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HSRI
Report No. HuF 6
Brake Force Requirement Study: Driver
Vehicle Braking Performance as Function
of Brake System Design Variables
R G
Mortimer L Segel
H
Dugoff
J.D. Campbell C.M Jorgeson R
W
Murphy
Highway Safety Research Institute
University o f Michigan
Huron Parkway and Baxter Road
Ann Arbor Michigan
481
05
April
10 1970
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The 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 ie w s
o f t h e Highway S a f e t y R e s e ar ch I n s t i t u t e wh ic h
i s r e s p o n s i b l e f o r t h e f a c t s an d t h e a cc u ra c y
o f t h e d a t a p r e s e n t e d h e r e i n
T he c o n t e n t s do
n 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 v ie w s o r
p o l i c y o f t h e D ep ar tm e nt o f T r a n s p o r t a t i o n .
T h i s r e p o r t d oe s n o t c o n s t i t u t e 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 l a t i o n .
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4
T~t le nd Su bt~t le
5.
Report Date
1
Brake Force Requirement Study: Driver -Vehic le
Braking Per formance a s a Func t ion of Brake
S ys te m D es ig n V a ~ i a b l e s
7 Author(s)
R . G .
Mortimer , L. Sege l ,
H
Dugoff,
J . D . Campbell, C.M. Jo r ge son ,
R.W.
Murphy
3. R ec~ p ~ en t satalog Xo
I
Report No.
9.
Performing Organ~zat~oname and Address
Highway S a f e ty R e se a rc h I n s t i t u t e
Univers i ty of Michigan
Ann Ar bo r, Mic hig an 48105
2 Government Access~onNo.
12. Sponsoring Agency Name and Address
Federa l Highway Adminis t ra t ion
Nat io na l Highway S af et y Bureau
Washington, D . C . 20591
Apr i l 10 , 1970
6
Perforrn~ngOrgan~zat~onode
8.
Performing Org an~z at~on eport No.
HuF-6
10.
Work Uut No.
11. Contract or Grant No.
FH-11-6952
13. Type of Report and Pe r~o dCovered
F i n a l R e p o r t
J u l v
1
1 9 6 8 - A ~ r i l10. 1970
14
Sponsonng Agency Code
15. Supplementary Notes
1 1 6 Abstract The o b j e c t i v e o f t h i s s t u d y was t o d e f i n e t ho s e b r ak e c h a r a c t e r i s t i c s ,
w i t h in t he spa c e bounded by th e r e l a t i on sh ip be twe en b r a ke pe da l f o r c e and
v e h i c l e d e c e l e r a t i o n , w hi ch l e a d t o a c c e p t a b l e d r i v e r - v e h i c l e p er fo rm an ce .
A
d r i v e r - v e h i c l e b r a k i n g t e s t was pe rf or me d i n w hi ch t h e d e c e l e r a t i o n / p e d a l f o r c e
r a t i o , t h e pe d al di s pl a ce m en t , t h e s u r f a c e - t i r e f r i c t i o n , a nd d r i v e r c h ar a c t er -
i s t i c s ( ag e, w ei g ht ) w ere s y s t e m a t i c a l l y v a r i e d i n o r d e r t o d et er mi n e t h e i n f l u -
e nc e o f th e s e v a r i a b l e s upon minimum s top p ing d i s t a nc e a nd o t he r pe rf o rm a nc e
v a r i a b l e s . The t e s t s t h a t w ere p er fo rm ed on a low c o e f f i c i e n t of f r i c t i o n s u r -
f a c e showed t h a t hi g h v a l u e s of d e c e l e r a t i o n / p e d a l f o r c e g a i n r e s u l t i n l a r g e
number o f whee l loc kups and lower mean de c e l e r a t i on i n b r ing ing th e ve h ic l e t o a
s t o p , compared t o i n t e r m e d i a t e o r low d e c e l e r a t i o n / p e d a l f o r c e g a i n l e v e l s .
T e s t s c o n d uc t ed on i n t e r m e d i a t e a nd hi g h c o e f f i c i e n t of f r i c t i o n s u r f a c e s s howed
t h a t h i g h a n d i n t e r m e d i a t e d e c e l e r a t i o n / p e d a l f o r c e g a i n s p ro du ce d g r e a t e r mean
de c e l e r a t i on s and g r e a te r f r e que n c ie s o f whe el loc kups tha n lowe r ga in sys t e m s .
The f re qu en cy o f l o s s o f l a t e r a l c o n t r o l was s i g n i f i c a n t l y g r e a t e r w it h t h e h i gh
d e c e l e r a t i o n / p e d a l f o r c e g a i n b ra k e s on a l l s u r f a c e s t h a n wi t h l o we r g a i n s .
There were minor be ne f i t s of 2 .5 inc h ped a l d isp lace ment compared t o ze ro inch es .
P o t e n t i a l b ra k e f a i l u r e s and t h e i r e f f e c t s upon p e d a l f o r c e r e qu i re m en t s we re
a n a l y ze d . The i m p l i c a t i o n s o f t h e f i n d i n g s f o r a v e h i c l e b r a k i n g s t a n d a r d w er e
shown i n t er m s o f de c e le r a t i on / pe d a l f o r c e ga in and pe da l f o r c e .
7
Key
Words
u n c l a s s i f i e d u n c l a s s i f i e d I xx 200
I
B R A K I N G , DECELERATION, STOPPING DISTANCE,
D R I V E R B R A K I N G ,
PEDAL FORCE, PAVEMENT
FRICTION, BRAKE FAILURE.
A v a i l a b i l i t y i s u n l i m i t e d . Document
may be r e l e a s e d t o th e C le a r inghouse
f o r F e d er a l S c i e n t i f i c and T e ch n ic a l
I n f o r m a t ion , S p r i ng f i e ld , Va. 22151
f r
s a l e t o t h e p u b li c .
19.
Secur~tyClass~f of thls report)
20.
Securlty Classif.(of thls page)
21 No, of Pages 22 Pr~ce
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TABLE
OF
CONTENTS
L i s t of T a b l es
L i s t of F i g u r e s
Acknowledgements
I n t r o d u c t i o n
ummary of Tasks
L i t e r a t u r e R e v i e w
F o o t F o r ce C a p a b i l i t y o f D r i v e r s
3 Dr i ve r Brak ing Pe r fo rmance a s a Func t i on o f
Pedal -Force and Pedal -Displacement Level s
D r i v e r B r a k i n g P r a c t i c e
F a i l u r e A n a l y s i s
Recommendations
asks
L i t e r a t u r e R e v i e w
n t r o d u c t i o n 5
eda l Force a s a Func t ion of Des ign Param eters 9
rake System Design
9
edal Force Des ign Goals 9
eda l Force/Brake Torque Re l a t i ons h ip 9
r a k e P r o p o r t i o n i n g 1 2
es ign Pr ac t i ce and Trends 13
r a k e u s a g e 1 5
rake Usage Measurements 15
ade and Fade Tes t ing
1 6
he Phenomenon of Fade 1 6
hermal Ana lys i s 17
i n i n g M a t e r i a l s 1 8
a de a nd P e d a l E f f o r t 1 8
rake Sys tem Degr ada t io n 19
ydr aul ic System 19
ear o f F r i c t i o n E l emen t s 2
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S ki dd in g a s R e l a t e d t o B r a k i n g 20
Impor tance o f Brak ing Cont ro l i n Acc iden ts 2
Braking Dynamics
2 1
R oad Tire F r i c t i o n C o e f f i c i e n t s
2 2
Skid Co ntr ol by Braking Modulation 23
Te st in g of t h e Vehicle Tire Brake System 25
Dec ele ra t ion Per formance 25
Stopping Dis t ance 5
De ce le ra ti on Measurement 5
Di r e c t i on a l C on t r o l 26
Cont ro l o f Brak ing Tes t s 6
r iver Response
7
S t a t i c D r iv e r V e hi c le R e l a t i o n s h ip s 27
Dr ive r T r a n s i e n t R esponse C h a r a c t e r i s t i c s 8
Dri ver Brak e Pe da l System Dynamics 31
Foot Force Capabi l i ty o f rivers 32
In t ro duc t io n 32
Method
34
Apparatus 34
P i l o t S t u d y
38
Procedure
3 8
R e s u l t s . .
38
M a i n s t u d y 4
Procedure
4
S ub je c t s 4 1
R e s u l t s 4 1
Discuss ion 4
Driver Braking Performance as
a
Funct ion of
edal Force ana Pedal Displacem ent 50
n t r o d u c t i o n 5
ethod
52
ub je c t s
52
he Tes t Vehic le 5 2
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G en er a l D es c r i p t i o n . . . . . . . . . . . 52
. . . . . . . . . . . .he Brake Syst em 55
Brake P ropor t ion ing
. . . . . . . . . .
58
B;ake System Pa ra me te rs . . . . . . . 60
Speed Control System . . . . . . .
6
D at a C o l l ec t i o n I n s t r u m en t a t i o n
. . . .
6 3
Independen t Var i ab les
. . . . . . . . . . . .
70
Dependent Variables . . . . . . . . . . . . .
7
P r oc e du r e: P i l o t S t u d i e s . . . . . . . . . . 71
. . . . . . . . . . . . . .
ydroplan ing 71
. . . . . . . . . . .roce dure: Braking Te st 74
Performance and Su bje ct ive Data Record ing 7 5
Experimental Design . . . . . . . . . . . . . 76
. . . . . . . . . . . . . . . . . . . . . .
esu l t s 76
Braking Dis tance . . . . . . . . . . . . . . . 76
D ece l e r a t i o n . . . . . . . . . . . . . . . . . 77
. . . . . . . . . . . . . . . . .
raking
T i m e
84
For ty Percen t Decrease in Speed . . . . . . . 85
Wheel Lockup Frequency . . . . . . . .
86
. . . . . . . . . . . .
heel Lockup Du ra ti on 86
Proportion of Wheel Lockup T i m e t o
. . . . . . . . . . . . . .
o ta l Brak ing T i m e 9 5
L os s o f L a t e r a l C o n t r o l . . . . . . . . .
9 5
. . . . . . . . . .a t in gs o f C o n t r o l l a b i l i t y 99
. . . . . . . . . . . .at ing s of Pe dal Force 9 9
Between Subject Performance Comparison . . . 00
Correlation Between Maximum Pedal Forces
Measured i n th e Vehi cle and th e Buck . . . . 00
. . . . . . . . . . . .
ub je ct Age and Weight 103
. . . . . . . . . . . . . . . . . . . .
i scuss ion 103
. . . . . . . . . . . .
ec ele ra t ion Measures 103
. . . . . . . . . . .
oss of Co ntr ol Measures 104
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Su bj ec t i ve Measures 06
D r ive r V e h ic l e Br a king E f f i c i e n c y 06
De r i va t i on of t h e PFG Envelope 09
evelopment of a Re vi si on t o MVSS 105
1 1 2
D r i v e r B r a k i n g P r a c t i c e
1 1 4
n t r o d u c t i o n
1 1 4
ethod 115
pparatus 115
Procedure 20
ub je c t s 120
esu l t s 120
i s c us s ion 1 2 4
F a i l u r e A n a l y s i s 1 2 5
n t r o d u c t i o n 1 2 5
a i l u r e Modes 125
i n e P r e s s u r e F a i l u r e 1 25
ower Boos t Fa i lu re 133
rake Fade 136
F a c to r s I n f lu e n ci n g t h e P a r t i a l F a i l u r e o f
rake Systems 140
onsequences of Fa i l ur e 147
f f ec t s on Vehic le Performance 147
I n f l u e n ce of P a r t i a l F a i l u r e s on D ri ve r
eh ic l e Braking Per formance 148
Genera l D iscuss ion 57
Recommendations 62
ppendices
Appendix
I
D e r iv a t io n o f Cons t a n t P e da l
D i s
p la c e m e n t /D e c e l e r a t i on Cha r a c t e r
s t i c
ppendix
I
I n s t r u c t i o n t o es t S u b j e c t s
1 6 6
n s t r u c t i o n s P r a c t i c e Run
ns t r uc t i on s O f f i c i a l Run 168
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A p p e n d i x
I
V e h i c l e I n s t r u m e n t a t i o n f o r
e c e l e r a t i o n R e c o rd i n g 7 5
D e c e l e r a t i o n M e as ure me nt 7 5
D e c e l e r a t i o n C a l i b r a t i o n 7 7
r ak e L i n e P r e s s u r e 8
r a k e L i n e P r e s s u r e C a l i b r a t i o n 8
S a m p le D a t a 8
n s t r u c t i o n s t o D r i v e r 8 3
T r i p S h e e t
8 4
R e f e r e n c e s 8 5
v
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L ST
O
T BLES
P a g ea b l e
2 1
PILOT STUDY MEDIAN RIGHT AND LEFT FOOT FORCE BY
THREE GROUPS OF SU BJ EC TS FOR STANDARDtf AND
INDUCED MOTIVATION INSTRUCTION. DATA ARE I N
OUNDS.
AGE DISTRIBUTION OF FEMALE AND MALE SUBJECTS.
WEIGHT DISTRIBUTION OF FEMALE AND MALE SUBJECTS
CUMULATIVE PERCENT RIGHT FOOT FORCE DISTRIBU-
T I O N : 2 7 6 FEMALE DRIVERS
CUMULATIVE PERCENT RIGHT FOOT FORCE DISTRIBU-
I O N : 3 3 M A L E D R I V E R S .
C H A R A C T E RI S T IC S O F T HE T E S T S U B J E C T S ( D R I V E R S ) .
EDAL FORCE GAINS
PILOT TEST: MEAN BRAKING
DISTANCE (FEET)
ON
DRY AND WET FOR POWER AND MANUAL BRAKE.
P I L O T T E S T : MEAN DECELERATION
f
/ ; e c 2 ON
DRY AND WET FOR POWER AND MANUAL BRAKE.
PILOT
T E S T : MEAN TIME
(SECONDS) TO DECREASE
SPEED BY
1 0
MPH FROM START OF BRAKING ON DRY
ND WET FOR POWER AND MANUAL BRAKE.
MEAN BRAKING DISTAN CE (F EE T) AS A FUNCTION OF
S P E E D , DECELEIIATION/PEDAL FORCE GAIN AND
R I C T I O N. DATA F OR
2 8
S U B J E C T S
ANALYSIS OF VARIANCE OF TRANSFORMED DECELERA-
TION PERFORMANCE.
GEOMETRIC MEAN DECEL ERATIO N, I N g FOR THE
INTERACTION OF
S P E E D ,
DECELERATION/PEDAL FORCE
AI N AND SURFA CE. DATA FOR
2 8
S U B J E C T S
NEWMAN-KEULS TEST
O F
MEAN DECELERATION FOR
DECELERATION/PEDAL FORCE GA INS AT EACH
SURFACE AND SPEED
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P a g e
a b l e
3 . 1 0 .
RANK ORDER OF DECE LERA TION /PEDA L FORCE GA IN S
D I F F E R I N G S I G N I F I C A N T L Y I N D R I V E R MEAN BRAK-
I N G D E C E L E R A T I O N . .
8 4
MEAN
TIME
TO REDUCE S PEE D BY 4 0 PERCENT FOR
MAIN EF FE CT S OF SPEED , PFG AND SURFACE.
MEAN NUMBER OF WHEEL LOCKUPS PER TRIAL FOR
MAIN E F F E C T S . 9 1
MEAN WHEEL LOCKUP TIME PER TRIAL FOR MAIN
E FF EC TS 9 3
PERCENT OF TRIALS INVOLVING LOSS OF LATERAL
CONTROL AS A FUNCTION OF BRAKE SYSTEM, SPEED
A N D S U R F A C E . . . . . . . . . . . . . . . . . . g g
MAXIMUM PEDAL FORCES I N THE ST AT IC T ES T AND
I N T HE T E S T V E H IC L E . C E L L V AL UE S IN D IC A T E
NUMBER O F S U B JE C T S . 0 3
CUMULATIVE FREQUENCY DISTRIBUTION OF
EAK DECELERATIONS (MANUAL BRA KES ). 1 2 2
CUMULATIVE FREQUENCY DISTRIBUTION OF
EAK DECELERATIONS (POWER BRAKES) 1 2 3
TYPICAL
IIECELERATION/ PEDAL FORCE
RATIOS.
HYDRAULIC LINE FAILURES FOR VEHICLES WITHOUT
O WE RB OO ST 1 2 8
TYP ICA L DECELERATION CHAR ACTE RISTI CS. HY-
D R A U L IC L IN E F A IL U R E F O R V E H IC L E W IT H
P O W E R B O O S T . 3 0
COMPLIANCE TE ST S, MVSS -105 FOR VEH ICL ES WITH NON-
P O W E R B R A K E S .
. . . . . . . . . . . . . . . . . .
4 1
C OM PL IA NC E T E S T S , MVSS - 1 0 5 F O R V E H IC L E S W IT H
P O W ER B R A KE S . 4 2
TABULATION OF REQUIRED PEDAL FORCES FOR FRONT
BRAKE CI R C U I T FA IL UR ES I N LOADED, MANUALLY
B R A K E D S E D A N . . . . . . . . . . . . a . m . . . . 1 5 5
P R O B A B IL IT Y O F FR ON T BRA KE C IR C U IT F A IL U R E
RESU LTING I N VEHI CLE DECELERATIO N LOWER THAN
D E S I R E D .
a 1 5 5
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Table Page
A.11.
Brake Force Modulation Study: Data Collection
Sheet Official Runs Displacement 0
.
. .
.
173
A.11. Brake Force Modulation Study: Data Collection
Sheet Official Runs Displacement 2.5
. . . . 174
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L IS T OF
FIGUR S
Figu re Page
1 1
T y p i c a l h y d r a u l i c b r a k e s y st e m drum t y p e ) . 6
1 2
T y p i ca l b r ak e p e rf o rm an ce cu r v es 0
1 . 3 . The b r ak i n g p r o ces s r ep r e s en t ed a s a f e edb ack
co n t r o l s y st em 29
2 1
Brake and ac ce le ra to r d imens ions . mean /s t andard
d e v i a t i o n . 1 96 8 ca r s 35
2 2 Foot pe da l fo rc e measurement buck 6
2 .3 . Hydra u l i c fo rc e gauge and fo o t pad 37
2 4
Cumula tive pe rc en t ped a l fo rc e f o r 276 females 3
2 . 5 . C u mu la ti ve p e r cen t p ed a l f o r ce f o r 323 m a le s
4
3 . 1. The h y d r a u l i c b r ak e co n t r o l s y s t em 4
3 .2 . Brake co n t ro l sy s t em 57
. 3 . B r ak in g e f f i c i e n c y o f t h e t e s t v eh i c l e 5 9
3 .4 . P e d a l f o r c e a nd d i s p la c e m e n t f o r e a c h d e c e l -
e r a t i o n / p e d a l f o r c e l e v e l 62
3 .5 . Peda l
displacenent decelration
r ange 2
3 . 6 . P er fo rm a nc e r e c o r d i n g d i s p l a y s i n t h e
t e s t
v e h i c l e 4
3 .7 . P er fo rm an ce d a t a c o l l e c t i o n i n s t r u m e n t a t i o n
bloc k d iagram 5
3 . 8 .
D e c e l e r a t i o n a s a f u n c t i o n o f l o ck e d w he el
v e l o c i t y and s u r f a c e
8
3 .9 . Tes t c a r i n t h e t r a c k . s ho wi ng l a n e m ar ke r
cones and s t imu lu s/g oa l lamps 69
. 10 . D e c el e ra t io n /p e da l f o r c e f o r p i l o t t e s t car s 72
3.11.
Mean b r a k i n g d i s t a n c e a s a f u n c t i o n o f d e c e l -
r a t i o n / p e d a l f o r c e g a i n . s p e ed and s u r f a c e 78
3 . 12 . Mean b r ak i n g d i s t an ce a s a f u n c t i o n o f s u r f ac e
and ped a l d i sp la cemen t 9
3 . 13 . G eom e tr ic mean d e ce l e r a t i o n a s a f u n c t i o n o f
e c e l e r a t i o n / p e d a l f o r c e g a i n . s p ee d a nd s u r f a c e 82
x i i
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Page
Mean t i m t o r e d u c e s p e ed by 40 a s a f u n c t i o n
o f d e c e l e r a t i o n / p e d a l f o r c e g a i n s p ee d a nd
s u r f a c e . . . . . . . . . . . . . . . . . . . . 87
Mean numbzr of wheel lockups as a f u n c t i o n o f
d e c e l e r a t i o n / p e d a l f o r c e g a i n a nd s p e ed . 88
Mean number of wheel lockups
as
a f u n c t i o n o f
d e c e l e r a t i o n / p e d a l f o r c e g a i n a nd d i s p l a c em e n t 89
Mean number of wheel lo ckups a s a fu nc ti on o f
d e c e l e r a t i o n / p e d a l f o r c e g a i n an d s u r f a c e . 90
Mean wheel lockup time a s a f u n c t i o n o f s p e ed
and su rf ac e . 92
Mean wheel lockup
t i m as
a f u n c t i o n o f d e c e l e r a -
t i o n / p e d a l f o r c e g a i n and s u r f a c e . 94
P e r c e n t wh ee l l o c ku p t i m e / t o t a l b r a k i n g time
a s
a
f u n c t i o n o f s u r f a c e and s pe e d
9 6
P e r c e n t wh ee l l oc k up t i m e / t o t a l b r a k i n g time
a s a f u n c t i o n of d e c e l e r a t i o n / p e d a l f o r c e g a i n
and su r f ac e . 97
P er ce n t of t r i a l s i n v o l vi n g l o s s o f l a t e r a l con-
t r o l a s a f u nc t i on of 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 s u r f a c e a nd s p e e d : 0 i n c h d i s p l a c em e n t . 98
P e r ce n t of t r i a l s i n v ol v i ng l o s s of l a t e r a l con-
t r o l a s a f u nc t i o n o f d e c el e r at i on / p ed a l f o r c e
g a i n s u r f a c e and s p e ed : 2 5 inche s d i sp lacement . 98
Mean c o n t r o l l a b i l i t y r a t i n g f o r 28 s u b j e c t s a s a
f u n c t i o n o f d e c e l e r a t i o n / p e d a l f o r c e g a i n and
peda l d i sp laceme l i t 01
Mean r a t i n g o f f o r c e r e q u i r e d f o r 28 s u b j e c t s a s
a f u n c t i o n of d e c e l e r a t i o n / p e d a l f o r c e g a i n and
d i s p l a c e m e n t 0 1
Braking performance of t h e b e s t s u b j e c t gr oup
mean a nd p o o r e s t s u b j e c t a s a f u n c t i o n o f d e c c l -
e r a t i o n / p e d a l f o r c e g a i n and s u r f a c e 02
x i i i
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Page
Mean braking ef f ic iency
as
a f u n c t i o n of d e c e l e r -
a t i o n / p e d a l f o r c e g a i n s pe ed and s u r f a c e . . l o 8
C u t - o f f PFG v a l u e s f o r s a t i s f a c t o r y d r i v e r -
veh ic l e b rak in g per fo rmance . 11 1
The recommended de ce le ra t i on /p ed al fo rc e space 11 3
Dat a r e co r d i n g i n s t r u m e n t a t i o n i n t r u n k o f
t e s t
c a r . 1 1 6
Deceler a t ion/p edal fo rc e f o r manual and power
ra ke mode. 1968 Plymouth 4-door se da n. .11 7
Lon git udi nal l o ca t i o n of manual and power brake
p e d a l s and a c c e l e r a t o r 1 1 8
Height and location of power and manual brake
p e d a l an d a c c e l e r a t o r . 1 19
Cumula tive pe rc ent of de ce le ra t i on s f o r manual and
power brakes 1 2 1
Dece lera . t ion /peda l fo rce f o r a loaded passenge r ca r
wi t h o u t vacuum a s s i s t : f r o n t b r a k e s o p e r a t i v e and
i n o p e r a t i v e 1 2 7
Dece le ra t ion /peda l fo rce fo r a loaded passenge r ca r
wi t h vacuum a s s i s t : f r o n t b r a k e s o p e r a t i v e and
i n o p e r a t i v e 1 3 1
B oo st c h a r a c t e r i s t i c s o f a vacuum a s s i s t b r a k e
system . I35
raking performance diagram. -137
Fade-e ffec t iv eness d iagram 13g
Cumula tive pe rcen t o f ve h ic l e s w i th lower
gain : manual bra kes 143
Cumulat ive pe r cen t o f ve h ic l e s w i th lower
ga in: power brak es 1 4 4
Cumula tive perc en t of ve hi c l es wi th lower
a in: 30mph. .145
Peak d e c e le r a t io n - c u m ul a t i ve d i s t r i b u t i o n o f
l l d a t a .149
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Page
P e d a l f o r c e
c p bilities
of male and female
d r i v e r s u s i n g r i g h t f o o t w i t h i nd uc e d m o t i v a t i o n. 150
C um ul at iv e p ed a l f o r ce d is t ri b ut A .o n s f o r f r o n t
a x l e br ak e c i r c u i t f a i l u r e i n a l oa de d se da n
w i t h m a n u a l b r a k e s . 1 5 2
C umu la ti ve pe d al f o r c e d i s t r i b u t i o n s f o r f r o n t
a x l e br a ke c i r c u i t f a i l u r e i n a l oa de d se da n w i t h
power b rak es . 1 5 3
C um ul at iv e p ed a l f o r ce d i s t r i b u t i o n s f o r power
a s s i s t f a i l u r e i n a l oa de d s ed an . 1 54
Sample of sp eed pe dal f or ce and wheel lock-
up time h i s t o r i e s f o r t h e b e s t and w o rs t s ub-
j e c t : d e c e l e r a t i o n / p e d a l f o r c e r a t i o 0 .06 5 g / l b . 70
Sample of spe ed ped al fo rc e and wheel lockup
time h i s t o r i e s f o r t h e b e s t a nd w o rs t s u b j e c t :
d e c e le r a t i o n /p e d a l f o r c e r a t i o 0.004 g / lb . I 7 1
Sample of sp ee d ped al f or ce and wheel lockup
t i m
h i s t o r i e s f o r t h e b e s t a nd w or st s u b je c t :
d e c e l e r a t i o n /p e d a l f o r c e r a t i o 0.012 g / l b 1 7 2
De ce le ra t ion magn.i.tude in s t ru me nt at io n . 76
Top: Acce leromete r ca l i b r a t i on check vo l t ag e .
Bot tom: Ac ce le ra t i on check and acc eler omet er
s t e p r e s p o n s e . I 7 8
V e l o c i t y d e r i v a t i v e acce l e r o m e t e r low p as s
f i l t e r d i f f e r e n t i a t o r f re qu en cy r es po ns e. 179
Top: B rake l i n e p r e s s u r e ca l i b r a t i o n check.
B ottom: D ece l e r a t i o n ca l i b r a t i o n check . I 8 0
r ak e l i n e p r e s s u r e and d ec e l e r a t i o n d a t a s am pl e. 182
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CKNOWLEDGEMENTS
T h i s r e s e a r c h p rog ra m i n d r i v e r an d v e h i c l e b r a k i ng p e r f o r -
mance was conducted by s t a f f of t h e Human Fac to rs and Phys ic a l
F a c t o r s D e pa r tm en t s a t t h e Highway S a f e t y R es e ar c h I n s t i t u t e .
The p rogram was unde r th e d i r e c t i on o f D r R G Mort imer
Human F ac t o r s Depar tment and
M r L .
S e g e l P h y s i c a l F a c t o r s
Department who were r e s p o n s i b l e f o r t h e o v e r a l l p l an n i ng and
e xe cu ti o n of t h e r e s e a r ch t a s k s a nd t h e f i n a l r e p o r t .
M r
H . Dugoff p r ov i de d a s s i s t a n c e i n o v e r a l l p r o j e c t p l a nn i n g
and i n many s p e c i f i c p r o j e c t t a s k s . The d e s i g n o f t h e e l e c t r o -
h y d r a u l i c s e r v o c o n t r o l s y s te m i n t h e b r a ki n g t e s t v e h i c l e an d t h e
d a t a r e c o r d i n g s y st e ms were c a r r i e d o u t by
Mr. J
Campbell. The
r e a r d i s c b r a ke c o nv e r si o n a nd o t h e r a s p e c t s o f m e c ha ni c al e n g i -
ne er in g w er e t h e r e s p o n s i b i l i t y o f
M r
J W i r t h . I n s t r u m e n t a t i o n
was co ns t ruc ted and ve h i c l e modi f i ca t io ns and ma intenance were
c a r r i e d o u t by Messrs. J B o i s s o n n e a u l t G Popp and R . S t e i n
a l l of t h e P h y s i c a l F a c t o r s D e pa rt me nt .
M r R.
Murphy and
M r R . L im pe rt w er e r e s p o n s i b l e f o r c o n d uc t i ng t h e e n g i n e e r i n g
segments o f th e Fa i l u r e Ana lys i s Sec t io n . The Review o f t he
L i t e r a t u r e was c a r r i e d o u t by Mr. D F i s h e r .
M r C . J o r g e s o n d i r e c t e d
t h e
o pe ra t i on s a t t h e t e s t i n g s i t e
and was r e s p o n s i b l e f o r t h e d a t a c o l l e c t i o n p h as e i n t h e b r a k i ng
t e s t . H e was a s s i s t e d by D r J Lower D r
D
Damkot M r H Sch ick
M r
G
Popp and Ilr.
R .
S t e i n . A na ly se s o f t h e d a t a wer e c a r r i e d
o u t by D r . J Lower and M r C . J o r g e s o n . M r S. S t u r g i s measured
t h e b r a k e p e d al c o n f i g u r a t i o n s an d an a l yz e d t h e p e d a l f o r c e a nd
peak d e c e l e r a t i o n d a t a .
S e c r e t a r i a l S e r v i c e s a s s o c i a t e d wi t h t h e p r o j e c t w ere admin-
i s t e r e d
by
Mrs.
M
Damberg Ilrs. D . Davis and Mrs S P o t t s .
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INTRODU TION
One of the most desirable and important characteristics of
motor vehicles is that they should have qood handling charac-
teristics in terms of directional response to steering inputs
and performance as affected by accelerator and brake applica-
tion. A previous report by the Highway Safety Research Institute
(HSRI) was concerned with evaluating the present status of vehicle
handling properties and of the potential role of these character-
istics upon collisions (HSRI, 1967). In that study, various
aspects of the man-vehicle interface were identified as having
safety significance.
For example, the ability of the driving population to exert
the pedal forces required to brake a car is one facet of this
interface. Further, this facet has both a static and dynamic
component. From a static viewpoint, it appears desirable to
build motor vehicles such that a specific percentile of the driv-
ing population can exert the maximum pedal forces associated
with peak decelerations on dry, high-friction surfaces. From a
dynamic standpoint, it appears that pedal-force characteristics
should also enable driving population to attain maximum
braking performance irrespective of the friction conditions pre-
vailing at the tire-road interface. By maximum braking perfor-
mance we mean the shortest distance to slow or stop that can
be obtained without excessive locking of the wheels in order
that sufficient control and stability prevail for holding the
vehicle in the desired lane of travel.
It is primarily the dynamic aspect of the man-vehicle
interface with which this study is concerned. The braking
process is viewed as a task in which the driver must control
and modulate his pedal force such that he achieves the shortest
braking distance possible under the prevailing road conditions,
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while further satisfying the requirement that the trajectory
of his vehicle be under reasonable control. The investigation,
as conceived, is concerned with both the static and dynamic
component of the ergonomics of braking. The major question
addressed is:
In the distribution of anthropometric character-
istics and perceptual-motor skills possessed by the driving popu-
lation, how do the relationship of pedal force and pedal dis-
placement to vehicle deceleration influence the braking perfor-
mance of the man-vehicle combination? . From the standpoint of
generating a braking standard, this question can be rephrased
to: What are the bounds on brake pedal force/vehicle decelera-
tion space wherein the bulk of the driving population shall find
it possible to maximize deceleration while making controlled
(i.e., well modulated) braking maneuvers on both,dry pavement
and surfaces with a reduced coefficient of friction? .
SUMM RY
O
T S K S
The study was conceived as having six major phases which
will be briefly discussed here so as to provide the reader with
a general orientation of the overall approach.
1 LITERATURE REVIEW
A review of the literature was carried out pertinent to
an analysis of the pedal force/vehicle deceleration character-
istics of an automotive vehicle. The factors considered impor-
tant in the review were brake system design, brake usage, skid-
ding, brake testing, and driver characteristics. The review was
submitted earlier as an interim report.
2 FOOT FORCE CAPABILITY OF DRIVERS
The vehicle braking system is actuated with the feet of
the driver and, therefore, it is essential to learn more of the
foot force capabilities of individuals comprising the driving
population.
A procedure was developed by which left and right
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f o o t maximum fo rc e e xe r t io n cou ld be measured f o r a l a r ge sample
o f f emale and male d r iv e r s . The pu rpose was t o de te rmine a
s u i t a b l e u pp er f o r c e
l m t
f o r t h e o p e ra t i on o f v e h i c l e s e r v i c e
b r a k e s .
3
DRIVER BRAKING
PERFORMANCE
AS
A FUNCTION OF PEDAL-FORCE AND
PEDAL-DISPLACEMENT LEVELS
The ma j or emp hasi s i n t h i s s t u d y was t o l e a r n more o f t h e
dynamic r e l a t i o n s h i p s betwe en t h e d r i v e r a s a c o n t r o l l e r of
t h e b r ak e sy st em a nd v a r i o u s c h a r a c t e r i s t i c s of t h a t sy st em
Ma jor v a r i a b l e s t o be c o n s i d e r e d w er e t h e r e l a t i o n s h i p be tw ee n
t h e f o r c e a p p l i e d t o t h e p e d a l and t h e r e s u l t i n g d e c e l e r a t i o n of
t h e v e h i c l e and t h e p e d al d i sp l ac e me n t i n a f f e c t i n g t h e st o p -
p i ng d i s t a n c e i n a b ra k in g t a s k r e q u i r i n g v e h ic l e d i r e c t i o n a l
co n t ro l . The co r e o f t h e exper imen t was t h e deve lopmen t o f a
s p e c i a l t e s t v e h i c l e i n which v a r i a t i o n s i n br a ke r e s po ns e c ha r-
a c t e r i s t i c s c o ul d b e r e a d i l y o b ta i ne d . A d r i v e r - v e h i c l e b r a k i n g
t e s t was d ev e lo p ed and m easu rement s t ak en t o d e t e rm i n e t h e e f f e c t
o f t h e v a r i a b l e s o f i n t e r e s t on b r a k i n g p er fo rm an ce .
4
DRIVER
BR KING
PRACTICE
t
was n e c e s sa r y t o o b t a i n e m p i r i c a l d a t a d e s c r i b i n g t h e
l e v e l s o f d e c e l e r a t i o n t h a t d r i v e r s employ u nd er no rma l d r i v i n g
co n d i t i o n s . Such d a t a w er e n eeded a s a p a r t of an o t h e r ph ase of
t h i s program t h e f a i l u r e a n a l y s i s s i n c e b ra ke d e c e l e r a t i o n
l e v e l s c an f or m o ne c r i t e r i o n m eas ur e o f r eq u i r ed p e rf o rm ance
b o t h u nd e r n or ma l an d f a i l e d co n d i t i o n s .
5.
FAILURE ANALYSIS
An a n a l y s i s was co n d uc t ed t o a s ce r t a i n t h e e f f e c t upon
v e h i c l e pe rf or ma nc e of v a r i o u s f a i l u r e s i n t h e b r a k i n g sy st em .
C o n di t i on s u nd er whi ch s u c h f a i l u r e s a r e l i k e l y t o o c c ur w er e
co n s i d e r ed and t h e co ns equ en ces e s t i m a t ed fr om t h e r eq u i r e d
d e c e l e r a t i on l e v e l p r ob a b i l i t y and t h e a b i l i t y of t h e d r i v e r t o
e x e r t t h e n ee de d p e d a l f o r c e .
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6.
RECOMMENDATIONS
Finally based upon the preceding work recommendations
for a modified brake performance standard were made utilizing
the information that had been gathered in the analytic and experi-
mental phases of the project.
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T S K S
1
LITERATURE REVIEW
INTRODUCTION
Brak ing i s a complex energy conve rs ion p r oces s whereby t h e
k i n e t i c en er gy of t h e v e h i c l e
i s c o n ve r t e d i n t o t h e r ma l e n er g y
a t t h e b ra k es and a t t h e r o a d- t i r e i n t e r f a c e . During t h e b r ak i ng
p r o c e s s , t h e p e d a l f o r c e a c t s th r o ug h a m e c h an i c a l- h y dr a u l ic s y s-
tem
t o a pp l y a r e t a r d i n g t o rq u e t o e ac h o f t h e f o u r w he el s of t h e
v eh i c l e . The b r ak i n g t o r q u e i s op po se d by t h e i n e r t i a o f t h e
w he el a nd t h e f r i c t i o n a l f o r c e between t h e
t i r e
a nd t h e r o a d , w i t h
t h e n e t r e s u l t be i ng t h e d e c e l e r a t i o n of t h e v e h i c l e . The c ha ra c-
t e r i s t i c s
o f t h e m ech an i ca l - h y dr au l i c s y s t em d e t e r m i n e t h e b r ak i n g
t o rq u e a v a i l a b l e a t e ac h w he el , w h i le t h e r o a d - t i r e f r i c t i o n c o ef -
f i c i e n t and t h e m ec ha nic s o f t h e v e h i c l e d e t er m in e t h e d e c e l e r a t -
i n g f o r c e s . C on se qu en tl y, t h e c o n t r o l of t h e d e c e l e r a t i o n o f a
v e h i c l e t h r o u gh b r ak e p ed a l f o r ce d epend s on t h e s t a t i c and d yn am ic
c h a r a c t e r i s t i c s o f t h e e n t i r e s ys te m, i n c l u d in g t h e d r i v e r .
I n t h e r e vi ew t h a t f o l l o w s , t h e t e rm in ol og y e s t a b l i s h e d by t h e
So c i e t y o f Automot ive Eng ineer s SAE) f o r t he au tomot ive ve h i c l e
b ra ki ng pr oc es s SAE J6 56 cI 1968; SAE J65 7a, 1968)
w i l l
be used .
By d e f i n i t i o n , b r ak i ng i s i n i t i a t e d i n a m ot or v e h i c l e by a d r i v e r
a p p ly i ng a f o r c e t o t h e f o o t - a c t u a te d l e v e r t e rm ed t h e b r a ke p e d a l.
The m ag ni tu de o f t h e a c t u a t i n g f o r c e a t any i n s t a n t i s
c a l l e d
th
p e d al f o r c e . The v a r i a t i o n o f t h i s f o r c e by t h e o p e r a t o r w i t h
time i s d e f i n e d h e r e a s p e d a l f o r c e m od u la t io n .
s e t
o f s i m p l i f i e d e q u a t io n s d e s c r i b i n g t h e o p e r a t i o n of a
t y p i c a l b r a k e s y st em
i s
p r e s e n t e d b el ow . The r e l a t i o n s h i p s
w l l
b e h e l p f u l i n e va l u at i n g t he l i t e r a t u r e p e r t i n e n t t o t h e br ak in g
p r o ces s , an d w l l s e r v e t o d e f i n e t h e v a r i a b l e s t h a t a r e i nv ol ve d.
F i g u r e
1 1
i l l u s t r a t e s a t y p i c a l b ra k e s ys te m C ro us e, 1 9 6 5) .
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rMaster C v l i n d e r
FRONT BR KES RE R BR KES
Fig u re 1 1 T y p i c a l h y d r a u l i c b r a k e s y s te m drum t y p e ) .
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Application of a pedal force, F, causes the brake pedal to be
displaced through a distance
XI.
The pedal linkage is designed
to yield a mechanical advantage of force, r, between the pedal
and the master cvlinder piston, resulting in a displacement of
the piston, X2, that is less than the displacement X1.
The master
cylinder, with an area Am, traps the oil in the brake line, there-
by developing a hydraulic pressure, p. Since there are frictional
losses, it is common to assume a pedal efficiency, E, that is
less than unity. The relationship between line pressure and pedal
force is actually nonlinear but may be simplified for illustration
purposes
Brown,
1965).
where
In power brakes, a vacuum-powered device assists the driver by
multiplying the force by a factor
K.
This power boost modi-fies
the above relationship to:
It should be noted that the factor
K
may be a nonlinear function
of the pedal force.
In the brake proper, a friction material having a coefficient
il is pressed against the brake drum or disc), resulting in a
brake torque,
T.
Measurements Brown, 1965, Shigley, 1963, Stroh,
1968) have shown that the relationship between brake torque and
line pressure is not necessarily linear and is very much dependent
upon the type of brake employed. Thus, we have:
T kp
where
k , brake type, brake geometry, A ~ ~ )
and
area of wheel cylinder.
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The development of v eh ic le de ce le ra ti on by means of bra ke
to rq u e fo l l o ws d i r e c t l y when wheel i n e r t i a
i s
neg lec ted and it
i s
assumed t h a t no sk idd ing occurs . The dec e le ra t io n of a
v eh i c l e du e t o ap p l i ed b rake t o rq u e can b e ex pre s s ed a s :
where
g a c c e l e r a t i o n of g r a v i t y
E
e f f e c t i v e r a di us of wheel i
i
v eh ic l e we ig h t
i
b rake t o rq u e a t wh ee l
i
On combinin g Eq ua ti on s
3 ,
4 and 5 t h e d e c e l e r a t i o n o f a v e h i c l e
can b e ex p re s s ed a s a fu n c t i o n o f p ed a l fo r ce v i z :
where
On t h e ot he r hand t h e maximum de c el er a ti on t h a t can be pro-
duced i n a locked-wheel s t op can be reduced t o th e
s imple expres -
s io n :
a
max Yt9
7 )
where
measured r o a d - t i r e f r i c t i o n c o e f f i c i e n t
v
under locked-wheel condi t ions .
t s h ou ld b e no t ed t h a t n e i t h e r Eq ua t io n 6 nor
7
h ol d s f o r t h e
brak ing reg ime i n which th er e i s s u b s t a n t i a l s l i p p i n g between
t i r e and r o ad w i t h t h e w he el s t i l l r o t a t i n g .
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PEDAL FORCE AS
A
FUNCTION OF DESIGN PARAMETERS
BR KE SYSTEM DESIGN.
Pedal Force Design Goals .
A
s u c c e s s f u l br a k e s ys t em d e s i g n
s h ou ld embody t h e c h a r a c t e r i s t i c s o f r e l i a b i l i t y , p r e c i s en e s s of
c o n t r o l , and t h e a b i l i t y t o w i t hs t a nd s h o r t pe r i o d s of e xt re me
o v e r l oa d V a l l i n , 1 9 6 8 ).
The s e l e c t i o n of com po nent s f o r t h e
b r a k e s ys t e m R obs on, 1 9 67 ) e s t a b l i s h e s t h e n om in al c h a r a c t e r i s -
t i c s
o f t h e p e d a l f o r c e / br a k e t or q u e r e l a t i o n s h i p . The f i n a l
produce, however, i s t h e r e s u l t o f many assumpt ions , compromises ,
a n d d e s i g n c h o i c e s .
Ped al Force/Brake Torque Re la t i ons h i p . One measure of b ra ke
performance i s t h e p l o t o f t h e b ra k e t o r qu e v e r s u s l i n e p r e ss u r e
Wi nge , 1 9 6 1 ) ; n am e ly , a g r ap h i ca l r ep r e s en t a t i o n o f E q u a t i on
f o r a c o n s ta n t l i n i n g c o e f f i c i e n t o f f r i c t i o n . T yp ic a l p e rf o r-
mance cu r v e s o b t a i n ed ex p e r i m en t a l l y w i t h d i s c and drum b r ak es
a r e shown i n F i g u r e 1 . 2 .
S e v e r a l f e a t u r e s of t h e p e rf o rm an c e cu r v e f o r t h e drum b r a k e
may be noted.
The o f f s e t of t h e c u r ve n e a r t h e o r i g i n i s
termed
t h e p u s h ou t p r e s s u r e and
i s
t h e b r ak e l i n e p r e s su r e n ec e ss a ry t o
o ve rcome t h e b r ak e s h o e r e t u r n s p r i n g s .
The co n c av i t y o f t h e p e r -
fo rma nc e c ur v e r e s u l t s f rom t h e d i s t o r t i o n o f t h e s h oe s and t h e
drum Winge, 1961) and t he r e f o r e may va ry con si de ra b l y f rom one
b r a k e d e s i g n t o a n o t h e r . A lt ho ug h drum d i s t o r t i o n h a s b ee n c a l -
c u l a t e d a n a l y t i c a l l y Winge, 1 9 6 1 ) , t h e i n f l u e n c e o f drum d i s t o r -
t i o n r em ai ns t o b e i n c o r po r a t e d i n t o t h e o r e t i c a l b ra ke t o r q u e
a n a l y s i s .
C o n s eq uen t l y , t h e co ncave f ea t u r e o f t h e d rum b r ak e
per fo rmance cu rve
i s
n o t p r e s e n t i n t h e o r e t i c a l l y d e r iv e d b ra ke
t o r q u e / l i n e p r e s s u r e r e l a t i o n s h i p s S h i g l e y , 1963; S t e e d s , 1960:
S t r o h , 1 9 6 8 )
W it h t h e a i d o f E q u a t i o n 5 ,
t
can b e shown t h a t t h e v eh i c l e
d e c e l e r a t i o n i s a p p r o xi m a t e l y p r o p o r t i o n a l t o t h e sum o f t h e
i n d i v i d u a l b r ak e t o r q u es when n o s k i d d i n g o ccu r s . C o ns eq uen t ly ,
any d e s ig n f e a t u r e a f f e c t i n g t h e l i n e a r i t y of t h e p ed a l f or c e /
v e h i c l e d e c e l e r a t i o n r e l a t i o n s h i p .
I f t h e drum b r ak e s a r e u t i l i z e d ,
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Line Pressure--.
A )
Disc
rake
B) r u m rake
Figure 1 2
Typical brake performance curves
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t h e n o n l i n e a r i t i e s a s s o c i a t e d w i t h t h e b ra ke t o r q u e / l i n e p r e s s u r e
cu r v e a r e i n t r o d u ced . T h er e
i s
some ev id ence Leah , 1964) t o in d i -
c a t e t h a t t h e b ra ke l i n e p r e s s u r e i t s e l f may n o t b e e x a c t l y pr o-
p o r t i o n a l t o t h e p e d al e f f o r t . I n t h e c as e o f power a s s i s t e d b r a ke s ,
t h e l i n e p r e s s u r 3 h a s b ee n r e p o r t e d S p u r r , 19 65 ) a s b e i ng d i r e c t l y
p r o p o r t i o n a l t o t h e p e d a l e f f o r t u p t o a l i n e p r e s s u r e co rr es po nd -
i n g t o s a t u r a t i o n o f t h e vacuum a s s i s t component,
D i s c b r ake s y s t em s a r e g en e r a l l y ack no wl ed ged a s h av i n g a
l i n e a r m odu la tion c h a r a c t e r i s t i c ; i . e . , t h e v e h i c l e d e c e l e r a t i o n
i s
a p pr o xi m at e ly p r o p o r t i o n a l t o t h e p e d a l e f f o r t .
t
has been
r e p o r t e d Brown, 1 96 5) t h a t t h i s f e a t u r e o f d i s c b r a ke s a ll o w s t h e
d r i v e r t o a v o i d u n i n t e n t i o n a l w he el l o ck u p and p e r m i ts h i g h d e c e l -
e r a t i o n r a t e s u nd er a d v e rs e r oa d c o n d i t i on s . t s h o u l d b e n o t ed ,
how ever, t h a t t h i s l a s t s ta t e m en t r e s u l t e d f rom q u a l i t a t i v e d r i v e r
e v a l u a t i o n s r a t h e r t h a n a n e x t e n s i v e s t u d y.
t was s t a t e d e a r l i e r t h a t t h e b r ak e t o r qu e i s d e pe nd en t on
t h e l i n i n g c o e f f i c i e n t of f r i c t i o n a s w e l l a s t h e br ak e l i n e p re s-
s u r e . A lt ho ug h t h e t o r q u e o u t p u t s o f b o t h drum and d i s c b r ak es
a r e , i n t h e main, p r o p o r t io n a l t o t h e br ak e l i n e p r e s s u r e , t h e s e
b ra ke s d i f f e r s u b s t a n t i a l l y i n t h e i r be ha vi or w i th r e s p e c t t o
c ha nge s i n t h e l i n i n g c o e f f i c i e n t of f r i c t i o n . The t o r qu e o u t -
p u t o f a d i s c b r a k e
i s
d i r e c t l y p r o po r t i o na l t o t h e c o e f f i c i e n t
o f f r i c t i o n f o r a g i v en l i n e p r e s s u r e The B en di x C or p. , 1 9 6 4 ) .
Most drum b r a ke d e s i g n s h av e a s e l f - e n e r g i z i n g f e a t u r e i . e . , t h e
b r ake a s s i s t s i n i t s own a c t u a t i o n ) whic h r e s u l t s i n t h e b r a ke
t o r qu e b e in g v e ry s e n s i t i v e t o ch ang es i n t h e c o e f f i c i e n t o f f r i c -
t i o n o f t h e b r ak e l i n i n g L ueck , 1 96 5; K i n ch i n , 1 96 1; F u r i a
e t
a l . ,
1967 ; Farob in , 196 8) . Thus , bo th f ade and t he normal day- to-day
v a r i a t i o n s i n t h e f r i c t i o n c o e f f i c i e n t Winge, 1961) o f t h e l i n i n g
c an ha ve a n e x a g g e r a t e d e f f e c t on t h e p e d a l e f f o r t of a drum b r a ke
s ys te m. F u r t h e r , t h e r e i s an i nc r ea s ed p o s s i b i l i t y of s i d e t o
s i d e v a r i a t i o n s i n b r ak i n g t o r q u e when a d rum b r ak e s y s t em i s
employed. Th i s phenomenon may r e s u l t i n an und es i rab le d i r e c t i o n a l
r e sp o ns e of t h e v e h i c l e F u r i a e t a l . , 1967; L i s t e r , 1 96 5)
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One factor affecting the choice between drum and disc brakes
is the self-energizing design of most drum brakes. In effect,
this self-actuation feature assists the driver in applying the
brakes, thus allowing relatively low pedal effort for a given
brake torque. Compared to the self energizing drum brake (Farobin,
1968; Winge, 1963) a typical disc brake requires four to five
times the wheel cylinder area and twice the hydraulic line pressure
to produce the same brake torque. In terms of brake pedal actua-
tion this implies higher pedal efforts and greater pedal displace-
ment (Shaw, 1965; SAE, 1963; Burke Prather, 1965). This problem is
often solved by using a power assist.
Brake Proportioning. When a four-wheeled vehicle decelerates
there is a transference of load (Parker, 1960; Taborek, 1957) onto
the front wheels because the body s center of qravity is above the
ground plane. To achieve optimum braking the proportioning of
the braking effort between the front and rear axles should match
the instantaneous load distribution (Taborek, 1957; Alexander,
1967). Manufacturers build in a front to rear proportioning
(Automotive Industries, 1968), but a brake system having a fixed
front to rear braking ratio can only achieve optimum performance
for a given rate of deceleration (chase, 1949; Hofelt, 1959;
Parker Newcomb, 1964). Typically, brake proportioning is fixed
with 60 percent to 70 percent of the braking occurring at the
front wheels (Automotive Industries, 1968).
A series of tests on vehicles having different weight dis-
tributions and different brake proportioning has shown (Alexander,
1967) that a front/rear brake ratio equalling the wheel load dis-
tribution of the vehicle at a deceleration of 1.0 g is desirable
if the car is to remain directionally stable when heavily braked
on all surfaces. On low coefficient surfaces, however, this
would result (Parker, 1960) in premature front wheel lockup and
a deceleration less than maximum for that surface. The overall
effect of proportioning on the driver-vehicle performance during
braking is that under certain conditions the driver may be able
12
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t o ap p l y h i g h e r p e d a l e f f o r t s and ach i ev e h i g h e r maximum d ece l -
e r a t i o n s w i t h o u t i n c u r r i n g w he el l oc ku p.
One s o l u t i o n t o t h e p r ob le m o f p r o p e r p r o p o r t i o n i n g i s t o
v a r y t h e br a k e pr o p o r t i o ni n g w i t h t h e d e c e l e r a t i o n of t h e c a r .
T hi s c an b e done by c o n t r o l l i n g t h e p r e s s u r e a t e ac h a x l e i n
c or re sp on de nc e t o t h e a x l e l o a d i n g ( F u r i a e t
a l , , 19 67 ; E at on
S c h r e u r , 1 96 6; E n g i n e e r i ng , 1 9 6 4 ) , b u t t h e u s u a l p r o c e d ur e i s t o
l m t
o r p r o p o r t i o n t h e r e a r b r a k e h y d r a u l i c p r e s s u r e a bo ve some
f i x e d u pp er l m t ( F u r i a e t a l . , 1 96 7) . A t b e s t , ho weve r, t h e s e
l a t t e r methods a r e on l y compromises a s a r e s u l t o f v a r i a t i o n s i n
v e h i c l e l o ad i n g and b r ak e p er f or m an ce.
DESIGN PRACTICE AND TRENDS. V eh ic l e b r a k i n g sy st em s may be
c l a s s i f i e d a s b e i ng e i t h e r fo ur -w he el drum, f r o n t d i s c and r e a r
drum ( h y b r i d ) , o r fo u r- w hee l d i s c . D i s c b r ak es hav e b een
common i n Eu ro pe f o r s ev e r a l y ea r s ( F u r i a e t a l . , 1 96 7; Hu n ti n gt o n,
1964; S t r i e n , 1 9 6 1 ) , b u t ha ve be en i n t r o d uc e d o n l y r e c e n t l y i n
t h e U n i te d S t a t e s ( Bu rk e e t a l . , 1965 ; T ignor , 1966 ; Thomas, 196 7) .
Only on e A m er ican - bu i l t c a r h a s f o u r -w h ee l d i s c b r ak es a s s t an d a r d
e q ui p me n t, a nd on e m a n uf a c t ur e r o f f e r s them a s a n e x t r a c o s t
o p t i o n . S e v e r a l a uto ma lt er s o f f e r f r o n t d i s c br a k e s a s s t a n d a r d
equipment whi le most l i s t them a s an o p t i o n ( Au to mo ti ve I n d u s t r i e s ,
1 96 8; Ki ng , 1 9 6 8 ) . D i s c b r ak e s w er e f i r s t made av a i l a b l e o U.S.
c a r s i n 1 96 5, an d 2.19 p e r c e n t of t h e c a r s s o l d were s o eq u ip p ed.
By 1 96 7 t h i s p e r c e n t a ge ha d r i s e n t o 6. 22 p e r c e n t , a nd t
i s
e xp ec te d t h a t t h i s t r e n d
w i l l
co n t i n u e .
I n r e c e n t y e a r s t h e r e h a s be en a t r e n d t o wa r ds l o we r p e d a l
e f f o r t s t o a ch i e ve a gi v en d e c e l e r a t i o n . I n 1 935 a d e s i gn bogey
( a s t a n d a r d of p e r fo r m ance ) was ad o p t ed ( C h ase , 1 94 9) f o r t h e
r a t e o f d e c e l e r a t i o n f o r a g iv en pe d al p r e s s ur e , s p e c i f y i n g t h a t
a b rake sys t em shou ld r eq u i re be tween 100 l b and
1 3
l b of p e d al
e f f o r t t o a c h i ev e a d e c e l e r a t i o n of
2
f t / s e c 2 . T h i s b og ey was
e x te n de d i n 1 949 ( C ha s e, 1 94 9) t o r ed u c e t h e r e q u i r e d p e da l e f f o r t
t o a s low a s 77 l f o r t h e same d e c e l e r a t i o n . The p ur po se of t h i s
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new bogey was t o se t an upper l i m i t of b ra ke s e n s i t i v i t y t h a t
would
s t i l l
exc lud e b rakes capab le o f lock ing th e wheel s on d r y
pavement e.g.
v
1 . 0 ) w i t h
l ess
t h a n 100 l b of p e da l e f f o r t .
t
was n ot ed t h a t a number o f t h e c a r s t e s t e d o b t a i n e d 20 f t / s e c 2
d e c e l e r a t i o n s w i t h p e d a l f o r c e s l ower t h a n t h o se a l lo we d by t h e
new d e s i g n b og ey . T h er e h a s b ee n n o l i t e r a t u r e p u b l i sh e d i n t h e
i n t e r i m d e a l i n g wi th t h e d e s i r a b i l i t y of v a r i ou s d e c e l e r a t i o n
v e r s u s p e da l e f f o r t c u r ve s.
Some recent ly publ i shed design summaries have
i n d i c a t e d
s a t i s f a c t o r y p er fo rm an ce w i t h v e h i c l e b r a k e s y st em s t h a t a c hi ev ed
2 0
f t / s e c 2 d e c e l e r a t i o n s w i t h 50 l b Brown, 1 9 6 5 ) ,
4 4
l h Winge,
1 96 31 , and 3 1 l b Va n s t e e n k i s t e , 19 63 ) of p e d a l e f f o r t , Data
p r e s e nt e d i n a b ra k i n g t e s t of f ou r d i f f e r e n t B r i t i s h b u i l t c a r s
Mack en zi e e t a l . , 1 96 6) a l lo we d t h e c a l c u l a t i o n o f c o mp a ra t iv e
d a t a : f o r a 20 f t / s e c 2 d e c e l e r a t i o n , t h e r e q u ir e d p e da l e f f o r t s
ranged from
48
l b t o 73 l b .
A l l
f o u r c a r s wer e e q ui pp ed w i t h
hybr id b rake sys t ems ,
Compara t ive da ta de r ived f rom o the r
p u b l i sh e d p er fo rm an ce c u r v e s i n c l u d e s ev en f o r e i g n s p o r t s c a r s
Motor ing Which?, 1968) re qu i r in g 40 l b t o
7 9
l b of p e da l e f f o r t ,
and s i x fo re ig n sedans Motoring Which?, 1968) re qu i r in g 39 l b
t o 50 l b , a l l f o r a 20 f t / s e c 2 d e c e le r a t i o n .
These c a r s were
e qu ip pe d w i t h a l l t h r e e t y p e s of b r a k e s y st em s .
A l l
but one of
t h e s e c a r s r e p r e s e n t pe rf or ma nc e i n e x ce s s of t h e l i m i t sugges ted
i n 1 94 9, i n d i c a t i n g t h a t t h e i n t r o d u c t i o n of power b r a k e s o r
advan ces i n br ak e s t a b i l i t y have r e s u l t e d i n a c o n s id e ra b le s h i f t
i n t h e d e s i g n b og ey to wa rd l ower p e d a l e f f o r t s . Motor Ve h i c l e s a f e t y
S t an d ar d No. 1 05 1 96 8) s p e c i f i e s t h a t t h e s e r v i c e b r a k e p e r-
f or ma nc e of m ot or v e h i c l e p a s se n g er c a r s m us t n o t b e l e s s t h a n
t h a t d es cr ib ed i n S e ct i o n D
of
SAE Recommended Practice J9 7
1 9 6 8 ) , when t e s t e d i n a c c o r d a nc e w i t h
SAE
Recommended P r a c t i c e
J843a 19 68 ). The
SAE
s t a n d a r d s p e r m i t
a
very b road range o f
braking performance
SAE
Pu bl ic a t io n SP-299, 19 67 ) . For example ,
on d r y P o r t l a n d c ement c o n c r e t e t h e y o n l y r e q u i r e t h a t t h e b r a ke
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p ed al e f f o r t f a l l i n t h e r ang e of 1 5 t o 120 l b i n o r d e r t o a c hi ev e
a d e ce l e r a t i o n of 20 f t / s e c2 f ro m 60 mph w h i l e m a i n t a i n i n g t h e
v e h i c l e w i t h i n a 1 2 f o o t l a n e .
BRAKE USAGE
BRAKE USAGE MEASUREMENTS. S e v e r a l s t u d i e s (C a rp en te r Lees ,
1 95 6) have b een made i n v es t i g a t i n g t h e u s e b r ak es r ec e i v e d u r i n g
n o r m a l d r i v i n g .
A l l
b u t one of t h e s t u d i e s
were
European and
i n v o l v e d a v a r i e t y of d r i v i n g c o n d i t i o n s . D ur in g a t e s t i n v o l v i n g
f o u r d r i v e r s o v e r a d i s t a n c e of 1400 m i l e s ,
t
was found th a t on ly
5
p e r c e n t o f t h e s t o p s ex ce ed ed an a v er a g e d e c e l e r a t i o n of . 3 g
and t h a t 50 p e r cen t o f a l l s t o p s we re made a t . 09 g o r l o w er
( C a r p e n t e r , 1 9 5 5 ) . I n a t e s t i n v o l v i n g 23 d r i v e r s o v e r 300 m i l e s
o f European d r iv in g , t h e average o f t he maximum de ce le ra t i on s
observed on a number of d i f f e r e n t t e s t r o u t e s v a r i e d b et we en 2 1 g
and .34 g, t h e mean bei ng .26 g. The s i n g l e maximum de c e le ra t io n
re cor ded was .6 g. A B r i t i s h s t u d y ( L i v s ey , 196 0- 61) i n v o l v i n g
16 v e h i c l e s co v e r i n g a w id e s e l e c t i o n of v e h i c l e t y p es was co n-
d u c te d o v er f o u r d i f f e r e n t r o u t e s , i n c l u d i n g f a s t m ai nr oa d, c r o s s
co u n t r y , w i n di n g r o ad s , and an a l p i n e d e s cen t . t was found t h a t
d e c e l e r a t i o n s on a l l t h e r o u t e s were u s u a l l y i n t h e r an ge o f 2
t o
3
g , and r a r e l y exceeded
4
g . T h i s
i s
i n ag reemen t wi th an
A meri can s t u d y w hi ch o b t a i n ed d e ce l e r a t i o n f r eq u en cy d a t a o n an
Itin town d r i v i n g co ur se (Kumrner Meyer, 19 65 ) . t was f e l t
t h a t t h e b r ak i n g l e v e l s ex p e r i en ced w e r e t h e maximum t h a t w ould
b e g e n e r a t e d by t h e g e n e r a l p u b l i c . A d d i t i o n a l l y
t
was found
t h a t t h e r o o t mean s q u a r e of t h e s p eed a t w hi ch b r ak i n g was
i n i t i a t e d f o r a l l t h e ro u te s i n cr ea se d i n d i r e c t pr op or t i on t o
t h e v e h i c l e s ' maximum s p e e d, b u t t h e p r o p o r t i o n a l i t y c o n s t a n t
was d i f f e r e n t f o r t h e f o u r r o ut e s .
Ano ther B r i t i s h s tud y (McKenzie e t a l . , 1962-63 ) was made
a t t e m p t i n g t o e s t a b l i s h m a th em at ic al r e l a t i o n s h i p s which
woul
en ab l e t h e p r e d i c t i o n o f b r ake u s ag e. By d e f i n i n g two p a r am e t e r s ,
vo
a c h a r a c t e r i s t i c sp ee d a s s o c i a t e d w i th t h e r o u t e , and M des -
c r i b i n g t h e manner o f d r i v i n g , t h e a v e ra g e d e c e l e r a t i o n c o ul d be
15
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e x p r e s se d a s
a ( .039 .09 3)g
where
M
)*45(i0) 2
max
i s
t h e a v e ra ge r o u t e s pe ed f o r t h e c a r ,
and
'max
i s
t h e t o p
speed o f which th e ca r i s c a pa b le . F or t h e c a r s t e s t e d , t h e
d e c e l e r a t i o n s r a n g e d f r o m 15 g t o . 28 g on a l l r o u t e s , and t h e
r o o t mean sq u a r e of t h e sp ee d a t whi ch b r a k i n g was i n i t i a t e d
c ou ld b e r e l a t e d t o Vo an d f o r e ac h r o u t e .
I n c o n cl u s i o n, t h e n , i t a p p e a r s t h a t t h e d e c e l e r a t i o n s e x p e r -
i e n c ed by a p a r t i c u l a r c a r d u r i n g r o u t i n e d r i v i n g dep en ds on t h e
d r i v e r a nd t h e t o p s p ee d p er fo rm an ce of t h e c a r , and t h a t t h e s e
d e c e l e r a t i o n s w i l l r a r e l y e x c e e d
. 3
g . S i n c e none o f t h e s t u d i e s
d e a l t wi th emergency b rak ing , th e f requency of occur r ence and th e
magn itu de o f t h e d e c e l e r a t i o n s i n s uc h a s i t u a t i o n a r e s t i l l unde-
te rmined.
FADE AND FADE
T E S T I N G .
The Phenomenon of Fade. Brake f a d e
SAE
J657a , 1968)
i s
t h e
g e n e r a l t e rm used t o d e s c r i b e any on e of s e v e r a l c o n d i t i o n s wh ic h
r e s u l t i n r ed uc ed br ak e t o r q u e / l i n e p r e s s ur e g a i n f o r a g iv en
v e h i c l e . Hea t f a d e r e s u l t s f ro m t h e c ha ng e i n b r a k e p a r a m et e r s
caused by t h e e ne rg y d i s s i p a t e d a t t h e l in in g- dr um i n t e r f a c e ;
w at er f a d e r e s u l t s from t h e r ed u ct i on i n t h e l i n i n g c o e f f i c i e n t
o f f r i c t i o n du e t o wa t e r c o n ta m i n a ti o n ; an d washo ut d e sc r i b e s
f a d e d ue t o any o t h e r c au s e ( ~ l e e t wner, 1966; P er c y,
1 . 9 5 2 .
H eat f a d e h as r e c e iv e d by f a r t h e g r e a t e s t a t t e n t i o n i n t h e
l i t e r a t u r e . A s n ot ed e a r l i e r , t h e b r ak e t o r q ue d ep en ds on t h e
l i n i n g f r i c t i o n c o e f f i c i e n t , t h e l i n e p r e s s u re , and t h e g eometry
of th e b rake . Hea t f a de r e s u l t s (Her r i ng , 1967) i n two ways--
t h e r m al d i s t o r t i o n o f t h e b r a k e ge om et ry a nd ch an g es i n t h e
a p p a re n t c o e f f i c i e n t of f r i c t i o n o f t h e l i n i n g due t o h i gh t emp era-
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t u r e s . The l a t t e r mode o f b r a k e f a d e
i s
a p p a r e n t l y t h e m os t
i n f l u e n t i a l , however no compar isons have been re po r t ed . Con-
s i d e ra b l e e f f o r t
i s
b ei ng d i r e c t e d t o wa rd s c o n t r o l l i n g t h e
tem-
p e r a t u r e
r i s e
i n t h e b r a k e a nd t o w a rd s d e v el o pi n g f a d e res i s -
t a n t l i n i n g m a t e r i a l s ( W ei nt ra ub B e r na r d, 1968; J a c k o
e t
a l . ,
1 9 6 8 ) .
T he rm al A n a l y s i s . T h e o r e t i c a l i n v e s t i g a t i o n s ha ve con-
ce r n zd t h em s e l v es w i t h t h e t em p e r a t u r e
r i s e
expec ted du r ing
br ak in g wi th bo th drum br ak es (F az ek as , 1953; Newcomb, 1958-59;
Bannister , 1957; Noon e t a l . , 1 96 4) and d i s c b ra k e s (Noon, e t a l . ,
1964; Newcomb, 195 9; Newcomb, 196 0; Ri ch ar ds on Sa un de rs , 19 6 3 ) .
An a n a l y s i s of drum and d i s c br ak es (Newcomb, 1960; Newcomb
M i l l n e r , 1965-66; P e t r o f , 1 96 5) i n d i c a t e s t h a t of t h e h e a t ge n-
e r a t e d d u r i n g a s i n g l e s t o p , 95 p e r c e n t i s d i ss i p a t ed a t t he
drum wh i l e 99 per ce n t
i s
d i s s i p a t e d a t t h e d i s c . Durincj a s i n g l e
s t o p t h e t e m p e r a t u re s a c h ie v e d a r e a p p ro x im a t el y t h e same f o r
b o t h drum and d i s c b r ak es . The i n c r ea s ed co n v ec t i o n co o l i n g
ca pa c i ty of d i s c b ra kes ( Van s te enk i s t e , 1963; Newcomb Mi l l ne r ,
1965-66; Newcomb, 19 60 ) , however, r e s u l t s i n lower avera ge
tem-
p e r a t u r e s d u r in g r e p e a t e d b r a k i n g . T h i s f a c t , combined w i t h t h e
d i s c b r a k e s lower s e n s i t i v i t y t o l i n i n g c o e f f i c i e n t c ha ng es ,
h a s be en t h e r e a s on f o r
i t s
adop t ion on h igh per fo rmance veh ic l es
(Lueck , 1965; Hun t ing ton , 1964 ; Ihn ac ik ,
J r
Meek, 1967; Kemp,
1 9 6 1 ) .
O th er d e s i g n v a r i a t i o n s s uc h a s b i m e t a l l i c b r a ke drums
(Automob il e Eng ineer , 1959; Eng i neer in g , 1959) and ve n t i l a t ed
d i s c s (K off man, 1 95 6) h av e a l s o been u s ed t o r ed u ce t em p e r a t u r e
r i s e (SAE 5971 , 19 68) . Var iou s pap ers have p re sen ted p roce dures
f o r c a l c u l a t in g th e a pp ro p r i a t e b rake s i z e (Newcomb, 1964; Newcomb,
1 96 4; R ab in ow i cz , 1 96 4) f o r v eh i c l e s on t h e b a s i s o f en e rg y
a b s o r pt i o n c o n s i d e r a t i o n s , and f o r e s t a b l i s h i n g t e s t s c h ed ul es
f o r e v a l u a t i n g b r a ke f a d e p er fo rm a nc e by d r i v i n g t e s t s (M ac ke nz ie
e t
a l . , 1962-63; L i vs e y e t a l . , 1960-61) .
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Lin ing Mat e r i a l s . Sev e ra l t h eo r i e s have been advanced
(H er rin g, 1967; Rabins Harke r, 1960; Garg Rabins , 1965)
dea l in g wi th t h e mechanism of fade caused by an in c re as e i n th e
l i n i n g t e mp e ra t ur e . The most r e c e n t o f t h e s e ( ~ e r r i n g , 9 67)
has proposed t h a t th e phenomenon i s du e t o a n e v o l u t i o n o f g a se s
from t h e l i n i n g m a t e r i a l which t e n d s t o s e p a r a t e t h e r u bb i ng s u r -
f a c e s . A d d i t i o n a l work h as b een done i n v e s t i g a t i n g t h e e f f e c t s
o f compos it ion (Weint raub Berna rd, 1968 ; Jacko e t a l . , 1968)
on t h e t he rm al s t a b i l i t y and f ad e c h a r a c t e r i s t i c s of f r i c t i o n
m a t e r i a l s .
I n v e s t i g a t o r s ha ve a l s o been c on c er n ed w i t h d e v e l o p i n g
a p p r o p r i a t e
t e s t
e qu ip me nt ( Wi lson e t a l . , 1 96 8; And erson e t a l . ,
1 9 6 7 ;
Clayton Manufactur ing Co. , 1967; Percy , 1951) f o r th e
e v a l u a t i o n o f l i n i n g m a t e r i a l s . C hoo sing an a p p r o p r i a t e f r i c t i o n
m a t e r i a l
s
a t ra de- of f ( F l ee t Owner, 1966; Autocar , 1965;
Burkman Hi gh le y, 1967; Mulvogue, 196 6) between good and bad
c h a r a c t e r i s t i c s of t h e a v a i l a b l e p r o d u ct s . Many S o c i e t y o f
Autom otive En gi n ee rs (SAE J6 61 a, 1968 ; SAE J8 40 a, 196 8; SAE 566 7,
1968) and Fe de ra l (F ed er al S p e c i f i c a t i o n No. KKK-L-370c, 1961;
In te r i m Fed era l Sp ec i f ic a t io n HH-L-00361d, 1965; Vi rg in ia
Equipment S af et y Commission, 1966; Fede ra l Sp ec if ic at i o n No.
HH-L-361b, 1952) st an da rd s have been gen era ted t o pro vid e guide-
l i n e s i n b ra ke l i n i n g e v a lu a t io n .
Fade and Peda l E f fo r t . The d r i v e r w l l sense any dec rease
i n t h e l i n i n g c o e f f i c i e n t of f r i c t i o n a s a de cr ea se d ga in i n t h e
s ys tem , i . e . , g r e a t e r p ed al e f f o r t w l l b e r e q u i r e d t o p ro du ce
t h e same v e h i c l e d e c e l e r a t i o n .
I ieat fade i s n o t a p ro ble m i n n or ma l d r i v i n g c o n d i t i o n s
(V al l i n , 1968; MacKenzie , 1966) a s th e brake tem pera tur es a r e
ge ne ra l l y below 300 F.
The performanc e s t an d ar d recommended by
SAE s p e c i f i e s (SAE J8 43 a, 1968; SAE J9 37 , 1968) t h a t i n f o u r
su c c e s s i v e s t o p s f r o m
6
mph no t more t h an
2
l b peda l e f f o r t
s h a l l be n ec e ss a ry t o a ch ie ve a d e c e l e r a t i o n o f 15 f t / s e c 2 .
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Normal brake effectiveness by the same standards is specified
as being 15 to 120 lb to achieve a 20 ft/sec2 deceleration.
Most drivers are more likely to encounter fade through
water contamination of the lining than through heat fade. The
mechanism is basically the same in that the effective lining
coefficient is reduced, thereby requiring higher pedal efforts.
SAE Recommended Practice (SAE Publication SP-299, 1967; SAE J937,
1968) specifies that 8 ft/sec2 decelerations should be obtain-
able from 25 mph with less than 200 lb pedal effort after a two
minute soaking of the vehicle s brakes.
BRAKE SYSTEM DEGRADATION. The performance of a brakinq
system may be decreased by wear or failure of any one of its
components. Failure here refers to both catastrophic failure
such as
a
ruptured brake line or to marginal performance caused
by the deterioration of a single component.
A comprehensive
brake system failure analysis of motor vehicles, such as that
normally performed in the aircraft industry (Glasenapp Gaffney,
1967), has never appeared in the literature. A qualitative dis-
cussion of many of the factors affecting brake performance is
available (White, 1963), but it is written from the viewpoint
of vehicle inspection. Some work has been done in determininq
the effectiveness of different dual braking arrangements (Vallin,
1968). Federal Standards (Federal Standard No. 515/9, 1965;
VSS No. 105, 1968) specify that following a pressure loss in a
portion of a hydraulic braking system, the remaining portion of
the system must be capable of stopping the vehicle from 60 mph
in less than 646 feet on dry Portland cement concrete while main-
taining the vehicle within a 12 foot lane. Deqradation of the
braking system will generally result in increased pedal forces
and/or greater pedal travel, or a total loss of braking.
Hydraulic System. Hydraulic brake fluid performance is
affected by its boiling point, water avidity, freezing point,
viscosity, and corrosive action on rubber parts (Markey, 1956;
SAE J664, 1968; Ker, 1968; Shiffler et al., 1968; Hanson Coryell,
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1960; Sharrard Hanson, 1956). Considerable legislation, based
mainly on SAE Recommended Practice (SAE J70b, 1968; Wright,
19651, has been passed (Richards, 1954; Lederer, 1955; Federal
Specification No. W-B-680a, 1967) to control the quality of brake
fluid reaching the consumer.
Motor Vehicle Safety Standard No.
116 (1969) sets federal specifications for hydraulic brake fluid
using the testing procedures set forth in SAE Standard J70b
(SAE J70b, 1968; Wright, 1965). Other SAE publications include
information on the storage and handling (Niehaus Shiffler, 1966;
Shiffler, 1966; SAE 575, 1968) of brake fluid, brake line hoses
(SAE J40d, 1968), and hydraulic cylinder seals (SAE J60, 1968;
SAE J65, 1968). Motor Vehicle Standard No, 106 (1968) sets federal
specifications for hydraulic brake hoses.
Wear of Friction Elements. The wear characteristics of the
lining and drum (or disc) depend on the particular combination
of the materials used (Willer, 1967; Lang, 1961). Lining wear is
compensated for on all current U.S. production vehicles by auto-
matic adjusters (Automotive Industries, 1968) and is therefore
only a problem when this device fails. Procedures for determin-
ing lining wear performance are specified in various SAE publi-
cations (SAE J661a, 1968; SAE J667, 1968) yet no specific level
of performance is suggested. Other SAE documents cover the
lining bonds (SAE J840a, 1968) and rivets (Csathy, 1964) for
attaching the brake lining or pad to the shoe.
SKIDDING AS RELATED TO BRAKING
IMPORTANCE OF BRAKING CONTROL IN ACCIDENTS. Skiddinq, as
used here, is any situation in which there is gross slippage
between one or all of the vehicle s tires and the road surface.
Skidding of
a
vehicle occurs when the limits of adhesion between
the tire and the road are exceeded, and frequently results in a
loss of directional control of the vehicle. The relationship
between braking, skidding, and highway accidents should not be
underestimated.
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In a study Grime, 1963) of 453 accidents involving one or
more vehicles, more than three-fourths of the accidents involved
skidding, and loss of control occurred following application of
the brakes in more than half of these. It was not made clear in
this study whether or not there was a cause and effect relation-
ship between braking and skidding or whether skiddinq was simply
symptomatic of the situation.
A
similar trend was indicated in
a survey of commercial vehicle accidents Starks, 1963). Althouqh
two-thirds of all accidents occur on dry roads, the incidence of
accidents involving skidding is two to seven times higher when
the roads are wet Grime Giles, 1954-55; Bulmer, 1962; Normann,
1953). It is felt that improved braking control Grime, 1963) in
many of these skidding accidents would have had a beneficial
effect.
BRAKING DYNAMICS. During braking there is a dynamic trans-
fer of weight onto the front wheels of a vehicle with a corres-
ponding decrease at the rear wheels. Consequently there is a
redistribution of usable braking torque between the front and
rear axles for every different vehicle deceleration Parker, 1960;
Newcornb Spurr, 1967; Lister, 1963; Ellis, 1963; Chandler, 1960).
Studies of vehicle dynamics during braking Lister, 1965; Odier,
1960; Radt Milliken, 1960; Jones, 1962-63) have shown that the
initial speed, braking characteristics, road surface friction
coefficient, and vehicle parameters are important factors deter-
mining the directional response characteristics of the car. If
a brake application should result in 100 percent longitudinal
slip of a tire, the lateral force capability of that tire is
reduced essentially to zero Francis, 1963). This means that
locking the f r o ~ t heels in a braking maneuver results in nearly
total steering loss. Locking the rear wheels causes the car to
slew around if the vehicle encounters a yawing moment disturbance.
If all wheels lock, the car can sideslip as well as rotate,
depending on whether or not there is an external disturbance con-
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sisting of a side force and a yawing moment.
ROAD-TIRE FRICTION COEFFICIENTS. As indicated by Equation
7
the deceleration produced in a locked-wheel braking maneuver
is determined by the locked-wheel coefficient of friction between
the tire and the road. It is possible, however, for the friction
coefficient achieved by a rolling, braked tire to allow decel-
erations in excess of those produced under locked-wheel condi-
tions. Considerable research has shown Csathy, 1964; Frood,
1962; Virginia Council of Highway Investigation and Research,
1959; Giles, 1963; ASTM Special Technical Publication No. 326,
1962; Texas Transportation Institute, 1962) that the effective
coefficient of friction achieved by a rolling tire is dependent
on many variables, the primary one being the longitudinal slip
of the tire.
Longitudinal slip is the ratio of the equivalent ground
speed of the tire to the actual vehicle speed. Measurements
have shown that the coefficient of friction reaches a maximum
at a longitudinal slip of 10 percent to 30 percent and then
decreases gradually to a value termed the sliding coefficient
at 100 percent slip. For a given tire, the shape of this curve
and its magnitude varies Hofelt, 1959; Kulberg, 1962) with
both the surface condition and the vehicle speed. It has been
found that both the peak and sliding coefficients generally
decrease with increasing speed, but the peak value decreases at
a lower rate Kulberg, 1962; Schulze Beckman, 1962). s the
speed increases, the value of slip at which the coefficient is
maximum tends to decrease Goodenow et al., 1968). The measured
coefficient of friction for a public road is by no means a con-
stant and varies seasonally Csathy, 1964; Kummer lleyer, 1967
and from lane to lane Mahone, 1962) on the highway. The tread
pattern, construction, and composition of the tire influence
the tire-road friction coefficient Easton, 1960; Mechanical
Engineering, 1968; Kelley, Jr. Allbert, 1968) whereas tire
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inflation pressure appears to have negligible effect DeVinney,
1967). Wet road surfaces change the frictional characteristics
of the road Kelley, Jr. Allbert, 1968; Maycock, 1965-66;
Obertop, 1962; Hoefs, 1961) and introduce the additional factor
of hydroplaning Allbert, 1968). It appears that vehicle speed
DeVinney, 1967) is the single most important variable in wet
surface conditions, the coefficient decreasing with increasina
vehicle speed. Investigations have also been made on winter
driving conditions NSC, 1966; Sapp, 1968; NSC, 1962) and tread
wear Leland Taylor, 1964).
Although the problem of accurately measuring the coeffi-
cient of friction Texas Transportation Institute, 1962; Goodwin
Whitehurst, 1962; Davisson, 1968) of a road surface has been
dealt with in many ways, basically three methods have been used:
skid trailers, vehicle stopping distance measurements Whitehurst,
1965), and portable testers ASTM Special Technical Publication
No. 366, 1965). Skid trailers of various designs Kulberg, 1962;
Goodenow et al., 1968) have been employed to measure both the
sliding and peak friction coefficients. Comparisons between the
British Portable Tester and automobile-stopping distance measure-
ments
Rizenbergs Ward, 1967) have shown a useful correlation
ASTM Special Technical Publication No.366, 1965) between their
results when patterned tires are used. In all skid resistance
measurements Frood et al., 1962) the conditions and methods
must be carefully controlled to obtain consistent results.
SKID CONTROL BY BRAKING MODULATION. An examination of the
frictional characteristics of the tire-road interface indicates
that maximum braking is obtained when tire slip is maintained
near the peak of the curve. Braking beyond this point Bulmer,
1962)
can result in significantly longer stopping distances
Lister Kemp, 1961) and the loss of directional control. Modu-
lating the pedal force so as to control the wheel slip at the
peak of the curve is a difficult task for the driver, so simpler
methods of pedal modulation have been suggested.
2
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Pumping and cadence braking Lister Kemp, 1961) are two
ways of improving braking performance and maintaining steering
control. The philosophy of these methods is that repeated on-
off braking will
cause the tire slip to pass repeatedly through
the peak of the coefficient/slip curve, Pumping consists of
applying and releasing the brakes rapidly, while cadence braking
is pumping the brakes in resonance with the pitch motion of the
car. Comparisons Lister Kemp, 1961) indicate that these two
methods can achieve shorter stopping distances than those of
locked-wheel stops on low coefficient surfaces. On high coeffi-
cient surfaces the stopping distances were the same, although
pumping or cadence braking also enabled the driver to maintain
directional control.
Anti-wheel-lock systems have been employed for several years
on large aircraft Collier, 1958) and a few systems have been
developed for automotive use
SAE
J840a, 1968; Autocar, 1962;
Automobile Engineer, 1958; Lister Kemp, 1958; SAE J660, 1968).
When using these systems, the driver operates as usual under
normal conditions; however, as the driver causes the wheels to
approach lockup in
a
panic s t o ~ , he device takes over. Using
an inertia switch Design News, 1959; Design News, 1957; Machine
Design, 1959) to sense impending wheel lockup, the anti-skid
device automatically pumps the brakes to maintain a slip rate
close to that value producing maximum adhesion. Control is
returned to the driver when he reduces the pedal effort. Other
anti-skid systems operate similarly but derive the wheel acceler-
ation signal from wheel velocity. It is claimed that several of
these systems Lister Kemp, 1958; Scafer Howard, 1968) pro-
vide performance superior to locked wheel stops by improving
driver steering control and shortening stoppinq distances, par-
ticularly in adverse driving conditions.
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TESTING OF THE VEHICLE-TIRE-BRAKE SYSTEM
DECELERATION PERFORMANCE. The deceleration performance
(Tignor, 1966; Harding, 1961) achieved during a stop can be
expressed as a function of (a) the stopping distance, (b) the
average deceleration produced during the stop, and (c) the degree
of driver control.
Stopping Distance. The total distance covered during a
braking maneuver includes the distance traveled during the driver s
response time and the actual deceleration of the vehicle. During
the driver s response period the vehicle would be continuing at
nearly the initial velocity. The deceleration experienced during
the braking interval depends on the rolling resistance, aerodynamic
drag, and engine drag as well as the braking torque and the coeffi-
cient of friction existing at the tire-road interface.
Stopping distance may be measured experimentally by integra-
tion of a fifth wheel velocity signal or by direct distance mea-
surement. The latter method can be made reasonably accurate by
employing an explosively-fired chalk pellet (Lister, 1959) to
indicate the point of brake application. Minimum driver response
times have been measured by recording the time period between the
flashing of a signal and the start of brake application (Normann,
1953, Konz Daccarett, 1967).
Deceleration Measurement. An average value of vehicle decel-
eration can be computed from a measurement of the initial speed
and stopping distance. An instantaneous value can be obtained by
differentiation of a fifth wheel velocity signal (Carpenter, 1956)
or by direct measurement with an
accelerometer on board the
vehicle (Harding, 1961). The computation of average deceleration
method is not useful, however, in correlating pedal force with
deceleration. Accelerometers on board the vehicle have the prob-
lem (Harding, 1961) of being sensitive to the vibrations caused by
normal road roughness, the pitch motion during braking, nd the
ascent and descent of grades. The latter two problems may be
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eliminated by mounting the accelerometer on a stabilized platform
within the vehicle.
The velocity signal differentiation method
avoids the difficulties encountered with the on-board acceler-
ometer and has been successfully used in brake usage tests
Carpenter, 1955). This method does, however, suffer from a loss
in frequency response because of the necessary electrical filter-
ing circuits.
Directional Control. Studies in which measurements have
been made of the steering control required during braking have
not appeared in the literature. The few quantitative measurements
of directional response that have been made consist of a deter-
mination of the final angular deviation of the vehicle from its
intended path after a straight line braking maneuver Lister,
1963). Although tests and evaluations of anti-skid systems
Traffic Institute, Northwestern University, 1960) have included
cornering maneuvers during severe braking, quantitative evalua-
tions of improvements in directional stability and control with
respect to co
