bus truck tractor brake performance

8/4/2019 Bus Truck Tractor Brake Performance

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Sunupy Final Report,

ContractFH-11-7290January 1970

BUS, TRUCK, TRACTOR/TRAlLERBRAKING SYSTEM PERFORMANCE

Ray W. MurphyRudolf LimpertLeonard Segel

Highway Safety Research InstituteThe university of MichiganAnn Arbor

forNational Highway Traffic Safety AdministrationU.S. Department of TransportationThe opinions, finding, and conclusions expressed in thispublication are those o f the authors and not necessarilythose of the National Highway Traffic Safety Administration.


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1 . Report No. 2. Government Accession No . 3. Recipient's Catalog No.I

4 . Title and Subtitle

7 . Author(s) 1 8. Performing Organnation Report No.

5 . Report DateBUS, TRUCK, TRACTOR-TRAILER BRAKING SYSTEMPERFORMANCE March 19716 . Performing Organization Cod e

Highway Safety Research InstituteUniversity of MichiganHuron Parkway & Baxter RoadAnn Arbor, Michigan 48105

R.W. Murphy, R. Limpert, L, Segel9. Performing Organization N ame and Address

1 2 . Sponroring Agency Name and Address

PF-lOla10. Work Unit No.

National Highway Traffic Safety AdministrationU.S. Department of TransportationWashington, D.C. 20591

1 1 . Contract or Grant No.FH-11-72901 3 . Type of Report and Period CoveredSummary Final Report7/69 - 3/7114. Sponsoring Agency Code

1 5 . Supplelnentary Note s

16 . Abstract

The objectives of this study were to determine the range of braking per-formance currently exhibited by buses, trucks, and tractor-trailers and toestablish the maximum braking performance capabilities of these vehiclesbased upon full utilization of the technology related to brake system design.Both vehicle testing and ana,lytical echniques, including dynamic modelingand simulation, were used to accomplish these objectives. Performance mea-sures were defined which serve to quantify the degree to which a. givenvehicle-braking system possesses those qualities necessary for adequatebraking performance. Using these measures, a braking performance standard isrecommended based upon a comparative analysis of (1) current braking perfor-mance, (2) the maximum performance achievable by full exploitation of exist-ing technology, and (3) performance a,s constrained by a host of associatedfactors.

I Unclassified I Unclassified I IForm DOT F 1700.7 (8-69)

17 . Ke y Wordsbrake performance, brakes, brakesystems, trucks, buses, tractor-trailers

18. Dlstribution StatementAvailability is unlimited. Docu-ment may be released to the Clear-inghouse for Federal Scientific In-formation, Springfield, Virginia;32151. f o r sale to the n u .

19 . Security Claasif.(of th ~ , eport) 20. Security Cl ass~f.( ofhis page) 21. No. of Pages 22 . Price


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ACKNOWLEDGMENTS

T he a u t h o r s w i s h t o ac kn ow le dg e t h a t m any i n d i v i d u a l s a nd o r g a n i z a t i o n smade o u t s t a n d i n g c o n t r i b u t i o n s t o t h i s p ro gra m , n o t o n l y by s u p p l y i n g v e h i -c l e s and t e c h n i c a l d a t a , b u t a l s o by p ro v i d in g t e c h n i c a l a s s i s t a n c e .


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TABLE OF CONTENTS

PageLIST OF TABLESLIST OF FIGURESABSTRACT1.0. INTRODUCTION

1.1. Statement of the Problem1.2. Object ives

2.0. SUMMARY OF TASKS2.1. Li te ra tu re Review2.2. Vehicle-Brake System Performance Te st s2.3. An aly tic al Program2 . . Recommenda.tions f o r a Saf ety Standard

3.0. RESULTS3.1 . Test Resul ts, Baseline Vehicles3.1 .1 . Brake effec t iv eness t es t s

3.1 .2 . Brake fa i l ur e t e s t s3.1.3. Fade and recovery t e s t s3.1.4. Brake ra t in g te s t s3.1.3. Brake response time te s t s3.1.6. System fai lures

3.2. Test Results, Vehicles Equipped With Advanced Systems3.2.1. Truck equipped with disc brakes3.2.2. Tr a,c tor -tr ail ers equipped with advanced

sys erns3.3 . Test Surfaces and Tire-Road Interface Tests3.4. Analyt ica.1 Re sult s3.4.1. Braking performance3.4 .2. Propo rtionin g schemes

4.0. RECOMMENDATIONS4 .1 . Performance Measures2 Maximum Achievable Performance4.3. Recommendations f o r a, Standard


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LIST OF TABLES

T a b l e1. E f f e c t i v e n e s s T e s t S um m ar y-B as eli ne V e h i c l e s2. Summary of Performance o f T r u c k s a n d B u s e s U n d e r F a i l u r e

C o n d i t i o n s3. Sum mary o f P e r f o r m a n c e o f T r a c t o r - T r a i l e r s U n de r F a i l u r eC o n d i t i o n s4. Summary o f F a d e T e s t R e s u l t s3. Sum mary o f B r a ke R a t i n g T e s t R e s u l t s - T r u c k s a n d B u s e s6. Sum mary o f B r ak e R a . t i ng T e s t R e s u l t s - T r a c t o r - T r a i l e r7 . Summary of A i r B r a k e R e s p o n s e T i m e T e s t s - T r a c t o r - T r a i l e rC o m b i n a t i o n s8. E f f e c t i v e n e s s T e s t S u r m a r y - J is c B r ak e T r uc k9. Te s t Summa r y , Pe r f o r ma nc e o f Ve h ic le s Equ ippe d W i th Adva nc e d

Systems on Dry Tra.ck10 . T e s t Summary , Pe r f o r ma nc e o f Ve h ic le s Equ ipped W i th Adva nc ed

S y s t e m s o n Low C o e f f i c i e n t S u r f a c e11. Summary of Brake Response Times2 S k i d N um bers a n d p e a k / s l i d i n g T i r e- R oa d C o e f f i c i e n t s f o r T e s t

S u r f a c e13. Pe r f o rma .nc e M e a s u r e slit. O v e r a l l Bra .k ing E f f i c i e ~ c i e s o r A dvanced S y st em s T e s t e d

Page7


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LIST OF FIGURES

Figure1. Interrelationship of project tasks and subtasks.2. Maximum deceleration capability, baseline vehicles.3. Effectiveness test results, disk brake truck.4. Braking performance diagram for 2-S1 tractor-trailer.5 . Braking efficiency for 2-S1 tractor-trailer.

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6. Maximum deceleration performance ranges, baseline and ad-vanced systems. 37

7 . Improvement in maximum deceleration performance with ad-vanced systems. 3

8. Deceleration-pedal force gain for disk brake truck and HSRIrecommended limits. 41


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1 .0 INTRODUCTION

This re po rt pre se nts find ings , conclusions, and recommendations derivedby t he Highway Sa fe ty Research In s t i t u t e (HSRI) of The Uni ve rs it y of Michiga,ni n a re sear ch program f o r the N ational Highway Tr aff ic Safe ty Administration(NHTSA) n t i t l ed , "BUS, Truck, Tr ac to r- Tr ai le r Braking System PerformanceStudy. " The broad o bje cti ves of t h i s program ar e the formulation of techniquesand th e production of data designed t o aid NHTSA i n f u l f i l l i n g i t s mandate t cissue reasonable and desirable safety standards.

1.1 STATEMENT OF THE PROBLEMThe complex rel at io ns hi p between cu rr en t brak ing c a p ab il i ti e s of commer-

c i a l v e hi c le s and th e frequency of accide nts with the se vehicles due t o defi -cien cies i n braking performance i s nei the r completely understood nor s t a t i s -t i c a l l y documented. There i s , neverthele ss, ample int ui t i ve bas is t o hypoth-es iz e th a t such a r e la t ion sh ip ex i s t s a.nd, f u r t h e r , t h a t t h e re a r e c e r t a i nspe ci f ic vehicle braking performance cha rac ter is t ics which, during ei th er t henormal dr ivin g process or emergency s i tu at i ons , cause th e po ten t ia l fo r lo ssof cont rol t o r i s e above a thres hold beyond which even the s k i l l and experi-ence of t h e p r of e s s io n a l d r i v e r a r e of l i t t l e av a i l . I t i s known th at th ebraking performance of buses, t ruc ks, a,nd tr ac to r- tr a, i l er s va,r ies si gn if i-ca.ntly over a. wide range, and i s , on the avera.ge, l e s s than tha,t fo r passen-ger ca ,rs , and c er t ai nl y l e s s than th e ma,ximum performance a.chieva.ble. Thusi t can be argaed t ha t th i s performance di ff er en ti al , i f gre at enough, cancons t i tu te a s ign i f i ca n t sa fe ty haza,rd s ince a l l veh ic le types a re subjectedto t he same tr a f f i c and phys ical environments. The performance demands ofthe se environments a.nd th e inte gra, tion of v eh icl e types with in tile environ-ments in di ca te a need f o r uniform a.nd high er bra.king performance le v el s t o beachiev ed by commercia,l ve hi cl es . The study described herein addresses thisneed by fccusing on the establishment of braking system performance require-ments fo r buses, t rucks , and t r a c t o r - t r a i l e r s ,

1 . 2 OBJECTIVESThe spec i f i c object ives of th i s s tudy ar e thre efol d:.To determine, by mea,ns of ve hic le t es ti ng , th e range of brakingperformance cu rre ntl y exhibited by buses, t ruc ks, and tr ac to r-t r a i l e r s .

.To e s ta bl i s h th e maximum brak ing performance ca p ab il it i es ofthese vehicles based on fu l l u t i l i za t i on of the technology


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r e l a t e d t o b ra k e s y st em d e s ig n ..T o recom mend a r a t i o n a l b r a k i n g p e r f o rm a n c e s t a n d a r d b a s e d u po na c o m p a r a ti v e a n a l y s i s o f ( 1 ) c u r r e n t b r a k i n g p e rf o rm a n c e, (2 )the maximum per formance achievable by f u l l e x p l o i t a t i o n of e x i s t -i n g t e c h n o l o g y , a nd ( 3 ) p e r f o r m a n c e a s c o n s t r a i n e d by a h o s t o fa s s o c i a t ed f a c t o r s .


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2 . 0 SUMMAEY OF TASKS

In order t o meet the obje ctiv es of th i s program, four major e~p eri rne nta ~la.nd ana lyti ca.1 ta sk s were c ar ri ed out.

2.1 LITERATURE REVIEWThe fore ign and domestic l i t e r a t u r e was surveyed with th e ob ject ive of

findi ng information per t in ent t o accurate analyses of braking systems, exper-imental test procedures, and mea.ns of measuring and evaluating the brakingperformance of buses, trucks, and tr ac to r- tr ai le rs . Factors considered i m -po rt an t i n th e review were brake system design, braking performance, brakeusage, brake testing, brake failure, and performance standards.

2 .2 VEHICLE-BRAKE SYSTEM PERFORMANCE TESTSIn order t o determine the braking performance ca pa bi l i t i es of vehic les

equipped with s tandard braking systems, three integral trucks, three buses,and four t ra ,c to r - t r a i le r combinat ions were subjected t o a se r i es of ef fec-tiv ene ss, fade and recovery, and brake ra ti ng t e s t s . So t h a t th e improvementi n performance through use of more ef fe ct iv e brakes and advanced brak ing sys-tems could be determined, th re e add it io na l veh icl es were te st ed , namely:

1. An in te gr al truc k equipped with dis k bra,kes a.nd a f u l l powerhydraulic brake actuation system.

2. A t r a ct or -t ra i l e r equipped with proportioning valves, ada,ptivebraking system,* and t r a i l e r bra.ke s~ynchroni zationdevice.

3. P. t r a c to r - t r a , i l e r equ ipped wi th a. wheel antilock system.**The disk brake tr uck was subjected t o effec tive ness , fade and recovery,and brake r a t ing t e s t s , whi le the two t r ac to r - t r a i l e r veh ic les were t es ted

primarily for effectiveness and minimum stopping capability.

*Designation used throughout th i s rep or t f o r the system developed by Bendix-Westinghouse.

**Designation used throughout this report for the system developed by Eaton,Yale, and Towne.


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2.3 ANALYTICAL PROGRAMThe analytical study was directed toward establishing mathematical and

computation procedures for predicting braking performance based upon vehicleand braking system design factors. Analysis of the performance of several ofthe test vehicles demonstrated the feasibility of making accurate predictionsof brake effectiveness, braking efficiency, pedal force gain, and thermalresponse. Dynamic modeling and simulation were employed to determine theextent to which vehicle braking performance can be improved with increasedeffectiveness, refinements in brake torque distribution, and load and decel-eration sensitive proportioning systems.

2.4 RECOMMENDATIONS FOR A SAFETY STANDARDBased upon the results of the three aforementioned tasks, performance

requirements are suggested which would effectively upgrade the braking per-formance of these vehicles to a level approaching that of passenger cars.Procedures and tests for ensuring compliance with the requirements are alsosuggested.

The interrelation of the tasks and important subtasks is shown in blockdiagram form in Figure 1.


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3 . 0 RESULTS

3.1 TEST RESULTS, BASELINE VEHICLESThe t e s t r e s u l t s f o r t h e t e n b a s e l i n e v e h i c l e s ( v e h i c l e s eq uip pe d w it h

s t a n d a r d b r a k i n g s y s t e m s ) a r e s um m arized i n t h i s s e c t i o n . F or s p e c i f i c d e t a i l son v e h ic l e s p e c i f i c a t i o n s and t h e r e s u l t s of e ac h i n d i v i d u a l t e s t , t h e r e a d e ri s r e f e r r e d t o t h e F i n a l R ep or t volum e.

3.1.1 B r a k e E f f e c t i v e n e s s T e s t sThe v e h i c l e s we re t e s t e d f o r b ra k e e f f e c t i v e n e s s , i n t h e l o a d e d and e mptyc o n d i t i o n t o t h e p o i n t of w h ee l l o ck u p i f t h e b r a k e s had s u f f i c i e n t t o rq u e

c a p a c i t y . I f t h e b e h i c l e r e m ai ne d s t a b l e beyond t h e p o i n t of f i r s t w he ell o c k up , p e d a l f o r c e was i n c r e a s e d t o l o c k u p a s many w h e e ls a s p o s s i b l e .R e s u l t s f o r a l l t h e e f f e c t i v e n e s s t e s t s a r e sum marized i n T ab le 1, and a graph-i c a l summary of t h e d e c e l e r a t i o n c a p a b i l i t i e s o f t h e b a s e l i n e v e h i c l e s i sg i v e n i n F i g u r e 2 ,

The a v e r a g e m aximum d e c e l e r a t i o n a c h i e v e d ( w i t h o u t w h e el l o c k o c c u r r i n g )w i th t h e t e s t e d t r u c k s i s 15.5 f t / s e c 2 i n t h e em pty c o n d i t i o n an d 1 7 . 0 f t / s e c 2i n t h e l oa d ed c o n d i t io n . I f w h ee l l o c k i n g i s pe r m i tt e d , t h e f i g u r e s becom e1 9 . 2 an d 1 7 .7 f t / s e c 2 , r e s p e c t i v e l y . T he se r e s u l t s s h o w t h a t b r a k e t o r q u ec a p a c i t y h a s b ee n p r o p o r ti o n e d t o g i v e b e s t p e r fo rm a nc e f o r t h e l o ad e d c o n d i-t i o n .

F o r t h e t e s t b u s e s , th e ave rage maximum de ce le ra t io n , whee l s un locked , i s18.5 f t / s e c 2 e m p t y a n d 19.9 f t , /s ec2 loaded , w i th th e pe r fo rmance o f th e emptyb u se s i n c r e a s i n g t o 2'1.7 f t / s ec 2 , when some whee l s a r e locked . It shou ld ben o t e d t h a t i t w as i m p o s s i b l e t o l o c k w h e e ls on an y of t h e b u s e s when f u l l yloaded .

F o r t h e t r a c t o r - t r a i l e r s t e s t e d , t h e a v e ra g e maximum d e c e l e r a t i o n , w h e el su n lo c k ed , i s 1 5 .0 f t / s e c 2 em pty a nd 16.5 f t / s e c 2 l o a d ed , w i t h t h e p e rf or m an c eem pty i n c r e a s i n g t o 1 9 . 4 f t / s e c 2 when w h ee ls a r e l o c k ed .

A l th o ug h t h e t r u c k s e x h i b i t a wid e r an g e i n p e rf o rm a n c e, t h e l o a d e d b u se se x h i b i t u n i f o r m p e r f o r m a n c e . A r a t h e r r em a rk a bl e v a r i a t i o n i n p er fo rm a nc e i sn o te d f o r t h e t r a c t o r - t r a i l e r c om b in at io n s, n o t w i th s ta n d i ng t h e f a c t t h a t t h esame t r a c t o r was used i n two of th e comb ina t ions (2 -S 1 and 2-52) and th e s amet r a i l e r w as u se d i n tw o c o m b in a ti o ns (2 -S 2 a n d 3 - ~ 2 ) .

O v e r a l l , t h e t e n v e h i c l e s a c h i e v e d an a v e r a g e maximum d e c e l e r a t i o n , w h e e l su n l o c ke d , of 1 6 . 5 f t / s e c 2 , e mp ty , an d 1 7 . 7 f t / s e c 2 , l o a d e d , a n d 1 9 . 4 f t / s e c P ,


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TABLE 1EFFECTIVENESS TEST SUMMARY-BASELINE VEHICLES

Minimum Stopp ing Dis tan ce , f t ry' ix imm Dec elera t ion , f t / se c2Nom . Empty Loaded Empty Loaded

Vehic les vo J No Some No Some No Some No Somemph Wheels Wheels Wheels Wheels Wheels Wheels Wheels WheelsLocked Locked Locked Locked Locked Locked Locked Locked

Light Truck 60 238 150 219 191 2 0 .0 28 .0 23 . O 25.0Medium Truck 60 322 282 30 7 --- 1 3 . 0 1 3 .7 12 .6 - -Heady Truck 60 316 248 26 3 262 13.2 17.8 15.5 15.5School Bus 40 108 8 4 119 - - - l g O 22.0 20.0 - -I n t e r c i t y BUS 6o 290 221 202 --- 1 6 . 3 21.5 19.5 --

-4 C i t y Bu s 40 93 72 89 - - - 20 .5 24 .0 20 .3 - -T r a c t o r - T r a i l e r

2-s1 60 291 258 292 -- - 15.5 1 7 .0 1 5 .2 --T r a c t o r - T r a i l e r

2-s2 60 428 220 222 --- 15 .0 20 .0 20 .0 - -T r a ct o r- T r a i l e r

3- s2 60 366 24 7 376 299 1 4 . 0 16. o 1 4 .0 16.1Tract or- Double

T r a i l e r 60 395 294 309 -- - 15 .0 18.8 16.6 --


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SUMMARY OF PEW'ORMANCE OF TRUCKS AND BUSES UNDER FAILURE CONDITIONS

V e h i c l e s T ype o f F a i l m e B r a k e s U se d Vo , Max. D e c e l . , f t / s e c 2 Min. Stopp ing D i s t . , f tmph Empty Loade d Empty ~ o a d e d

TrucksL i g h t

MediumHeavy

P0 BusesSchoo lI n t e r c i t yC i t y

Fron t BrakesRear BrakesPower Boost{ ower BoostS e r v i c e B r a k e s

S e r v i c e B r a ke sS e r v i c e B r a k e s

Rear Brakes OnlyFron t Brakes On lyS e r v i c e B r a ke sS e r v i c e B r a ke sHand Brake

knergenc yEmergencyHand BrakeRear Brakes--

*Ind i c a t es l ock -up o f one o r more wheel s .


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SUYMARY OF PERFOMNCE OF TRACTOR-TRAILERS UNDEB FAILURE CONDITIONSMaximum Minimumvo 9 LoadingVehi c les Type of Fa i l u r e Brakes Used Dece l . , S topp ing Wheel Lockupmph Condition f t / s e c2 D i s t ., f t

2 -S l O p en in g T r a i l e rEmergency A i r L in e T r a i l e r 20 Empty 6 .5 94 B o th T ra i l e r Wh e e l sO p e n in g T ra i l e rEmergency A i r L in e T r a i l e r 2 0 L oa de d 7.5 85 None

2-S2 Opening T ra i l e rEmergency A i r L in e T ra i l e r 2 0 E mp ty 9 5 70 A l l T ra i l e r Wh e e l sOpening T ra i l e r Lef t Tandem, Rig h tEmergency A i r L in e T r a i l e r 2 0 L oa de d 1 2 . 0 45 Rear

PP 3-S2 Opening T ra i l e rEmergency A i r Line T r a i l e r 20 Empty 7 .0 102 A l l T r a i l e r W h e e l sOpening T ra i l e r Le ft Tandem, Rig h tEmergency A i r Line T ra i l e r 20 Loaded 9 .0 100 TandemSer v ic e Sys tern Tr ac to r Spr ing Brakes 20 Empty 8 .0 83 A l l T r a c t o r D r i v eWheelsSer v ic e Sys tem Tr ac to r Spr ing Brakes 20 Loaded 6 .2 110 None

2-S1-2 Opening T ra i l e r R i g h t F r o n t a n d L e f tEmergency A i r L in e T r a i l e r 2 0 E mp ty 1 3 . 3 48 R ea r On F u l l T r a i l e rO p en in g T r a i l e rEmergency A i r L i ne T r a i l e r 20 Loaded 7.8 91 NoneS e rv i c e S y ste m T ra c to r S p r in g B ra k e s 20 E mpty 5 .4 1 0 0 B o th R e ar T r a c t o rWheelsSer v ic e Sys tem Tr ac t or Spri ng Brakes 20 Loaded 3 .2 155 None


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TABLE 4SUMMARY OF FADE TEST RESULTS

T es t Numb e Ve lo ci ty Average Average High estV e h ic l e s D e c e l. , o f S nu bs ~ n i t i a l / F i n a l , A tP e l , A t l m 2 > Lining Temp. , H o t t e s t B r a k e

f t / s ec 2 Ach iev ed se c sec sec OFTrucks

Lig h t 1 5 1 2 60110 5 - 9 55 520 Le f t RearMedium 1 2 3 60/10 7 - 5 125 335 Rig ht RearHeavy 14 5 60110 5. 2 213 405 Le ft Tandem

BusesSchool 1 5 9 4 0 1 1 ~ 5.9 29 445 Le f t Fron tI n t e r c i t y 15 9 60/10 6 . 3 85 600 L e f t D r i v e

I-Jrc C i t y 1 5 10 40110 3 . 4 48 3 15 R ig h t F ro n tT r a c t o r - T r a i l e r s

2 -S l 15 1 0 5o / lo 5.6 2 22 3 90 Le f t R ear Tra c to r2-S2 15 10 45/10 4 -3 l o 5 3 80 L e f t R e a r T r a c t o r3- ~ 2 1 4 . 5 10 50/10 5 - 3 157 4 05 R ig h t R ear Tra c to r2- Sl- 2 1 2 . 0 10 50/10 7 . 0 1 4 4 460 L e f t R e a r T r a c t o r

A t 2 - l i s t h e t im e fr om i n i t i a l br ak e p e d al a p p l i c a t i o n a t i n i t i a l v e l o c i t y t o b r ak e p e d al r e l e a s e a t l ow ersn u b v e lo c i ty .

A t l - 2 i s t h e a c c e l e r a t i o n t im e f ro m l ow er s nu b v e l o c i t y b ac k t o i n i t i a l t e s t v e l o c i t y .


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a c h i e v e 15 f t / s e c 2 ) t o 1 0 mph an d a c c e l e r a t e d a s r a p i d l y a s p o s s i b l e b a ck t ot e s t s pe ed , t h e s nu bs c o n t i n u in g u n t i l t h e s p e c i f i e d d e c e l e r a t i o n l e v e l c o ul dn o l o n g e r b e m a i n ta i ne d . The r e c o v e r y t e s t a s s p e c i f i e d i n SAE 5786 was con-d u ct ed i m m e di at el y a f t e r t h e f a d e t e s t .

I n c o n du c ti ng t h e f a d e t e s t on t h e c i t y bu s and t h e t r a c t o r - t r a i l e r s , i ts oo n becam e o b v i ou s t h a t t h e s p e c i f i e d p r o c e d u r e wo uld n o t f a d e t h e b r a k e se ve n a f t e r t e n s nu bs . The t e s t s were t e r m i na t ed a t t h i s p o i n t . The t e s t p r o -c e d u re a l s o p ro ve d i m p r a c t i c a l i n t h a t i t was n o t p o s s i b l e t o m a i n t a i n t h er e q u i r e d 15 f t / s e c 2 d e c e l e r a t i o n on t h e 3-S2 t r a c t o r - t r a i l e r c om b in at io n o r ont h e d o u b l e s c o m b i n a t i o n b e c a u s e of i n c i p i e n t w h e el l o c ku p .

3.1.4 B r a k e R a t i n g T e s t sThe pr oc ed ur e s p e c i f i e d f o r t h i s t e s t , r e q u i r ed t h a t t h e t e s t v e h ic l e be

towed on a f l a t s u r f a ce a t a c o n s t a n t v e l o c i t y w i th b r a ke s a p p l i e d a t a l i n ep r e s s u r e e q u i v a l e n t t o t h a t r e q u i r e d t o m a in t a i n c on s t a n t v e h i c l e v e l o c i t y ona 7% d e s c e n d i n g g r a d e . The v e h i c l e was t o be t ow ed w i t h l i n e p r e s s u r e h e l dc o n s t a n t u n t i l t h e t o w ba r f o r c e a s m easu re d b y a l o a d c e l l i n t h e t o w ba r de -c r e a s ed ( d u e t o f a d e ) b y 15%. The to w in g f o r c e o v e r t h e d u r a t i o n o f t h e t e s twas a ve ra g ed , t h e r o l l i n g r e s i s t a n c e s u b t r a c t e d , and t h e a v er a g e b r a k i n g f o r c ec a l c u l a t e d , f ro m w h ic h t h e e n er gy a b s o r p t i o n r a t e of t h e b r ak e ( h o r s ep o w e r )a n d t h e t o t a l e n e r g y a b s o r b e d w e r e d e t e r m i n e d . Two m e a s u re s w e r e f o r m u l a t e df o r r a t i n g t h e b r a ke s . The f i r s t m ea su re i s b as ed on h or se po we r and l i n i n ga r e a g i v i n g some i n d i c a t i o n o f t h e r a t e of e c e rg y a b s o r p t i o n , t h e s ec on d i sb a s ed u po n t o t a l e n e rg y a b so r b ed a nd t h e w e i g ht o f t h e v e h i c l e . A summaryof t e s t d a t a f o r t h e t r u c k s a nd b us es i s g i ve n i n T ab le 5 and f o r t r a c t o r san d t r a i l e r s i n T ab l e 6.

3.1.5 Bra k e Re s p o n s e T ime T e s t sTo m ea su re t h e br a k e re s p o n s e t i m e of t h e t r a c t o r - t r a i l e r c o m bi na ti o ns

p r e s s u r e t r a n s d u ce r s w er e f i t t e d t o t h e o u t p u t of t h e b r a k e - c o n tr o l ( t r e a d l e )v a l v e , a n d a t e a c h a x l e o f t h e v e h i c l e on w h i ch b r a k e s w e r e m o un te d, e x c e p to n l y o n e t r a n s d u c e r w as m ou nt ed a t a t an de m a x l e . R e s u l t s o f t h e r e s p o n s e -t i m e t e s t s a r e sum marized i n T a b le 7 b o t h f o r a p p l i c a t i o n and r e l e a s e .

The b r a k e a p p l i c a t i o n t i m e s shown i n T a b le 7 w e re m e as ur ed f r o m t h e i n s t a n tp r e ss u r e s t a r t e d t o r i s e a t t h e o u tp u t of t h e t r e a d l e v al ve t o t h e i n s t a n t a tw h i c h t h e p r e s s u r e r e a c h e d 60 p s i a t a g iv e n ax l e . * R e l ? a s e t i m e s w e r e m e a s u r e d

"Thi d e f i n i t i o n i s s l i g h t l y d i f f e r e n t f ro m t h a t g i v en i n SAE 5982, w hichs p e c i f i e s t h a t a p p l i c a t i o n t im e b e m e a m re d "from t h e s t a r t o f p e d a l movementt o a p r e s s u r e b u i l d u p o f 60 p s i . "


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TABLE 5SUMMARY OF BWKE FLAYING TEST RESULTS-TRUCKS AND BUSES

V eh ic le T es t T es t Average Power/ ~ n e r ~ ~ /aximum H o tt e stVehicle s Weight, Velo ci ty , Durat io n, Brake Power, L inin g Area, Weight , Brake Temp.,

h p / f t 2 f t - l b / l b OF Brakel b mph se c hpT r ucks .

Light 10,732 45 165 66 .5 28 .7 563 470 L e f t F r on tMedium 25,5 00 22 85 85.8 2 0 . 8 1 5 6 275 R i gh t R ea rHeavy 39, coo 23 90 146 . 0 24.4 186 425 Le ft Tandem

BusesSchool 24,500 25 120 l o g . 0 32.0 293 466 L e f t F r o n tI n t e r c i t y 3 5,9 40 2 3 1 9 5 1 5 1 .0 1 9 . 9 4 5 1 435 L ef t Dr i ve

P C i t y 32 ,145 24 240 124 .0 20 .3 510 425 L e f t F r on t+-


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f rom the i ns ta nt pressure began to drop a t the output of the t rea dle valve t othe in st an t a t which the pressure a t a g iven axle dropped t o 5 p s i . Thesemeasurements ind ica te th at the average response t ime fo r the r ea r ax le of th et r ac to r (when the t r ac to rs were t es ted i n combination wi th a semi t ra i ler ) was0.24 sec. Average response time f o r th e axl e on the se mi tr ai le rs was 0.28sec. Release times were cons ider ably longer, averaging 0.55 sec f o r the re araxle on the t r ac to r and 0.79 sec fo r the axle on the sem it ra i lers . Applica-ti on and rele as e times were consi dera bly longer on the doubles combinationdue t o the la rg er volumes of a i r t h at had t o be moved through th e system.

3.1.6 System FailuresSeveral ve hicl e suspension and brake system fa il u re s occurred during th e

te s t in g of the basel ine vehic les . None of t he se fa i l u r e s was conside redextraordina ry since th e ve hicl es and systems were subjected t o a se r i es ofhigh lev el decelera t ions th at generated s t re ss es and cycles of s t re ss notnormally encountered i n se rvic e.

3.2 TEST RESULTS, VEHICLES EQUIPPED WITH ADVANCED SYSTEMS

3.2 .1 Truck Equipped with Disc BrakesThe medium tr uc k equipped wi th Bendix d is k brak es and f u l l power hyd ra ul ic

actu at io n system was subjected t o the same s er ie s of t e s t s as th e basel inevehi cles : effec t ive nes s, brake fa i l ur e, fade and recovery, and brake rat ing .The r es u l t s of the e f f ec t iveness t e s t a r e g iven i n Figure 3. These da ta poi ntup two important factors:

1, The v ehi cle has a maximum dec ele rat ion ca pa bi li ty ( wi th no wheelslocked on the g iven t e s t sur face ) of be t t er than 21 f t / s ec2 , t h i sperformance being better than nine of the t en basel ine ve hicl es andcons ider ably be tt e r t han both th e medium or heavy tru ck.

2. The vehic le has the c apa bi l i ty of locking up a l l four wheels i n theloaded condit ion, demonstrating th at i t does have brake torque suf-f i c i e n t t o u t i l i z e t h e f r i c t i o n a l f o r ce s i n t h e t i re - ro a d i n t er f a cet o a maximum e xt en t.

With sep ara te ly powered f ro n t and re ar brake systems, i t was poss ible t of a i l ei th er system and maintain braking ca pa bi l i ty on one axle. Table 8summarizes the minimum stopping distances and maximum decelerations achievedi n the brake ef fect iven ess and brake fa i l ur e tes ts . Loss of the rear brakesdecr ease s the maximum dec ele rat ion ca pa bi li ty of the empty vehicl e by 35% and


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of th e loaded vehicle by 4376, while lo ss of the f r on t brakes decreases themaximum dece le ra ti on c ap ab il it y by 47% and 43%, empty and loaded, respectively.

Two fad e t e s t s were conducted on th i s veh icl e, with th e r e s u l t s demon-s t r a t i ng the super ior f ade r es i s t an ce of d i sk b rakes. I n t he f i r s t t e s t t hepedal f orce requi red t o mainta in 15 f t / se c2 decelera t ion from 60-10 mphincreased from 47 l b on t h e f i r s t snub t o 65 l b on th e 1 5th snub. In thesecond test the pedal force increased from 47 l b on the f i r s t snub t o 84 l bon the 15th snub. In bot h cas es t he bra ke tempera ture, a s measured by thermo-couples i n the l in in g pads, was i n excess of 1270F. To make a f a i r comparisonwith the r es u l t s of the base l ine veh ic l es , i t should be noted th at f o r the twotests the average snub decelerat ion t ime was 5.8 sec and 5.7 sec and the averaget ime t o accelera te back t o t e s t speed was 76.4 sec and 81.8 sec.

In the hrake r at in g t e s t an average power inpu t t o the brakes of 91.3 hpwas maintained f o r 670 sec with l e s s than 2w0 decrease i n brake for ce a t aconstant pedal pressure. The te s t was terminated a f t e r 670 se c because, byt h i s time, the tempera tures of a l l th e brakes exceeded 1200CF. In comparisonwith the basel ine veh icles , t h i s vehi cle produced an energy and parer r at in gof 1215 f t - l b of therm al energy absorbed per pound of ve hi cl e weight and 71.4hp fo r square fo ot of l in ing , respect ively .

3.2.2 Tr ac to r- Tr ai le rs Equipped wi th Advanced SystemsFor the two tractor-trailer combinations equipped with advanced systems,

changes i n some of the t e s t procedures were made because t he t e s t obj ect ive swere di ff er en t from those of th e other eleven vehicles. For example, i ntes t ing the Whi le 6 x 4 t r ac to r and the Fruehauf tandem axl e platform t r a i l e r(he re i na f t s r r e f e r r ed t o as Vehicl e #12), f iv e d i f f e r en t veh ic l e and load ingconfigurat ions were used, th e effect ive ness of the tra ct or f ro nt and re arbrakes was varie d, and the aforementioned brake co nt ro l systems, i .e ., : pro-port i oning valve s, adaptive braking, and t r a i l e r brake synchronizat ion, wereemployed i n va ri ou s combinations. The purpose i n te s ti ng Vehicl e #12 was t odetermine the best braking performance which could be achieved using a standardt e s t vehic le, equipped with commercially av ai la bl e brakes, and employing st at e-of- the -ar t brake contr ol systems. The eff ect ive nes s of th e tr ac to r brakeswas varied by changing the wedge angles and brake chambers on the front brakes,and t h e s lack adjuster lengths on the rear brakes . F ive d i f ferent vehic le andload confi gurat ions were te st ed using Vehicle #12, namely,

1. t r a c t o r - t r a i l e r , e m p t y2. t r ac to r - t r a i le r , loaded, high center of gravi ty3. t r ac to r - t r a i le r , loaded, low center of gravi ty4. t r ac to r only, bob ta i l conf igurat ion5. t r ac to r only, loaded t o gross axle weight ra t i ng


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P r i o r t o com mencem ent of t e s t s w i t h V e h i c l e # 12 , t h e f o l l o w i n g a dv an c edb r a k e c o n t r o l s ys te m s w ere i n s t a l l e d :

* P r o p o rt io n i n g v a lv e s f o r t h e t r a c t o r r e a r b ra ke s a nd t r a i l e r b r ak es ,suppl ied by Borg-Warner .

* A d a p t i v e b r a k i n g s y st em , w i t h a s e n s o r an d c o n t r o l l e r m ou nted on e a c h oft h e t e n w h e e ls of t h e c o m b in a ti o n t o p r e v e n t w h ee l l o c k u p d u r i n g b r a k i n g ,suppl ied by Bendix-West inghouse .T r a i l e r b r ak e s y n c h r o n i z a ti o n ( ~ y n c r o n ) y st em , w hich e f f e c t i v e l y a p p l i e st h e t r a i l e r b r ak e s a s s oo n a s t h e t r e a d l e v a l v e i s de p re s se d , s u pp l ie d bythe Berg Manufactur ing Company.

D u r i n g t h e c o u r s e of t h e t e s t p ro gr am c o n d u ct ed w i t h V e h i c l e # 12 , t h eb a s i c a i r b ra ke s y ste m of b o t h t h e t r a c t o r a nd t r a i l e r was a l t e r e d b y i n s t a l -l a t i o n o f a h i g h c a p a c i t y b ra ke c o n t r o l ( t r e a d l e ) v a l v e, q u ic k r e l e a s e v a l ve son ea ch t r a i l e r b r a ke a c t u a to r , l a r g e r c a p a c i ty l i n e s t o th e f r o n t t r a c t o rb r a k e s , a nd r e p la c e m e n t of c o n n e c t o r s , f i t t i n g s , a nd t e e s w hic h t e n d ed t or e s t r i c t a i r f lo w .

To ev a l ua te th e b rak ing per fo rmance of V eh ic le #12 unde r va r iou s cond i -t i o n s of l o a d i n g an d b r a k e e f f e c t i v e n e s s , a s m o d i fi ed by t h e v a r i o u s b r ak ec o n t r o l s y s t e m s a n d c o m b i n a t i o n s o f s y s t e m s , 59 minimum-s topping-dis tancet e s t s * a nd f i v e e f f e c t i v e n e s s t e s t s were c o nd uc te d.

The l a s t v e h i c l e t e s t e d i n t h e p ro gra m w as a t r a c t o r - t r a i l e r c o m bi na ti on ,fu rn is he d by Eaton, Yale , and Towne. T h i s v e h i c l e ( h e r e i n a f t e r d e s i g n a t e dV e h i c l e # 1 4 ) , a 4 x 2 Brockway COE tr a c t o r w it h an Arrow lowboy 35 f t t r a i l e r ,was ex t en s iv e l y modi f i ed f o r us e i n development o f th e Ea ton , Ya le , and Townew h e e l a n t i l o c k sy st em , w i t h w h ic h t h e v e h i c l e was eq u ip p ed . T hr ee e f f e c t i v e -n e s s t e s t s , f i v e m i ni m um - st op p in g -d is ta nc e t e s t s , and f o u r br a k e r e s p o n s e t i m et e s t s w er e c o n d u c te d on V e h i c l e # 14 .

S um m aries of t h e t e s t r e s u l t s f o r V e h i c l e s #12 a nd # 14 a r e g i v e n i n T a b le9 f o r s t o p s on t h e d r y s u r f a c e a nd T a b le 1 0 f o r s t o p s on t h e lo w c o e f f i c i e n ts u r f a c e . I n T a b le 9 r e s u l t s f r om t h e e f f e c t iv e n e s s t e s t s a r e a l s o in c lu d ed ,a l o n g w i t h r e s u l t s f ro m t h e d i s k b ra ke t r u c k . ( A c o m p l e te c o m p i l a t i o n o f t h ed a t a f ro m t h e e f f e c t i v e n e s s and m inimum s t o p p i n g d i s t a n c e t e s t s a r e i n c l u d e di n t h e F i n a l R e p or t volum e. )*In t h e minimum s t o p p i n g d i s t a n c e t e s t t h e d r i v e r w as i n s t r u c t e d t o a p p ly

maximum b r a k e p e d a l f o r c e a s r a p i d l y a s p o s s i b l e t o s t o p t h e v e h i c l e i n t h es h o r t e s t d i s t a n c e f r om t e s t s pe ed w i t h o u t l o c k i n g u p an y w h e el s, T e s t s w e r ec o nd u ct ed on b o t h d r y ( h i g h c o e f f i c i e n t ) an d w et s l i p p e r y ( l ow c o e f f i c i e n t )s u r f a c e s .


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braking efficie ncy , pedal forc e gain, and thermal response. Analyt ica lmethods found i n the l i t e r a t u r e were evaluated and extended fo r use i n thestudy.

The te s t r es ul t s ind ica ted t ha t the loaded vehic les exper ienced consider-ab le br ake f ade when being d ece ler ate d from speeds of 60 mph. The te s t da taa l s o showed th a t wheel lockup occurred prematu rely on ax le s of a tandem-axlepa i r mounted on e i t he r a t r ac to r or t r a i l e r . The ana lys i s developed t o pre-d ic t maximum decelera t ion cap abi l i t y necessar i ly had t o inc lude fade e ff ec tsas we ll a s dynamic load tr an sf er occurr ing on tandem-axle suspensions. Sinceth i s an a lys i s re su l t ed i n a ra th e r ex t ens ive se t of a lgebra i c and d i f fe r en t i a lequatio ns, d i g i t a l , analog, and hybrid computer techniques were developed t operform the ca lcula t ions .

3.4.1 Braking PerformanceFor eleven of th e th ir te en vehicl es t est ed , braking performance diagrams

were constru cted, i n which by use of an al yt ic al expressions, r ela t io ns hip sbetween pedal force , brake l i n e pressure , vehic le dece lera t ion cap abi l i ty( loaded and unloaded) and t i re -road f r ic t i o n co eff ic i en t requi red t o preventwheel lockup on a given axle ar e depicted." Figure 4 shows the brake perfor-mance diagram f o r th e 2-S1 combination t es te d, wherein t he experimental po int sar e denoted by smal l c i rc le s . Curves fo r braking eff i c ien cy f o r th i s vehic lea re g iven i n F igure 5 . I n th es e diagrams good agreement i s shown betweentheo re t i ca l and t e s t r e su l t s which a re gene ra l l y t yp i ca l of t he re su l t s f o rthe o the r ve hic les te st ed . The major conclusion th at can be drawn from t h i sphase of t he an al yt ic al program i s tha t accurate pred ict io ns of braking per-formance can be made based upon veh ic le and brak e system des ign d at a f o r buses ,t r uc ks , and t r a c t o r - t r a i l e r s .

The re la t i on sh ip between brake for ce dis t r ib ut io n and braking eff ici en cywas al so considered i n asse ssin g the braking performance of veh icles tes ted .It was demonstrated that those vehicles which exhibited the best performance,a s measured by maximum de ce le ra ti on a chi evab le, were tho se whose brake fo r c ed is tr ib u ti o n was a t or near optimum, whereas those veh icles showing le s se rperformance c ap ab i l i t ie s could have achieved be t t er performance with a di f-fe ren t b rake force d i s t r i bu t ion . For example, the 2-S1 combination had a brakeforce d i s t r i bu t ion g iven by

*Exceptions were th e heavy tru ck and th e c i t y bus, f o r which diagrams couldnot be constructed because necessary design information was not a vai lab lefrom the vehicle manufacturers.


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1F F i rst Tractor Axle1R Rear Tractor Axle2R Rear Tra iler Ax le

Pedal Force (Ib)

0.6 0.8 1 O

&Tra iler Wheels Locked

F ig u r e 4. Braking performance diagram for 2 - S 1 t r a c t o r - t r a i l e r .


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where

$1, = brake force on f r on t ax l e l t o t a l b rake force

4 R= brake for ce on rea r ax le / to ta l brake force

8, = brake force on t r a i l e r ax l e / t o t a l b rake forceThe optimum brake f or ce d is t r ib ut io n f o r t h i s vehicle was determined t o be0.20 : 0.42 : 0.38 ind i ca t ing in suf f i c i en t b rak ing e f fo r t on t he f ron t ax l eand too much on the t r a i l e r axle . Test da ta support th i s ana lys i s in tha tone tr ac to r r e ar wheel and one t r a i l e r wheel approached wheel s li d e when t h eempty vehicle dec elerated a t 17 f t / s e c 2 . Although better performance can beachieved by modi fying the brake for ce dis t r i bu t io n, a f ixed d is t r ib ut i onshould not be expected t o produce braking ef fi ci en ci es f o r combination veh iclesmuch higher than 75%.

Also included i n t h i s s tudy are e f fo r t s th a t have been di rec te d towardsdeveloping techniques t o pre di ct brake thermal response. The analysis washindered by severa l fac to rs , namely: ( a ) the ins t rumenta t ion used i n theexper imenta l program did not lend i t s e l f t o an accura te corre la t io n of ca l -cula t ion s and exper imenta l re su l t s , ( b ) l i ni ng mater ia l parameters such a sthermal condu ctiv ity, sp ec if ic heat, and densi ty, could not be measured andthus had t o be es t imated, ( c ) the lo ca t io n of the thermocouples in s t a l le d i nth e brake li n in g was not known with s uf fi ci en t accuracy, and ( d ) th e convectiveheat t r an sf er c oe ff i c ie nt could be ne i ther accura te ly computed nor accura te lydetermined from experiment. Notwithstanding thes e d if f ic u l t ie s , cal cu lat ion swere made t o pr ed ic t brake temperatures a s functio ns of t in e.

F i n i t e di ff er en ce methods were employed t o compute th e tempera tures a sa func tion of t ime a t var ious po in t s i n t he l i n ing , T = f ( x , t ) , t ha t wouldensue i n three of the t e s t procedures , namely, the fade te s t , the recoveryt e s t , and t he b ra ke r a t i ng t e s t . This procedure was di cta ted by th e nec ess i tyt o int roduce the convect ive hea t t ran sfe r co eff i c ien t , h , as a var iable dependentupon vehicle speed. The thermal pro pe rt ie s needed f o r the a na ly si s were ob-tain ed from published da ta and a d i g i t a l computer program was wr i t t en t o fa c i l -i t a t e the numerical work involved. Although reasonably good agreement wasobtained between th eor y and te s t f o r one of th e veh icle s, agreement betweentheory and t e s t was l e s s sa t i s fac t ory fo r o the r veh icl e s , t he re being , i ngen eral , d iscrepan cies of t he order of 2%. It can be concluded from thesere su l t s t ha t i n o rde r t o develop t echniques fo r p red i c t i ng brake t he rmal re s -ponse based upon veh icle and brake system design data, fu rt he r an al yt ic alwork supported by care fu l ly instrumented t e s t s w i l l be required.


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3.4.2 Proportioning SchemesDynamic modeling and sim ulat ion were employed i n t h i s study t o in ve st i-

gate th e effec tive nes s of load and decelerat ion -sensit iv e brake proportioningschemes i n improving brak ing performance over a wide range of loa din gs andtes t su r faces .

The dynamic si mula tion developed f o r t h i s study i s based on a model t h atrepresents e i ther a two-axle t ruck or t ractor , o r a th ree-ax le t r ac t o r - t r a i l e rcombination. Motions ar e constrained t o the plane of symmetry ( v e r t i c a l plan e).Spe cif ica lly th e wheels can bounce and s pin, the cha ss is can heave and pit ch ,and the vehicle can acceler ate (d ecel erat e) in s t ra ight - l ine motion. Thebraking system i s modeled i n a manner such th a t v ari ab le time lag s and dela ysi n torque response can be introduced. Any desired brake for ce di st ri bu ti oncan be specified,

To determin e th e improvement i n performance t h a t could be expected f o r athree-axle t rac tor - t r a i l e r combination ( 2 - ~ 1 ) s in g s t a t i c l oad- s en s i t i v e p ro-por t ioning valves, a ty pic al two-axle t r ac to r and 27 f t van t r a i l e r were usedas a basis fo r the s tudy. Brake torque for each axle was calculated fromty pi ca l design information. In th i s prel iminary inve stig atio n, no account wastaken of the lo ss of brake effectiveness th at i s caused by fade during a sin glestop . The foll owin g fin din gs were obtained:

1. Without proportioning the loaded vehicle achieves a maximum decelera-t ion o f 16.7 f t / sec 2 before the t r a i l e r wheels lockup,2. The empty vehicle without proportioning achieves a maximum deceleration

of 16.9 f t / sec 2 before the tr a i l e r wheels lockup.3. The empty vehicle wi th the brakes proportioned t o the s t a t i c loads on

the axles can achieve a maximum deceleration of 23.8 f t / sec2 wi thoutencounte ring wheel lockup, an improvement i n s topping ca pa bi li ty ofabout 4M0.

A second study was conducted t o determine i f a fe as il be p roportioni ngscheme could be developed which would d i s t r i b u t e th e brake fo rc es f o r maximumbraking ef fi ci en cy over a wide range of loadi ngs and surfa ce condi tion s. The2-S1 combination vehi cl e te st ed i n the program, namely, th e Ford F-7000 tr a c t o ri n combination with the Trailmobile 35 f t van t r a i l e r , was se lected as theprototype for th is s tudy. Parameter data defining the vehicle and brake systemwere determined and introduc ed i n t o th e simulation. The e ff ec ts of brake re s-ponse time, f ade, and pushout pres sure s were included . Prelimina ry runs weremade t o check th at the simulation re su lt s matched t e s t re su lt s obtained fo rthe ve hic le i n the empty and loaded co ndition.


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induced. However, i f t he r equi rement s fo r the i n i t i a l and f i na l snubve loc it i es ar e changed t o correspond with those of SAE 5786, h a t i s , 40mph t o 20 mph i nste ad of 60 or 50 mph t o 1 0 rnph, fa de can be induced anda measure of thermal cap ac ity could be obtained. One such performancemeasure i s th e number of snubs t ha t can be produced a t a spe cif ie d le ve lof decelera t ion with the t ime to ac cele rate back to t e s t speed betweensnubs not t o exceed a s peci fied pericd. From th e po in t of view of sa fe ty ,a thermal cap aci ty t e s t should be based upon th e most severe duty cycleth a t a vehicle might encounter i n general use.

Brake c ont ro l lab i l i ty i s used here i n the sense of t he dr iv er beingab le t o modulate brake forc e under a wide va ri et y of load ing and roadsurf ace c onditions t o minimize stopping dista nce while preventi ng wheellockup. ~ e cl e r a t i o n l ~ e d a l forc e g ain i s a performance measure th at canbe used t o character i ze brake cont ro l lab i l i ty .

Braking eff ici ency i s a measure of the a b il i t y of a vehicle t ou t i l i z e t h e f r i c t i o n f o r ce s a v a i la b l e a t t h e t i r e - ro a d i n te r fa c e. S t r i c t l yspeaking, i t i s defin ed a s th e r a t i o of th e maximum wheels-unlocked de-cele ra t i on capabi l i ty of t he vehic le on a g iven surface to the peak t i r e -road f r ic t i on coef f ic i ent of tha t surface . When braking eff ic ie nc y i sdetermined experimentally, th e su rface on which the v ehic le i s te st ed mustbe measured t o determine the peak t i re-roa d fr ic t i on coe ff ici ent . A t t h epr es en t time, adequat e means do not ex is t f o r making such a measurementwi th large t ruck t i re s . Therefore , as an a l t ern at i ve , it i s proposed th atbraking efficiency be determined by calc ulat ing th i s measure, using vehicleand brake system design data a s was accomplished f c r th e t e s t vehi cle s i nt h i s program.

Brake response time i s defined as t he time required fo r a brake t oreach a given le ve l of effect ive ness from the t ime th at the brake con tro l(pe dal ) i s acti vate d. Measurements of response time i n an ac tu al stopwould therefore require torque sensors on each braked wheel. Consequently ,a more common means f o r det erm ini ng th e re sponse t ime of pneumatic brak esystems i s t o measure th e t ime from the ins tan t of pedal appli cat i on ( re -su l t ing i n a f u l l , f a s t opening of the t r ead le va lve) t o the ins t an t agiven pressure le ve l i s reached i n t he brake chamber. Measurements showth at the response times ty-pically exhibite d by air-brake systems a resu f f i c i e n t t o infl uenc e th e braking performance of commercial veh icl es, a smeasured e it he r by average decel erat ion or stopping distance. Synchroni-zat ion of brake timing i s impor tant f or prevent ing in st ab i l i t ie s i nar t icula ted vehic les . Brake-release t ime i s s ign if i can t when the dr ive ri s atte mpting t o modulate th e brake forc e t o prevent wheel lockup. There-fo re i t i s apparent th at brake response i s a s ign i f ica nt measure ofcommercial vehicle brake system performance.

The performance measures de fi ne d above ar e summarized i n Table 13.


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c a n t l y l e s s t h a n t h a t o f p a ss en ge r c a r t i r e s . The e f f e c t i v e n e s s t e s t sp o i n t up c o n s i d e ra b l e v a r i a t i o n i n t i r e - r o a d f r i c t i o n c o e f f i c i e n t amongt h e v a r i o u s t y p e s of t r u c k t i r e s . F o r ex am p le , t h e l i g h t t r u c k w as c a p a b l eo f l o c ke d w h e el d e c e l e r a t i o n s of a s h ig h a s 28 f t / s e c 2 , w h i l e t h e d i s k

2b r a k e t r u c k w as c a p a b l e o f o n l y 2 2 f t / s e c w i t h t h e w h e e l s l o ck e d . S i n c et e s t s w i t h b o t h t h e M ic hig an D e pa rt me nt o f S t a t e Hig hw ay s' s k i d t r a i l e r an da n i ns tr um en te d c a r i n d i c a t e d t h a t t h e f r i c t i o n c o e f f i c i e n t o f t h e t e s tt r a c k v a r i e d o nl y s l i g h t l y ov er t h e e n t i r e p e ri o d o f t e s t i n g , t h e d i f f e r -e nc e i n lo ck ed w he el d e c e l e r a t io n m ust b e a t t r i b u t e d t o d i f f e r e n c e i n t i r et r a c t i o n . The i m p o r t a n c e o f t h i s o b s e rv a t i o n c a n n o t b e o v e re m p h as i ze ds i n c e it i s p o i n t l e s s t o i n c r e a se t h e e f f e c t i v en e s s o f b ra ke s i f f r i c t i o n a lf o r c e s c o r re s po n d in g t o t h e i n c r e a s e d br a ke t o r q u e c a p a b i l i t i e s c an n otb e p ro du ce d a t t h e t i r e - r o a d i n t e r f a c e . The r e a d e r i s t h e r e f o r e c a u t i on e dt o j ud ge t h e pe rf or ma nc e c a p a b i l i t i e s of t h e l a r g e r t r u c k s a nd t r a c t o r -t r a i l e r s t e s t e d i n t h i s program i n l i g h t o f t h e s e f in d i n g s on t r u c k t i r et r a c t i o n .

Th e maximum w h e e l s u n l o c k e d d e c e l e r a t i o n a c h i e v a b l e , a s d e t e rm i n e de i t h e r from a n e f f e c t i v e n e s s t e s t o r a minimum s t o p p in g d i s t a n c e t e s t ,i s a m ea su re of n o t o n l y t h e t o r q u e c a p a c i t y o f t h e b ra k es , b u t a l s o o fhow w e l l t h e v e h i c le u t i l i z e s t h e f r i c t i o n a v a i l a b le a t t h e t i r e - ro a di n t e r f a c e . The maximum d e c e l e r a t i o n o b t a i n e d o n t h e d r y s u r f a c e i ss um m ar iz ed i n F i g u re 6 f o r a l l o f t h e v e h ic le s t e s te d . Wi th pe r formanced e p ic t ed i n t h i s f as h i on , i t i s i m m ed ia te ly o b v io u s t h a t t h e v e h i c l e se q u ip p e d w i t h a d v an c ed b r a k e a nd b r a k e - c o n t r o l s y st e m s d i d n o t e x h i b i tb r a k in g p e rf o rm a n ce t h a t w as s i g n i f i c a n t l y b e t t e r t h a n w as a c h i ev e d by t h eb a s e l i n e v e h i c le s . I n f a c t it c a n b e a rg u ed t h a t a t r u c k , b us , o r t r a c t o r -t r a i l e r w i t h b r a k e s b a l a n c e d f o r m aximum b r a k i n g p e r f or m a n ce c a n e x ce edt h e p e rf o rm a n c e a c h i e v e d o n a d r y s u r f a c e b y a dv a nc ed b r a k e c o n t r o l s ys te m s.F o r e xa mp le , t h e l i g h t t r u c k , a l l t h e b use s, and t h e 2-S2 t r a c t o r - t r a i l e rc o m bi na ti on , e x h i b i t e d p e r f o r n a n c e o n t h e d r y s u r f a c e s a s good a s V e h ic l e# 12 an d c o n s i d e r a b l y b e t t e r th a n V e h i c l e #14. The d i s k b r a k e t r u c k , hcw-e v e r , w as o u tp e rf or m ed o n l y by t h e l i g h t t r u c k , a nd t h i s r e s u l t may s te mfrom a d i f f e r e n c e i n t i r e t r a c t io n . F i g u r e 7 p er ha ps b e t t e r d e p i c t s t h ei mp ro ve m en t i n p e r fo rm a n c e t h a t c a n b e e x p e c t e d by u s e o f a d v a n c ed b r a k e -c o n t r o l s y s t e m s . S i n c e w h e el l o ck u p i s v iew ed a s u n d e s i r a b l e , o n l y t h emaximum d e c e l e r a t i o n s o b t a i n e d up t o t h e p o i n t o f w h e el l o c k u p a r e p r e s e n t e di n t h i s f ig u r e . C o n s i d e r a b l e i mp ro ve m en ts i n p e rf o rm a n c e a r e a c h i e v e d f o rV e h i c l e #12 t h ro u g h u s e o f t h e a d a p t i v e b r a k in g s y st em , e s p e c i a l l y i n t h et e s t c on d uc te d on t h e low c o e f f i c i e n t s u r f a ce , an d i n t h e t e s t s c on d uc te dw i t h t h e e mpty v e h i c l e on t h e d r y s u r f a c e . I t s h o ul d b e n ot e d t h a t t h ee f f e c t i v e n e s s o f t h e b r ak e s o n V e h ic l e #12 was i n s u f f i c i e n t t o l a c k up a l lt h e wh ee ls on t h e d r y s u r f a c e i n t h e l o ad ed c o n d i t i o n .

V e h i c l e #14 d i d n o t p e rf or m a s w e l l a s V e h i cl e #12, e i t h e r on t h e d r ys u r f a c e ( e ve n a f t e r m aking a ll ow a n ce f o r t h e s l i g h t l y l ~ w e r i r e - r o a d f r i c -t i o n c o e f f i c i e n t o f t h e s u r f a c e upon w hic h V eh i cl e #14 was t e s t e d ) o r t h e


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wet surface. The empty tr ac to r (b ob ta il ) of Vehicle #14 produced a maximumdecelera t ion of 19 ft /s ec 2 on the d ry s urface while th e combination, empty2o r loaded, produced 15 f t / s e c . Although the antilock system did improveperformance of t he combina tion a s measured by th e maximum de ce le ra ti onac hie vab le with out wheel locki ng, th e improved performance was cons ider abl yle ss than th at which could have been achieved by be tt er ut i l iz at io n cf thefo rces in the t i re - road in t e r f ace .

Using the eAqer imenta ldata a t hand, it i s i mp ossi bl e t o c a l c u l a t eexactly the braking eff ici ency of the te st ed vehicles s ince the peak valuesof t i r e-r oad fr ic t i on coef f ic ien t , upon which eff icie ncy calculat ions mustne cessar il y be based, have been determined i n an approximate manner. How-ever , using these approximate coeff ic ient s, th e eff ic ienc y exhibited by eachvehicle and system te st ed i s tabulated i n Table 14 . Using eff ic ienc y a s thecr it er io n, th e di sk brake tru ck showed bes t performance on the dry surface.The system which produced t he bes t ov er al l e ff ic ie nc y measure was t he adapt-ive braking system installed on Vehicle #12. Next bes t was th e ant il oc ksystem installed on Vehicle #14. The sm all es t performance g ai ns wereachieved with the prop orti onin g system. As pointed out ea r l i e r , minor ad-justments i n the proport ioning valves, plus design ste ps th at prevent pre-mature lockup of t he wheels on th e lea ding ax le of th e t r a i l e r would havemarkedly improved th e performance obt ained with t he pro por tio ni ng system.The simulation study described e a rl ie r has in dica ted t he magnitude of per-formance improvements that can be expected from use of proportioning systems.

TABLE 1 4

OVERALL B . N K I N G EFFICIENCIES FOR ADVANCED SYSTEMS TESTED

Dry SurfaceVehicles Low Co ef fi ci en tSystem Empty Loaded Empty LoadedDisk Brake Truck - - 9 88 - - - -#12 Tra ctor -Tra iler Standard 48 67 - - 54

Proportioning 47 67 - - - -Adaptive 83 70 81 96

#12 Tractor Standard 46 70 4 3 47Proportioning 82 70 3 47Adaptive 82 80 80 7

#14 Tract or- Tr ai le r Standard - - 45 4 - -Ant il oc k 66 68 66 72

#14 Tra ctor Ant il oc k 86 - - 6 - -


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I f a vehicle i s equipped with an adaptive o r wheel a nti loc k brakesystem, th e dr ive r does not have t o modulate the pedal for ce t o preventwheel lock. However, fo r ve hi cl es not so equipped, a recent study a t HSRIshowed tha t t he d r i ve r ' s a bi l i t y i n modulating th e brake-pedal force t oprevent wheel lockup i s di re ct ly dependent upon th e decelerat ion-pedal forc echaract ,er ist ic of the vehicle." Of a l l the vehicles tes t ed , the disk braketru ck achieved t he g re at es t maximum de ce le ra ti on without lockup of wheels.Figure 8 shows the decelerat ion-pedal force gain cha rac ter is t ics of thedi sk brak e tr uc k alo ng with t he recommended upper and lower l im it s deri vedfrom t h e HSRI study. I t i s in te re s t in g t o note tha t those basel ir ie veh i-c l e s i n which the dri ver was a ble t o modulate th e pedal pressur e such tha ta high dece lera t ion was possible without wheel lockup a l l had dece lera t ion -pedal fo rce ch ar ac te ri st ic s which were with in the l im it s suggested by thefISRI stud!. These ve hic les include d t he l i g h t tr uc k, t he th re e buses, and.the 2-22 ractor - t ra i ler combinat ion. I t ther efor e appears reasonable t oconclude t . ha t these l i mi t s may a l so se rve as a gu ide l ine i n se l ec t i t~ g hedecelerat ion-pedal force cha rac ter is t ics for buses , t r ucks, and t ra cto r -t , r a i l e r s .

Measurements of brake re spo nse ti me given i n Table 11, show that t,hestandard braking systems of ' Vehicles #12 and #14 had application/releasetimes t h a t were approximate1.y th e same or s l i g h t l y longer tha n those o f t h eb ase l i n e t r a c t o r - t r a i l e r s . However when using S::ncron on Vehi cle {l2 andthe antilock system on Vehicle 1/14) the r esponse times o f the t r a i l e rbrakes were improved considerably.

1 . 3 RECOMMENDATIONS FOR A Sfl'ANllARI)The above discuss ed fi ndin gs, a s derived both from ana lys is and t e s t ,

ind ica te th a t th ree major s t eps w i l l have t o be taken t o s ign if i can t ly up-grade the maximum braking performance of comrne~cialvehicles.

F i r s t . The basi c braking systems of th e majo rity of th es e vehi cle sw i l l have t o be improved by use of more e ff e ct iv e brakes, b et te r balanc e,and f a s t e r system response or1 a i r braked vehic les.

Second. The tr ac ti on ch ar ac te ri st ic s of t i r e s used on the majori tyof medium and h e a ~ommercial vehicles w i l l have t o be improved so thatt he advantage of improved brake effe ctiv enes s can be fu ll y uti l i ze d a t th et i r e - road in te r f ace .*E. G. Mortimer, - -t a l . ,--rake Force Requirement Study: Driv er-Vehicle- -raking Performance - -s a Func tion of Brake System Design Varia ble s, Con-

t r a c t No. FH-11-6972, pre par ed f o r t he Nationa l Highway Safet y Bureau,U. S. Department of Transportation, by the Highway Safety Research IR-s t i t u t e , The University of Michigan, Ap ri l 10, 1970.


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1.2 - I I I1 II I

1 o.. -.

-..8,-0-c0.-w22 0.6,-a -.$n

-.

10.0 I I I20 40 60 80 100 120Pedal Force (Ibs)

F i g u r e 8. D e c e l er a .t i o n -p e d a l f o r c e g a i n f o r d i s k br a k e t r u c k a n dHSRI recommended l imi t s .


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compromise has been t o produce ve hi cle s which a r e s af e and r el ia b le wi thi nt h e i r performance range, and which a r e char act eri zed by high payload/vehi-c l e weight r a t i o s and minimal oper atin g and maintenance costs . To in tr o -duce a requirement f c r sev erely increased braking cap ab i l i t y int o the com-m e r c i a l f l e e t , a s i t has evolved, w i l l necessa r i l y requi re a reeva lua t imof th e design of th e en t i re system. The following points are suggestedfcr ser ious considera t ion.

1. More ef fe ct iv e brakes w i l l require st ronger suspensions andst ronger adjacent vehic le s t ruc tures .

2 Large brakes on the fro nt axle s of t r ac to rs could req uir e newfr on t axle and s tee rin g system designs, and, i n many cases, th euse of power steering.

3. With increased dece le ra t i cn capabi l i ty , methods of cargo res t r a i ntw i l l have t o be reevaluated. On buses, passenger re st ra in t sys-tems may have t o be ut i l i ze d.

4. The re la t i ve ly high r a t i o of cen ter of gra vi t y height above road-way t o t ru ck width, th a t i s common t o s t r a i gh t t rucks , causedvehic le s t ab i l i t y problems t o be encountered a t moderate dece lera-t i ons i n t h i s t e s t program. It i s expected th a t th e problem w i l lbe worse a t higher d ecelerat io ns. This problem may be alleviatedby use of ant i lo ck systems. However, a t t h i s poin t i n t ime, t heproblem i s not c le ar ly def ined and requ ire s much more stu dy be-fo re a de f i ni te s olut ion can be sugges ted.

5. I f p ropor ti oning and/or ant i lock systems ar e t o be widely used,cognizance should be taken of maintenance and re l i a b i l i t y problemsass oci ate d with each system. Load se n si ti v e propo rtionin g systemsre qu ir e mechanical, pneumatic, or o the r means of s ens ing changesi n load. Due t o wear, c orr osio n, and other degrading fa ct or s, th ele v el of coulomb fr i c t i o n i n th e suspension system may change,thus r equ irin g per iodi c inspect io n and adjustment o f the l inkage .S ince an t i l ock sys tems for a i r b raked veh ic l es a re s t i l l i n t hedevelopmental s tage, r e l i a b i l i t y problems with both mechanical andel ec tr on ic ccmponents were encountered i n the t e s t program. It i smandatory that ant i lock systems have a high degree of rel iabi l i tybecause of th e human fa ct or s irvolved. The t e s t program haspointed out th a t reg ardless of load or surf ace condi tion, thed r i ve r w i l l make rapid, high-l evel brake app l icat ion s i f he knowsth e ant i lock sys tem i s opera t ional , whereas he w i l l be extremelyse ns i t iv e t o load and surfa ce con dit ions when applying the brakeswi thout the ant i l ock system i n opera t ion. Ser ious s t ab i l i t yproblems a re poss ib l e i f t he dr ive r app l i e s t h e brakes rap id lythinkin g th a t the ant i lock sys tem i s ope ra t ional where, indeed, i t


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i s n o t , du e t o a com ponent f a i l u r e .I n m ak in g s p e c i f i c re co m m en da tio ns f o r a s t a n d a r d , c a r e f u l c o n s i d e r a -

t i o n h a s b ee n g i v e n t o t h e n e c e s s i t y t o u p g ra d e c om m er cia l v e h i c l e b r a k i n gp er fo rm a nc e a s q u i c k l y a s p o s s i b l e t o a c c e p t a b l e l e v e l s . C a r e f u l c o n s i d e r a -t i o n h as a l s o b ee n g iv e n t o t h o s e p o i n t s d i s c u s se d i n t h e p a ra g ra p h above.T a ki ng i n t o a c c o u n t t h e s y st em d e s i g n p r ob le m s w h ic h w i l l r e s u l t f r o m i n -c r e a s e d p er f or m a n ce r e q u i r e m e n t s , a nd t h e s t a t e of d ev el op m en t o f a d v an c edsys tems , it i s r ecom men ded t h a t r u l e s b e p r o m u l g at ed w h ic h r e q u i r e u p g r a d i n gt h e p erf orm an ce o f t ru c k s , b u se s, and t r a c t o r - t r a i l e r s i n t h r e e d i s c r e t es t e p s , s e p a r a t e d by a p p r o p r i a t e p e r i o d s o f t im e .* As a f i r s t s t e p , i t i sr e c o m m e n d e d t h a t t h e r u l e s r e q u i r e i m m e d i a t e a c t i o n t o u p g r a d e b r a k i n g p e r -fo rm an ce t o a l e v e l a c h ie v a b l e by c u r r e n t d e s ig n p r a c t i c e , t h a t i s , t h eb e s t p e rf o rm a n ce a l r e a d y d e m o ns tr a te d by b a s e l i n e v e h i c l e s t e s t e d . F o r t h es e c o n d s t e p , i t i s recom mended t h a t t h e r u l e s r e q u i r e p e rf or m an c e t o b e i m -p ro ve d t o t h e l i m i t of t h e t i r e - r o a d i c t e r f a c e t r a c t i v e c a p a b i l i t i e s o ft r u c k t i r e s now a v a i l a b l e w i t h du e r e g a rd t o r e a l i s t i c b r ak i ng e f f i c i e n c i e s .The s ec o nd s t e p may r e q u i r e u s e o f l o a d s e n s i t i v e p r o p o r t i o n i n g s y st e m s onc e r t a i n v e h i c l e s , an d t h e r e f o r e s u f f i c i e n t l e a d t im e s h ou ld b e a ll ow e d f o rf u r t h e r d e ve lo pm e nt an d t e s t i n g o f t h e s e d e v i c e s . A f t e r a n a p p r o p r i a t e ti m ei n t e r v a l t o al lo w f o r de velo pm en t and t e s t i n g of a r e l i a b l e a n t i l o c k s y s t e m ,t h e d evelopm ent o f t r u c k t i r e s w it h b e t t e r t r a c t i v e c h a r a c t e r i s t i c s , and t h en e c e s s a r y d e s i g n m o d i f i c a t i o n s of v e h i c l e b r ak e , s u s p en s i o n , an d s t r u c t u r a lsys tems , it i s recom mended a s a t h i r d s t e p t h a t p e rf or m an ce e q u a l t o o ra p p ro a ch i n g t h a t o f p a s se n g er c a r s b e r e q u i r e d a l o n g w i t h u s e o f a n a n t i l o c ks ys te m t o i n s u r e v e h i c l e s t a b i l i t y o v er a w id e r a n g e of v e h i c l e l o a d i n g s an dro a d s u r f a c e c o n d i t i o n s . S um m ar ie s o f t h e s u g g e s t e d p e r fo rm a n ce r e q u i r e m e n t sf o r e ac h s t e p a r e g i ve n a s f o l l o w s :

Maximum d e c e l e r a t i o r . c a p a b i l i t y - 16 f t / s e c 2 * *M i n i m u m b ra k i n g e f f i c i e n c y - 65$ f o r s u r f a c e s h av in g pe a k t r u c kt i r e - r o a d f r i c t i o n c o e f f i c i e n t s betw een 0.2 and 0.8

* Re co mm en da tio ns f o r a s p e c i f i c t i m e f r a m e o r s c h e d u l e t o i m pl em e nt t h e s es t e p s c a n n ot b e made s i n c e i n f o r m a t i o n on l e a d t i m e s f o r i n t r o d u c t i o n o fde s ig n chan ges , deve lopment o f new hardware and ne ce ssa ry man ufac tur ingt e c h n i q u e s i s n o t g e n e r a l l y a v a i l a b l e f ro m v e h i c l e , b ra ke , a nd h a k e com-p o n e n t m a n u fa c t u r e r s .

**It i s presu med t h a t t h i s d e c e l e r a t i o n w ou ld b e m e as ur ed on a s u r f a c e h a v i n ga peak t ru c k t i r e - r o a d f r i c t i o n c o e f f i c i e n t of a t l e a s t 0.75.


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pedal force gain char acte r is t ics , and s ta t i c response t ime of a i r brakesystems. I f stopping dis tan ce t e s t s are required, they should be made i nconjunct ion with t he eff ect ive nes s t e s t s t o ensure maximum unlocked wheeldecelerat ions are achieved. Compliance with braking efficiency requirementscan be made by requir ing desigc calculat ions s imi lar t o those ~ u t l i n e d nth e Fin al Report volume, and by val ida tin g the c al cul at ion s by effectivenesst e s t s on high co eff ici ent and low coe ff i ci ent surfaces.

Testing of tr ac to r- tr ai le r combinat ions presents a sp eci al challenge.Because of brake balan ce problems, a t r a c t c r which may perform very we llwith one t r a i l e r may perform poorly with another. Conversely, a tr a i l e rwhose brakes were t e s t ed on a brake dygamometer and deemed adequate mayperform wel l with one tra c to r but perform poorly with another. Also atr ac to r could perform well a s a loaded s tr ai gh t truck, but poorly i n corn-binat i. cn wi th a t r a i le r . For t h i s reasan i t i s recommended t h a t tr ac to rsbe cer t i f i ed t o pu l l only those t r a i l e r s wi th which i t has been demonstratedby design calc ula tio n and t e s t t ha t performance of the combination vehi clei s adequate.


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bus truck tractor brake performance

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