转向.docx
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转向.docx
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转向
"Ackerman?
Ornot?
Doesitmatter?
".
DaleThompsonfromRacingCarTechnologylooksforsomeanswers.What'syourview?
Haveyouanytestdata?
EmailDaleon racetech@.au
Ackerman?
Anti-Ackerman?
OrParallelSteering?
Ackermansteeringgeometryisusedtochangethedynamictoesetting,byincreasingfrontwheeltoeoutasthecaristurnedintothecorner.Racersareinterestedbecauseofthepotentialtoinfluencethehandlingofthecaroncornerentryandmidcorner.
OurinterestatRacingCarTechnologyistolookforfurtherdevelopmentsinracingcarsetupforourcustomers.Wehavebeensettingupracingcarswithour"WeightTransferWorksheet"(WTW)forafewyearsnow.Bytracktestingwegotconfirmingdata,andshowedthatifyouhaveabaselinesetupthatisclose,youcanmakechangesthedriverfeels,andimprovethecarfurther.Springs,anti-rollbarsandshockabsorberscomefirst.EventhoughWTWvehicledynamicstheorywillremainthebackboneofoursetups,wehopetodevelopprocedureswithrespecttosteeringandsuspensiongeometrythathaveequallygeneralapplication.
AckermanSteeringGeometry
Figure1
Thetypicalsteeringsystem,inaroadorracecar,hastie-rodlinkagesandsteeringarmsthatformanapproximateparallelogram,whichskewstoonesideasthewheelsturn.Ifthesteeringarmsareparallel,thenbothwheelsaresteeredtothesameangle.Ifthesteeringarmsareangled,asshowninFigure1,thisisknownasAckermangeometry.Theinsidewheelissteeredtoagreateranglethentheoutsidewheel,allowingtheinsidewheeltosteeratighterradius.Thesteeringarmanglesasdrawnshow100%Ackerman.Differentdesignsmayusemoreorlesspercentagepro-Ackerman,anti-Ackerman,orAckermanmaybeadjustable.(InfactadjustableAckermanisrare.Thiscouldbethecardesignersayingtous,"Donotmesswiththis.")
FullAckermangeometryrequiressteeringangles,innerwheelandouterwheel,asperFigure1.Theanglesareafunctionofturncentreradius,wheelbaseandtrack.
Inpractise,thesteeringanglesachievedarenotperfectAckermangeometry.InfactAckermangeometryisjustamathematicalconstructofnoparticularsignificanceforcarscorneringatspeed.Weareonlyinterestedinthefactthatwecanhaveincreasingdynamictoeoutandthatitisexponentiallyincreasingwithsteeringangle.SeeFigure7belowforsomeexamplecurves.Soweshallconsider"Ackerman"atermtodescribeanyprogressionofdynamictoeoutgeneratedbythesteeringgeometry.IfitisourchoicetouseAckerman,wemustuseahighpercentagebecause,forsmallsteeringangles,Ackermanisminimal.
Wewillalsolookatthestatictoesetting,becauseofit'sinteractionwithAckerman.
Suspensionmovementmayalsocausechangesintoe(bumpsteer).Toecouldchangewithrollangleofthecar,butprobablynotinanycontrolledwaywecoulduse.Usually,bumpsteerwillbesetatzeroaspartoftheworkshopsetup.Inadditiontotoechanges,effectivesteeringratioisquitevariableinmoststeeringsystems.Driversdonotappeartohaveproblemswiththis.
TyreSlipAngle-themajorvariableintheAckermanstory
Tyreslipangleissimplythedifferencebetweenthesteeredangleofthewheelandthedirectionthetyrefootprintistaking.Themechanismresponsibleforcreatingtheslipangleinteractswithanumberofthesuspensionsettingsonthecar.Forinstance,therollingtyredeformationatthetyrefootprint,resultsinareactiveforce,thesocalled"pneumatictrail",thatappliesa"selfaligningtorque"onthesteeringaxis.Thedrivercanfeelthisthroughthesteering,inadditiontoany"castertrail"thatmaybebuiltintothesuspensiongeometry.Herethough,ourinterestistheinteractionofslipanglewithdynamictoe.
Figure2
Whenthecariscorneringatracingspeeds,steeringAckermangeometryismodifieddramaticallybythetyreslipangles,asperFigure2.WithracingtyresatmaximumlateralG,wemightbelookingat5,6,7or8ormoredegrees,andgenerallymoreslipangleagainfordotroadlegalracingtyres.Lowprofiletyresworkatlesserslipangles.Currently,thestiffestracingtyres,asusedinIRL,operateataround2degreesslipangle.
Figure3
Clearly,slipanglevarieswithtyreload,andcouldinfluencethetoesetting.Asthecarcorners,thetyreloadvariessidetoside,andtheslipanglesincreaseanddecreaseinresponsetotheloadings.Verticaltyreloadingvarieswithcorneringweighttransfer,andalsothetyreloadingandunloadinginresponsetobumpsintheroadsurface.LateraltyreloadingvariesaccordingtothelateralGforce.
Figure3isanexamplegraphofLateralForcevsSlipAnglefromClaudeRouelle'sracecarengineeringseminar.Ifwearegoingtogetahandleonhowtoeangleswork,tyredatalikethishelps.Ascorneringforcebuildsonthetyre,theslipangleisincreasingquickly.Theslopeofthispartofthecurve,the"tyrestiffness",isameasureoftheresponsivenessofthetyretosteeringinputs.Whenmaximumcorneringforceisreachedthecurveflattensout.Ifthedriveriseasyonthetyreshewilldriveinthisareaofthecurve.Ifthedriverstressesthetyresmore,heuseshigherslipangles,withsimilarcorneringforce(lateralforce,grip),butwiththepossibilityofoverheatingthetyres.Thegraphalsoshowstheaffectofchangingloadonthetyre.The300lbbluecurvemightrepresenttheinsidetyre.Ithasahighco-efficientoffriction,2.Thusmaximumlateralforceis2timesverticalload.The900lbcurvemightrepresentthemoreheavilyloadedoutsidetyre.Theco-efficientoffrictionislessat1.6andthereforethemaximumlateralforceisonly1.6timesverticalload.
Firstthingofinterestisthatasthefrontoutsidetyreisloadedupinacorneritwilladoptahigherslipanglethanthemorelightlyloadedinsidetyre.Intuitivelyweknowthismusthappen.Theloadedtyrewilltoeoutmorethanthelightyloadedinsidetyre.Weexpectthatthemoreheavilyloadedtyrewillcontrolthetrajectoryofthecarinthecorner,soallthetoeoutgeneratedwillbeseenattheinsidetyre.Ackermangeometrywillalsoproducetoeout.Addtothisthestatictoeoutyougenerallyrunonaracingcar.Howmuchtoeoutcanthecartakebeforeitstartsdraggingtheinsidewheel?
Willtheinsidetyrebegivingawaygrip?
Itisapparent,thatgainorlossingripwillbeattheinsidetyre,assumingthatoutsidetyregripisatamaximum,andthatthecarisbalanced.
Thereareanumberofobservationswecanmakeatthispoint:
∙SaythecariscorneringatmaximumlateralGandthedifferencebetweentheoutsideandinsideslipanglesisonedegree.Thisequatestoanincreaseintoeoutof6mm.Thisisasignificantchangeintoethatwemightexpecttoinfluencehandling.
∙Asthetyretraversesthecorner,anychangeintyreloadingduetodriverinputorroadsurfacewillresultintoechanges(duetotheslipanglechanges).ThesechangesareadditionaltoanyAckermanandbumpsteerresultingfromthesteeringandsuspensiongeometry.Theinterdependenceofslipanglewithallthevariablesishardtovisualise.Butfortunately,itseemswedohavealargewindowwheretheinsidetyregripwillbeOK.ThetyreinFig3showsprettyconstantgriplevelwhenlightlyloadedbetween4and8degrees,indicatingtheinsidetyreparticularly,canhandlealotofslipanglevariation,andstilloffernearmaximumgrip.Thismeansthatmidcorner,eventhoughthetoeanglesmightbeprettywild,wecanhavenearmaximuminsidegrip.Lookingatthetoeandslipangles,itappearsasifwemightbedraggingtheinsidetire,butnotsowhilewemaintainnearmaximumgrip.
∙Atcornerentry,weexpecttherewillbegreaterneedforprecisioninthedynamictoesetting.Initially,thereisnoAckerman,soweareonlylookingatthestatictoesetting,plusthedevelopingslipangles.
WhattheGuru'sSay
Costin&Phipps,"Racing&SportsCarChassisDesign",1961.Forperformanceandracingcars,theyrecommendedasmallamountofanti-Ackerman,anddidnotdiscussanycircumstancewhereAckermanmightbeused."Owingtoweighttransfer,theoutsidewheelalwaysrunsatahigherloadingthantheinsidewheel,andthereforehigherslipangles,whichnecessitategreaterlock".
CarrollSmith,"TunetoWin",1978.Referringtoanti-Ackerman,hewritesit"cannotberight".HesuggestedthatracingcarsteeringanglesaregenerallytoosmallforAckermantobuild,andthatinthemidcorner,theinsidetyreisnotsufficientlyloadedforittohavemuchaffectanyway(meaningforAckermaneffect-ingeneralconsiderationofinsidetyregripisamajorfocusforsetup).Forcornerentryhepreferedtousesmallamountofstatictoeoutand/or,interestingly,smallamountofbumpsteertoeoutinbump.Becauseofthedifficultyofpredictingdynamicrideheightsidetoside,itmaybepreferabletorunthestatictoeoutrequiredwithzerobumpsteer.Thoseteamswithwheelpositionsensorsanddataloggingcouldtellforsure."EngineerinYourPocket",1998.NomentionofAckerman.Thisissignificant.Twentyyearsafter"TunetoWin"CarrollSmithmusthaveconsideredAckermanadjustmentstillonlyasmallpartofsetup.
DonAlexander,"PerformanceHandling",1991.Hewritesthatanti-Ackermanwasusedinearlieryears.Butthatbythe90's,"Ackermansteeringhasreturned,oftenexceeding100percentgeometry",egforvehicleswithhighaerodownforce.However,hehasgothisexplanationoftheslipangleeffectthewrongwayround,anddoesnotexpandfurther.Finally,hesaysAckermanisadesignelement,notatuningtooltheracerwilluse.
PaulValkenburg,"RaceCarEngineering&Mechanics",1992.Takin
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