversduringcornering.2THEVISCOUSCOUPLINGTheviscouscouplingisawellknowncomponentindrivetrains.Inthispaperonlyashortsummaryofitsbasicfunctionandprincipleshallbegiven.Theviscouscouplingoperatesaccordingtotheprincipleoffluidfriction,andisthusdependentonspeeddifference.AsshowninFigure1theviscouscouplinghasslipcontrollingpropertiesincontrasttotorquesensingsystems.Thismeansthatthedrivetorquewhichistransmittedtothefrontwheelsisautomaticallycontrolledinthesenseofanoptimizedtorquedistribution.Inafront-wheeldrivevehicletheviscouscouplingcanbeinstalledinsidethedifferentialorexternallyonanintermediateshaft.TheexternalsolutionisshowninFigure2.Thislayouthassomesignificantadvantagesovertheinternalsolution.First,thereisusuallyenoughspaceavailableintheareaoftheintermediateshafttoprovidetherequiredviscouscharacteristic.Thisisincontrasttothelimitedspaceleftintoday’sfront-axledifferentials.Further,onlyminimalmodificationtothedifferentialcarrierandtransmissioncaseisrequired.In-houseproductionofdifferentialsisthusonlyslightlyaffected.Introductionasanoptioncanbemadeeasilyespeciallywhentheshaftandtheviscousunitissuppliedasacompleteunit.Finally,theintermediateshaftmakesitpossibletoprovideforsideshaftsofequallengthwithtransverselyinstalledengineswhichisimportanttoreducetorquesteer(shownlaterinsection4).Thisspecialdesignalsogivesagoodpossibilityforsignificantweightandcostreductionsoftheviscousunit.GKNViscodriveisdevelopingalowweightandcostviscouscoupling.Byusingonlytwostandardizedouterdiameters,standardizedplates,plastichubsandextrudedmaterialforthehousingwhichcaneasilybecuttodifferentlengths,itispossibletoutilizeawiderangeofviscouscharacteristics.AnexampleofthisdevelopmentisshowninFigure3.3TRACTIONEFFECTSAsatorquebalancingdevice,anopendifferentialprovidesequaltractiveefforttobothdrivingwheels.Itallowseachwheeltorotateatdifferentspeedsduringcorneringwithouttorsionalwind-up.Thesecharacteristics,however,canbedisadvantageouswhenadhesionvariationsbetweentheleftandrightsidesoftheroadsurface(split-μ)limitsthetorquetransmittedforbothwheelstothatwhichcanbesupportedbythelow-μwheel.Withaviscouslimited-slipdifferential,itispossibletoutilizethehigheradhesionpotentialofthewheelonthehigh-μsurface.ThisisschematicallyshowninFigure4.Whenforexample,themaximumtransmittabletorqueforonewheelisexceededonasplit-μsurfaceorduringcorneringwithhighlateralacceleration,aspeeddifferencebetweenthetwodrivingwheelsoccurs.Theresultingself-lockingtorqueintheviscouscouplingresistsanyfurtherincreaseinspeeddifferenceandtransmitstheappropriatetorquetothewheelwiththebettertractionpotential.ItcanbeseeninFigure4thatthedifferenceinthetractiveforcesresultsinayawingmomentwhichtriestoturnthevehicleintothelow-μside,Tokeepthevehicleinastraightlinethedriverhastocompensatethiswithoppositesteeringinput.Thoughthefluid-frictionprincipleoftheviscouscouplingandtheresultingsofttransitionfromopentolockingaction,thisiseasilypossible,Theappropriateresults2obtainedfromvehicletestsareshowninFigure5.Reportedaretheaveragesteering-wheeltorqueTsandtheaveragecorrectiveoppositesteeringinputrequiredtomaintainastraightcourseduringaccelerationonasplit-μtrackwithanopenandaviscousdifferential.Thedifferencesbetweenthevalueswiththeopendifferentialandthosewiththeviscouscouplingarerelativelylargeincomparisontoeachother.However,theyaresmallinabsoluteterms.Subjectively,thesteeringinfluenceisnearlyunnoticeable.Thetorquesteerisalsoinfluencedbyseveralkinematicparameterswhichwillbeexplainedinthenextsectionofthispaper.4FACTORSAFFECTINGSTEERINGTORQUEAsshowninFigure6thetractiveforcesleadtoanincreaseinthetoe-inresponseperwheel.Fordifferingtractiveforces,Whichappearwhenacceleratingonsplit-μwithlimited-slipdifferentials,thetoe-inresponsechangesperwheelarealsodifferent.Unfortunately,thiseffectleadstoan