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Foot Brake Pedal Force Capability of DriversR. G. MORTIMER aa Highway Safety Research Institute, University of Miohigan, Ann Arbor, Mich, USAPublished online: 25 Apr 2007.

To cite this article: R. G. MORTIMER (1974) Foot Brake Pedal Force Capability of Drivers, Ergonomics, 17:4, 509-513, DOI:10.1080/00140137408931381

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ERGONOMICS, 1974, VOL. 17, No.4, 509-513

Foot Brake Pedal Force Capability of Drivers

By R. G. MORTIMERHighway Safety Research Institute,

University of Michigan, Ann Arbor, Mich., U.S.A.

An experimont was conducted to provide distributions of foot force for U.S. driversin order to derive guidelines for the maximum force requirement for actuation of anautomobile service brake. The results of this study are compared with three othersconducted at about the same time. Since the 5th percentile female maximum brakepedal force is about 400 newtons. it is recommended that no more than this force berequired to attain near maximum braking capability from a passenger cer.

1. IntroductionAn important consideration in the design of an automobile braking system

is the relation between the force applied on the brake pedal and the decelerationwhich results. Too high a deceleration/brake pedal force gain will lead to abrake control which is difficult to modulate smoothly on dry roads and produceswheel lock-up on wet roads, increasing the likelihood that the driver will losepath control of the vehicle. Too low a deceleration/pedal force gain willproduce a brake control that cannot be modulated as rapidly as desirable,and may impose brake pedal application force requirements on drivers that arebeyond their foot force capabilities.

Studies by Brigham (1968) and Segel and Mortimer (1970) have providedreasonably complete guidelines for bounds on deceleration/pedal force gain forbrake modulation. A a part of the latter research, data were obtained(Mortimer et al. 1970) on the maximum pedal force application of drivers, forwhich no suitable values were then available.

In order to set a limit on brake pedal force, the distribution of foot forceof drivers must be known. Until quite recently, most measurements of footforce have been made on military populations (Damon et al. 1966). Anexception is the study by Aoki (1960), who found that the 5th percentile youngJapanese female could exert a pedal force of only 165 N. Thus, on a drysurface, she would be unable to obtain the maximum braking capability of,for example, any 1968 model American car that is not equipped with power-assisted brakes (Mortimer et al. 1970). Because of population differences suchdata could not be applied to American drivers. This paper describes onestudy in which foot force measures were obtained for U.S. drivers and comparesthe findings to those of three other studies, all completed at about the sametime (Stoudt et al. 1969, Radlinski and Price 1970, Eaton and Dittmeier 1970).

2. Method2.1. A pparala

Subjects were seated on a wood seat, 071 m wide and 041 m deep, coveredwith a nonslip vinyl surface. The seat was raised or lowered by means of ahydraulic lift. The chair back was 043 m high, mounted at an angle of

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044 radians from the vertical. A hydraulic force gauge, 1334 N full-scale,equipped with a ribbed circular steel pad, 0045 m in diameter, was mountedat an angle of 061 radians, which is within the range commonly used for brakepedals, and was horizontally and vertically adjustable.

2.2. ProcedureThe equipment was taken to a shoe store in a shopping centre and subjects

were recruited from patrons and passers-by. It was later moved to a DriverLicense Bureau of the Michigan Department of State where, with thecooperation of officials, subjects of a greater age range could be tested.

Prior to data collection, the pad of the force gauge was adjusted in heightand distance for each subject to yield a thigh angle of zero and a knee angleof 279 radians, to allow maximum force to be applied (Damon et al. 1966).This adjustment was facilitated by computing these settings in advance as afunction of all likely combinations of driver foot length and knee height thatmight be encountered.

When the subject was seated and all adjustments had been made, the, standard' motivation instructions were given: 'push the pedal as hard asyou can and hold it for three seconds '. A measurement of the maximum forcesustained for about three seconds was taken with each foot. The procedurewas immediately repeated using the' induced' motivation instructions: 'thistime really push as hard as you can, like you are driving a car and have toavoid a serious uccident '. Subjects were allowed to see the gauge whileapplying force.

2.3. SubjectsThe sample consisted of 276 female and 323 male drivers. The females were

H;-70 years of age with a mean age of 32'5 years, and weighed 40-102 kgwith a mean of 62 kg. The males were 16-89 years of age with a mean age of318 years, and weighed 54-129 kg with a mean of 81 kg. Young drivers,16-24 years, were over-represented in the sample. Accordingly, the dis-tribution of foot forces is not likely to be an underestimate of the capabilityof the driver population.

3. ResultsFigure I shows the percent cumulative frequency distributions of maximum

force achieved by female and male subjects with the right foot. For thestandard motivation instruction, or first trial, the 5th and 50th percentilesof maximum force achieved by the 276 females are respectively, 311 Nand681 N. For the induced motivation instruction, or second trial, the 5th and50th percentiles are equivalent to 454 Nand 863 N. Males attained a 5thpercentile force of 592 N and a 50th percentile level of 1241 N with standardmotivation instruction on the first trial, and a 5th percentile level of 840 Non the second trial with induced motivation instruction. Performance ofmales at the 50th percentile could not be measured in the induced motivation

Results arc given only for the right foot, since a. pilot study showed B high correlation(r=0'93) between left and right foot force capability.

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Foot Brake Pedal Force Capability of Drivers 511

instruction since more than half of them exceeded the 1334 N limit of theforce gauge.

The correlation between body weight and maximum foot force on the firsttrial was found to be low (rw.f.=024) based on a random sample of 57 maleand 43 female subjects.

100,.-------------------,--- "Standard" Motivation---- "Induced" Motivation

Figure 1. Cwnulative percent distribution of right foot pedal force for 276 female and 323 maledrivers.

4. DiscussionThe results of this study can be compared most directly with those obtained

by Stoudt et al. (1969) at Harvard University. They used a representativesample of 50 U.S. females in a mock-up consisting of an adjustable upholsteredseat, steering wheel, brake and accelerator pedal. Subjects adjusted the seatthemselves for a comfortable position, and grasped the steering wheel whiledepressing the brake pedal. Measurements were made of the average brakepedal force maintained over ten seconds, on five consecutive trials, after eachof which subjects were urged to apply as much force as possible. The5th percentile pedal forces were 383, 489, 543, 583 and 623 N respectively,on the five trials.

The 105 female subjects of Radlinski and Price (1970) each adjusted theseat in an automobile for a comfortable driving position, fastened the lap belt,and were told to depress the brake pedal with maximum force. Themeasurement taken was the mean maximum force applied for five seconds.The 5th percentile pedal forces achieved in the two cars used, on the secondof two trials in each condition, were 400 and 556 N, and with a simulatedsplit-braking system malfunction in the first car, resulting in a pedal havingmore than normal displacement, the 5th percentile pedal force was 289 N.

Each of the above studies, and ours, were done in a static environment inwhich the subjects could carefully position their foot on the pedal and preparethemselves for exertion of force. On the other hand, they were under no stressand there was little incentive to maximize the foot force.

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A study by Eaton and Dittmeier (1970) of the Ford Motor Company used48 females representative, in stature, of the U.S. population. Their brakepedal application force was measured in a potentially stressful driving situation.Subjects drove either a full-size or mid-size automobile around a track,occasionally braking to a stop in a sealed-off lane. On the last of these trialsthe brake power assist booster was disabled, unknown to the driver, and pedalforce during 05 seconds was measured as she tried to stop the car. The5th percentile brake pedal force values achieved with the intermediate andfull-size cars, respectively, were 334 and 423 N. The difference in the valueswas attributed by the authors to the greater braking effectiveness of themid-size car compared to the full-size car, when the booster was disabled.If that is the case, the value found for the full-size car may better representthe performance of motivated drivers.

Table 1. Summary of subject sample characteristics and 5th and 50th percentilepedal forces (N) of female drivers, in four studies

Stationary tests1\fortimerclal. Stoudtctal. Radlinski and Price

(1970) (1969) (1970)Hurd seat Soft seat

Steering wheel Car A Car BPercentile Percentile Percentile Percentile

Dynamic testEaton and Dittmeier

(1970)Mid size Full size

ear carPercentile Percentile

Trial 5 50 5 50 5 50 5 50 5 50 5 50I 311 676 383 729 343 641 409 734 334 620 423 7072 41;4 858 489 863 400 778 556 S45

Sample size 276 50 105 48Mean age (yra) 33 38 35Min-max age 16- 79 18- 76 18- 62Mean weight

(kg) 62 60 60Min-max

weight (kg) 40-102 41-102 46- 93

The findings of all four studies are summarized in Table 1. They showthat repeated trials and urging-on by the experimenters can raise the forcelevels. Although a hard seat was used in our test somewhat lower valueswere obtained than by Stoudt et al, (1969) who used a compliant seat.However, their subjects may have obtained some advantage from the steeringwheel. When actual cars were used (Radlinski and Price 1970) one car allowedgreater forces to be generated than the other. There is no evident reasonfor this. But, the 5th and 50th percentile pedal force values achieved arewithin the range of those obtained in the laboratory tests. The study byEaton and Dittmeier (1970) was particularly valuable because it helps to settlearguments concerning the likely forces to be exerted in an actual brakingmanoeuvre. Their results show that there is no reason to suppose that driversin a moving car will generate force levels that are higher than in laboratoryor stationary car tests.

Overall the 5th and 50th percentile pedal force values obtained by thesestudies are remarkably similar considering the different methods that wereused.

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Foot Brake Pedal Force Gapahility of Drivers 513

Based on the findings it would seem reasonable that the brake pedal effort,required to obtain the maximum or a high level of deceleration of a vehicle,should not exceed about 400 N. This will ensure that no more than 5% offemale drivers, and about 1% of male drivers (Mortimer et al. 1970), are unableto apply adequate brake pedal force. The current SAE J-937 brake systemperformance recommended practice allows up to 534 N of brake pedal forceto be applied for a deceleration of 61 m/s 2 from 97 km/h. Such a pedal forceis certainly higher than warranted based on the results of the recentlycompleted studies of foot force capability of female drivers. In practice, mostpassenger cars meet the requirement on deceleration with less than 445 N ofpedal force. In the fourth stop in the brake fade test, the SAE recommendedpractice allows up to 890 N of brake pedal force to be applied to achieve46 m/s 2 deceleration. This could not be achieved by 76% and 55% of ourfemales and 19% and 7% of our males, on the first and second trials,respectively.

The foot force distribution of U.S. drivers appears to be documentedsufficiently well by these studies to assist with revisions to brake pedal forcestandards for braking systems in normal and in degraded conditions such asdue to fade, power booster failure or loss of pressure in the front or rear brakeline circuit, and for foot-operated parking brakes.

L'objectif de cette experience etait d'etablir la distribution des forces d'eppui du pied pourles conducteurs americains eftn d'eteblir Ies exigences maximales pour l'actionnement des freinsd'une voiture. Lee resultats de cette etude eteiene compares A ceux de trois autres etudeseffectuees Apeu pres e le meme epoque. Puisque le 56 percentile pour Is. force maximale d'appuisur une pedale de frein est ait.ue, pour 180 femme, A environ 400 newtons, il est recommende quepa.s plus que oette force ne devrait etra neceasaire pour ectionner efficacement Ie frein d 'WlCvoiture de tourisme.

In einer Versuchsreihe wurde die Verteilung der Fusskriifte von U.S. Kraftfahrern ermittelt,urn Richtlinien fur die maximalen Kraftanforderungen bei der Betiitigung amerikeniecherAutomobil-Servo-Bremsen .abzuleiten. Die Resultate dieeer Studie wurden mit drei anderenetwa gleichzeitig ausgefUhrten Untersuchungen verglichen. De, die rnaximale Pedel-Bremakreftvon Frauen in 95% der Falle ungefahr 40 Kg erreioht, wird empfohlen, fUr einen Peraonenkraft-wagen keine grossere Bremskraft zu verlangen.

ReferencesAOD, K., 1960, Human factors in braking and fade phenomena for heavy applications-problems

to improve brake performance. Bulletin oj the Japanu8 Society oj Mechanical Engineer8,8,587-594.

BRIGHAM, F. R., 1968, A human factors etudy of vehicle braking systems. Univermy oj Aston,Birmingham (England), MSc. Th eeie.

DAMON, O. A., STOUDT, H. W., and McFARLAND, R., 1966, The Human Body in EquipmentD68ign (Cambridge: HARVARD UNIVERSITY PRESS.)

EATON, D. A., and DrI'TMEIER, H. J., 1970, Braking and steering effort capabilities of drivers.Society oj Amomotive Engineer