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HSRI Report No. PF-100a

VEHICLE HANDLING TEST PROCEDURES

Howard DugoffR.D.EninLead el

Highway Safety Resetzrch InstituteUniversity of MichiganHuron Parkway and k t e r RoadAnn Arbor, Michigan 48105

November 1970

Summary Final Report

Contract FH-11-7297

Prepared forNational Highway Safety BureauU.S. Department of Transportation- - - washinson, D.C.20591 -

Availability is unlimited. Document may bereleased to the Cleamghouse for FederalSoientzc and Technical Information,Springfield, Virginia 22151, for sale to thepublic.

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T h e c o n t e n t s o f t h i s r e p o r t r e f l e c t t h e v iew so f t h e Highw ay S a f e t y R e s e a r c h I n s t i t u t e w h i chi s r e s p o n s i b l e f o r t h e f a c t s and t h e a c cu ra cyo f t h e d a t a p r e s e n t e d h e r e i n . The c o n t e n t s don o t n e c e s s a r i l y r e f l e c t t h e o f f i c i a l view s o rp o l i c y o f t h e D e pa rt m en t of T r a n s p o r t a t i o n .T h is r e p o r t d oes n o t c o n s t i t u t e a s t a n d a r d ,s p e c i f i c a t i o n o r r e g u la t i o n .

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'. R e p ~ r tNo.

' . T ~ t l e n d Subtitle I/ VEHICLE HANDLING TEST PROCEDURESiI

i; H. D u g o f f , R . D. E r v i n , a n d L. S e g e l / P F - 1 0 0 - a

2. Government Accession No.

i5 . Report D te~ o v e m t e r1 9 7 0 I6. Performing Organization Code i

11 7 . Author(s1

3. Recipient's Catalog No.II

8. Perform~ngOrganlzat~onReport No.1

N a t i o n a l H i gh wa y S a f e t y b u r e a u' U .S . D e p a r t m e n t o f T r a n s p o r t a t i o ni W a s h i n g t o n , D . C . 2 0 5 9 1

9 . Perform~ngO r g a n ~ z a t ~ o nam e and Address1 H ig hw ay S a f e t y R e s e a r c h T n s t i t u t e1 U n i v e r s i t y o f M ic h ig a nH u r o n P a r k w a y 6 B a x t e r R o a d 11. Contract or Grant No.I F H - 1 1 - 7 2 9 7 IA nn A r b o r , M i c h i g a n 4 81 0 5 -

7 / 6 b - 1 1 / 7 01 14 . Sponsoring Agency CodeI

10. Work U n ~ t o .

! 12. Sponsoring Agency N a n ~ e nd Address

I--L1 Supplelnentary Notes! 13. Type of Report and Period CoveredSummary F i n a l R e ~ o r tA s e t of s a f e t y - r e l e v a n t p e r fo r i n a n c e q u a l i t i e s w e r e d e f i n e d f o r t h ep a s s e n g e r c a r a s a f i r s t s t e p i n t h e d ev e l o p m en t o f o b j e c t i v e m e a s u r es o f

p r e c r a s h s a f e t y p e rf o r m a n c e . 3 le a s u r es w e re s o u g h t t h a t s t r e s s t h e p e r -f o rm a n c e p r od u c e d by a p a s s e n g e r v e h i c l e when i t i s o p e r a t e d u n d e re m e rg en cy , c r a s h - a v o i d a n c e c o n d i t i o n s . T h i s g o a l l e d t o t h e i d e n t i f i c a t i o n 'o f s i x l imit m a n e u ve r s , an d a s s o c i a t e d l i m i t r e s p o n s e s , t o s e r v e a s af i r s t - o r d e r m ea n s o f a s s e s s i n g t h e s a f e t y q u a l i t y o f a m o t o r v e h i c l e . Twoo f t h e m a n eu v er s i n v o l v e d c o n t r o l i n p u t s s o c om p le x t h a t a d r i v e r c o u l dn o t p er f o rm th e m w i t h a c c e p t a b l e f i d e l i t y . T h e se m a ne u v er s a c c o r d i n g l yw e re p e r f o r m e d u s i n g a n a u t o m a t i c c o n t r o l s y s t e m t o m a n i p u l a t e t h e Is t e e r i n g , b r a k i n g , a n d a c c e l e r a t o r c o n t r o l s . T he v i a b i l i t y an d d i s -c r i m i n a t o r y po we r o f t h e p r o p o s e d t e s t p r o c e d u r e s w e re d e m o n s t r a t e d bya p p l v i n g t h e s e p r o c e d u r e s t o f o u r s e p a r a t e v e h i c l e s r e f l e c t i n g w id e lyd i f f e r e n t d e s i g n p h i l o s o p h i e s a n d t r a s n p o r t o b j e c t i v e s .I

- ---pp! 1 7 K L ~ioro,I v e h i c l e d y n a m i c s , v e h i c l e p e r f o r m a n c e ,v e h i c l e h a n d l i n g , c r a s h a v o i d a n c e , e m e r-g e nc y m a n e u v e r s , t e s t p r o c e d u r e s--I . I c iun ly Clnsr ~ rot ,hi$ report) 20. Securlty Class~f.(of this page)I

18. Dist r~bur ionStatementA v a i l a b i l i t y i s u n l i m i t e d . D o c u m e n t ,may b e r e l e a s e d t o t h e C l e a r i n g h o u s e !f o r F e d e r a l S c i e n t i f i c an d T e c h n i c a l ;I n f o r m a t i o n , S p r i n g f i e l d , V i r g i n i a i2 2 1 5 1 , f o r s a l e t o t h e p u b l i c . ,

I U n c l a s s i f i e d U n c l a s s i f i e d

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TABLE OF CONTENTSPage

I n t r o d u c t i o n . . . . . . . . . . . . . . . . . . . . . . . . 1Safe ty-Relevant Performance Qual i t ies of Motor Vehic les . . . 2. . . . . .otor Vehicle Performance and Highway Safety 2. . . . . . . . . . . . . .i m i t Performance Maneuvers 4. . . . . . . . . . . .i m i t Braking (No St ee ri ng ) 4

Response t o Rapid. Extreme Steering (No Braking) . . 4Braking i n a Turn (Fixed. Non-Zero S t ee r Angle) . . 5. . . . . . . . . . . .urning on a Rough Surface 5

. . . . . . . . . . . . . . . .apid Lane Changing 5"D ra st ic t t St ee r and Brake Maneuver . . . . . . . . 6. . . . . . . . . . . . . .est Vehicles and Equipment 7

L i m i t Braking Performance . . . . . . . . . . . . . . . 14Response t o Rapid . Extreme St ee r i ng . . . . . . . . . . 18. . . . . . . . . . . . . . . . . . .raking i n a Turn 23Turning on a Rough Surface . . . . . . . . . . . . . . . 2 5. . . . . . . . . . . . . . . . . .apid Lane Changing . 28Drastic Steer and Brake Maneuver . . . . . 3

Concluding memarks . . . . . . . . . . . . . . . . . . . . . 36References . . . . . . . . . . . . . . . . . . . . . . . . . 38

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INTRODUCTION

This report summarizes the findings of a study performedby the Highway Safety Research Institute (HSRI) for the NationalHighway Safety Bureau (NHSB) entitled, "Vehicle Handling TestProcedures." The major purpose of the study was the develop-ment of objective procedures for measuring safety relatedaspects of the dynamic performance of passenger cars.

At present, the complex relationship between vehicle per-formance and highway safety is neither theoretically understoodnor experimentally documented. There is nonetheless ampleintuitive basis to hypothesize that such a relationship existsand, further, that there are certain specific performancecharacteristics of motor vehicles which, during either thenormal driving process or during emergency situations, causethe potential for loss of control to rise above a thresholdbeyond which driver skill and experience are of little avail.The problem remains to (1) identify such safety-relevantperformance qualities, and (2) develop reliable, objectiveprocedures for their measurement.

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SAFETY-RELEVANT PERFORMANCE QUALITIES OF MOTOR VEHICLES

MOTOR VEHICLE PERFORMANCE AND HIGHWAY SAFETYAlthough the broad spectrum of highway vehicles categorized

as passenger cars differ markedly in their dynamic performancecharacteristics, these qualities do not influence the highwaysafety record in a directly recognizable manner.

Notwithstanding the fact that pneumatic-tired vehicleshave been designed and produced for many years, there are, asyet, no commonly accepted qualities or attributes associatedwith the concept of pre-crash safety performance. In attempt-ing to formulate a complete catalog of such qualities, we haveconcluded that they generally are subsumed within the followingfour major categories:

(1) controllability sufficient for evasive action;(2) limit maneuver capabilities, i,e., the upper

bounds of maneuvering performance achievableunder braking, traction, and cornering con-ditions

( 3 ) dynamic response characteristics bearing onthe ability of a driver to close the controlloop in a stable manner during an emergencymaneuver

(4) insensitivity of limit-maneuver capabilitiesand dynamic response characteristics to externaldisturbances, environmental conditions, and ser-vice factors.

Research has shown that the motor vehicle constitutes amechanical system amenable to control by a population ofwidely ranging skills [I]. Further, it has been demonstratedthat the human operator possesses levels of flexibility andadaptability which allow him to adjust easily to a broad range

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of vehicle characteristics [ Z ] .Given that the above statements are true, it appears

logical to define pre-crash safety qualities related to thelimiting maneuver characteristics of a vehicle (as may becalled upon in an emergency situation) as opposed to qualitiesrelated to the dynamic interaction between man and his vehicle.This is not to say that properties of the man-vehicle combinationdo not possess implications with respect to "safety performance,"for indeed they do. However, the subtleties of this interaction,and our knowledge of man's adaptability, suggest that first at-tempts to assess the safety performance of vehicles be restrictedto defining and measuring qualities that reside "within" thevehicle. Such attempts should recognize the limited role ofthe vehicle in accident causation and should thereby stressthose qualities that give drivers increased opportunity foravoiding catastrophic consequences of an improper decision oract. Vehicle attributes that result in less demands being placedon drivers as a result of operations in an unfavorable environmentshould also be considered as having safety relevance.

It appears that particular emphasis should be given to"service factors" in any assessment of roadworthiness. Thesefactors describe the degree to which an operational vehicledeparts from a reference design condition, Vehicle loadingsranging from empty to full represent a commonplace example ofservice factor variations. So too do deviations from thenominal (manufacturer's recommended) distribution of tireinflation pressures. It is clear that service factors exertsignificant influence on performance as achieved in the fieldand that there are comhinations of service factors which mitigateagainst achieving the levels of performance exhibited by thebaseline or reference vehicle.

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LIMIT PERFORMANCE MANEUVERSThe premises and conclusions discussed above constitute

a basis for defining a family of safety-relevant handlingtest procedures. These procedures derive from a concept of"limit performance maneuvers," i.e., extreme, yet realistic,prototypical maneuvers in which vehicle performance qualitiesplay a significant and clearly defined role. Six limit per-formance maneuvers, as discussed below, have been selectedto produce a first-order assessment of safety performance.

LIMIT BRAKING (NO STEERING), Braking effectiveness isa roadworthiness component with an intimate and well recognizedconnection to safety - - all other things being equal, theshorter the distance a car can stop in, the safer it is. How-ever, stopping distance (or deceleration) per se is but apartial descriptor of a motorcar's braking performance. Itis also important to take into account the vehicle's control-lability characteris tics as it approaches and attains thebraking limit. This can be done very concisely by using theconcept of braking efficiency, i.e., the ratio of maximumdeceleration achievable without wheel locking to the pre-vailing pavement friction coefficient. Since wheel locking isa condition which either makes it impossible to apply effectivesteering control (front wheel locking) or dramatically degradesdirectional stability (rear wheel locking), braking efficiencycan be regarded as a measure of vehicle performance having directsafety implications.

RESPONSE TO RAPID, EXTREME STEERING (NO BRAKING). Thismaneuver is performed by applying a quasi-step steering inputto a vehicle initially coasting on a straight line path. Thespiral trajectory characteristically produced is hardly repre-sentative of a realistic highway maneuver. Its initial "J-turn" phase, however, is similar to the initial phase of atypical obstacle-avoidance maneuver. If the maneuver isrepeated with progressively increasing steering inputs (ata fixed value of initial speed), the results may be interpreted

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in terms analogous to the braking efficiency measure dis-cussed above. The limit response condition can correspondto lateral force saturation either of the vehicle's fronttires (drift-out), or of its rear tires (spin-out) [ 3 ] . Thepoint at which the limit occurs, its basic character, and howit is affected by realistic variations in service factors areperformance characteristics of potential safety-relevance.

BRAKING IN A TURN (FIXED, NON-ZERO STEER ANGLE). Thebraking effectiveness achievable at the point of wheel lockup(normalized with respect to the prevailing level of pavementfriction) appears to be a significant roadworthiness componentrelative to any fixed-steer braking maneuver. Important alsois the order of tire shear force saturation, which dramaticallyinfluences the nature of the limit trajectory response. Itfollows that the performance qualities manifested in this maneuvercan profitably be assessed in terms of a generalized interpreta-tion of braking efficiency, namely, one that takes account notonly of the limit longitudinal deceleration, but also of thedeviation from the nominal circular trajectory produced as thelevel of deceleration is increased towards its limit value.

TURNING ON A ROUGH SURFACE. A maneuver serving to pro-vide an objective roadholding evaluation can be performed bydriving a coasting vehicle in a curved path (with the steeringwheel held fixed) over a series of pavement irregularities.Qualitatively, the maneuver is similar to real world situationscommonly experienced in driving over washboard roads. The degreeto which the path-curvature response of the fixed-steer vehicleis influenced by the road disturbance would appear to representa significant component of an overall road-worthiness assessment.

RAPID LANE CHANGING*. At very low speed, a lane changecan be executed perfectly by applying a steering input of sin-usoidal form. -4s the speed increases, however, the steering-* A lane change is defined here as a maneuver in which steeringinput causes a vehicle to be displaced laterally with zeronet change in its heading angle.

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inp ut r eq ui re d t o produce a su cc es sf ul lan e change becomes con-s id er ab ly more complex. By th e same tok en, th e respo nse t o ape r f e c t l y symme t r i c a l s t e e r i npu t , suc h a s a s i nuso i d , be c ome spr og re ss iv el y more asymmetr ica l . For an inp ut of given s i z e ,the magnitude of t h i s a symmetry appears t o rep re se n t a ve h i c ler es po ns e c h a r a c t e r i s t i c d i r e c t l y r e l a t e d t o t he d i f f i c u l t y ad r i v e r would e nc ount e r i n c l o s i ng t he l oop t o produc e a suc c e ss -f u l l a ne ch ange. I n p a r t i c u l a r , i t i s hypo t he s i z e d t ha t t heg r e a t e r t h e d e p a r tu r e from a t r a j e c t o r y whose f i n a l and i n i t i a lp a t h s a r e p a r a l l e l , and t h e g r e a t e r t h e e vi de nc e o f o s c i l l a t o r yo r uns t a b l e beha v i o r , t he ha rde r i t w i l l be f o r t y p i c a l d r i v e r st o m od ul at e t h e i r s t e e r i n g i n emergency s i t u a t i o n s .

"DRASTIC" STEER AND BRAKE MANEUVER . The followingseque nc e o f c o n t ro l i n pu t s , w hi ch f a l l w i t h i n t h e un i ve r se ofp o t e n t i a l e mergency c o n t r o l a c t i o n s , p r o vi d es a s t r i n g e n t t e s to f t h e s u s c e p t i b i l i t y o f a s u s pe n s io n s ys te m t o " j a ck i n g I '" t uc k in g , " o r o t h e r u n d e s i r a b l e r e s p o n s e: ( 1) a h a l f s i n ewave of s t e e r i ng i npu t ( t h e f i r s t ha l f o f a l a ne changemaneuver) and ( 2 ) a ha rd pu l s e brake input o f one ha l f seconddura t ion appl ied when the yaw response to the s t ee r input i sapproaching i t s peak magni tude . This combinat ion of co nt ro linputs simulates a real-world emergency maneuver where thed r i v e r a t t e mpt s t o a vo i d a n obs t a c l e by s i mul t ane ous s t e e r i ngand b ra k i ng , t he n r e l e a s e s t he b r a kes upon pe rc e i v i ng t he s k i d -d i ng r e su l t i n g from wheel l oc k i ng , The c r uc i a l j unc t u re i n t hemaneuver i s the in s ta n t immedia tely fo l lowing brake re l ea s e ,when t he locked wheels suddenly s pi n up, and t he magnitudesof t h e l a t e r a l t i r e f o r c e s i n c r ea s e from n e ar z e ro t o r e l a t i v e l yhi gh v a l u e s . As t h e maneuver becomes more extrem e a motorv e h i c l e w i l l e x h i b i t a n in c r e a s i n g l y s e v e r e r e sp o n se , p o s s i b l yt o t h e p o i n t of r o l l o v e r . The c h a r a c t e r of t h e r e s u l t i n g r e -sponse and t he maneuver se ve r i ty l e ve l a t which i t oc c ur s a r ep er fo rm an ce c h a r a c t e r i s t i c s h av in g d i s t i n c t s a f e t y - r e l e v a n c e ,

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TEST PROGR.MI

TEST VEHICLES AND EQUIPMENTThe object of the test program was to refine andevaluate the proposed test procedures and equipment. To

this end, four vehicles reflecting widely differing designphilosophies and transport objectives were selected for test.The selection was guided by the requirement to assess thediscriminatory power of the procedures, not a requirement tomeasure the performance of specific vehicles, per se.

The four vehicles were the following:(1) 1967 Ford Country Sedan Station Wagon - - a full size

station wagon whose static directional stabilityvaries over a wide range as a function of servicefactors

( 2 ) 1970 Toyota 2000 GT Sports Coupe - - a sports carcharacterized by high steering gain, lowheight to track ratio, and a high horse-power to weight ratio

(3) 1961 Chevrolet Corvair - - a compact sedan with arearward weight bias, and an independent rearsuspension (swing axle).

(4) 1969 Mercedes 250 Sedan - - an expensive intermediatesedan with independent rear suspension whose costreflects a substantial expenditure of performance-oriented, engineering effort

The Corvair and Fords were used cars and thus requiredextensive overhauling before testing. New brake linings,shock absorbers, and, where necessary, ball joints and wheelbearings were provided. Outrigger arms were installed to preventroliovers (see Figure 1).

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FIGURE 1, OUTRIGGER INSTALLATION ON THE CORVAIR

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F ou r o f t h e s i x t e s t p r o c e d u r e s w e re e x e c u t e d by t e s td r i v e r s , w i t h t h e a i d o f p a s s i v e b r a k e a nd s t e e r i n g l i m i t e r s ,( s e e F i g u r e s 2 a n d 3 ) , a n d t w o w e r e p e r f o r m e d u s i n g a n a u t o -m a t i c c o n t r o l s y t em . The l a t t e r two t e s t s ( c o r r e s p o n d i n g t or a p i d l a n e c h a n g i n g an d " d r a s t i c " s t e e r an d b r a k e m a n eu v er s)i n v o lv e d c o n t r o l i n p u t s s o co m ple x t h a t a d r i v e r c o u l d n o tp e r f o rm them w i t h a c c e p t a b l e f i d e l i t y .

The a u t o m a t i c v e h i c l e c o n t r o l l e r p r o v i d e s c o n t r o l i n p u t st o t h e s t e e r i n g w h e e l , b r a k e p e d a l , a n d a c c e l e r a t o r i n t h r e ed i s t i n c t o ~ e r a t i o n a lm o d e s :

1. a d r o n e mode, w h er eb y t h e c o n t r o l l o o p o n v e h i c l ed i r e c t i o n a n d s p e e d i s c l o s e d m a n u a l ly by a no p e r a t o r i n a f o l l o w i n g v e h i c l e , u s i n g a p u l s e -m o d u l a t i ng r a d i o t r a n s m i t t e r .

2 . a p r o g r a m - e x e c u t io n m ode , i n w h ic h t h e s t e e r , b r a k e ,an d a c c e l e r a t o r c o n t r o l i n p u t s o r i g i n a t e f r o m a n o n -b o a r d prog ra mm ed f u n c t i o n g e n e r a t o r w i t h no l o o pc l o s u r e o n v e h i c l e r e s p o n s e .

3. a n a b o r t mode , i n w h ic h t h e b r a k e s a r e a p p l i e ds p o n t a n e o u s ly u p o n t h e o c c u r r e n c e o f c r i t i c a l f a i l -u r e s , o r c a n b e commanded bv t h e t e s t o p e r a t o r t h r o ug ht h e t r a n s m i t t e r l i n k .

The c o n t r o l l e r i s c o m p r i se d o f n i n e m a jo r s u b a s s e m b l i e s[ s e e F i g u r e 4 ) , w hose t o t a l w e i g h t i s 2 6 0 p o u n d s . T h e s y s t e mi s l i m i t e d t o u s e w i t h v e h i c l e s h a v i n g a u t o m a t i c t r a n s m i s s i o n s .I t i s d e s i g n e d t o f i t i n t o a n y s i z e p a s s e n g e r c a r .

The s t e e r i n g a n d b r a k e s e r v o s a r e e l e c t r o - h y d r a u l i cp o s i t i o n f e e d b a c k d e v i c e s . A h y d r a u l i c m o t o r m ou nte d t o t h es t e e r i n g c olu m n d r i v e s a p u l l e y m ou n te d o n t h e s t e e r i n g s h a f tt h r o u g h a t i m i n g b e l t ( s e e F i g u r e 5 ) . A s m a l l h y d r a u l i ca c t u a t o r w i t h m a n i f o l d a nd v a l v e a s se m b ly a l s o m ou nte d o nt h e s t e e r i n g co lu m n p u s h e s d i r e c t l y on t h e b r a k e p e d a l , a sshow n i n F i g u r e 5 . The a c c e l e r a t o r s e r v o i s a DC t o r q u e

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F I G U R E 2 , A D J U S T A B L E S T E E R I N G W H EE L S TO P D E V I C E A ND S T E E R I N GW HE EL R O T A T I O N P O T E N TI O M E TE R I N S T A L L E D I N ME RC ED ES

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- - - - - - - - -INSTRUMENTSANDRECORDERHYDRAULIC

POWER STEERINGPACKAGE SERVO

MASTERLO G I C r: - BRAKINGC E I V E R ' NETWORK ' SERVO-

CONTROLV E H I C L E .

ACCELERATORFUNCTION SERVO

GENERATOR 8

ABORT

ITEST VEHICLE- ---

F I G U R E 4 1 B L O C K D I A G R A M O F A U T O M A T I C V E H I C L E C O N T RO L L E R

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FIGURE 5, STEER AND BRAKE SERVOS IN ST AL LE D I N FORD ST AT IO N WAGON

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motor which uses simple pinion and rack gearing to obtaina rectilinear output.

The function generator (see Figure 6) is the key componentof the automatic control sytem. It is an electro-mechanicalinstrument which stores the maneuver program and, ultimately,commands the servos to execute a programmed steering, brakeand accelerator control input. The steering time history canassume any functional shape, analytic or nonanalytic. Thebrake time history is confined to a ramp-fronted step ofvariable ramp slope, step height, and duration.

The tests were conducted on the East Ramp of the Universityof Michigan Willow Run Airport. This ramp is a 3300 x 425 feetconcrete-paved pad, a portion of which has been resurfaced withasphalt. Some straight braking tests were performed on a paintedportion of the asphalt strip, wetted down to provide a low frictioncoefficient surface. All other testing took place on the concretepavement. Experiments in which severe maneuvers (e.g., "drastic"steering/braking) were performed at speeds of 60 mph or morerequired just about all of the available area for accelerationto speed, test execution, and runout.

All four test vehicles were exposed to the driver-controlledexperiments. Only three (all but Mercedes) were tested with theautomatic controller.LIMIT BRAKING PERFORMANCE

These tests involved the measurement of straight-linebraking effectiveness, from an initial speed of 30 mph, witha program of brake inputs designed to permit the precisedetermination of deceleration at the point of incipient wheellockup (hence braking efficiency).

Each test vehicle was subjected to straight-line brakingtests on two yavement surfaces: dry concrete and wet, paintedasphalt. Tests were run on all vehicles in an "unloaded" con-

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FIGURE 6 , FUNCTION GENERATOR FOR AUTOMATIC VEHICLE CONTROLLER

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dition (test driver plus instrumentation package). In addition,the Ford Station Wagon was tested with a load of 450 lb in thecargo compartment.

Effectiveness data from the straight-line braking testsare presented in Figure 7, Stops in which wheel locking wasencountered are so indicated. Absolute braking efficienciescomputed from the plotted data on the basis of nominal frictioncoefficients for the different test surfaces (obtained fromtraction measurements on a single, arbitrarily selected tire)are given in Table 1.

TABLE 1ABSOLUTE BRAKING EFFICIENCIES

Absolute 11 I Braking Efficiency 1l~ehicle onfiguration/ Ford, empty

Ford, loadedToyo taCorvair

The results summarized in Figure 7 and Table 1 appearto provide a meaningful assessment of a safety-relevant aspectof vehicle performance. Substantial differences in the maximumability of the vehicle to decelerate without gross degradationof stability and/or controllability have been perceived andquantified.

The influence of speed on absolute braking efficiencyderives from its influence on the peak coefficient of tire-roadfriction. Although this can certainly be a significant effect

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Ford

Wheels Locked?Test Conditions No Yes

Corvair

Wet Painted AsphaltDry ConcreteDry Concrete, Vehicle Loaded,

Mercedes1.01 I I I I 1

A A0o I

Brake Line Pressure, psi Brake Line Pressure, psi

F I G U R E 7 , BRAKING EFFECTIVENESS DETERM INED FROM STRAIGHT-LINEBRAKING TESTS

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( p a r t i c u l a r l y i n t h e c a s e o f w e t pa v em en t when h y d r o p l a n i n gi s a p o s s i b i l i t y ) , i t i s m ore a p p r o p r i a t e l y m e a s u r e d d i r e c t l yt h r o u g h t i r e t r a c t i o n t e s t s t h a n th ro u g h v e h i c l e t e s t s w hic ha r e c on fo u nd e d b y many o t h e r f a c t o r s .

The i n f l u e n c e of p av em en t f r i c t i o n on b r a k i n g e f f i c i e n c yi s b o t h i m p o r t a n t an d c om p le x. O ve r t h e lo n g t e r m , a t t e m p t ss h o u l d b e made t o d e v e l o p a c o m p r e h e n s iv e p r o c e d u r e f o re v a l u a t i n g a b s o l u t e b r a k in g e f f i c i e n c y a s a f u n c t i o n o fp av em en t f r i c t i o n , by m eans of a s y s t e m a t i c p ro g ra m o f t i r et r a c t i o n t e s t i n g o n a s e r i e s o f d i f f e r e n t s u r f a c e s p l u s al i m i t e d num ber o f v e h i c l e t e s t s . An i n t e r i m p r o c e d u r e w h e r e ins t r a i g h t l i n e b r a k in g t e s t s a r e r e p e a t e d on a s many d i f f e r e n tp a v e m en t s u r f a c e s a s p r a c t i c a b l e a p p e a r s t o b e a r e a s o n a b l e ,p r a g m a t i c a p p r o a c h .

T r a c t i o n d a t a o b t a i n e d r e c e n t l y [ 4 ] o n s e v e r e l y o v e r l o a d e d /u n d e r i n f l a t e d t i r e s i n d i c a t e t h a t s i g n i f i c a n t d e g r a d a t i o n i ne f f e c t i v e f r i c t i o n c o e f f i c i e n t s a r e p r od u c ed u nd e r s u c h c i rc u m -s t a n c e s . S h o u l d f u r t h e r t e s t i n g i n d i c a t e t h a t r e a l i s t i c o f f -d e s i g n c o m b i n a t i on s m i g h t s i g n i f i c a n t l y a f f e c t b r a k in g e f f i c i e n c yr e s u l t s t h r o u g h t h i s me c h a n i sm , s t r a i g h t - l i n e b r a k i n g t e s t s w i t hm u l t i p l e v a l u e s o f i n f l a t i o n p r e s s u r e w i l l b e i n d i c a t e d . P e n d in gs u c h a d e v e l o p m e n t , i t a p p e a r s a p p r o p r i a t e t o c o n d u c t t e s t s u s i n go n l y n om in a l i n f l a t i o n p r e s s u r e s . S i n c e v e h i c l e l o a d i n g h a s al a r g e i n f l u e n c e o n b r a k in g e f f i c i e n c y , b r a k in g t e s t s s h o u l d b ep e r fo rm e d w i t h l o a d i n g s t h a t a c t t o d e g r a d e d e c e l e r a t i o n p e r -f o r m a n ce . T h e s e l o a d i n g s may b e d e r i v e d f ro m s e r v i c e f a c t o rd a t a s u ch a s h a v e be e n r e p o r t e d i n r e f e r e n c e [ S ] .RESPONSE TO K 4 P I D , E X TR E M E S T E E R I N G

I n t h e s e t e s t s , t h e v e h i c l e was s u b j e c t e d t o a q u a s i - s t e pd i s p l a c e m e n t o f t h e s t e e r i n g w h e e l , c o n c u r r e n t w i t h t h e r e l e a s eo f t h e a c c e l e r a t o r , w i t h t h e v e h i c l e m oving i n i t i a l l y i n as t r a i g h t p a t h a t a s p e e d o f 3 0 mph, A t e s t s eq u e nc e c o n s i s t e do f s u c c e s s i v e r u n s w i t h s y s t e m a t i c a l l y i n c r e a s i n g v a l u e s o fn o r m a l i z e d s t e e r a n g l e ,

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where Asw is the steering wheel displacement, k is the vehiclewheelbase, and NG is the overall steering ratio as determinedexperimentally by measuring front-wheel and steering-wheeldisplacements with the front wheels lifted clear of the ground.It can be shown from geometric considerations that A A w representsthe zero-speed-limit value of path curvature corresponding to thesteering wheel angle Asw. Accordingly, differences in the pathcurvature response of vehicles operating with equal values ofA' are entirely attributable to the influence of dynamic effects.SW

Each test vehicle was subjected to quasi-step steeringresponse tests under at least two sets of service factor con-ditions: (1) nominal - vehicle loaded with driver plus instru-mentation, and tires inflated as per manufacturer's recommendation,and (2) off-design - a rear-.biased load, underinflated rear tires,and overinflated front tires.

The peak values of lateral acceleration (A ) produced inYPthe step steering maneuvers are plotted in Figure 8 as a functionof steering wheel displacement for each vehicle configurationtested. These results are somewhat analogous to the brakingeffectiveness measurements (Figure 7) discussed earlier. Thereis one important difference, however. The steering responseis not characterized by a clearly recognizable limit conditionsuch as the occurrence of wheel locking. The limit responses of"spinout" and "driftout" occur, but it is not straightforward toidentify or to quantitatively characterize these limit phenomenaon the basis of the objective data that are obtained.

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Ford (NG = 22.8) Toyota (N G = 14.4)

No Yes

Corvair (N G = 23.8) Mercedes (NG = 15.3)

NominalOff-design

Steering Wheel Displacem ent, deg. Steering Wheel Displacem ent, deg.

0 -A

F I G U R E 8 , P E A K LATERAL A C C E L E R A T I O N S ATTAINED IN S T E P S T E E R I N GR E S P O N S E T E S T S

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Qualitatively, the limit responses produced in the stepsteering manuever involve a tire reaching a saturation con-dition wherein the prevailing level of tire-road frictioneffectively bounds the maximum achievable lateral force.Hence an increase in the tire sideslip angle does not pro-duce an increased force. The relative extent to which frontand rear tires are affected by this phenomenon dictates thegeneral nature of the limit response. When saturation is en-countered at the rear axle, there obtains an effective deficiencyin the yaw moment offsetting that produced by the steered fronttires, and the vehicle experiences a sharp increase in yaw rate,or " s p i n ~ u t . ~ ~onversely, when the force deficiency appears atthe front axle, the effectivene~s f the front tires in pro-ducing yaw rate decreases and the vehicle "drifts out" of the turn.

To quantify these limit steering responses on the basisof the data at hand, a normalized yaw rate can be defined as

lahere r is the yaw rate in radians per second. The peakmcasured values of this response variable, designated as1' I , are plotted in Figure 9 as a function of AYP Thenensuing result appears to represent a significant descriptionof the nature and severity of the limit response that isobserved. Discrimination of vehicle configurations experiencingspinout (e.g., Corvair, off-design Toyota) is positive.

Qualitative aspects of the step steering maneuver arehasically independent of speed. Accordingly, performance as-sessments derived from the 30 mph tests are also meaningful(in a relative sense) for the same maneuver performed athigher speeds.

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Ford Toyota

No Yes

Corva i Mercedes

NominalOff-Design

F I G U R E 9, O R M A L I Z E D P E A K YAW R A T E V ER S US P E A K L A T E R A L A C C E L E R A T I O N -S T E P S T E E R I N G R E S P ON S E T E S T S

o -A

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By and large, the quantitative influence of vehicle sus-pension characteristics on response to step-steer inputs appearsto he greater on higher friction surfaces where load transfereffects are more pronounced. Thus, experiments on a dry pave-ment surface may be expected to maximize the discriminatorypower of the test procedure.

Variations i n service factors significantly influence theresults of the step steering response test. Specifically, changesin load and tire pressure distribution that tend to degrade staticstability are found to increase the tendency for spinout. Con-versely, service factor variations which increase the staticnzrgin tend to inhibit spinout and produce a greater tendencyto c'yift.BRAKING TN A TURN

These tests involved the same procedure as the straightline braking tests, but the initial condition (instead of beinga straight course equilibrium) was a 30 m-ph steady turn pro-ducing a lateral acceleration of 0.3 g , The steering-wheeldi s~ la ce me nt equired to establish the steady turn was heldf xecl throughout the maneuver.

Th e principal results of the braking-in-a-turn tests are,,resented in Flgure 10, where the 9eak normalized yaw rate, r ' ,Pha. been plgtted as a function of the effective longitudinal(iecelcrntion, \ , for each vehicle configuration tested.

F o r those configurations which are characterized byinitial locking of the rear wheels, producing spinout (i,e.,Ford, Corvair), the r 1 versus plots provide an effectiveP Xderxonstration of how the directional response is affectedql~nificantly y braking, even at longitudinal accelerationssubstantially lower than the wheel-locking limit. In caseswhere wheel locking is first encountered at the front axle[l'oyotn, \Iercedzs), thcre is no increase in yawing velocityas a result of braking. Consequently there is no peak yaw

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Wheels Locked? No Yes AI INominal o UOff-Design A A &

i

Ford Toyota

Corvair Mercedes

F I G U R E 10, N OR M AL 1 Z E D P E A K YAW R A T E V E R S US L O N G I T U D I N A L D E C E L E R A T I O NA S P RO DU CE D B Y B R A K I N G I N A S T E A D Y T U RN

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r a t e a n d r 1 , a s p l o t t e d , m e r e l y c o r r e s p o n d s t o t h e i n i t i a lPt u r n i n g r a t e . . 4 lt h ou gh e f f o r t s h a v e b e e n m ade t o q u a n t i t a t i v e l yc a t e g o r i z e d r i f t o u t r e s p o n s e by g r a p h i c a l l y d i f f e r e n t i a t i n gt h e y a w - r a t e t im e h i s t o r y , t h e s e a t t e m p t s h a ve n o t b ee n s u c c e s s -f u l . F e n d in g t h e de v e lo p m en t o f a s a t i s f a c t o r y m ean s f o rm e a su r i n g t h e s i d e s l i p v e l o c i t y o f a m o t o r v e h i c l e ( a n d /o r i t sr a t e of c h a n g e ) , i t a p p e a r s d e s i r a b l e t o e m plo y b r a k i n g e f -f i c i e n c y a s o ne i n d e x o f p e r f o r m a n c e i n t h i s m a n eu ve r a nd r 1- Pv e r s u s A x a s a s e c on d i n d e x o f p e r f o r m a n c e .T U R N I N G O N A R O U G H S U R F A C E

R o a d h ol d i ng t e s t s w e re pe r f o rm e d by d r i v i n g t h e t e s tv e h i c l e s , i n i t i a l l y i n a s t e a d y t u r n w i t h 0 . 4 g l a t e r a la c c e l e r a t i o n , a c r o s s a p r e f a b r i c a t e d g r i d o f s t e e l p i p e c o n-s t i t u t i n g a f i x e d d i s t u r b a n c e o f r o a d r o u g h n e s s . S t e e r i n gd i s p l a c e m e n t was h e l d f i x e d . A r a n z c o f t e s t s p e e d s w ass e l e c t e d t o a s s u r e t h a t t h e c o r r e s p o n d i n g r a n g e o f p i p e -t i r e c o n t a c t f r e q u e n c i e s w o u ld c i r c u m s c r i b e t h e r a n g e o fw h e e l- h o p f r e q u e n c i e s p o s s e s s e d by t h e t e s t s a m pl e o f v e h i c l e s .

E a c h ~ r e h i c l ew as s u b j e c t e d t o r o a d h o l d i n g t e s t s w i t h7. i res i n f l a t e d a t e a c h of tw o d i f f e r e n t s e t s o f i n f l a t i o np r e s s u r e s : (11 a c c o r d i n g t o t h e m a n u f a c t u r e r ' s r ec om m e nd a t io n ,a n d ( 2 ) o v e r i n f l a t e d by a c o n s t a n t i n c r e m e ~ l ton a l l f o u r t i r e s .T he F o rd S t a t i o n Wagon w as a l s o t e s t e d w i t h u n i f o r m l y u n d e r -i n f l a t e d t i r e s .

The r o a d h o l d i n g t e s t r e s u l t s a r e p r e s e n t e d i n F i g u r e s 11a n d 1 2 i n t h e f o rm o f t h e p ea k d e c r e m e n t s i n l a t e r a l a c c e l e r a t i o na n d y a r , i - r a t e , AA a nd Ar , n o r m a l i z e d w i t h r e s p e c t t o t h e i n i t i a lYs t e a d y v a l u e s o f l a t e r a l a c c e l e r a t i o n a n d yaw r a t e , A a n d r o eY OI t \ < i l l b e n o t e d t h a t n e i t h e r d a t a p r e s e n t a t i o n i s c h a r a c t e r i z e db y c l e a r l y d e f i n e d a n d s y s t e m a t i c r e s p o n s e s h a v i n g a r e s o n a n tc h a r a c t e r . I t i s c l e a r , h o w ev e r , t h a t t h e r e a r e s i g n i f i c a n td i f f e r e n c e s ainong t h e d i f f e r e n t v e h i c l e s .

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Tire Pressure:' Nominal o

Overinflated AUnderinflated a

Ford Toyota

Corvair Mercedes

Speed, mph Speed, rnph

FIGURE 11, ATERAL-ACCELERATION DECREMENT CAUSED BY ROAD ROUGHNESSIN A STEADY TURN

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Ford

Tire PressureNominal oOverinflated *Underinflated

Toyota

Corvair Mercedes

Speed, mph Speed, rnph

F I G U R E 12, A W - R A T E D E C R E M E N T C A U S E D BY R O A D R O U G H N E S S I N AS T E A D Y T U R N

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The r o a d h o l d i n g t e s t d a t a p r o v e t o b e q u a l i t a t i v e l y c o n-s i s t e n t . F or ex am p l e , i n c r e a s e d t i r e i n f l a t i o n p r e s s u r e i n -v a r i a b l y d e g r a d e s p e r fo r m a n c e , w h e t h e r m e as ur ed i n t e rm s o fa c c e l e r a t i o n o r yaw r a t e . I t f o l l o w s t h a t a n y t e s t p r o c e d u r et o e v a l u a t e t h e r o a d h o l d i n g a b i l i t y o f a m o t o r v e h i c l e s h o u l di n c l u d e m e a s u re m e n t s w i th t i r e s o v e r i n f l a t e d t o r e a l i s t i c l e v e l s .

T h e re i s c o n s i s t e n c y , t o o , w i t h r e s p e c t t o t h e r e l a t i v ep e r fo r m a nc e o f t h e f o u r v e h i c l e s t e s t e d . I f we e l e c t t oq u a n t i f y p e rf o rm a n c e on t h e b a s i s o f t h e pe a k m ea su re d v a l u eo f e i t h e r ( 1 ) t h e l a t e r a l a c c e l e r a t i o n d e c r e m e n t , (AA /A ) ,Y Y O Po r ( 2 ) t h e yaw r a t e d e c r e m e n t , ( A ~ / T ~ ) ~we f i n d t h a t t h e r a n ko r d e r o f t h e f o u r s p e ci m e n s i s t h e sam e f o r b o t h p e r fo r m a n c em e a su r es ( s e e T a b l e 2 ) .

RAPID LANE CHANGING

-TABLE 2

ROADHOLDING PERFORMANCE MEASURES

The t e s t v e h i c l e s w e r e s u b j e c t e d t o s i n u s o i d a l s t e e ri n p u t s w i t h a f i x e d p e r i o d o f 2 .0 s e c o n d s an d s y s t e m a t i c a l l yv a r i e d a m p l i tu d e s . T e s t s w e r e c o n d u c t ed a t n om in al i n i t i a ls p e e d s o f 3 0 , 4 0 , 5 0 , a nd 60 mph. ( A c t u a l t e s t s p e e d s v a r i e dso me wh at f ro m t h e n o m i n al v a l u e s ) . A t e a c h s p ee d , r u n s w e re

V e h i c l e

F o r dT o y o t aC o r v a i rM e r c e d e s

AA

(g)Y O- 7 4

1 . 0 6. 6 0. 5 2

Ar(f-)0

. 5 7$ 5 9. 3 4- 2 8

I

R a n k i n g

3421I

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m a de w i t h s t e e r i npu t s o f p r og r e s s i ve l y i nc r e a s i ng m a gn i t ude( f ol l ow i n g t h e r e l e a s e o f t h e a c c e l e r a t o r ) u n t i l one of t wol i m i t i n g c ond i t i ons was e nc ount e r ed , a t w hi ch po i n t t he spe edwas i ncr em en te d t o t he ne x t h i ghe r l e ve l . S p e c i f i c a l l y , t hespe ed was i nc re me n te d i f ( 1) t he s t e e r a m pl i tude c a l l e d f o r bythe incrementing scheme exceeded 3 6 0 de g r e e s , o r ( 2 ) t he nega-t i v e yawing ve l oc i t y c aused by l e f t s t e e r was obse r ve d t o r e -main ne ga t i ve f o l l ow i ng t h e i n i t i a t i o n and c om ple t ion o f t h er i g h t s t e e r p o r t i o n of t h e i n p u t wave form. The f i r s t l i m i tcor responds t o th e peak magnitude of s t ee r i ng whee l d i sp lac e -ment cons ide red l i k e l y i n a rea l -wo r ld emergency l a ne change( ba se d on d r i v e r - v e h i c l e t e s t s ) ; t h e s ec on d l i m i t r e p r e s e n t sa l i m i t performance co nd i t ion we de f in e as a l td ive rgen tVresponse .

The r e s u l t s of t h e s i n u s o i d a l s t e e r i n g t e s t s a r e summarizedi n F igur e 13 i n th e form of p l o t s o f g ros s head ing change , A$ ,ve r sus no r ma li ze d s t e e r i ng a m p l i t ude ,

O f t h e t h r e e t e s t ve h i c l e s , two ( i . e . , t h e Cor va ir and T oyot a)exper ienced l i m i t (o r "d ive rgen t I1) re sponses wi th in the a l lowedrange of s t ee r i ng-wh ee l d i sp lacement ( i . e . , 36 0 degrees) whereast he t h i r d d i d no t . I n o r de r t o make some qu a n t i t a t i ve s t a t e m e n t sw i t h r e s p e ct t o f a c t o r s a f f e c t i n g t h e l i m i t response, we note( s e e F i g ur e 1 3 ) t h a t t h e r e i s a d e f i n i t e a s s o c i a t i o n between g r o s sheading change and the occur rence of a d i ve rg en t res pon se , Fort he ve h i c l e s t e s t e d , a d i ve r ge n t r e sponse occ u r s w henever t hegross heading change i s g r e a t e r t h a n 50 de g r e e s , w i t h t he r e sponsebeing nondivergent whenever the heading change i s l e s s t h a n 50deg r e e s . Le t us de f i ne t he nor ma li ze d s t e e r a m p l i tude r e qu i r e dt o produce a gr oss heading change of 5 0 degrees as (Azw)lim .

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I Nominal Velocity, mph 130 40 50 60 1I Vehicle Empty, Tire Inflation Nominal 1 o A O 0

FordI I I '1

Vehicle Empty, Tire Infl atio n Off-DesignVehicle Loaded, Tire Inflation Off-Design

As"wToyota

A 0 6A 4

Corvair

Points with X: Divergent Response

F I G U R E 1 3 , G R O S S H E A DIN G C H A N G E V E R S U S N O R M A L I Z E D STEER AMPLITUDEP RO DU CE D I N T H E S I N U S O I D A L S T E E R M A NE UV ER

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When t he da t a p l o t t e d i n F igu r e 13 a r e i n t e r p o l a t e d t odetermine (AEw) im , t h e f i n d i n g s t a b u l a t e d i n T ab le 3 a r eob t a i ne d , On t he ba s i s of t he se r e s u l t s ( and o t he r da t ao b t a i ne d from e x p l o r a t o r y t e s t s w i t h t h e C o r va i r i n a d ifwf e r e n t o f f - d e s i g n c o n d i t i o n ) i t appears reasonable t o conc ludet h a t t h e i n f l u e n c e of t e s t s pe ed on t h e l i m i t r e s p o n s e v a r i a b l e ,(A:w)lim , i s no t sy s t e m a t i c . Hence the normalizat ion schemeemployed a ppe a r s t o c o ns t i t u t e a n e f f e c t i ve p r oc edu r e t o a cc oun tf o r t h e sp ee d e f f e c t , and i t i s m ea ni ng fu l t o c o n s i d e r t h ea ver age va l ue s t a b u l a t e d i n t he l a s t column o f T a bl e 3 asi n d i c a t i v e of t h e l i m i t performance achieved i n t h i s maneuver .Re sponse- l i m i t bounda r ie s c ons t r uc t e d on t he ba s i s of t h i sp re mi se a r e p r e se n t e d i n F i gu r e 14 . A ls o p l o t t e d on t h i s f i g u r ea r e d a t a p o i n t s f rom a l l of t h e l i m i t ( i . e . , d i ve r ge n t ) and ne a r -l i m i t t e s t r uns . The de g r e e o f p r e c i s i on w i t h w hich t he s t a t e -ment t h a t A$ - 5 0 de g r e e s de f i ne s a l i m i t boundary i s apparen t .

The i n f l u en c e o f s e r v i c e f a c t o r v a r i a t i o n s on t h e l i m i tr e s po n s e d e p i c te d i n F i gu re 14 i s s u b s t a n t i a l . Not s u r p r i s i n g l y ,v a r i a t i o n s i n lo ad i ng and t i r e i n f l a t i o n pr e s s u r es which t en d

T A B L E 3

LIMIT NORMALIZED STEER AMPLITUDE (Aw)lim; SINUSOIDALSTEERING RESPONSE TESTS.

4 0Conf i gu r a t i on 5 0

5 . 13 . 22 . 32 . 0

Toyota, Nominal Se rv ic e Fa ct or sToyota , Of f -des ign S erv ice Fac tor sCor vai r , Nominal S er vi ce Fac tor sCor va i r , O f f - de s ign S e r v i c e F a c t o r s

5.52 . 62 . 71 . 9

6 0

5 . 7-1.8-

Avg.

5 . 42 . 92 . 32 . 0

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CONVERGENT TEST DA TA oDIVERGENT TEST DAT A

ai'u3w.-

CORVAIR TOYOTA

Off-Design Service

Nom inal Service Nom inal Service

Factors--- Limi t

Factors Limit:-----

Divergent-0- Convergent jI

I 1 I I I I

400

300

200

100o c

Off-Design Service

10 20 30 40 50 60 10 20 '30 40 50 60

- Factors Limit 400'-------\Divergent-

- Convergent 1 100I

I I 1 I I ' 0

actors--- Limit

I

Velocity, mph Velocity, mph

FIGURE 1 4 , LIMIT-RESPONSE BOUNDARIES -- SINUSOIDAL STEERINGRESPONSE TESTS

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t o r e d u c e s t a t i c m a rg in t e n d a l s o t o r e d u c e t h e l e v e l o fm an eu ve r s e v e r i t y r e q u i r e d t o p r o d u ce d i v e r g e n t r e s p o n s e t os i n u s o i d a l s t e e r i n p u t s . T h i s e f f e c t s h o u l d c e r t a i n l y bec o n s i d e r e d i n d e s i g n i n g a t e s t s c h e d u le f o r e v a l u a t i n g t h ee m erg en cy l a n e - c h a n g in g p e r f o rm a n c e o f a p a r t i c u l a r v e h i c l ec o n f i g u r a t i o n .DRASTIC STEER AND BRAKE MANEWER

The t e s t v e h i c l e s w er e s u b j e c t e d t o s t e e r i n g an d b r a k i n gi n p u t s o f t h e f o r m shown i n F i g u r e 1 5 . L i m i t r e s p o n s e s w e r es o u g h t i n a m an n e r s i m i l a r t o t h a t e m p lo y e d i n t h e s i n u s o i d a l -s t e e r t e s t s . S t e e r i n g a m p l i tu d e was i n c re m e n te d a t a g i v e ns p e e d i n a c c o r d a n c e w i t h t h e p r o c e d u r e d e s c r i b e d e a r l i e r . How-e v e r , t h e i n c r e m e n t a t i o n sch em e w as made l e s s c o m p l e te i n o r d e rt o m i ni m iz e t h e t o t a l num ber o f s t e e r - b r a k e r u n s w h ic h p r ov e dt o b e e x t r e m e l y h a r d o n t h e t e s t v e h i c l e s f ro m t h e s t a n d p o i n t o fm e c h a n i c a l s t r e s s i n g .

The b r a k i n g i n p u t l e v e l w as h e l d f i x e d t h r o u g h o u t a t e s ts e q u e n c e w i t h a g i v e n v e h i c l e . I n e a c h c a s e , t h e m a g ni tu d e o ft h e b r a k e i n p u t w as s u f f i c i e n t l y g r e a t t o p r o d uc e w h ee l l o c k i n g .T he l i m i t r e s p o n s e c o n d i t i o n d e f i n e d f o r t h i s m an e u v e rc o r r e s p o n d s t o t h e c a s e w h er e t h e r o l l i n g m o t i o n f o l l o w i n gr e l e a s e o f t h e b r a k e i s s o g r e a t a s t o c a u s e t h e v e h i c l e t or o l l o v e r . Of t h e v e h i c l e s t e s t e d , t h i s l i m i t r e s p o n s e w ase n c o u n te r e d o n l y w i t h t h e C o r v a i r . *

A r o l l o v e r l i m i t r e s p o n s e was o b s e r v ed i n t h r e e s e p a r a t er u n s w i t h t h e C o r v a i r o p e r a t e d i n a n om i n a l s e r v i c e f a c t o rc o n d i t i o n . T e s t c o n d i t i o n s i n t h e s e r u n s w er e s o s i m i l a r t h a ti t was n o t p o s s i b l e t o i d e n t i f y a ny s o r t o f l i m i t r e s p o n s eb o un d ar y su c h a s was d on e i n t h e s i n u s o i d a l s t e e r i n g r e s p o n s et e s t s . N e v e r t h e l e s s , t h e p o s i t i v e i d e n t i f i c a t i o n o f a s i n g l e

* B ec a us e o f t h e p r e s e n c e of t h e r o l l - l i m i t i n g o u t r i g g e r s , t h et e s t v e h i c l e c o u l d n o t i n f a c t r o l l c o m p le t e l y o v e r . I n t e s t sw h e r e i n " r o l l o v e r " i s r e p o r t e d , t h e r e i s no d o u b t t h a t t h ev e h i c l e wo u ld h a v e o v e r t u r n e d i f n o t s o r e s t r a i n e d .

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l i m i t r e s p o n s e p o i n t i s c o n s i d e r e d t o c o n s t i t u t e a t h o ro u g h l ya d e q u a t e d e m o n s t r a t i o n o f t h e d i s c r i m i n a t o r y power o f t h i s u n iq u et e s t p r o c ed u re . I t was fo un d , a l s o , t h a t i n t e s t s p er f o rm e d w i t ho f f - d e s i g n t i r e p r e s s u r e s , a g r e a t e r s e v e r i t y of r e s p o n s e waso b t a i n e d t h a n was o b s er ve d i n t e s t s w i t h n o m in al t i r e p r e s s u r e sf o r c o m p ar ab le c o n t r o l i n p u t s an d s p e e d s . T h i s f i n d i n g demon-s t r a t e s t h e d e s i r a b i l i t y of t e s t s w i t h o f f - d e s i g n s e r v i c e f a c t o r swhen e v a l u a t i n g m o t o r v e h i c l e s w i t h t h e d r a s t i c s t e e r an d b ra k em an eu v e r .

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CONCLUD I NG REMARKS

The specific objectives of the study have been achieved.A set of performance characteristics postulated to reflectthe pre-crash safety quality of the motor vehicle have beendefined, Methods of testing and data analysis have been demon-strated which provide objective and discriminating proceduresfor measuring these safety-relevant characteristics. Particularlynotable is the development of an automatic controller which permitsthe conduct of severe handling tests heretofore impossible becauseof the limitations of the human controller.

With respect to the ultimate goal of the study, viz., im-plementation of a vehicle handling performance standard, muchwork remains to be done. A logical first step is the conductof a more general test program to measure the performancecharacteristics defined here for a much broader cross-sectionof passenger vehicles, The purpose of such a program wouldbe threefold:

1. To refine and augment the developed procedures withthe aid of additional test experience and data.

2. To more precisely define the precision and discrim-inatory power of the proposed performance measures,

3. To produce a data base defining the performancecharacteristics of the passenger vehicle population,which would serve as a basis for accident causationstudies and, ultimately, for the establishment ofminimum performance requirements.

It is to be hoped that additional objective measures ofmotor vehicle performance and associated test procedures willbe developed in follow-on studies, Requirements also existfor the advancement of instrumentation techniques, for example,the development of a reliable procedure to measure v'ehiclesideslip angle under dynamic conditions.

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The efforts described herein appear just to scratch thesurface of a virtually limitless domain of investigationsthat are made possible with the aid of an automatic controller.Other combinations of steering and braking time histories couldand should be programmed on the function generator and evaluated.

It should be noted that the performance measures defined inthis study provide a meaningful focus for new analytical workand simulation activity. Mathematical models of the mechanicsof the motor vehicle should be extended and refined to permitaccurate simulation of these maneuvers, The existence of refinedsimulations would facilitate sensitivity analyses to guide theefforts of designers and researchers and to provide new depthsof understanding of the pre-crash dynamics of the tire-roadway-vehicle system.

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REFERENCES

1. "Basic Vehic le Handl ing Proper t ies - Phase I , " F i n a lRepor t , Federa l Highway Adminis t ra t ion Contrac t FH-6528, Highway Sa fe ty Research I n s t i t u t e of The Univ-v e r s i t y of Michigan, Nov. 1967,2 . L . Seg el , "Display Versus Veh icl e Augmentat ion a s aMeans f o r Impro ving Man-Vehicle Per fo rm ance ," ASMEPu bl ic at io n 65-WA/HUF-10, paper pr es en te d a t WinterAnnual Mee tin g, Nov. 7-1 1, 1965.3 . I . D . Neils on, "An Intr od uc tu ry Discu ssion of theFa ct or s Af fe ct in g Car Handling," Road Research La-bora to ry , Repor t L R 133, 1968.4. P . S . Fancher, J r . , e t . a l : , "E x p e r i m e n t a l S t u d i e sof Ti re Shear Force Mechanics - - a Su m ~ ~ ~ a r yeport ,"F i n a l Repor t NBS Con tr ac t CST-928-5, Highway S a f et yR esea rc h I n s t i t u t e , Ju l y 19 70 .5 . J . L . Harvey and F. C . Brenner, "Tire Use Survey -The Physical Condition, Use, and Performance ofPassenger Car Ti r es i n th e Uni ted St a t es of America"National Bureau of Standards Technical Note 528, May1970.

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