汽車轉(zhuǎn)向系統(tǒng)外文原文及翻譯

本文摘于《RaceCarVehicleDynamics》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.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.TierodlocationNotethatinfigure19.1ashadedareaisshownforthesteeringtierodlocation.Cambercomplianceunderlateralforceisunavoidableandifthetierodislocatedasnoted,theeffectonthesteeringwillbeintheundersteer(steeroutoftheturn)directionbecomesmuchmorecomplexthancanbecoveredhere.19.2AckermansteeringgeometryAsthefrontwheelsofavehiclearesteeredawayfromthestraight-aheadposition,thedesignofthesteeringlinkagewilldetermineifthewheelsstayparallelorifonewheelsteersmorethantheother.ThisdifferenceinsteerAnglesontheleftandrightwheelsshouldnotbeconfusedwithtoe-inortoe-outwhichareadjustmentsandaddto(orsubtractfrom)Ackermangeometriceffects.Forlowlateralaccelerationusage(streetcars)itiscommontouseAckermangeometry.asseenontheleftoffigure19.2,thisgeometryensuresthatallthewheelsrollfreelywithnoslipAnglesbecausethewheelsaresteeredtotrackacommonturncenter.Notethatatlowspeedallwheelsareonasignificantlydifferentradius,theinsidefrontwheelmuststeermorethantheouterfrontwheel.Areasonableapproximationtothisgeometrymaybeasshowninfigure19.3.Accordingtoref.99,RudolfAckermanpatentedthedoublepivotsteeringsystemin1817andin1878,CharlesJeantaudaddedtheconceptmentionedabovetoeliminatewheelscrubbingwhencornering.AnotherreasonforAckermanngeometry,mentionedbyMauriceolley,wastokeepcarriagewheelsfromupsettingsmoothgraveldriveways.Highlateralaccelerationschangethepictureconsiderably.NowthetiresalloperateatsignificantslipAnglesandtheloadsontheinsidetrackarelessthanontheoutsidetrack.Lookingbacktothetireperformancecurves,itisseenthatlessslipangleisrequiredatlighterloadstoreachthepeakofthecorneringforcetoahigherslipanglethanrequiredformaximumsideforce.DraggingtheinsidetirealongathighslipAngles(aboveforpeaklateralforce)raisethetiretemperatureandslowsthecardownduetoslipangle(induced)drag.Forracing,itiscommontouseparallelsteeringorevenreverseAckermannasshownonthecenterandrightsideoffigure19.2.ItispossibletocalculatethecorrectamountofreverseAckermannifthetirepropertiesandloadsareknown.Inmostcasestheresultinggeometryisfoundtobetooextremebecausethecarmustalsobedriven(orpushed)atlowspeeds,forexampleinthepits.AnotherpointtorememberisthatmostturnsinracinghaveafairlylargeradiusandtheAckermanneffectisverysmall.Infact,unlessthesteeringsystemandsuspensionareverystiff,compliance(deflection)undercorneringloadsmaysteerthewheelsmorethananyAckermann(orreverseAckermann)builtintothegeometry.ThesimplestconstructionthatgeneratesAckermannngeometryisshowninfigure19.3for“rearsteer”.Here,therack(crosslinkorrelayrodinsteeringboxsystems)islocatedbehindthefrontaxleandlinesstaringatthekingpinaxis,extendedthroughtheoutertierodends,intersectinthecenteroftherearaxle.Theangularityofthesteeringknucklewillcausetheinnerwheeltosteermorethantheouter(toe-outonturning)andagoodapproximationof“perfectAckermann”willbeachieved.Thesecondwaytodesign-indifferencesbetweeninnerandoutersteerAnglesisbymovingtherack(orcrosslink)forwardorbackwardsothatitisnolongeronalinedirectlyconnectingthetwooutertierodballjoints.Thisisshowninfigure19.4.with“rearsteer”,asshowninthefigure,movingtherackforwardwilltendmoretowardparallelsteer(andeventuallyreverseAckermann),andmovingittowardtherearofthecarwillincreasethetoe-outonturning.Athirdwaytogeneratetoewithsteeringissimplytomakethesteeringarmsdifferentlengths.Ashortersteeringarm(asmeasuredfromthekingpinaxistotheoutertierodend)willbesteeredthroughalargeranglethanonewithalongerknuckle.Ofcoursethiseffectisasymmetricandappliesonlytocarsturninginonedirection—ovaltrackcars.RecommendationWiththeconflictingrequirementsmentionedabove,theauthorsfeelthatparallelsteerorabitofreverseAckermannisareasonablecompromise.Withparallelsteer,thecarwillbesomewhatdifficulttopushthroughthepitsbecausethefrontwheelswillbefightingeachother.atracingspeeds,onlarge-radiusturns,thefrontwheelsaresteeredverylittle,thusanyackermanneffectswillnothavealargeeffectontheindividualwheelslipangles,relativetoareferencesteerangle,measuredatthecenterlineofthecar.文獻(xiàn)翻譯摘自《RaceCarVehicleDynamics》第19章轉(zhuǎn)向系統(tǒng)序言：本章以轉(zhuǎn)向幾何參數(shù)的討論為開始，包括主銷后傾角，后傾拖距，主銷內(nèi)傾角，主銷偏置量。接下來的局部討論了轉(zhuǎn)向齒輪齒條以及阿克曼轉(zhuǎn)向幾何關(guān)系。跳動轉(zhuǎn)向和側(cè)傾轉(zhuǎn)向之間是緊密相關(guān)的，如果沒有柔性這兩種情況是等同的。最后討論了車輪的調(diào)整。這一章與第17章的懸架幾何形狀密切相關(guān)，在設(shè)計新的底盤系統(tǒng)時，轉(zhuǎn)向和懸架幾何參數(shù)是優(yōu)先考慮的因素。19.1轉(zhuǎn)向幾何關(guān)系〔定位參數(shù)〕在整體式車橋上轉(zhuǎn)向節(jié)主銷是轉(zhuǎn)向時的樞軸。1932年MauriceOlley在Cadillac首次提出了現(xiàn)在的非獨(dú)立懸架，主銷因此而被兩個球絞連接定義的轉(zhuǎn)向軸線代替。因?yàn)楦鞣N原因這根軸并不是垂直的也不在輪胎接地中心處。主銷的位置表示見圖19.1。·在前視圖中，主銷偏轉(zhuǎn)的角度被稱為主銷內(nèi)傾角，轉(zhuǎn)向主銷與地面的交點(diǎn)至車輪中心平面與地面相交處的距離稱之為主銷偏置量。在前軸所在水平面內(nèi)，從主銷軸心到車輪中心平面的距離稱為主銷偏距〔spindlelength〕?！ぴ趥?cè)視圖中，主銷偏轉(zhuǎn)角度稱為主銷后傾角。如果主銷軸線沒有通過車輪中心那么就有了側(cè)視的主銷偏距〔sideviewkingpinoffset〕，就像大局部的摩托車前輪一樣。在地平面內(nèi)測量從主銷到輪胎接地點(diǎn)中心的距離稱為主銷后傾拖距。前視圖中的主銷定位參數(shù)正如在17章中提到，主銷內(nèi)傾角，主銷偏距還有主銷偏置量在裝配以及性能滿足時往往是互相妥協(xié)的。一些需要考慮的因素包括以下：當(dāng)主銷偏距是正的時〔一般的車都是正主銷偏距，如圖19.1中一樣〕那車輪轉(zhuǎn)離中心位置的時候車會有一個抬升效果。主銷內(nèi)傾角偏離豎直平面越大前輪轉(zhuǎn)向時車被抬起的效果越明顯。不管車輪往哪個方向轉(zhuǎn)都會是一個抬升的效果，除非主銷是完全垂直的。這個效果只有在主銷后傾角為零時才是兩邊對稱的。見后面關(guān)于主銷后傾角局部。對于一個給定的主銷內(nèi)傾角來說，主銷偏距越大轉(zhuǎn)向時的抬升量也越大。主銷內(nèi)傾角和主銷偏距將車子前端抬起的效果對于自身來說是有助于低速轉(zhuǎn)向的。在高速轉(zhuǎn)向時，只要有主銷后傾拖距就可能會掩蓋掉轉(zhuǎn)向時抬升和下落的效果。主銷內(nèi)傾角影響轉(zhuǎn)向時車輪的外傾角特性。如果主銷向內(nèi)傾斜〔主銷上端傾向車輛中心〕當(dāng)車輪轉(zhuǎn)向的時候，車輪上端將會向外傾斜，趨向正的車輪外傾角。左右轉(zhuǎn)向都會導(dǎo)致正的車輪外傾。如果跑道有比擬緊的彎這個作用效果是比擬小但卻是有重要意義的。當(dāng)車輪滾過顛簸不平的路面時，滾動半徑是不斷變化的，將會導(dǎo)致輪速的改變。這將會增加車輪中心的縱向力。這些力的反作用與主銷偏距的大小成比例，成為反沖效果進(jìn)入轉(zhuǎn)向系統(tǒng)。如果主銷偏距為零，那么將不會有由此引起的反沖。在前面提到的一輛通用“P”型車〔菲羅車〕中做出設(shè)計改動，與較早的一輛“P”型車模型相比，減小了主銷偏距，因此而減少了不平路面上的反沖。如圖19.1中所示的主銷偏置量是負(fù)的，正如下面這輛前輪驅(qū)動車用的一樣。來自地面的驅(qū)動和制動力與主銷偏置量成比例的轉(zhuǎn)化成轉(zhuǎn)向力矩。如果左右輪的制動或者驅(qū)動力是不等的，那么駕駛者將會感受到的到這個轉(zhuǎn)向力矩〔假設(shè)轉(zhuǎn)向器有較高的逆效率〕。只有在主銷偏置量為零時才不會有這個力矩產(chǎn)生因?yàn)榇藭r制動力或驅(qū)動力對主銷的作用力臂為零。如果輪胎比擬寬的話輪胎力通常并不是作用在輪胎中心平面內(nèi)的，因?yàn)檩p微的外傾角變化、路面不平、輪胎有一定圓錐度、或者其他的不對稱因素存在。這些不對稱因素可能導(dǎo)致轉(zhuǎn)向反沖，即使沒有前輪的各個定位參數(shù)作用。裝配要求通常會與中心點(diǎn)轉(zhuǎn)向要求沖突因而很多賽車在較平整的賽道上是采用較大的主銷偏置量也是可以的。對于前輪驅(qū)動來說，一個負(fù)的主銷偏置量有兩個重要的穩(wěn)定作用：固定方向盤，如果一個驅(qū)動輪打滑，另外一個輪將會外張一定角度，因?yàn)檗D(zhuǎn)向系統(tǒng)內(nèi)有變形。即使兩側(cè)的牽引力不等，不同的牽引力使車輛產(chǎn)生一個偏航角，這個負(fù)的主銷偏置量作用也會使車輛回復(fù)到直線行駛。有良好的反應(yīng)作用情況下駕駛員從來不會真正的固定住方向盤。在這種情況下方向盤可能在不等的車輪縱向牽引力作用下而轉(zhuǎn)動，因此而增加了負(fù)主銷偏置量的穩(wěn)定效果。制動的情況同樣適用。負(fù)的主銷偏置量能使車子回正，即使是在左右輪制動力不等的情況下〔左右輪的制動情況或者路面情況不同時〕。(fsae沒人用吧)主銷后傾角和后傾拖距如圖19.1中所示，在有后傾拖距時，側(cè)視圖中輪胎接地點(diǎn)是在主銷之后的。或許最簡單的例子就是辦公室座椅上的小腳輪〔？〕——不管移動多遠(yuǎn)，輪子總會校正使其自身在樞軸之后。主銷拖距越大意味著輪胎側(cè)向力在主銷軸上作用有更大的力臂。這會產(chǎn)生更明顯的回正作用，并且是作用在主銷上最主要的回正力矩。在選擇主銷后傾角和主銷拖距時需要考慮的因素如下：主銷后傾拖距越大轉(zhuǎn)向力也越大。對于所有的車來說，小的后傾拖距都將會減小轉(zhuǎn)向力。在某些情況下，如果后傾拖距減小接近零的話，人力轉(zhuǎn)向也可能被用于重型轎車〔代替助力轉(zhuǎn)向〕。像主銷內(nèi)傾角一樣，主銷后傾角伴隨著轉(zhuǎn)向過程也會引起車輪的抬起和回落。與內(nèi)傾角不同的是，后傾角對兩側(cè)的影響是相反的。在有對稱定位參數(shù)時〔包括左右輪有相等的正的主銷后傾角〕，左轉(zhuǎn)的效應(yīng)是使車向右側(cè)傾，導(dǎo)致一個對角線的重量轉(zhuǎn)移。在這種情況下，左前——右后對角線會承受更大的載荷，有一個左轉(zhuǎn)時的過度轉(zhuǎn)向效應(yīng)。使用的彈簧越硬對角線的重量轉(zhuǎn)移效果也會越明顯因?yàn)檫@個是幾何效應(yīng)。每個車輪被抬起〔或者下落〕的距離是恒定的但是重量抬起量和底盤側(cè)傾角是前后側(cè)傾剛度的作用結(jié)果。這個對角線的載荷轉(zhuǎn)移可以通過把車放在秤上和定位板上來測量。記住在實(shí)際比賽中前輪并沒有轉(zhuǎn)過很大的角度，除非是非常緊的發(fā)夾彎。例如，在一個半徑是100英尺〔時速在40-50英里〕的彎，一個10英尺的軸距的中性轉(zhuǎn)向車輛轉(zhuǎn)彎時