转向几何参数的讨论汽车外文文献翻译中英文翻译外文翻译.docx

转向几何参数的讨论汽车外文文献翻译中英文翻译外文翻译.docx

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转向几何参数的讨论汽车外文文献翻译中英文翻译外文翻译

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Introduction

Thischapterbeginswithadiscussionofsteeringgeometry-casterangle,trail,kingpininclination,andscrubradius.ThenextsectiondiscussesAckemanngeometryfollowedbysteeringracksandgears.Ridesteer（bumpsteer）androllsteerarecloselyrelatedtoeachother,withoutcompliancetheywouldbethesame.Finally,wheelalignmentisdiscussed.ThischapteristiedtoChapter17onSuspensionGeometry-whendesigninganewchassis,steeringandsuspensiongeometryconsiderationsarehighpriorities.

19.1SteeringGeometry

Thekingpininasolidfrontaxleisthesteeringpivot.Inmodernindependentsuspensions,introducedbyMauriceOllyatCadillacin1932,thekingpinisreplacedbytwo（ormore）balljointsthatthatdefinethesteeringaxis.Thisaxisisnotverticalorcenteredonthetirecontactpatchforanumberofreasons.SeeFigure19.1toclarifyhowkingpinlocationismeasured.

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

Insideviewthekingpinangleiscalledcasterangle;ifthekingpinaxisdoesnotpassthroughthewheelcenterthensideviewkingpinoffsetispresent,asinmostmotorcyclefrontends.Thedistancemeasuredoffsetispresent,asinmostmotorcyclefrontends.Thedistancemeasuredonthegroundfromthesteeringaxistothecenterofthetireprintisthetrail（calledcasteroffsetinRef.1）

KingpinFrontViewGeometry

AsmentionedinChapter17,kingpininclination,spindlelength,andscrubareusuallyacompromisebetweenpackagingandperformancerequirements.Somefactorstoconsiderinclude:

1.Withapositivespindlelength（virtuallyeverycarispositiveasshowninFigure19.1）thecarwillberaisedupasthewheelsaresteeredawayfromcenter.

Themorethekingpininclinationistiltedfromverticalthemorethecarwillberaisedwhenthefrontwheelsaresteered.Thiseffectalwaysraisesthecar,regardlessofwhichdirectionthewheelissteered,unlessthekingpininclinationistruevertical.Theeffectissymmetricsidetosideonlyifthereisnocasterangle.SeethefollowingsectiongonCasterAngle.

Foragivenkingpininclination,alengerpositivespindlelengthwillincreasetheamountofliftwithsteer.

2.Theeffectofkingpininclinationandspindlelengthinraisingthefrontend,byitself,istoaidcenteringofthesteeringatlowspeed.Athighspeedanytrailwillprobablyswampouttheeffectthatriseandfallhaveoncentering.

3.Kingpininclinationaffectsthesteer-cambercharacteristics.Whenawheelissteered,itwillleanoutatthetop,towardpositivecamber,ifthekingpinisinclinedinthenormaldirection（towardthecenterofthecarattheupperend）.Positivecamberresultsforbothbeftandright-bandsteer.Theamountofthiseffectissmall,butsignificantifthetrackincludestightturns.

4.Whenawheelinrollingoverabumpyroad,therollingradiusisconstantlychanging,resultinginchangesofwheelrotationspeed.Thisgivesrisetolongitudinalforcesatthewheelcenter.Thereactionoftheseforceswillintroducekickbackintothesteeringinproportiontothespindlelength.Ifthespindlelengthiszerothentherewillbenokickfromthissource.DesignchangesmadeinthelastmodeloftheGM”p”car（Fiero）shortenedthespindlelengthandthisresultedinlesswheelkickbackonroughroadswhencomparedtoearlymodel”p”cars.

5.ThescrubradiusshowninFigure19.1isnegative,asusedonfront-wheel-drivecars（seebelow）.Drivingorbrakingforces（attheground）introduceisdifferentonleftandringhtwheelsthentherewillbeanetsteeringtorquefeltbythedriver（assumingthatthesteeringgearhasgoodenoughreverseefficiency）.Theonlytimethatthisisnottrueiswithzeroscrub（centerpointsteering）becausethereisnomomentarmforthedrive（orbrake）forcestogeneratetorqueaboutthekingpin.

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

6.Forfrontdriv,anegativescrubradiushastwostrongstabilizingeffects:

First,fixedsteeringwheel_ifonedrivewheellosestraction,theopposingwheelwilltoe-outanamountdelerminedbythesteercomplianceinthesystem.Thiswlltendtosteerthecarinastraightline,eventhoughthetractiveforceisnotequalside-to-sideandtheunequaltractiveforceisapplyingayawmomenttothevehicle.

Second,withgoodreverseefficiencythedriver’shandsnevertrulyfixthesteeringwheel.Inthiscasethesteeringwheelmaybeturnedbytheeffectofunevenlongitudinaltractiveforces,increasingthestabilizingeffectofthenegativescrubradius.

Underbrakingthesameistrue.Negativescrubradiustendstokeepthecartravelingstraightevenwhenthebrakingforceisnotequalontheleftandrightsidefronttires（duetodifferencesintheroadwayorthebrakes）

CasterAngleandTrail

Withmechanicaltrail,showninFigure19.1,thetireprintfollowsbehindthesteeringasisinsideview.Perhapsthesimplestexampleisonanofficechaircaster-withanydistanceoftravel,thewheelalignsitselfbehindthepivot.Moretrailmeansthatthetiresideforcehasalargermomentarmtoactonthekingpinaxis.Thisproducesmoreself-centeringeffectandistheprimarysourceofself-centeringmomentaboutthekingpinaxisatspeed.Someconsiderationsforchoosingthecasterangleandtrailare:

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

2.Casterangle,likekingpininclination,causesthewheeltoriseandfallwithsteer.Unlikekingpininclination,theeffectisoppositefromsidetoside.Withsymmetricgeometry（includingequalpositivecasteronleftandrightwheels）,theeffectofleftsteeristorollthecartotheright,causingadiagonalweightshift.Inthiscase,moreloadwillbecarrideontheLF-RRdiagonal,anoversteereffectinaleft-handturn.

Thediagonalweightshiftwillbelargerifstifferspringingisusedbecausethisisageometriceffect.Thedistanceeachwheelrises（orfalls）isconstantbuttheweightjackingandchassisrollanglearefunctionsofthefrontandrearrollstiff-ness.Thisdiagonalloadchangecanbemeasuredwiththecaronscalesandalignment（Weaver）plates.

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

1steeringratiothisisabout90atthesteeringwheel）.

Forcarsthatturninonedirectiononly,casterstagger（differencesinleftandrightcaster）isusedtocausethecartopulltoonesideduetothecarseekingthelowestrideheight.Casterstaggerwillalsoaffectthediagonalweightjackingeffectmentionedabove.

Ifthecasterisopposite（positiveononesideandnegativethesamenumberofdegreesontheotherside）thenthefrontofthecarwillonlyriseandfallwithsteer,nodiagonalweightjackingwilloccur.

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

Inskidrecovery,”oppositelock”（steeroutoftheturn）isusedandinthiscasethesteer-camberresultingfromcasterangleisinthe“wrong”directionforincreasedfronttiregrip.Conveniently,thisconditionresultsfromverylowlateralforceattherearsolargeamountsoffrontgriparenotneeded.

4.AsdiscussedinChapter2,tireshavepneumatictrailwhicheffectivelyaddsto（andathighslipanglessubtractsfrom）themechanicaltrail.Thistireeffectisnonlinearwithlateralforceandaffectssteeringtorqueanddriverfeel.Inparticular,thefactthatpneumatictrailapproacheszeroasthetirereachesthelimitwillresultinloweringtheself-centeringtorqueandcanbeasignaltothedriverthatthetireisnearbreakaway.

Thepneumatictrail”breakawaysignal”willbeswampedoutbymechanicaltrailifthemechanicaltrailislargecomparedtothepneumatictrail.

5.Sometimesthetrailismeasuredinadirectionperpendiculartothesteeringaxis（ratherthanhorizontalasshowninFigure19.1）becausethismoreaccuratelydescribesthelever（moment）armthatconnectsthetirelateralforcestothekingpin.

Tierodlocation

NotethatinFigure19.1ashadedareaisshownforthesteeringtierodlocation.Cambercomploanceunderlateralforceisunavoidableandifthetierodislocaterasnoted,theeffectonthesteeringwillbeintheundersteer（steeroutoftheturn）direction.Ifthesuspensionandrackaremountedonsomsortofflesiblesubframe,thesituationbecomesmuchmorecomplexthancanbecoveredhere.

19.2Ackermannsteeringgeometry

Asthefrontwheelsofavehiclearesteeredawayfromthestraight-aheadposition,thedesignofthesteeringlinkagewilldetermineifthewheelsstayparallelorifonewheelsteersmorethantheother.Thisdif