Steering Drift and Wheel Movement During Braking: Parameter Sensitivity Studies

Steering drift and wheel movement during braking: parameter sensitivity studies

Item Type Article

Authors Klaps, J.; Day, Andrew J.

Citation Klaps, J. and Day, A.J. (2003). Steering drift and wheel movement during braking: parameter sensitivity studies. Proceedings of the Institution of Mechanical Engineers D: Journal of Automobile Engineering. Vol. 217, No. 12, pp. 1107-1115.

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Link to Item http://hdl.handle.net/10454/882 1107

Steering drift and wheel movement during braking: parameter sensitivity studies

J Klaps1 and A J Day2* 1Ford Motor Company,Belgium 2School of Engineering,Design and Technology, University of Bradford, Bradford, UK

Abstract: In spite of the manysigni cant improvements in carchassis design over the past two decades, steering drift during braking where the driver must applya corrective steering torque in order to maintain course canstill be experienced under certain conditions whiledriving. In the past, such drift, or ‘pull’, would havebeen attributed to side-to-side braking torque variation[ 1], but modern automotive friction brakes and friction materialsare now ableto provide braking torque with such high levelsof consistency that side-to-side braking torque variationis no longer regarded asacause of steering drift during braking. Consequently, other inuences must be considered. This paper isthe rst of two papers to report on anexperimental investigation into braking-related steering drift in motor vehicles.Parameters that might inuence steering drift during braking include suspension compliance and steering o set, and these havebeen investigated to establish the sensitivity of steering drift to such parameters. The results indicate how wheelmovement arisingfrom compliance in the front suspension and steering system of apassenger carduring braking canbe responsible for steering drift during braking. Brakingcauses changes inwheelalignment which in turn a ect the toe steer characteristics of eachwheel and therefore the straight-line stabilityduring braking. It is concluded that arobust design of suspension is possible in which side-to-side variationin toe steer is not a ected bychanges in suspension geometry during braking, and that the magnitude of these changes and the relationships between the braking forces and the suspension geometry and compliance require further investigation, which willbe presented in the second paper of the two.

Keywords: vehicle,automotive, steering, braking, drift, robust design

1INTRODUCTION serious fault that requires immediate attention. However,the causes of steering drift during braking need ‘Steering drift during braking’refers to anyminor devi- to be understood atthe design stage,and this paper ation of avehiclefrom astraight-line whilebraking. By starts to quantify the parameters that cana ect suspen- today’s standards of vehicleperformance, handling and sion and steering geometry changes under wheelbraking drivability,even minor deviation of this type isunaccept- torque loads asa possible cause of steering drift during able.In contrast, the term ‘brake pull’has been used to braking in amodern car,in order to be ableto minimize describe amajor deviation of the vehicleduring braking, the sensitivity of the vehicleto its operational environ- and has, in the past, often been identied with unequal ment. It is generallyconsidered that wearof the suspen- torque generated bythe friction brake on eachof the sion system, particularlyjoints and bushings, could front (steered)wheels of amotor vehicle.The potential increase the sensitivity of the vehicleto its operational danger of brake pull has been recognized byvehicle environment in terms of steering drift during braking, designers for manyyears, and the consistency of per- and this is one of the reasons whyannual vehicletests, formance of modern friction brakes together with required bylaw in manycountries, require the recti- modern designs of suspension and steering geometry cation of worn suspension components. mean that, if brake pull occurs nowadays,there is a The friction brakes on eachwheel of amotor vehicle haveto meet the legalrequirements of stopping in asafe, controlled and predictable fashion. Most modern pass- TheMS wasrecei vedon 12 February 2003 and was accepted after enger cars and light commercial vehicles are tted with revisionfor publication on 29 August 2003. *Correspondingauthor: School of Engineering, Design and Technology, front disc brakes and reardisc or drum brakes. Universityof Bradford,Bradford BD7 1DP, UK. Important factors for controlling braking stability

D02403© IMechE 2003 Proc. Instn Mech. EngrsVol. 217Part D:J. Automobile Engineering 1108 JKLAPSAND AJDAY include control of the clearancebetween the friction rolling is achievedonly in the centre of the tread. This materialand rotor, temperature stabilityof the friction produces camber steer: positive camber willmake the pair such as[ 2]brake fade, excessivethermal distortion wheels turn awayfrom each other, i.e.toe-out, and surface crackingand rotor composition, e.g.trace whereasnegative camber willmake the wheels turn elements of titanium and vanadium[ 3]. towards eachother, i.e.toe-in. The wheeltrack must Eachbrake generates abraking torque which is be set to match the design of suspension to counteract reacted through the suspension components bythe the inherent tendency of the wheels to either move subframe or chassis system [ 2].Although the suspension awayfrom or towards eachother [ 6]. components maybe symmetrical side to side, the 3.Caster is employed to provide stabilitythrough self- subframe and /or chassis system aregenerally not. The aligningof the steered wheels. Decreasing caster will suspension, subframe and chassis systems arecompliant reduce directional stability(and decrease steering to agreateror lesser extent: compliance in the suspension e ort). In the steered wheels of amotor car,uneven system maybe necessary to achievea good ride charac- road surfaces cause alternatinglateral forces atthe teristic, but anundesirable side e ect canbe ‘compliance centre of the tyre contact patch which cancause steer’, which results from the application of lateralor moments about the steering axis,resulting in steering longitudinal forces atthe tyre contact patch. These forces disturbances. deect the suspension bushings and changethe camber 4.Scrub radius is the geometric distance between the and toe angles[ 4, 5],and thus areconsidered to be one centre-line of the tyre contact patch and the steering of the biggestcontributors to straight-line stability axisat the ground plane [ 7].Anegativescrub radius during braking. canimprove the straight-line stabilityof avehicle Compliance canbe introduced into vehiclesuspension whilebraking on split í conditions or during failure systems by: of one brake circuit.

(a) elastomeric (rubber) suspension pivots, The e ects of changes in wheelalignment and steering (b)rubber-mounted cross-members, geometry parameters canbe further compounded bytyre (c)rubber-mounted steering racks and other steering performance parameters. Tyrelateral force and self- joints, aligningtorque arisingfrom pneumatic traildetermine (d)component deection under load, including suspen- the e ectiveslip angleand total axlelateral forces [ 8]. sion links, steering links and the chassis mounts for A di erent lateralsti ness between tyres of the left and suspension and steering. right track of avehicleleads to apermanent steering drift, independent of the drivingsituation. Fivetyre- Deections resulting from the braking forces and torques related parameters that areconsidered to be associated cantherefore be responsible for di erent wheelmove- with steering drift are:tyre ination pressure, tyre tem- ments on eachside of the carbecause, eventhough the perature, conicity, plysteer and residual self-aligning forces maybe the same side to side, the compliances may torque (RSAT): not. The kinematic e ect of this canbe to create dynamic changes in wheelalignment and steering geometry during 1.Higher tyre pressure increases the cornering sti ness: braking, particularlyon the front wheels where braking for agivensmall slip angle,an increase in pressure loads arehighest. The alignment or steering geometry willgive an increase in lateralforce. As tyre pressure parameters of the front (steered)wheels then needs to is decreased, the contact patch becomes longer and be considered. the centre of lateralforce moves rearward,increasing Four parameters associated with steering geometry self-aligningtorque. that could a ect steering drift aretoe steer, camber, 2.Tyre in ation pressure is temperature dependent. caster and scrub radius: 3.Conicity is the lateralforce o set atzero slip angle [8]and acts inthe same direction whether the tyre is 1.The toe setting is designed to compensate for the rolling forwards or backwards. An o -centre belt amount the tyres turn awayfrom straight ahead causes 80per cent or more of conicity forces, the force under drivingconditions. Poor adjustment of the toe sensitivity being typically30 N /mm of belt o set [9]. setting cancause unstable straight-line drivability, Ply-steer forces result from the angleof the belt and asteering reaction is generated if the toe steer plies; the outer plyexerts the dominant e ect, and anglesare di erent between left and right track. The the direction of the ply-steer lateralforce depends on amount of toe-in or toe-out canalso depend on the the direction of rolling. suspension position relativeto the steering gear. 4.Ply-steer forces result from the angleof the belt Toe-in or toe-out is usuallythe same in the design plies; the outer plyexerts the dominant e ect, and the ride position but canchange in opposite directions direction of the ply-steer lateralforce depends on the during body roll, thereby causingthe vehicleto pull direction of rolling [ 10]. to one side. 5.For avehicleto move in astraight line, the sum of 2.Wheel camber generates camber scrub, because true the lateralforces actingon the axlemust equal zero.

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However,even though the sum of the lateralforces The front suspension wasa McPherson strut design, with maybe zero, the sum of the self-aligningtorques the lower wishbone pivoted to the front subframe via actingon the two steering tyres maynot be zero [ 9]. rubber bushes to provide lateraland longitudinal Residual self-aligningtorque (RSAT)is ameasure of location. The subframe wasmounted to the vehiclebody tyre contribution to steering pull; if the total RSAT viarubber mounts. The top of the strut wasmounted iszero, there is zero torque steer from the tyres, and directly to the vehiclebody viarubber bushing atthe therefore no pull. RSATis related to tyre conicity suspension turrets. This design of suspension o ered the and plysteer and is the self-aligningtorque atthe slip opportunity to adjust certain parameters for the pur- anglefor which the lateralforce becomes zero [ 8]. poses of investigation. Instrumentation and equipment to measure the following parameters wereinstalled on Torque steer canbe generated when the outboard con- each carfor initialroad tests to investigatethe sensitivity stant velocityjoint of afront wheeldrive caris not in of the veparameters Ato Eexplained next. line with the wheelspindle because of ride height changes caused byvehicle loading, body roll or weighttransfer. As long asthe system is symmetrical, the e ects on each side of the carwill be equal and opposite. However,if the driveshafts areunequal in length, the articulation 2.3Experimental design angleseach side willbe di erent and aresultant steering An experiment wasdesigned according to astatistical torque willbe generated [ 11].Torque steer e ects can design of experiment methodology, based on ave- be important under vehicleacceleration, but arenot factor two-levelL16 orthogonal array[ 12].This enabled considered here. the main e ects to be identied and gaveno confounding between the major two-wayinteractions. The vefactors areexplained in Table1. The levelsselected werethe 2PARAMETERSENSITIVITY INVESTIGATION largestpractically possible, considered within the con- straints of safetyand /or clearancebetween, for example, 2.1Objective the tyres and the body in the caseof the wheelo set. The purpose of the rst test work wasto nd out which Side-to-side variationin the braking force (e.g.from parameters in the front suspension /steering/braking di erent brake discs or friction materials) wasnot system might havethe greatest inuence on steering drift included since this wasa known e ect. during braking. The tyre and brake temperature (factor A)wascon- sidered to be dependent upon braking under di erent drivingstyles. Hot brakes /tyres could produce di erent 2.2Test equipmentand procedure braking torques which could inuence steering drift during braking. Twoidentical specication test cars wereselected of a The suspension geometry toe-steer curve (factor B), front wheeldrive familysaloon. This particular model the steering gearhousing /engine subframe reinforcement of vehiclewas selected because it wasa standard car cover (factor C)and the front suspension lower wish- manufactured bythe company sponsoring the research. bone rearbush sti ness (factor D)wereselected because

Table 1 Experimentalfactors and settings for parameter sensitivity tests

Factor LevelSetting

Tyreand brake temperature A Tyresand brakes cold A+ Tyresand brakes hot Suspensiongeometry toe-steer B Standard(without spacer) between the steering gear housing curve andsubframe B+ 2.5mm spacerbetween the steering gear housing and subframe onthe left-hand side Steeringgear housing /engine C Withoutreinforcement subframereinforcement cover C + With reinforcement Frontsuspension lower wishbone D ‘Voidedrear bush’ ( This bushhad voids moulded in to provide rearbush sti ness di erent sti nessin two orthogonaldirections: 1200 N /mm in the x directionand 4000 N /mm in the y direction.This was thestandard arrangement.) D+ ‘Non-voided’bush ( 5000N /mmradialsti ness) Wheel o set (y direction) E Standard( 49.5mm wheel o set) E+ With 6mmthick spacer between the wheel spider and hub (55.5mm wheel o set,right-hand side only)

D02403© IMechE 2003 Proc. Instn Mech. EngrsVol. 217Part D:J. Automobile Engineering 1110 JKLAPSAND AJDAY they imparted compliance to the front suspension alternate directions alongthe track to eliminate any assembly. directional e ects. Acooling lapof approximately6 km The wheel o set in the y direction (factor E)was wasdriven after each brake application to return the chosen to investigatethe e ect of scrub radius. Two tyres and brakes to the ‘cold’condition (A ). The ‘hot’ levelsfor eachfactor Ato Ewereset asshown in Table1. condition test wasconducted immediately after the The tests on the two cars werecarried out on a1.2km preceding ‘cold’test. long by8 mwidestraight test track [ 13].The measured response in the experiment wasthe lateraldisplacement of the vehicle,measured bythe number of carriageway 2.4 Results lanes moved from the straight-ahead position during the braking manoeuvre [ 13]. The results aresummarized in Table2. The test procedure wasdesigned to be representative of actualroad usage.After checking the setting and test conditions, the vehicle(unladen weight +driver and 3TESTSTO INVESTIGATESIGNIFICANT observer), tted with newbrakes in ‘virgin’condition, PARAMETERS wasbraked to rest in neutral gearfrom 100km /h at a set deceleration of 30,50, and 70% g (allbrakes oper- 3.1Objective ational) with a‘free’( hands-o )steering wheel.The deviation from straight-line braking wasmeasured, and Havingestablished (section 2)that factor D(the the procedure wasrepeated to atotal veruns from a suspension compliance) and factor E(wheel o set) virginbrake condition. The test runs wereconducted in had asignicant e ect, further tests werecarried out to

Table 2 Resultsof parameter sensitivity investigation

Factor E ect

A Tyreand brake temperature Insignicant B Suspensiongeometry toe-steer curve Insignicant C Steeringgear housing /enginesubframe reinforced C was signicantly worse thanC +,i.e.drift was cover increasedwithout the perimeter frame reinforcement cover.Best suitable level was + (with perimeterframe reinforcementcover) D Frontsuspension lower wishbone rear bush Most signicant e ect. D+ (non-voidedrear bush) sti ness gavesmallest steering drift E Wheel o set Secondmost signi cant e ect A×BInteractionbetween ( A)thetyre and brake Insignicant temperatureand ( B)thesuspension geometry toe-steercurve A×CInteractionbetween ( A)thetyre and brake Insignicant temperatureand (C )thesteering gear housing / enginesubframe reinforced cover A×DInteractionbetween ( A)thetyre and brake Minor interactione ect.When D andA wereboth +, temperatureand ( D)thefront suspension lower A×D became + aswell, whichmeant a reduction wishbonerear bush sti ness inthe steering drift. Thee ectwas similar toDalone, butlarger than A alone(which was insignicant), indicatingthat the interaction e ectwas minor A×EInteractionbetween ( A)thetyre and brake Insignicant temperatureand ( E)thewheel o set B×CInteractionbetween ( B)thesuspension geometry Insignicant toe-steercurve and (C )thesteering gear housing / enginesubframe reinforced cover B×DInteractionbetween ( B)thesuspension geometry Insignicant toe-steercurve and (D) thefront suspension lowerwishbone rear bush sti ness B×EInteractionbetween ( B)thesuspension geometry Insignicant toe-steercurve and ( E)thewheel o set C×DInteractionbetween (C )thesteering gear housing / Insignicant enginesubframe reinforced cover and ( D)the frontsuspension lower wishbone rear bush sti ness C×EInteractionbetween (C )thesteering gear housing / Insignicant enginesubframe reinforced cover and ( E)the wheel o set D×EInteractionbetween ( D)thefront suspension lower Largest signi cant interaction e ect.When D is +, wishbonerear bush sti nessand ( E)thewheel and E is +,thenthe interaction also becomes +. o set This hasa negativee ecton the response (greater steeringdrift )

Proc. Instn Mech. EngrsVol. 217Part D:J.Automobile Engineering D02403© IMechE 2003 STEERINGDRIFT AND WHEEL MOVEMENT DURINGBRAKING 1111 investigatehow steering drift during braking could be (c)increased front suspension lower wishbone rear reduced or eliminated byadjusting these two factors. bush sti ness, standard wheelo set. In each conguration adynamicstraight-line braking test wascarried out with ‘free control’, which required 3.2Test equipmentand instrumentation the driver to applyno correction to anyperceived steer- Another carof the same model was tted with instru- ingdrift during braking. The test procedure wasas mentation that allowedthe dynamiclongitudinal deec- follows: tions and forces in the suspension components, the brake 1.Drive the test carin astraight line ata speed of forces and the dynamicsteering behaviour to be meas- 100 km/h. ured. Aportable computer with A /Dconverter and 2.Apply abraking deceleration of 7m /s2 to rest (engine Ò measuring acquisition software (DIA /DAGO ) was braking is included; the clutch is disengagedjust used to logthese data[ 13].The instrumentation is before the vehiclecomes to rest). summarized below: Two di erent steering methods wereemployed: free con- 1.To measure the longitudinal and lateralacceleration trol and xedcontrol. Freecontrol meant that the and the yawvelocity, a gyrostabilized platform was vehiclewas driven ‘hands-o ’the steering wheel;in this installed near the centre of gravitywhich provided casethe steering wheeltorque waszero. Under xed measurements independent of the roll and pitch angle control the steering wheelangle was held atzero. The of the vehicle. recording of the measured parameters wasstarted 2s 2.The steering wheeltorque and anglewere measured before braking commenced, and nished 2safter the with aninstrumented steering wheel,which replaced vehiclecame to rest. Adeceleration of 7m /s2 was chosen the originalsteering wheel:a positive steering wheel because this showed the most repeatable results without anglesigni es steering to the right (clockwise rotation anyin uence of the anti lock braking system. The con- of the steering wheelby the driver). The same conven- trol and measured parameters aresummarized below. tion applied to the measurement of the steering torque, namelyto steer to the right, apositive (clock- Control parameters: wise) torque must be applied bythe driver. However, 1.Longitudinal velocity. itshould be noted that, if the vehiclewere to drift to 2.Longitudinal deceleration. 3.Steering wheelangle xedcontrol. the right, then anegative(counterclockwise) torque ) 4.Steering wheeltorque free control. would haveto be applied bythe driver to maintain ) course. This is important in the ‘xed’and ‘free’ Measured parameters: control testing explained in section 3.3below. 1.Vehicle response: 3.To measure the steering tyre rod forces of the rack (a) lateralacceleration, and pinion power steering, the originaltie rods were (b)yawvelocity, equipped with strain gaugesto sense only tension ( +) (c)steering wheelangle free control, ) and compression ( ) forces. (d)steering wheeltorque xedcontrol. ) 4.A Datron Correvit sensor wasmounted atthe rear 2.Steering system: of the vehicleto measure the longitudinal speed of (a) toe-steer angles,front axle, the vehicle. (b)camber angles,front axle, 5.To measure the dynamictoe steer and camber (c)steering wheeltorque and angle, angles,the vehiclewas equipped with aZimmer (d)steering tie rod forces. Autokollimator mounted on alightweightframe. This optical devicemeasured the deection of alight beam which wasprojected on to amirror mounted 3.4 Results atthe wheelrim. 3.4.1Standard front suspension lower wishbone rear Fulldetails of the test equipment and instrumentation bush sti ness,standard wheel o set canbe found inreference [ 13]. In the standard conguration, the test carexhibited some steering drift to the left during braking asshown in 3.3Test procedure Fig.1; note that data-loggingstarted attime zero, and the braking manoeuvre commenced atapproximately2 s The test carwas set up inthree test congurations: into the loggingtime. The steering wheelangle measured (a) standard front suspension lower wishbone rear bush under free control [ 13]also showed anegativesteering sti ness, standard wheelo set; wheelangle during braking. Under xedcontrol the yaw (b)standard front suspension lower wishbone rear bush velocityinitially increased and then decreased towards sti ness, increased wheelo set; the end of the deceleration, but remained positive

D02403© IMechE 2003 Proc. Instn Mech. EngrsVol. 217Part D:J. Automobile Engineering 1112 JKLAPSAND AJDAY

ation. Under deceleration the steering tie rod forces were in tension (positive), and, assuming no side-to-side brake force variation,the relativechange in the left /right steeringtie rod forces indicated that the steering o set varied during the braking test, although it remained negative. The higher tension (positive) force in the left tie rod between the start and approximately2 sinto the braking under xedcontrol (2and 4sinto the data-loggingtime shown in Fig.3 )indicated that the left wheeltoe steer (inwards) exceeded the right wheeltoe steer (inwards). This would appear to cause the vehicleto steer to the right during braking (assuming no other compliance Fig. 1 Typicalyaw velocity measurements, xedand free control steer inuences and equal braking torque side to side), and not to the left asexperienced bythe driver and indicated in Fig.1. The corresponding measured steering throughout, indicating acontinuous drift to the left. wheeltorque under xedcontrol is shown in Fig.4; a Under free control the yawvelocity characteristic also positive steering wheeltorque over the same time period remained positive throughout, showing aninitial under xedcontrol indicates that aclockwise torque was increase, then asharp decrease and then anincrease required atthe steering wheel,which againis consistent before decreasing towards the end of the deceleration. with steering drift during braking to the left. Againthis represented adrift to the left, but less continu- Twoseconds after applyingthe brakes, the force in ous. The brake pressures weremeasured and found to the left tie rod decreased to less than that in the right be higher atthe left front wheelthan atthe right front tie rod, indicating toe steer to the left. The corresponding wheel,but, when the brake pipe connections were measured steering wheeltorque then switched from swapped from left to right, the steering drift to the left negativeto positive 2sinto the braking (Fig.4 ),indicat- remained. This conrmed that the drift wasnot aresult ingthat the drift commenced asdrift to the right, but of side-to-side variationin brake actuation pressures. changed during the brake application to drift to the left. Figure2 shows the steering geometry of the test car, This wasconsistent with restraining the left toe steer and and Fig.3 shows the measured steering tie rod forces conrmed the relationship between tie rod force and toe during the test under xedcontrol, standard congur- steer angle. When the vehiclewas driven under free control, no signicant di erence between left and right tie rod forces wasseen (Fig.5 ).In this case,the vehiclewould drift to the direction of the toe steer during braking, assuming that there wereno other inuences. While investigatingthe standard conguration, the e ect of front wheelorientation wasalso investigated more deeply. Any dynamicchange in the caster angleis directly related to the longitudinal sti ness of anyfront suspension. The x and z axisde ections ( x axisrefers to the longitudinal direction and z axisrefers to the vertical Fig. 2 Steeringlayout direction) of the wheelcentre-point weremeasured during braking, together with the x deection of the top of the suspension strut, relativeto the zero position

Fig. 3 Steeringtie rod forces during the braking test, xed control Fig. 4 Steeringwheel torque, xedcontrol

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positive to negativecaster angleduring braking willnot improve vehiclestability.

3.4.2Standard front suspension lower wishbone rear bush sti ness,increased wheel o set

The steering o set of the carwas altered from 6.5 to +1.5mm bychanging the wheelo set by tting aspacer between both front wheels and the hubs, thereby changingthe tyre contact patch (scrub radius) centre but not the kingpin geometry. Figure7 shows the steering tie rod forces; during braking the tie rods werestill in tension, though the magnitude of the tie rod forces waslower. The vehicleyaw velocity response waslittle changed Fig. 5 Steeringtie rod forces, free control from that shown in Fig.1. before the application of the brakes. Knowing this and 3.4.3Increased front suspension lower wishbone rear the body pitch angleallowed the absolute caster to be bush sti ness,standard wheel o set calculated. The compliance of the lower wishbone of the front sus- The dynamicchange in the right wheel(rearward ) pension (both sides) wasdecreased by tting asti er longitudinal deection wasfound to be initiallylarger, bush to the rearpivot. The steering tie rod forces and which would cause adrift to the right, asshown in Fig.6. the toe-steer angleswere measured during braking, and From these measurements the corresponding maximum the results areshown in Fig.8. The toe-steer angles caster anglechanges werecalculated, and areshown in with the originalbushes and the sti er bushes arecom- Table 3. pared in Figs9 and 10.In this conguration, no steering Under dynamicbraking, the right caster angle drift during braking wasgenerated that wasdiscernible changed from 1.68 to 0.8 ,equivalent to adynamic + ° to the driver. changein the caster o set from 9.38to 3.8 mm. The left caster o set changed from +1.60 to 0.45 mm. A negativecaster angledoes not automaticallylead to a changein steering direction, but adynamicchange from

Fig. 7 Steeringtie rod forces with additional spacers, xed Fig. 6 Longitudinalde ection of the wheel centre-point control

Table 3 Maximumvalues of thedynamic caster

Static Static Dynamic Dynamic measuredmeasured measured measured value,value, castervalue, caster value, De nition Nominalvalue left track right track left track right track

Casterangle (deg )3.00 1.60 1.68 0.45 0.8 Castertrail at wheel0 2.96 1.48 Not Not centre (mm) measurablemeasurable Caster o set(mm) 14.64 10.46 9.38 1.5 3.8 Scrubradius (mm) 6.05 6.695 6.55 Not Not measurablemeasurable

weretherefore considered for further investigation. A third factor, the tment of asteering gearhousing /engine subframe reinforced cover, wasfound to be signicant but wasnot investigated. The temperature of the tyres and brakes (cold or hot) wasnot found to be signicant. This indicated that there wereno side-to-side di erences in braking torque generated bybrakes in either the hot or the cold condition. The results clearlyindicated that, for this particular design of suspension and steering geometry (6mm nom- inalnegative o set), side-to-side variationin the braking forces atthe front wheels wasnot asignicant factor in the generation of steering drift during braking. This Fig. 8 Steeringtie rod forces, with sti erlower wishbone rear means that anyattempt to enhance straight-line braking bushes, xedcontrol stabilityby close control of the frictional performance of the brakes would not, in this instance, be appropriate. Negative o set steering is therefore conrmed to have minimum sensitivity to side-to-side brake torque variation. The suspension geometry toe-steer curve [dened as the wayin which the toe-in setting of the front (steered) wheels changes asthe suspension deects in bounce or rebound]had no reproducible e ect, which conrmed that the verticalde ection of the front suspension during braking did not a ect steering drift. This wasinitially considered to be apossible important inuence, because of the suspension movement of the steered wheels relativeto the xedsteering rack. Further investigation of the two most signicant parameters found conrmed their importance in a ecting Fig. 9 Toe-steerangles, lower wishbone with standard rear steering drift during braking. What became veryclear bushes from the test datawas rstly that changes in wheelpos- ition and orientation did occur during braking, and sec- ondly that the magnitude of such changes variedduring the braking manoeuvre. Thus, the drift experienced by the driver, and measured in terms of anumber of parameters, wastime-dependent, and appeared evento change from one direction to the other (right to left on the particular cartested ). The more detailed study of the front wheelgeometry under braking loads enabled dynamicvalues of wheel orientation (caster) to be calculated and compared with the static measured and design values.Comparisons of caster angleshowed that the measured valuewas below the design value( but still in specication), but under dynamicconditions it went veryslightly negative. This willnot directly a ect steering drift during braking but Fig. 10 Toe-steerangle, lower wishbone with sti er rear is anindication of the magnitude of wheelde ection due bushes to suspension compliance e ects. Changingthe steering o set of the carby tting a 4DISCUSSION OFRESULTS spacer between the front wheels and the hubs changed the tyre contact patch centre but not the kingpin The parameter sensitivity studies showed that suspension geometry. However,this produced no changein the per- compliance (as dened bythe front suspension lower ceiveddrift of the carduring braking. The spacer tted wishbone rear bush sti ness) and the steering o set (as changed the steering o set from negativeto slightlyposi- dened bythe wheelo set) werethe two most signicant tive,but this appeared to be insu cient to create any parameters of the veinvestigated. These parameters signicant improvement. Concern would be raised if

Proc. Instn Mech. EngrsVol. 217Part D:J.Automobile Engineering D02403© IMechE 2003 STEERINGDRIFT AND WHEEL MOVEMENT DURINGBRAKING 1115 such achangewere to be suggested asa route to In this particular case,the lower wishbone rear bush improvement because of the stabilityassociated with the wasfound to playa major role in controlling the wheel negative o set design. deections, and, interms of steering drift during braking, Addressing suspension compliance byinserting a reducing the compliance byinserting asti er bush in the sti er bush in the rear pivot of the lower suspension arm rear pivot of the wishbone o ered agreaterdegree minimized the deection and controlled the wheelorien- of robustness in the steering drift e ects for this type tation better during braking, and reduced toe-steer of MacPherson strut suspension. Understanding the e ects to anegligiblelevel. Not only did the deection deection characteristics of other types of suspension of this component relativeto the subframe appear to under braking o ers asubstantial challengeto the generate largechanges in suspension geometry, mainly automotive design engineer. in the side-to-side camber and steering o set, but the e ect wasalso to changetoe-steer anglesduring braking which actuallyreversed side to side during the braking ACKNOWLEDGEMENTS manoeuvre. This result indicates that the major cause of steering drift during braking cantherefore be concluded This paper presents research carried out aspart of an to be side-to-side dynamicvariation in the deformation MPhil study with the University of Bradford, United and deection of suspension and steering components. Kingdom. The authors aregrateful to allwho contrib- Dynamictoe-steer changes area direct result of such uted to the research, including sta in the Ford Motor variation,and therefore minimizing di erential defor- Company,IKA (Aachen) and supplier companies. mation and deection inthe suspension geometry isvery Thanks alsogo to the Directors of the Ford Motor important. Companyfor permission to publish this paper.

5CONCLUSIONS REFERENCES

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