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Steering Drift and Wheel Movement During Braking: Parameter Sensitivity Studies

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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. Rights © 2003 IMechE. Reproduced in accordance with the publisher's self-archiving policy. Download date 24/09/2021 19:01:02 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 suspen- sion 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 each wheeland therefore the straight-line stability during braking. It is con- cluded 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 rear disc 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 A JDAY include control of the clearancebetween the friction rolling is achievedonly in the centre of the tread. This materialand rotor, temperature stability of 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 each other [ 6]. components maybe symmetrical side to side, the 3.Caster is employed to provide stability through 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 alternating lateralforces 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 stability of 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, even though 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
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