汽车转向系统外文原文及翻译.docx

汽车转向系统外文原文及翻译.docx

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汽车转向系统外文原文及翻译

本文摘于《RaceCarVehicleDynamics》

作者：

WilliamF.MilikenandDouglasL.Miliken

Steeringsystems

Introduction

Thischapterbeginswithadiscussionofsteeringgeometry—casterangle,trail,kingpininclination,andscrubradius.ThenextsectiondiscussAckermanngeometryfollowedbysteeringracksandgears.Ridesteer（bumpsteer）androllsteerarecloselyrelatedtoeachother;withoutcompliancetheywouldbethesame.Finally,wheelalignmentisdiscussed.thischapteristiedtochapter17onsuspensiongeometry–whendesigninganewchassis,steeringandsuspensiongeometryconsiderationsarehighpriorities.

19.1steeringgeometry

Thekingpininasolidfrontaxleisthesteeringpivot.Inmodernindependentsuspensions,introducedbyMauriceolleyatCadillacin1932,thekingpinisreplacedbytwo（ormore）balljointsthatdefinethesteeringaxis.Thisaxisisnotverticalorcenteredonthetirecontactpatchforanumberofreason.seefigure19.1toclarifyhowkingpinlocationismeasured.

Infrontview,theangleiscalledkingpininclinationandtheoffsetofthesteeringaxisfromthecenterofthetireprintmeasuredalongthegroundiscalledscrub（orscrubradius）.Thedistancefromthekingpinaxistothewheelcenterplane,measuredhorizontallyataxleheight,isthespindlelength.

Insideviewthekingpinangleiscalledcasterangle;ifthekingpinaxisdoesnotpassthroughthewheelcenterthensideviewkingpinoffsetispresent,asinmostmotorcyclefrontends.Thedistancemeasuredonthegroundfromthesteeringaxistothecenterofthetireprintisthetrail（calledcasteroffsetinref.1）

Kingpinfrontviewgeometry

Asmentionedinchapter17,kingpininclination,spindlelength,andscrubareusuallyacompromisebetweenpackagingandperformancerequirements.Somefactorstoconsiderinclude:

1.Withapositivespindlelength（virtuallyeverycarispositiveasshowninfigure19.1）thecarwillberaisedupasthewheelsaresteeredawayfromcenter.

Themorethekingpininclinationistiltedfromverticalthemorethecarwillberaisedwhenthefrontwheelsaresteered.Thiseffectalwaysraisesthecar,regardlessofwhichdirectionthewheelissteered,unlessthekingpininclinationistruevertical.theeffectissymmetricsidetosideonlyifthereisnocasterangle.Seethefollowingsectiononcasterangle.

Foragivenkingpininclination,alongerpositivespindlelengthwillincreasetheamountofliftwithsteer.

2.Theeffectofkingpininclinationandspindlelengthinraisingthefrontend,byitself,istoaidcenteringofthesteeringatlowspeed.Athighspeedanytrailwillprobablyswampouttheeffectthatraiseadfallhaveoncentering.

3.Kingpininclinationaffectsthesteer–cambercharacteristic.whenawheelissteered,itwillleanoutatthetop,towardpositivecamber,ifthekingpinisinclinedinthenormaldirection（towardthecenterofthecarattheupperend）.Positivecamberresultsforbothleft–andright-handsteer.theamountofthiseffectissmall,butsignificantifthetrackincludestightturns.

4.Whenawheelisrollingoverabumpyroad,therollingradiusisconstantlychanging,resultinginchangesofwheelrotationspeed.Thisgivesrisetolongitudinalforcesatthewheelcenter.Thereactionoftheseforceswillintroducekickbackintothesteeringinproportiontothespindlelength.Ifthespindlelengthiszerothentherewillbenokickfromthissource.DesignchangesmadeinthelastmodeloftheGM“P”car（fiero）shortenedthespindlelengthandthisresultedinlesswheelkickbackonroughroadswhencomparedtoearlymodel“P”cars.

5.Thescrubradiusshowninfigure19.1isnegative,asusedonfront-wheel–drivecars（seebelow）.drivingorbrakingforces（attheground）introducesteertorquesproportionaltothescrubradius.Ifthedrivingorbrakingforceisdifferentonleftandrightwheelsthentherewillbeanetsteeringtorquefeltbythedriver（assumingthatthesteeringgearhasgoodenoughreverseefficiency）.Theonlytimethatthisisnottrueiswithzeroscrub（centerpointsteering）becausethereisnomomentarmforthedrive（orbrake）forcetogeneratetorqueaboutthekingpin.

Withverywidetiresthetireforcesoftenarenotcenteredinthewheelcenterplaneduetoslightchangesincamber,roadsurfaceirregularities,tirenonuniformity（conicity）,orotherasymmetriceffects.Theseasymmetriescancausesteeringkickbackregardlessofthefrontviewgeometry.Packagingrequirementsoftenconflictwithcenterpointsteeringandmanyracecarsoperatemoreorlessokayonsmoothtrackswithlargeamountsofscrub.

6.Forfrontdrive,anegativescrubradiushastwostrongstabilizingeffects:

first,fixedsteeringwheel–ifonedrivewheellosestraction,theopposingwheelwilltoe–outanamountdeterminedbythesteercomplianceinthesystem.Thiswilltendtosteerthecarinastraightline,eventhoughthetractiveforceisnotequalside-to–sideandtheunequaltractiveforceisapplyingayawmomenttothevehicle.

Second,withgoodreverseefficiencythedriver’shandsnevertrulyfixthesteeringwheel.Inthiscasethesteeringwheelmaybeturnedbytheeffectofunevenlongitudinaltractiveforces,increasingthestabilizingeffectofthenegativescrubradius.

Underbrakingthesameistrue.Negativescrubradiustendstokeepthecartravelingstraightevenwhenthebrakingforceisnotequalontheleftandrightsidefronttiresome（duetodifferencesintheroadwayorthebrakes）.

Casterangleandtrail

Withmechanicaltrail,showninfigure19.1,thetireprintfollowsbehindthesteeringaxisinsideview.Perhapsthesimplestexampleisonanofficechaircaster–withanydistanceoftravel,thewheelalignsitselfbehindthepoint.Moretrailmeansthatthetiresideforcehasalargemomentarmtoactonthekingpinaxis.Thisproducesmoreself-centeringeffectandistheprimarysourceofself-centeringmomentaboutthekingpinaxisatspeed.Someconsiderationsforchoosingthecasterangleandtrailare:

1.Moretrailwillgivehighersteeringforce.withallcars,lesstrailwilllowerthesteeringforce.Insomecases,manualsteeringcanbeusedonheavysedans（insteadofpowersteering）ifthetrailisreducedtoalmostzero.

2.Casterangle,likekingpininclination,causethewheeltoriseandfallwithsteer.unlikekingpininclination,theeffectisoppositefromsidetoside.Withsymmetricgeometry（includingequalpositivecasteronleftandrightwheels）,theeffectofleftsteeristorollthecartotheright,causingadiagonalweightshift.Inthiscase,moreloadwillbecarriedontheLF–RRdiagonal,anoversteereffectinaleft-handturn.

Thediagonalweightshiftwillbelargerifstifferspringingisusedbecausethisisageometriceffect.Thedistanceeachwheelrises（orfalls）isconstantbuttheweightjackingandchassisrollanglearefunctionsofthefrontandrearrollstiffness.Thisdiagonalloadchangecanbemeasuredwiththecaronscalesandalignment（weaver）plates.

Keepinmindthatthefrontwheelsarenotsteeredverymuchinactualracing,exceptontheverytightesthairpinturns.Forexample,ona100-ft.radius（a40-50mphturn）,a10-ft.wheelbaseneutralsteercarneedsonlyabout0.1rad.（5.7）ofsteeratthefrontwheels（witha16:

1steeringratiothisisabout90degreeatthesteeringwheel）.

Forcarsthatturninonedirectiononly,casterstagger（differencesinleftandrightcaster）isusedtocausethecartopulltoonesideduetothecarseekingthelowestrideheight.casterstaggerwillalsoaffectthediagonalweightjackingeffectmentionedabove.

Ifthecasterisopposite（positiveononesideandnegativethesamenumberofdegreesontheotherside）thenthefrontofthecarwillonlyriseandfallwithsteer,nodiagonalweightjackingwilloccur.

3.Casterangleaffectssteer-camberbut,unlikekingpininclination,theeffectisfavorable.Withpositivecasterangletheoutsidewheelwillcamberinanegativedirection（topofthewheeltowardthecenterofthecar）whiletheinsidewheelcambersinapositivedirection,againlearningintotheturn.

Inskidrecovery,“oppositelock”（steeroutoftheturn）isusedandinthiscasethesteer–camberresultingfromcasterangleisinthe“wrong”directionforincreasedfronttiregrip.conveniently,thisconditionresultsfromverylowlateralforceattherearsolargeamountsoffrontgriparenotneeded.

4.Asdiscussedinchapter2,tireshavepneumatictrailwhicheffectivelyaddsto（andathighslipAnglessubtractsfrom）themechanicaltrail.Thistireeffectisnonlinearwithlateralforceandaffectssteeringtorqueanddriverfeel.Inparticular,thefactthatpneumatictrailapproacheszeroasthetirereachesthelimitwillresultinloweringtheself-centeringtorqueandcanbessignaltothedriverthatthetireisnearbreakaway.

Thepneumatictrail“breakawaysignal”willbeswampedoutbymechanicaltrailifthemechanicaltrailislargecomparedtothepneumatictrail.

5.Sometimesthetrailismeasuredinadirectionperpendiculartothesteeringaxis（ratherthanhorizontalasshowninfigure19.1）becausethismoreaccuratelydescribesthelever（moment）armthatconnectsthetirelateralforcestothekingpin.

Tierodlocation

Notethatinfigure19.1ashadedareaisshownforthesteeringtierodlocation.Cambercomplianceunderlateralforceisunavoidableandifthetierodislocatedasnoted,theeffectonthesteeringwillbeintheundersteer（steeroutoftheturn）directionbecomesmuchmorecomplexthancanbecoveredhere.

19.2Ackermansteeringgeometry

Asthefront