汽车转向系统外文原文及翻译

本文摘于《RaceCarVehicleDynamics》作者：WilliamF.MilikenandDouglasL.MilikenSteeringsystemsIntroductionThischapterbeginswithadiscussionofsteeringgeometry—casterangle,trail,kingpininclination,andscrubradius.ThenextsectiondiscussAckermanngeometryfollowedbysteeringracksandgears.Ridesteer(bumpsteer)androllsteerarecloselyrelatedtoeachother;withoutcompliancetheywouldbethesame.Finally,wheelalignmentisdiscussed.thischapteristiedtochapter17onsuspensiongeometry–whendesigninganewchassis,steeringandsuspensiongeometryconsiderationsarehighpriorities.19.1steeringgeometryThekingpininasolidfrontaxleisthesteeringpivot.Inmodernindependentsuspensions,introducedbyMauriceolleyatCadillacin1932,thekingpinisreplacedbytwo(ormore)balljointsthatdefinethesteeringaxis.Thisaxisisnotverticalorcenteredonthetirecontactpatchforanumberofreason.seefigure19.1toclarifyhowkingpinlocationismeasured.Infrontview,theangleiscalledkingpininclinationandtheoffsetofthesteeringaxisfromthecenterofthetireprintmeasuredalongthegroundiscalledscrub(orscrubradius).Thedistancefromthekingpinaxistothewheelcenterplane,measuredhorizontallyataxleheight,isthespindlelength.Insideviewthekingpinangleiscalledcasterangle;ifthekingpinaxisdoesnotpassthroughthewheelcenterthensideviewkingpinoffsetispresent,asinmostmotorcyclefrontends.Thedistancemeasuredonthegroundfromthesteeringaxistothecenterofthetireprintisthetrail(calledcasteroffsetinref.1)KingpinfrontviewgeometryAsmentionedinchapter17,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).CasterangleandtrailWithmechanicaltrail,showninfigure19.1,thetireprintfollowsbehindthesteeringaxisinsideview.Perhapsthesimplestexampleisonanofficechaircaster–withanydistanceoftravel,thewheelalignsitselfbehindthepoint.Moretrailmeansthatthetiresideforcehasalargemomentarmtoactonthekingpinaxis.Thisproduces