NBSIR 78-1467

Influence of Some Wetting

Parameters on Bicycle Braking

Performance

Leonard Mordfin

Institute for Materials Research

National Bureau of Standards

Washington, D C. 20234

January 1978

Final Report

Prepared for Office of Consumer Product Safety

Center for Consumer Product Technology

Institute for Applied Technology

National Bureau of Standards

Washington, D.C. 20234

NBSIR 78-1467

INFLUENCE OF SOME WETTING

PARAMETERS ON BICYCLE BRAKING

PERFORMANCE

Leonard Mordfin

Institute for Materials Research

National Bureau of Standards

Washington, D.C. 20234

January 1978

Final Report

Prepared for Office of Consumer Product Safety

Center for Consumer Product Technology

Institute for Applied Technology

National Bureau of Standards

Washington, D.C. 20234

U.S. DEPARTMENT OF COMMERCE, Juanita M. Kreps, Secretary

Dr. Sidney Harman, Under Secretary

Jordan J. Baruch, Assistant Secretary for Science and Technology

NATIONAL BUREAU OF STANDARDS, Ernest Ambler, Acting Director
'
CONTENTS

1. Introduction 2 2. Hose-Wetting and Rainy-Weather Tests 3 3. Trough-Wetting Tests 4

4. Concluding Remarks 5 5. References 6

Tables 7
INFLUENCE OF SOME WETTING PARAMETERS ON BICYCLE BRAKING PERFORMANCE

Leonard Mordfin

ABSTRACT

One approach toward evaluating the braking capability of a bicycle

in wet weather involves riding tests in which the stopping distance of

the bicycle with wet brakes is measured from a preselected initial

speed. The results of some domestic and foreign riding tests of this

kind are reviewed. It is found that the amount of water available at

the brake surfaces, above some small minimum, is not significant. There

are some sparse data that suggest that the manner of wetting (e.g.,

hose, trough or rain) may affect the test results but this evidence is

questionable due to the uncharacterized influences of other test parameters.

Recommendations for additional tests are given.

This report is an addendum to NBSIR 78-1416.

Key Words: Bicycles; braking, bicycle; braking, wet; consumer product

safety; measurements, bicycle braking; riding tests; safety, bicycle;

standards, bicycle safety; test methods, bicycle; wet braking.

1 . INTRODUCTION

The U.S. Consumer Product Safety Commission (CPSC) and the International

Organization for Standardization (ISO) are interested in methods for

evaluating the braking performances of bicycles in wet weather. The

approach which is presently under consideration involves riding tests,

under simulated wet-weather conditions, in which the bicycle is braked to

a halt from a preselected speed and the measured stopping distance is

used as the indicator of braking performance. Some aspects of this

approach were examined in NBSIR 78-1416 [l] 1 with regard to their relevance

to a possible standard test method for wet-braking performance.

One of the findings of the earlier report was that the braking

performance of a caliper-braked bicycle depends quite heavily on whether

the braking surfaces are wet or dry, but that the specific means by

which the surfaces are wetted is immaterial. Neither the film thickness

of the water (exceeding some relatively small minimum) nor its manner of

application was found to exert a significant influence on stopping

distance. This finding -- which has obvious implications on the amount

of detail needed to properly specify a standard test method -- was deduced from theoretical considerations and from the results of various

laboratory tests.

In this report the results of several series of actual riding tests

under wet conditions are reviewed in order to test the validity of the

Numerals in brackets denote the similarly numbered references listed

in Section 5 of this report.

2
earlier finding. As such, this report may properly be considered an addendum to NBSIR 78-1416. The reader is advised to consult that report

for much of the detail that has been omitted here in the interest of

brevity.

This report was prepared for the NBS Office of Consumer Product

Safety at the request of CPSC.

2. HOSE-WETTING AND RAINY-WEATHER TESTS

One of the experimental methods which has been used for intentional

wetting of bicycle brakes employs small-diameter plastic tubing to

direct a flow of water to the brake pads and the wheel rims. The water

supply is carried onboard the bicycle and four tubes emit a flow of 4

ml/s (0.14 fl oz/s) 2 to each of the four caliper brake pads for a period of time sufficient to ensure complete wetting prior to braking.

A series of tests was performed by bicycle manufacturers, under ISO

auspices, to examine the effect of the interval (expressed in distance

traveled) between the cessation of flow and the initiation of braking

[2-8]. The intervals examined were 0, 5, 15, and 30 m (0, 16, 49 and 98

ft). In addition, comparison tests were performed (1) with dry brakes,

(2) with water flow allowed to continue throughout the braking process,

and (3) in actual rainy weather without added water. Each bicycle was braked from an initial speed of 16 km/h (10 raph) and, except as noted

hereafter, the total mass of each bicycle, its rider, onboard equipment

and ballast was 90 kg (198 lb). The test results, in terms of measured

stopping distances, are given in Table 1 where each result is the average

of five test runs. Although all of the seven bicycles listed were different from each other, and were each tested by a different organization,

they were all fitted with nominally identical wheels having chromeplated-steel rims and nominally identical brakes.

The results show that the stopping distances under wet conditions

are considerably greater than they are under dry conditions, which is

not surprising. Of greater interest, at this point, is the fact that the

interval between water cutoff and brake initiation did not exhibit any

consistent, significant effect on stopping distance. This tends to

support the thesis that the amount of water at the brake surfaces is not

an important parameter.

The test results in Table 1 which were obtained in actual rain are somewhat more difficult to interpret. While the stopping distances in

rain were not consistently greater or smaller than the stopping distances

with simulated wetting, the differences between the two are sufficiently

great, in some cases, to justify further examination.

2 About 1 cup per minute.

3
Table 2 lists the stopping distances in rain as a function of the

severity of the rainfall, as described by the investigators, and presents

the ratios of the rainy-weather stopping distances to the dry-weather

stopping distances. It may be seen that there is no correlation between

rainfall severity and the stopping-distance ratio. This supports the

contention that the amount of water at the brake surfaces is not relevant.

How, then, can the inconsistent differences in observed braking performance

between rainy weather and simulated wet-weather conditions be explained?

To answer this question it must be recognized that the simulated

wet-weather tests and the dry tests were performed in dry weather and were all carried out, by each testing organization, at one time (i.e.,

in a single day or, perhaps, in two successive days). In order to

conduct the rainy-weather tests, on the other hand, it was necessary to

wait for the appropriate weather conditions to prevail. This may have

entailed delays of several weeks and, in some cases, months [2,6]. It

is not unreasonable, therefore, to surmise that the two sets of tests

involved different rider reaction times -- and perhaps even different

riders -- as well as different pavement conditions, wind speeds, ambient

temperatures, etc., all of which influence stopping distances [1,9].

In short, it is not inconsistent to conclude that the differences in the

stopping distances between the rainy-weather and the simulated wetweather conditions may not, in fact, reflect effects of the different

means of wetting.

An additional set of tests conducted by one of the testing organizations

is of particular interest [7]. The brakes on Bike No. 206 were hosewetted as in the previous tests and a braking test run was performed

with zero interval between the cessation of flow and the initiation of

braking. Then, nineteen additional braking test runs were carried out

at 40-s intervals with no further wetting. The stopping distances were measured for all twenty runs and then the entire series was repeated.

Table 3 shows, in abbreviated form, the manner in which the stopping

distances varied as the number of test runs increased. The data show

that more than five braking test runs had to be performed before any

sensible braking recovery was observed, and that full recovery of dry

braking performance was not obtained even after twenty runs. This is

consistent with the results of laboratory tests cited previously [1],

and supports the thesis that very little water need actually be present

at the brake surfaces in order to destroy dry braking capability.

3. TROUGH-WETTING TESTS

An effective method of intentionally wetting the brake surfaces,

which is simpler than the hose method, involves riding the bicycle

through a water trough before it is accelerated to the desired test

speed and then braked. The depth of water in the trough is sufficient

to cover the wheel rims. This wetting method was used in order to

investigate the effects of relatively long intervals (i.e., 50 and 100 m

(164 and 328 ft)) between wetting and braking [10-13].

4
The test results are summarized in Table 4, where each stopping- distance entry represents the average of five test runs, except the 50-m

data for Bike No. 212, which represents the average of three test runs. These results extend those given in Table 1 and show, once more, that

the interval between wetting and braking does not appear to exert any consistent, significant influence on braking performance.

Unfortunately, no tests were conducted to specifically compare

braking performances with hose wetting and with trough wetting. It

appears from information available [5,11] that Bike No. 204 (Table 1) and

Bike No. 212A (Table 4) were probably identical, at least nominally, and

if this is indeed the case, then the data suggest that stopping distances

were substantially shorter in the tough-wetting tests -- in which the

intervals between wetting and braking were relatively longer -- than

stopping distances in the hose-wetting tests. However, these two sets

of tests were conducted more than seven months apart and, as mentioned

earlier, differences in rider reaction times and environmental factors

tend to render the validity of such comparisons questionable.

In an earlier report [1] it was pointed out that it is quite difficult

to assess the systematic errors involved in stopping-distance tests

because of the absence of test data by different laboratories on the same bicycle. To this shortcoming might be added the need implied by

the above discussion, i.e., a need for systematic tests by a single

laboratory on a single bicycle at widely disparate times.

4. CONCLUDING REMARKS

The results of several series of riding tests, which were conducted

in order to investigate the wet-braking performances of caliper-braked

bicycles, were reviewed. It was found that:

1. Distance intervals up to at least 100 m (328 ft), between the

wetting and the actuation of bicycle brakes, exert no consistent,

significant influence on stopping distance.

2. The severity of a rainfall exerts no consistent, significant effect

on stopping distance provided that the duration of the rainfall and

the distance traveled have been sufficient to wet the wheel rims.

3. Once caliper brakes become wet, dry braking performance is not

recovered even after a substantial number of braking sequences.

4. There is some evidence which suggests that the manner in which the

brakes are wetted may influence braking performance. However, the

paucity of this evidence renders it inconclusive.

In short, there is no rational basis at this time to refute the contention

that the wet-braking performance of a bicycle is independent of the

means by which the brakes are wetted or the amount of water used (beyond some very small minimum)

5
In order to properly characterize the uncertainties involved in

riding tests to measure wet-braking performance, it is recommended that

nominally identical tests be performed by different laboratories on the same bicycles, and by individual laboratories at different times.

5 . REFERENCES

[1] Mordfin, L. , Analyses of Riding Tests for Evaluating the Wet Braking

Performances of Bicycles, NBSIR 78-1416 (April 1977).

[2] Proprietary data.

[3] Anon, Braking Tests Dry, and Braking Tests Wet (Wasserzufuhr 4 ml/s),

Weinmann, Feb. 7, 1977.

[4] Edinger, K. , private communication.

[5] Proprietary data.

[6] Proprietary data.

[7] Okajima, R. , Braking Test, Mar. 4, 1977.

[8] Proprietary data.

[9] Mordfin, L. , The CPSC Road Test of Bicycle Braking Performance -- Kinetic and Error Analyses, NBSIR 75-786 (Oct. 1975).

[10] Proprietary data.

[11] Proprietary data.

[12] Proprietary data.

[13] Cooper, T. Z., Wet Brake Test Report, CPSC ref. 540644, Nov. 4, 1977.

6
Table 1. Delayed Braking Following Hose Wetting (a)

Average stopping distance

Wet

Bike

No. ^ Dry

Water cutoff distance before braking,~7bT~

» m

Rain Ref None 0 5 15 30 m m m m m m m (ft) (ft) (ft) (ft) (ft) (ft) (ft)

201 2.39 12.1 - 12.9 10.8 11.9 8.4 2

(7.84) (39.7) (42.3) (35.4) (39.0) (28.)

202 2.53 11.41 12.55 12.13 12.58 12.30 3

(8.30) (37.43) (41.18) (39.80) (41.27) (40.35) -

203 2.67 - 10.97 10.75 9.75 10.5 12.25 4

(8.76) “ (35.99) (35.27) (31.99) (34.4) (40.19)

204 2.39 12.98 12.76 13.37 12.51 12.23 8.43 5

(7.84) (42.59) (41.86) (43.86) (41.04) (40.12) (27.7)

( e 205 ^ ') 1.8 - - 11.9 12.5 12.5 10.5 6

(5.9) “ “ (39.0) (41.0) (41.0) (34.4)

206 1.88 12.93 13.42 13.91 13.74 13.34 12.21 7 (6.17) (42.42) (44.03) (45 . 64) (45.08) (43.77) (40.06)

(f) 201 2.2 13.7 13.7 13.5 13.5 12.8 13.5 8

(7.2) (44.9) (44.9) (44.3) (44.3) (42.0) (44.3)

Avg. 2.27 12.62 12.68 12.64 12.20 12.22 11.38

(7.45) (41.40) (41.60) (41.47) (40.03) (40.09) (37.34)

(a) The special conditions under which these tests were run are described

in Section 2.

(b) Refers to distance the bicycle was allowed to travel between water

cutoff and initial application of brakes.

(c) Numbers assigned by author.

(d) Water flow allowed to continue throughout braking process.

(e) Tested with total mass of 101.2 kg (223.1 lb).

(f) Tested with total mass of 93.0 kg (205.0 lb).

7
Table 2. Rainy Weather Tests

Hi ku x No.

a Rainfall Avg. stopping distance

No. of runs Ratio

,

rain/dry

m (ft)

201 very It shower 8.4 (28) 4 3.5

203 not specified 12.25 (40.19) 5 4.5

204 heavy rain 8.80 (28.9) 1 3.7

medium rain 8.86 (29.1) 3 3.7

medium It rain 8.52 (28.0) 1 3.6

light rain 8.37 (27.5) 1 3.5

drizzle 8.63 (28.3) 1 3.6

light drizzle 7.91 (26.0) 2 3.3

very It drizzle 8.07 (26.5) 1 3.4

205 2.75 in in 48 h 10.5 (34.4) 5 5.7

(avg. 1.46 mm/h)

206 heavy rain 12.21 (40.06) 5 6.5

207 0.88 mm/h 13.5 (44.3) 5 6.1

(a) See Table 1.

(b) Ratio of stopping distance in rain to stopping distance under dry

conditions

.

8
Table 3. Braking Recovery Tests [7]

Run No. Average stopping distance

m (ft)

1 13.30 (43 . 64)

5 13.14 (43.11)

10 10.24 (33.60)

15 5.60 (18.37)

20 4.29 (14.07)

9
Table 4. Delayed Braking Following Trough Wetting (a)

Average stopping distance (b)

Bike Initial

speed ( 0 ) Payload^

Wet

(c) Trough-to-test distance, m Dry 50 100 Ref.

km/h kg m m m

(mph) (lb) (ft) (ft) (ft)

208 16 84.4 2.2 21.3 20.9 10

(10) (186) (7.1) (69.9) (68.7)

209 16 74.4 2.2 21.5 22.8 10

(10) (164) (7.2) (70.4) (74.7)

210 16 71.2 2.0 15.1 14.8 10

(10) (157) (6.6) (49.7) (48.7)

211 16 89.4 2.3 24.5 24.4 10

(10) (197) (7.5) (80.5) (80.1)

212A 16 77.9 - 7.43 8.62 11

(10) (172) (24.4) (28.3)

212B 16 77.9 - • 5.91 3.45 11

(10) (172) (19.4) (11.3)

212C 16 77.9 - 7.77 9.00 12

(10) (172) (25.5) (29.5)

212D 16 77.9 - 10.03 9.64 12

(10) (172) (32.9) (31.6)

212E 16 77.9 - 7.11 6 . 45 12

(10) (172) (23.3) (21.2)

212F 16 77.9 - 4.23 6.01 12

(10) (172) (13.9) (19.7)

213A 24 83.4 2.1 22.6 25.0 13

(15) (184) (6.8) (74.1) (82.0)

213B 24 83.4 2.9 17.1 14.5 13

(15) (184) (9.4) (56.1) (47.5)

214A 24 83.4 3.2 31.5 35.7 13

(15) (184) (10.6) (103.2) (117.2)

214B 24 83.4 2.8 15.4 15.1 13

(15) (184) (9.3) (50.4) (49.6)

continued

10
Table 4. Continued

(a) The special conditions under which these tests were run are described in Section 3.

(b) The stopping distances listed for Bikes 213 and 214 were corrected

for payloads exceeding 70 kg (154 lb) at the rate of 0.3 m/4.5 kg

(1 ft/10 lb).

(c) Distance between exit from trough and initiation of braking.

(d) Numbers assigned by author. Postscript letters designate different

brake pads on the same bicycle.

(e) Values listed for payload represent rider mass for Bikes 208 thru

211, and rider plus onboard instrumentation mass for Bikes 212 thru

214.

11
MBS-1 14A 1 R E V. 11-771

u.s. DEPT. of COMM. 1 . PUBLICATION OR REPORT NO. 2. Gov’t Accession

BIBLIOGRAPHIC DATA No. SHEET

3. Recipient’s Accession No.

4. TITLE AND SUBTITLE

Influence of Some Wetting Parameters on Bicycle Braking

Performance

5. Publication Date January 1978

6. Performing Organization Code

7. AUTHOR(S)

Leonard Mordfin

8. Performing Organ. Report No.

9. PERFORMING ORGANIZATION NAME AND ADDRESS

NATIONAL BUREAU OF STANDARDS

DEPARTMENT OF COMMERCE

WASHINGTON, D.C. 20234

10. Project/Task/Work Unit No.

11. Contract/Grant No.

12. Sponsoring Organization Name and Complete Address (Street, City, State, ZIP)

Office of Consumer Product Safety

Center for Consumer Product Technology

Institute for Applied Technology

National Bureau of Standards, Washington, DC 20234

13. Type of Report 8t Period Covered

final

14. Sponsoring Agency Code

15. SUPPLEMENTARY NOTES

16. ABSTRACT (A 200-word or less factual summary of most significant information. If document includes a significant

bibliography or literature survey, mention It here.)

One approach toward evaluating the braking capability of a bicycle in wet weather

involves riding tests in which the stopping distance of the bicycle with wet brakes

is measured from a preselected initial speed. The results of some domestic and

foreign riding tests of this kind are reviewed. It is found that the amount of

water available at the brake surfaces, above some small minimum, is not significant.

There are some sparse data that suggest that the manner of wetting (e.g., hose,

trough or rain) may affect the test results but this evidence is questionable due

to the uncharacterized influences of other test parameters. Recommendations for

additional tests are given.

17. KEY WORDS (six to twelve entries; alphabetical order; capitalize only the first letter of the first key word unless a proper name; separated by aemicolona)

Bicycles; braking, bicycle; braking, wet; consumer product safety; measurements,

bicycle braking; riding tests; safety, bicycle; standards, bicycle safety; test

methods, bicycle; wet braking.

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