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.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
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