转向.docx

转向.docx

《转向.docx》由会员分享，可在线阅读，更多相关《转向.docx（19页珍藏版）》请在冰点文库上搜索。

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