NBSIR 78-1416

Analyses of Riding Tests for

Evaluating the Wet Braking

Performances of Bicycles

Leonard Mordfin

Institute for Materials Research

National Bureau of Standards

Washington, D C. 20234

April 1977

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 78-1416

ANALYSES OF RIDING TESTS FOR

EVALUATING THE WET BRAKING

PERFORMANCES OF BICYCLES

Leonard Mordfin

Institute for Materials Research

National Bureau of Standards

Washington, D C. 20234

April 1977

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
r

'

I
ANALYSES OF RIDING TESTS FOR EVALUATING THE WET

BRAKING PERFORMANCES OF BICYCLES

Leonard Mordfin

ABSTRACT

The Consumer Product Safety Commission has expressed

interest in the development of a riding test method for

evaluating the braking performances of bicycles in wet weather.

In this report three different testing approaches for caliperbraked bicycles are examined using kinetic analyses, a review

of the literature, and an evaluation of available domestic and

foreign test results. On the basis of the findings it is recommended that the riding test include the intentional wetting of

both the bicycle brakes and the test pavement; the former to

obtain meaningful results and the latter to enhance the repeatability of the test results. A tentative pass-fail criterion is

also offered, based on a maximum wet stopping distance which, at

this time, appears to be generally attainable only with bicycle

wheels having aluminum-alloy rims. Error analyses of the test

methods are presented.

Key Words: Bicycles; brakes, bicycle; braking, wet; error

analysis; friction, brake; friction, tire/pavement; kinetics,

bicycle; measurements, bicycle braking; road tests; standards,

bicycle safety; test methods, bicycle; wet braking.

CONTENTS

Page

1. INTRODUCTION 1

1.1 Terminology 1

1.2 Scope 2

2. ANALYSIS: WET PAVEMENT, DRY BRAKES 2

2.1 Kinetic Analysis 2

2.2 Tire/Pavement Friction Coefficients 4

2.3 Systematic Errors 6

2.4 Random Errors 7

3. ANALYSIS: DRY PAVEMENT, WET BRAKES 8

3.1 Kinetic Analysis 8

3.2 Brake Friction Coefficients 8

3.3 Error Analysis 10

4. ANALYSIS: WET PAVEMENT, WET BRAKES 11

4.1 Kinetic Analysis 11

4.2 Error Analysis 11

4.3 Test Method Considerations 12

4.4 Discussion 13

5. RIDING TEST RESULTS 13

5.1 Effects of Wheel Rim Material 13

5.2 Error Analysis 16

6. CONCLUSIONS 18

7. REFERENCES 20

APPENDIX 23

1 . INTRODUCTION

The current bicycle safety regulations promulgated by the

U. S. Consumer Product Safety Commission (CPSC) include requirements governing braking performance in normal (dry) weather [1].*

These requirements specify a riding test** wherein the stopping

distance, which a bicycle is capable of achieving, is measured

under stipulated test conditions. Some of these test conditions

are based on analyses reported in NBSIR 75-786 [2]

,

which were

carried out by the National Bureau of Standards (NBS ) on an

earlier version of the safety regulations [3].

Requirements for a similar riding test method are being

developed by the International Organization for Standardization

(ISO) [4]

.

In addition, ISO is pursuing a modification of the

test method which would allow bicycle braking performance to

be evaluated under simulated wet-weather conditions. CPSC has

also expressed interest in this approach, and this report is

intended to provide the Commission with some basic analyses of

the kinetics and the errors involved in riding tests for measuring

the braking performances of bicycles under wet conditions.

This study was carried out for the NBS Office of Consumer

Product Safety at the request of CPSC.

1.1 Terminology

Since this report is an extension of NBSIR 75-786, the

terminology and definitions developed therein are also used here.

Some of these are explained here; for other relevant definitions,

and for the rationales behind them, the reader is advised to

consult the earlier report.

For the sake of specificity, this study is principally concern

ed with a typical bicycle having handlever-operated caliper brakes

on both wheels, and marginal braking capability. The essential

characteristics of the typical bicycle [2] are reproduced in Table 1

3

Numerals in square brackets refer to similarly numbered

references listed in Section 7 of this report.

* The term "riding test" is used in this report to distinguish

it from dynamometric or other performance tests which

are usually conducted in a laboratory. The preferred term,

"road test", is not used in order to avoid confusion with

the specific road test which is cited in the CPSC regulations

for a different purpose.

- 1-
Table 1. Characteristics of the Typical Bicycle [2]

Mass of bicycle 18 kg

Mass of bicycle frame 14 kg

Mass of bicycle wheel 2 kg

Wheel diameter 660 mm

Wheel base 1000 mm

(39.7 lb)

(30.9 lb)

( 4.4 lb)

(26.0 in)

(39.4 in)

Coordinates of the center of gravity of the bicycle with an

average 68.1-kg (150-lb) rider:

Distance aft of the

front wheel axis

Distance above the pavement

600 mm

8 50 mm

(23.6 in)

(33.5 in)

- 2 -
A bicycle exhibits marginal braking capability [2] when it is

brought to a stop over a distance of exactly 4.57 m (15.0 ft)

under the standard test conditions. (This is the pass-fail

criterion specified by the bicycle regulations [1] for dry conditions.)

The standard test conditions [2] include: an average 68.1-kg

(150-lb) test rider; an initial speed of 24 km/h (15 mph) ; the

application of 178-N (40-lbf) forces to the handbrake levers; and

a dry, clean, level, paved test course which offers a uniform

coefficient of friction of 0.75 at the tire/pavement interface

of a locked bicycle wheel. (These conditions are identical to

those specified in the bicycle regulations [1] except that the

latter permit both heavier riders and a range of friction coefficients .

)

An average test rider is one of average physical characteristics who provides an average reaction time of 0.15 seconds [2],

Reaction time is the interval between the instant at

which the rider first begins actuating the handbrake levers

and the instant at which the prescribed force levels are

reached [2] .

1 . 2 Scope

Within the scope of a riding test under wet conditions,

several alternatives are available. The pavement may be

wetted, the wheel rims (which are engaged by the caliper brake

pads) may be wetted, or both the pavement and the wheel rims

may be wetted. Sections 2, 3 and 4 of this report analyze each

of these three options, in turn, on the basis of theoretical considerations and available laboratory-type test data. The

results of actual riding tests are then examined in Section 5.

2. ANALYSIS: WET PAVEMENT, DRY BRAKES

2.1 Kinetic Analysis

Consider, first, an idealized riding test in which the

pavement is wet but the brakes are dry. The presence of the

water changes the friction coefficient at the tire/pavement

interfaces from that which prevailed under dry conditions. For

a typical bicycle which has marginal braking capability under

dry conditions, the effect of the friction coefficient on

stopping distance is illustrated in Figure 1. The curve plotted

in the figure was calculated from Equations (20), (25) and (28)

of NBSIR 75-786.

The curve is characterized by a discontinuity in the slope

at a friction coefficient of 0.547. Under the standard test

- 3 -
conditions this point represents the changeover, for the front

wheel, from locked-wheel skidding (on the left) to rolling

without slipping (on the right) . The rear wheel skids without

rolling throughout the range of coefficients shown. Note

that variations of the friction coefficient above the changeover

point do not cause major changes in stopping distance. However,

for friction coefficients less than 0.547, small variations of

the coefficient cause large variations of stopping distance.

In view of- the desirability of achieving repeatable test

results, it is clearly of interest to know whether the wet

friction coefficients which are likely to be encountered in

actual test situations are smaller or larger than 0.547.

0

:

2.2 Tire/Pavement Friction Coefficients

Friction coefficients for bicycle tires on wet pavements

have not been widely reported in the literature. However, data

obtained with automotive and aircraft tires provide some

guidance, particularly in view of the fact that tread patterns

do not exert a significant influence on the wet braking

capabilities of bicycles [5]

.

Mechanical engineering handbooks state that for rubber tires

a friction coefficient of 0.7 is commonly used with dry asphalt

and a value of 0.8 with wet asphalt [6, 7]. However, more

thorough treatments of the subject, based on test results obtained

with automobiles, reveal that the situation is not nearly so

well defined, and that the friction coefficients are generally

lower when wet than when dry. According to the British Road

Research Laboratory [8], for example, wet asphalt and concrete

pavements typically offer friction coefficients of 0.5 "to 0.7,

but values as low as 0.1 have occasionally been observed. On

the other hand, a value of 0.8 is considered to be characteristic

of a good dry road surface.

A review by Whitt and Wilson [5] suggests that friction

coefficients of the order of 0.1 are generally indicative of

icy, rather than wet, pavements. They report that for motor

cars on concrete or asphalt pavements, typical friction coefficients range from about 0.8 to 0.9 under dry conditions and

from about 0.4 to 0.7 when wet.

Obviously, the observed variations in wet friction coefficients result, in part, from differences in the pavement surface

characteristics. However, even for a specific road surface there

is no single value of wet friction coefficient that can be assigned

to it. Rather, as described below, it has been found that the

wet friction coefficient varies with the speed of the vehicle,

the depth of the water, the ambient temperature, and the cleanliness of the surface. For this reason, the friction values

- 4 -
cited above, while helpful in a general sense, do not provide

the guidance that is needed to estimate wet friction coefficients

in specific situations.

It has been well established, from tests with both automotive

and aircraft tires, that wet friction coefficients increase

with decreasing speed [5,8,9,10]. Thus, the friction coefficient

may be expected to vary continuously during a braking test from a given initial speed. However, it has been shown that stopping

distances may be calculated with reasonably good accuracy by

using, as an effective coefficient, that which corresponds to

two-thirds of the initial speed [8] . Thus, to calculate the

stopping distance of a bicycle from 24 km/h (15 mph) , it would

be appropriate to use the tire/pavement friction coefficient

which prevails at 16 km/h (10 mph)

.

The effect of water depth is more complex. Slightly dampened

pavements offer friction coefficients which are significantly

less than those obtained with dry pavements. However, as the

water depth is increased there is sometimes a tendency for the

effective friction coefficients to rise, due to fluid drag. Thus,

for example, with aircraft tires on a concrete test runway which

offered a dry coefficient of approximately 0.4 at 16 km/h

(10 mph),* reductions of the order of 0.1 were obtained with less

than 0.3 mm (0.01 in) of water. But with depths of 5 to 8 mm

(0.2 to 0.3 in) the measured coefficients were higher than this,

approaching the dry values [9] . A smaller increase in friction

with increasing water depth was observed in another, similar,

investigation. In that work the average friction coefficient

at 16 km/h (10 mph)* was found to be 0.56 on a damp runway with

no visible standing water, and 0.59 when the runway was dammed

and flooded to a depth of 6.4 mm (0.25 in) [10].

It would appear that if riding tests are to be performed on

wet pavements it would be desirable to maintain a uniform water

depth. In this connection, it has been reported that on a well-drained road, during and after a rain, the thickness of the

water film in a typical instance may vary between 0.5 and 0.08 mm

(0.02 and 0.003 in) [8], Perhaps with this in mind, the ASTM

method for measuring the skid resistances of highways calls

for a uniform water depth of 0.5 mm (0.02 in) [11]. Unfortunately,

NBS researchers were forced to conclude, after several years of

experimentation, that a completely satisfactory pavement-wetting

system does not appear to be available at this time [12].

The effect of temperature on wet friction coefficients has

not been studied extensively, but one investigation showed

changes of almost 0.05 for only a 10°C (18°F) change in

temperature [8] .

it These data were interpolated from test results at lower and

higher speeds.

- 5 -
J

It is intuitively obvious that mud, loose aggregate and

similar kinds of dirt can affect the coefficient of friction

at tire/pavement interfaces. Quite appropriately, therefore,

the CPSC and ISO test methods require that the road surfaces

be clean [1,4]. It is, perhaps, not quite so obvious, however,

that the tenacious rubber deposits, which accumulate on pavements

from braking tests, can also influence the friction coefficients.

Typical results from one investigation [13] , for example,

showed that on a runway with significant rubber deposits the

average dry and wet friction coefficients were 1.0 and 0.7,

respectively. After the deposits were removed the dry and wet

coefficients were 0.9 and 0.8, respectively. It would appear that

allowing rubber deposits to build up on a test pavement could

adversely affect the reproducibility of test results within a given laboratory.

test

to-i

cari

2 . 3 Systematic Errors

The CPSC test method for dry braking requires that the

coefficient of friction at the tire/pavement interfaces be less

than 1.0, yet large enough to avoid front-wheel locking [1].

For the typical bicycle this establishes the permissible range

as 0.547 to 1.0. (A typical value of 0.75 was used for calculation purposes [2].) For test pavements which meet these requirements under dry conditions, it must now be estimated what the

applicable friction coefficients are when the pavements are wet.

The data reviewed above suggest that when the dry coefficient

is low, the wet coefficient tends to be only about 0.1 lower.

However, when the dry coefficient is high, the reduction due to

wetting is sometimes greater. An extreme example of the latter,

from the work of Mortimer and Ludema*, shows a 50-percent

reduction, from 0.70 for a dry pavement to 0.35 for the same

pavement when wet. On the basis of these observations it does

not appear to be inconsistent to expect the range of wet friction

coefficients of bicycle test pavements to extend from about 0.4

to 0.8, with a typical value (corresponding to 0.75 when dry)

of about 0.6. This selection of a relatively broad range of wet

coefficients is supported by data which reveal that a smooth

concrete surface and a fine-textured asphalt surface, both of

which exhibited dry coefficients of approximately 0.75, showed

wet coefficients that differed by about 0.2 [8]. Furthermore, the

most sophisticated measuring techniques, when applied to a series

of specially prepared highway test surfaces, revealed a distribution

of wet coefficients ranging from about 0.57 to about 0.83 at

16 km/h (10 mph) [15]

.

J

As cited in Reference 14.

-6-
The differences in stopping distances obtained on different

test surfaces, as a result of differences in wet friction coefficients, represent systematic errors which inhibit the labto-lab reproducibility of test results. The stopping distances

which correspond to the probable range of wet friction coefficients

can be determined from Figure 1 for the typical bicycle with

marginal braking capability:

Wet coefficient

of friction Stopping distance

m “TTtT

0 . 4 (min .

)

6.23 (20.4)

0.6 (typ.) 4.65 (15.3)

0.8 (max.) 4 . 55 (14.9)

Note that the changes in stopping distance due to variations of

the friction coefficient above 0.6 are relatively small, but

that low coefficients produce large increases in stopping

distance. These data can be combined to indicate an average

systematic error of the order of ±0.84m (±2.8 ft) about the

typical stopping distance.

2.4 Random Errors

There is reason to expect that the wet coefficient of

friction of a bicycle test pavement may change from one test

to the next even though precautions are taken to keep the

pavement clean. Five measurements made with a single bicycle

on a single wet test surface revealed tire/pavement friction

coefficients ranging from 0.48 to 0.61 [16]. While some of this

variation was probably only apparent, due to variabilities in

rider reaction times, some real friction variations are probably

unavoidable, due to changes of temperature and water depth. A

variability of ±0.05, centered about the mean wet friction

coefficient of the surface, would appear to be a reasonable

estimate

.

If the mean coefficient is on the flat part of the curve

in Figure 1, the resulting variations in stopping distance

are not serious. However, if the mean coefficient is less

than 0.547, then the test-to-test variations in stopping distance

are significant. For example, if the mean wet coefficient is

0.45, then a variability of ±0.05 in the coefficient produces a variation of approximately ±0.58m (±1.9 ft) in stopping distance.

The effects of this test-to-test variability are fortunately

mitigated by the CPSC requirement that only the average of four

test results need be reported [1]; the average of four independent

measurements has twice the precision of a single measurement [17].

- 7 -
3. ANALYSIS: DRY PAVEMENT, WET BRAKES

3.1 Kinetic Analysis

Now consider an idealized riding test in which the caliper

brake pads and the wheel rims are wet but the tires and the

pavement are dry. The presence of water at the brake surfaces

reduces the brake friction and thereby results in increased

stopping distances.

Note that the important parameter here is not simply the

wet coefficient of friction at the brake surfaces but, rather,

the ratio of the wet coefficient of friction to the dry coefficient

of friction. Even a braking system with a low friction coefficient

can be made effective by providing a suitably high mechanical

advantage in the braking system linkage. This mechanical advantage

is fixed by the design of the bicycle and, unless the wet-to-dry

brake friction ratio is close to unity, a bicycle designed to

provide adequate dry braking will exhibit inadequate braking

performance under wet conditions. Conversely, a bicycle designed

for adequate wet braking will be overly sensitive under dry

conditions if the friction ratio is low. Excessive dry braking

capability introduces a pitchover hazard. Thus, for good braking

performance, both wet and dry, it is desirable that the brake

friction ratio be as close to unity as possible.

For a typical bicycle which has marginal braking capability

under dry conditions, the effect of the brake friction ratio, wet

to dry, is shown in Figure 2. The curve plotted in the figure

was calculated from Equations (1), (15), (20) and (28) of

NBSIR 75-786.

The curve is somewhat similar in appearance to that shown

in Figure 1 in that it contains a discontinuity in the slope. In

this case, however, the discontinuity represents a changeover in

the mode of braking for the rear, rather the front, bicycle wheel.

The changeover occurs at a brake friction ratio of 0.422. For

lower ratios the rear wheel rolls without slipping while for

higher ratios it skids without rolling. The front wheel rolls

without slipping throughout the range of brake friction ratios

plotted

.

Since small variations in the brake friction ratio below

the changeover point cause rather large changes in stopping

distance, it is of interest to ascertain whether the

brake friction ratios in actual tests are below or above the

changeover point.

- 8 -
3.2 Brake Friction Coefficients

The coefficient of friction between caliper brake pads

and bicycle wheel rims depends, of course, on the brake pad

and wheel rim materials and on their surface conditions, as

well as on whether they are wet or dry. Handbook-type

information specifies dry coefficients of 0.35 to 0.40 for

asbestos-fabric brake material against a cast iron brake drum

[7], 0.3 to 0.5 for leather to metal [5], and approximately 0.7

for brake-lining materials against cast iron or steel [5]

.

A

more thorough study of dry friction in caliper brakes was provided

by a bicycle manufacturer who tested brake pads of ten different

materials against a steel bicycle wheel rim [18] . Using the

data from these tests it may be shown by calculations that the

dry coefficients of sliding friction ranged from 0.22 for the

least effective brake material to 0.61 for the most effective.

These two materials were both proprietary products. Three

different kinds of rubber brake pads exhibited coefficients of

0.48.

Several investigations of bicycle brake pad materials have

been conducted to evaluate the reduction in the friction coefficients that are produced by wetting. In one of these [19] , seven

brake pad materials were examined against a plated steel wheel

rim and the results were not entirely consistent with those reported

above. In this case, rubber brake pads were found to offer the

highest dry friction coefficient (0.95) and the lowest wet friction

coefficient (0.05). The brake friction ratios, wet to dry,

varied from a low of 0.05 for rubber to a high of 0.50 for

a proprietary product.

The difference between the dry coefficients for rubber

brake pads, as given in the above two paragraphs, may stem from

differences in the shapes, compositions or surface conditions

of the brake pads or the wheel rims. This is evident from the

results of a set of tests, involving twenty different kinds of

rubber brake pad materials, which was performed on a dynamometer

bench [20]. Comparative stopping distances, wet and dry, were

measured. Since rider reaction times do not enter into this

type of measurement, the observed stopping distances are

inversely proportional to the brake friction coefficients. Thus,

it may be shown that the brake friction ratios, wet to dry,

varied from 0.29 to 0.44 for the twenty materials against a smooth steel wheel rim, and from 0.32 to 0.61 for the same

materials against a smooth aluminum-alloy wheel rim.

Another set of dynamometer tests, involving four different

kinds of unidentified brake pad materials, was carried out

using a striated steel rim [21] . In this case the brake

friction ratios, wet to dry, varied from only 0.13 to 0.19.

Intuitively, it would seem that the introduction of striations,
or other kinds of roughening, onto wheel rim surfaces would

tend to provide increased brake friction coefficients. It

has been reported, for example, that rust on the wheel rims

tends to raise the friction coefficients [22], and that unplated

wheel rims give 20-percent shorter braking distances than

chrome-plated rims under wet conditions [23]

.

This matter is

addressed further in Section 5.1.

3.3 Error Analysis

The scant data available on wet-to-dry brake friction

ratios show values ranging from 0.05 to 0.61. With such a wide

range it is difficult to select a "typical" value which can

be used as the basis for an error analysis that might be applicable to a typical bicycle. For want of a more meaningful

approach, the safety standard for motorcycles [24] has been used

as guidance. This standard requires the wet stopping distance

not to exceed 2.25 times the dry stopping distance. For bicycles,

the dry stopping distance from an initial speed of 24 km/h

(15 mph) is required to be no more than 4.57m (15.0 ft) under

the standard test conditions [1], If, therefore, the wet stopping

distance is taken as 2.25 this value, or 10.28m (33.7 ft), then

Figure 2 shows that the applicable wet-to-dry brake friction ratio

is 0.25.

As pointed out above, the brake friction ratio depends

on the brake pad and wheel rim materials, and these, of course,

are independent of the laboratory conducting braking performance

tests. Hence, for a given bicycle, it is not expected that there

would be any systematic error that would be attributable to

variabilities in brake friction from one lab to another. This, of

course, assumes that the testing procedure will have been

adequately standardized with respect to the means by which

wetting will be achieved.

On the other hand, it may be expected that there would be

random variations in the brake friction ratio from test to

test, due to minor changes in water film thicknesses, wear,

and cleanliness conditions. It would seem that a random

variation of approximately ±0.05, about the average brake friction

ratio, would be reasonable. This would introduce variations

of the order of ±2.0m (±6.6 ft) about an average 10.28-m (33.7-ft)

wet stopping distance. Such test-to-test variations are

disturbingly high, even when mitigated by a requirement that only

the average of four test results need be reported.

- 10 -
4. ANALYSIS: WET PAVEMENT, WET BRAKES 4.1

Kinetic Analysis

When both the pavement and the brakes are wet, reductions

in the friction coefficients may be expected at both the tire/

pavement interfaces and the brake surfaces. According to the

ISO working group which is concerned with wet weather braking,

"...the water film between the [brake] block and the rim [is]

by far the most important factor having a much larger effect

on the stopping distance than water on the road braking surface,"

and the group decided, therefore, that a dry surface would be

used for all wet braking tests [25]

.

At least one foreign

delegate to this group expressed a dissenting opinion, namely,

that the coefficient of friction at the road surface is the

most important parameter [26]

.

Performance calculations, based on Equations (1), (15),

(20) and (28) of NBSIR 75-786, tend to support the majority

ISO opinion. The effects of both friction sources, on the wet

stopping distance under otherwise standard test conditions, is

shown in Figure 3 for the typical bicycle which exhibits marginal

braking capability when dry. The location of the discontinuity

in the curve, in this case, is seen to depend upon the friction

coefficient y at the tire/pavement interfaces. For low values

of the wet-to-dry brake friction ratio both wheels of the bicycle

roll without slipping and the stopping distance is independent

of y. For higher brake friction ratios the rear wheel skids

without rolling and the front wheel rolls without slipping.

In this situation the stopping distance is influenced by both

the brake friction ratio and the tire/pavement friction coefficient,

with the former having the greater effect. Another discontinuity

appears in the curve for y = 0 . 4 at a brake friction ratio of

0.63. For brake friction ratios above this value both bicycle

wheels skid without rolling and the stopping distance is

independent of the brake friction ratio.

4.2

Error Analysis

An estimate of the errors involved in testing with both

wet pavement and wet brakes can be obtained by combining the

error estimates which were developed for wet pavements (Sections

2.3

and 2.4) and for wet brakes (Section 3.3). Using a squareroot-of-the-sum-of-the-squares approach, it is found that the

combined systematic error is ±0.84m (±2.8 ft) and the combined

random error is ±1.8m (±5.9 ft). The systematic error

influences the lab-to-lab reproducibility and the random error

affects the degree of reproducibility within a single laboratory.

- 11 -
(These error estimates do not include the variabilities in

rider reaction times and rider masses which would be expected

to be the same in wet braking tests as in dry braking tests.)

4

4.3 Test Method Considerations

Almost all of the riding test methods, which have been used

or proposed to evaluate the wet braking capabilities of bicycles,

have called for the wetting of the brakes, with or without

simultaneous wetting of the pavement. These methods include:

a. Riding the bicycle through a long water trough prior to

commencement of the braking test [27, 28].

b. Spinning the wheels in a water trough prior to the test [29]

c. Spraying the brakes and rims prior to the test [30, 31].

d. Spraying the wheels and the pavement prior to the test [15]

.

e. Spraying the brakes and rims during the test [25]

.

f. Spraying the brakes and rims during the test and conducting

the test on a previously moistened pavement [32].

g. Spraying the bicycle and the pavement both prior to and

during the test [33],

While the differences between these various testing approache:

would appear to offer difficulties in comparing test results,

this is not entirely the case. Even without any conscious

intent to moisten the test pavement, but only the brake surfaces,

it is likely that some water will flow onto the tires from the

wheel rims, and penetrate the tire/pavement interface. As shown

in Section 2.2, very little moisture need be present at the

interface in order to change the friction coefficient from the

dry value to the wet value. In fact, photographs show considerable

water accumulation on the pavement surface after a number of

tests have been run [27]. For this reason, tests involving wet

brakes and dry pavement are actually only idealizations; in

reality, wet brake tests should generally be considered as

involving wet pavements as well.

It has been suggested that when brakes are wetted

before rather than during tests, small-wheeled vehicles may

show better braking performance because more wheel revolutions

for a given speed tend to wipe more water off [31]. However, tests

have shown that when wet brakes are applied with full pressure,

and no additional water is added, the brake friction coefficient

does not begin to recover until the wheel has completed at least

-12-
four revolutions and frequently many more [19] . For a bicycle with

26-in (660-mm) wheels this represents a minimum distance traveled

of 8.3 m (27 ft). Substantially longer distances (40 m (30 ft) or

more) are required in order to attain 90-percent recovery of the

brake friction coefficient [19]

.

4.4 Discussion

The kinetic analysis (Section 4.1) showed that the presence

of water at the brake surfaces is generally more detrimental to

braking performance than water at the tire/pavement interfaces.

On this basis it is recommended that road tests for evaluating

the wet braking performances of bicycles should include the

wetting of the brake surfaces.

The considerations discussed in Section 4.3 suggest that it

is also of no major significance whether the water is introduced

onto the brake surfaces before or during the test so long

as these surfaces are wet when the brakes are applied. In

fact, it would seem that all of the seven test methods listed

in Section 4.3 might be expected to yield essentially equivalent

results if applied to a given bicycle, assuming that all of

the other aspects of the testing procedure are the same. This

observation provides the basis for the following section of this

report, which examines the results of riding tests carried

out with a variety of wetting practices to evaluate the wet-braking

performances of bicycles.

5. TEST RESULTS

Table 2 presents a compilation of available riding test

data on the wet braking performances of bicycles. The test

methods used were those cited in Section 4.3. Where directly

comparable data were acquired under dry conditions, these are

included in the table.

5.1 Effects of Wheel Rim Material

The effects of the wheel rim material can be assessed

from the test results shown in Table 2 where different rims

were used on the same bicycles. Considering, first, rims which

had been neither serrated** nor embossed, it is found that

Additional data, received more recently, involve some rather

specific test conditions and will be addressed in a subsequent report.

* * Strictly speaking, serration is an edge treatment. However,

when applied to wheel rims, the term is commonly used in

the bicycle industry to describe a surface treatment.

- 13 -
Table

2.

Results

of

Wet

(and

Dry)

Riding

Tests

. 1

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15

(e)

Data

listed

for

Bike

Nos.

117

through

121

are

midpoints

rather

than

averages.

(f)

Ratio

of

wet-to-dry

stopping

distances.

(g)

Material

not

identified.
aI aluminum rims provided substantially shorter stopping distances

than the various kinds of steel rims, whether plated or not.

In fact, of the seven bicycles tested with aluminum rims, six

showed average wet stopping distances not exceeding 10.28 m

(33.7 ft), which is the tentative criterion set forth

in Section 3.3. By contrast, all but one of the bicycles

with other rim materials exhibited average wet stopping

distances which were longer than this; for many of them

the wet stopping distances exceeded 18.29 m (60.0 ft).

The one bicycle without aluminum rims that had a wet

stopping distance less than 10.28 m (33.7 ft), Bike No.

108 A-Cr , had a dry stopping distance of only 3.05 m

(10.0 ft). This implies a deceleration of 0.9 g, well

above the pitchover threshold for many bicycles [2]. In

general, the dry stopping distances tended to be slightly

longer with aluminum rims than with steel rims although

they were nevertheless acceptable (i.e., less than 4.57 m

(15.0 ft) )

.

Looking at the data in a different way, it is seen that of

the four bicycles with aluminum rims that were tested both v;et

and dry, three had wet-to-dry stopping distance ratios of

less than 2.25. By contrast, no bicycle with another rim

material had a wet-to-dry stopping distance ratio of less than 2.7.

As pointed out earlier, wet stopping distances of 10.28 m

(33.7 ft) or less correspond to wet-to-dry brake friction ratios

of 0.25 or greater. If, as the data suggest, brake friction

ratios greater than 0.25 are achievable with aluminum-alloy

rims, then Figure 3 shows that it is possible for the friction

coefficient at the tire/pavement interfaces to affect stopping

distance even when the brake surfaces are wetted. In the interest

of achieving consistent test results, therefore, it would seem

advisable to wet the pavement intentionally in order to avoid

a situation where the first test is conducted on dry pavement

and successive tests on increasingly wetted pavements.

Consider, now, the effects of embossing and serrations

on the wheel rims. The data in Table 2 show that, on the

whole, these treatments did not improve wet braking performance.

This conclusion has also been put forth by a manufacturer, who

found that "embossed rims showed no pronounced advantage

and the most effective shredded the brake pads" [29]. Dimpling

of rims has also been found to be ineffective in wet conditions

[23] .

5.2 Error Analysis

The variations in the wet stopping distances of bicycles

with plain aluminum-alloy rims are given in Table 3 in terms

of the observed ranges (or spreads) in the measured stopping

distances. (Two of the bicycles listed in Table 2 with

aluminum-alloy rims are not included in Table 3 because the

data available for these two were inadequate to determine the

ranges in the wet stopping distances.)

- 16 -
Table 3. Variations in Wet Stopping Distances

Caliper brakes, front and rear.

Plain aluminum-alloy wheel rims.

Initial speed, 24 km/h (15 mph)

.

Wet stopping distance

Bike No. of

No

.

tests average range m (ft) m (ft)

107B 3 7 . 90 (25.9) 1.83 (6.0)

108A 10 6.06 (19.9) 0.32 (1.0)

108B 10 12.78 (41.9) 0.16 (0.5)

109 (a) 3 7.53 (24.7) 3.03 (9.9)

109 < a ) 3 5.76 (18.9) 0.98 (3.2)

112A< b ) 5 9.8 (32. ) 1.8 (6. ) 112A(b) 5 8.2 (27. ) 1.5 (5. ) 112A(b) 5 9.1 (30. ) 1.5 (5. )

( a ) The six wet braking tests of this bicycle with plain aluminumalloy rims, reported in Table 2, were performed in two groups

of three each, as shown here

.

(b) The fifteen wet braking tests of this bicycle with plain

aluminum-alloy rims, reported in Table: 2 , were performed in

three groups of five each, as shown here.

- 17 -
The variations in stopping distance for the test

series conducted with Bike No. 108 are exceedingly small,

and may well constitute an example of what is attainable with

good testing technique and careful attention to detail. The

other test series listed in Table 3 exhibit wet stoppingdistance ranges of approximately 1 to 3 m (3 to 10 ft) for

series of three to five tests each. These variations are somewhat smaller than the random error estimate developed in Section

4.2 and suggest, therefore, that the test-to-test variability of

the wet-to-dry brake friction ratio is not as large as had been

estimated (i.e., ±0.05). On the other hand, the observed

variations in wet stopping distance generally exceed those for

dry braking tests performed with reasonably good testing technique** and thus provide evidence that the random variability

in the brake friction ratio, while small, is not negligible.

An estimate of the lab-to-lab reproducibility of test

results requires an assessment of the systematic errors involved

in the test method. The available test results do not permit

such an assessment since the different laboratories tested

different bicycles. On the basis of the discussion in Section 3.3,

however, it would appear that the systematic errors in wet

braking tests may not be significantly different from those

in dry braking tests. As shown in NBSIR 75-786, the probable

overall range of the systematic errors is 1.24 m (4.1 ft) for the

CPSC test method.** Most of this error band, specifically 1.05 m

(3.4 ft), is attributable to allowed variations in test rider

mass. In this connection it may be noted that wet braking tests

carried out under ISO auspices call for all tests to be performed

with the same total mass of bicycle, rider and onboard instrumentation [26,39]

.

This may be expected to lead to rather good labto-lab reproducibility for a given bicycle.

6. CONCLUSIONS

The study reported herein was concerned with riding tests

to evaluate the wet-braking performances of bicycles having

caliper brakes on both wheels. The kinetics of three different

testing approaches were analyzed theoretically, relevant

literature was reviewed, and available test results were critically

examined. The following conclusions appear to be justified.

1. The stopping distances of bicycles are increased under

wet-weather conditions because the friction coefficients

at the brake surfaces and at the tire/pavement interfaces

are reduced when wet.

System C [ 2 ]

.

* *System B [2 ]

.

- 18 -
2 . The presence of water at the brake surfaces generally has

a greater effect on wet stopping distance than water at the

tire/pavement interfaces. On this basis it is recommended

that road tests to evaluate the wet braking performances

of bicycles should, if they are to be meaningful, include

the wetting of the brake surfaces. With wet brake

surfaces it is generally of no consequence whether or not

the pavement is also wetted, unless the caliper brakes

show a high degree of water resistance. In the latter

instance, water at the tire/pavement interfaces can also

influence the stopping distances. Since test pavements

tend to become wetted due to water runoff from the

bicycle wheel rims, it is further recommended, in the

interest of reducing random errors in the test method,

that the test pavements be intentionally wetted as well.

3. Aluminum-alloy wheel rims provide substantially better

braking performance, when wet, than steel or chrome-plated

rims. Embossing, serrating or dimpling of rims does not

consistently improve wet braking performance.

4. A stopping distance of 10.28m (33.7 ft) is tentatively offered as a criterion for evaluating caliper-braked bicycles

under wet conditions. This assumes a test method conforming to present CPSC requirements except for the wetting.

At the present state of the art this wet stopping distance

appears to be safely attainable only with bicycles having

aluminum-alloy wheel rims. In other words, a conventional

bicycle with steel wheel rims that could meet this wet

braking criterion would probably exhibit excessive braking

capability when dry.

5. The random variations in the wet stopping distances of

bicycles are greater than the random variations that

exist under dry conditions. The differences appear to

be attributable to a test-to-test variability of the

friction coefficients at the wet brake surfaces.

6. There is some reason to expect that the lab-to-lab

reproducibility of wet braking test results may not be

significantly different, for a given bicycle, from the

reproducibility of dry braking test results. However,

there are no test data available to confirm or refute this

conclusion

.

- 19 -
7

.

REFERENCES

1. Dunn, S. E., Bicycle Safety Regulations as Amended, Federal

Register 41, No. 19, pp. 4144-4154 (National Archives of

U.S., Washington, Jan. 28, 1976).

2. Mordfin, L., The CPSC Road Test of Bicycle Braking Performance

Kinetic and Error Analyses, NBSIR 75-786 (Oct. 1975) with

Addendum (Dec. 1975).*

3. Dunn, S. E., Bicycle Safety Regulations, Federal Register

40 , No. 220, pp. 52815-52825 (National Archives of U.S.,

Washington, Nov. 13, 1975).

4. International Organization for Standardization, 4th Draft

Proposals ISO/DP 4210 for Safety Requirements of Bicycles,

Document No. ISO/TC 149/SC 1 (Secretariat - 29) 77E

(British Standards Institution, London, June 1976).

5. Whitt, F. R. and Wilson, D. G. , Bicycling Science (MIT

Press, Cambridge, Mass., 1974).

6. Doughtie, V. L. , Mechanisms and Machine Elements, Section 7

of Kent's Mechanical Engineers' Handbook , Design and Production

Volume, C. Carmichael (ed) , 12th edition, p. 7-30 (John Wiley

& Sons , N.Y. , 1961)

.

7. Fuller, D. D. , Friction, Chapter 3 in Marks' Standard

Handbook for Mechanical Engineers , T. Baumeister (ed) , 7th

edition, p. 3-39 (McGraw-Hill Book Co., N.Y., 1967).

8. Road Research Laboratory (U.K.), Research on Road Safety

(Her Majesty's Stationery Office, London, 1963)

.

9. Dreher, R. C. and Yager, T. J., Friction Characteristics

of 20x4.4, Type VII, Aircraft Tires Constructed with

Different Tread Rubber Compounds, NASA TN D-8252 (July 1976).

10. Dreher, R. C. and Tanner, J. A., Experimental Investigation

of the Braking and Cornering Characteristics of 30x11.5-14.5,

Type VIII, Aircraft Tires with Different Tread Patterns,

NASA TN D-7743 (Oct. 1974).

11. American Society for Testing and Materials, Standard Method

of Test for Skid Resistance of Paved Surfaces Using a Full-Scale Tire, ASTM Designation E274-70, 1976 Annual

Book of ASTM Standards, Part 15 (1976).

T

See errata sheet, page 27.

- 20 -
12. Kearns, R. W. , and Ward, J. F., Skid Resistance Measurement

Test Procedures for Intercomparing FHWA National and Regional

Reference Systems, NBSIR 76-1174 (Dec. 1976).

13. Hiering, W. A., and Grisel, C. R. , Runway Friction Changes

due to High-Pressure Water-Jet Cleaning Operations,

Houston Intercontinental Airport, Houston, Texas, Report

No. FAA-RD-75-12 9 (Federal Aviation Administration,

Washington, Sept. 1975).

14. Kearns, R. W. , and Ward, J. F., Skid Resistance Measurement

Tests of FHWA Reference Systems at the Western Field Test

Center, NBSIR 76-1133 (Oct. 1976).

15. Kearns, R. W. , and Ward, J. F., Skid Resistance Measurement

Tests at the FHWA Eastern Field Test Center, NBSIR 75-972

(July 1976)

.

16. Anon., Wet Braking Performance of Cycle Braking Systems,

Report Reference BR 36 H, Raleigh Industries Ltd.

(Nottingham, England, March 4, 1976).

17. Eisenhart, C., Realistic Evaluation of the Precision and

Accuracy of Instrument Calibration Systems, J. Res. NBS

67C, No. 2, pp. 161-187 (April-June 1963).

18. Hesketh, G. P., An Investigation into Performance Testing

of Bicycle Brakes, Raleigh Industries Ltd., Report int. 90

(Jan. 22, 1975) with continuation sheet (May 17, 1977).

19. Hanson, B. D., Bicycle Rim Brake: Investigation of a Promising Friction Material and Development of Performance

Specifications, Bachelor of Science thesis, MIT (Jan. 1971).

20. Anon., Bremswirkung von ver schiedenen Gummiprof ilen

,

[Weinmann] , Dec. 12, 1975.

21. Anon., [Report to] ISO/TC 149/SC 1/WG 2, T.6126 [France],

April 26, 1976.

22. Asbell, O. D., Jr., Bicycle-Brake Pads: Friction and Wear

Characteristics, Bachelor of Science thesis, MIT (June 1970).

23. Starks, H. J. H., and Lister, R. D. , Some Safety Aspects

of Pedal and Motor-Assisted Cycles, Road Research Technical

Paper No. 38 (Her Majesty's Stationery Office, London, 1957).

24. U.S. Department of Transportation, Motorcycle Brake

System, Federal Motor Vehicle Safety Standard No. 122

(Washington, DC, April 1972).

- 21 -
25. International Organization for Standardization, Report of

Working Group 2 on Wet Weather Brake Testing held Sunday,

9 May 1976, Document No. ISO/TC 149/SC 1 (Working Group 2)

73 (British Standards Institution, London, May 1976).

26. Edinger, K., unofficial minutes of meeting of ISO/TC

149/SC 1 held on Oct. 25, 1976.

27. McConnell, W. H. , and Parks, J. T. , Capability Criteria

of Bicycle Braking Systems, Engineering Report on Contract

FDA 73-6 (U. Of Iowa, Iowa City, Dec. 7, 1972).

28. International Organization for Standardization, Classification of Bicycles, Document No. ISO/TC 149/SC 1 (Canada - 1)

51 (British Standards Institution, London, July 1975).

29. Buttars , K. , Field Braking, File No. 241, Schwinn Bicycle

Co. (Chicago, No. 1973).

30. Parks, J. T., Letter to CPSC, Aug. 30, 1973 (U. of Iowa,

Oakdale)

.

31. Lewis, G. D., The Manoeuvrability and Braking Performance

of Small-Wheeled Bicycles when Ridden by Children, Report

LR 500, Transportation and Road Research Laboratory

(Crowthorne, Berkshire, U. K. , 1973).

32. Ibayashi, H. : Letter to Raleigh Industries Ltd., March

29, 1976 (Japan Bicycle Industry Association, Tokyo).

33. Anon., Proposal for Wet Braking Test Procedure, Weinmann

Ltd., May 7, 1976 (Schaffhausen , Switzerland.

34. Parks, J. T., Letter to Bureau of Product Safety, Dec. 13,

1972 (U. of Iowa, Oakdale).

35. Proprietary data.

36. Anon., Wet Braking Performance of Cycle Braking Systems,

Report Reference BR 36 H (Additional Report) , Raleigh

Industries Ltd. (Nottingham, England, April 8, 1976).

37. Anon., ISO - Wet Brake Tests, Predom-Romet (Poland, undated).

38. Edinger, K., private communication.

39. Anon., Wet Weather Brake Testing, document No. ISO/TC

149/SC 1/WG 2 (Secretariat 1) 1 (undated)

.

- 22 -
Table

A1

.

Bicycle

and

Testing

Details

for

Tables

2

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- 23 -
TIRE/PAVEMENT FRICTION COEFFICIENT

Figure 1. Effect of the tire/pavement friction coefficient

on the stopping distance of a typical bicycle which has

marginal braking capability under standard test conditions.

The rear bicycle wheel skids without rolling throughout

the range of coefficients plotted.

- 24 -

STOPPING

DISTANCE,

ft
80

70

60

50

40

30

20

10

0

Figure 2. Effect of the wet-to-dry ratio of brake friction

coefficients on the stopping distance of a typical bicycle

which has marginal braking capability under standard

test conditions. The front bicycle wheel rolls without

slipping throughout the range of ratios plotted.

- 25 -

STOPPING

DISTANCE,

ft
80

70

60

50

40

30

20

10

0

Figure 3. Combined effects of the tire/pavement friction

coefficient (y) and the wet-to-dry ratio of brake

friction coefficients on the stopping distance of a

typical bicycle which has marginal braking capability

under standard test conditions.

- 26 -

STOPPING

DISTANCE,

ft
Errata in NBSIR 75-786 [2]

2 .

In Equation 21, page 9, the parenthetical condition should

read

Reference 5, page 46, should read

International Organization for Standardization, Interim

Report from WG1 on Criteria of Brake Performance Test

as Agreed at the WG1 Meeting, February 1975, Document

No. IS0/TC14 9/SCI (Working Group 1-1) 38 (British

Standards Institution, London, undated).

I

-27-

NBS-1 14A iRtv 7-73)

U.S. DEPT. OF COMM.

BIBLIOGRAPHIC DATA

SHEET

1. PU'HLK ATI ON OR REPORT NO. 2. Gov't Accession

No.

3. Recipient’s Accession No.

4 . TITLE AND SUBTITLE 5. Publication Date Analyses of Riding Tests for Evaluating The Wet

Braking Performances of Bicycles

April 1977

6. Performing Organization Code

7. AUTHOR(S) 8 . Performing Organ. Report No. Leonard

9. PERFORMIN(

Mordf i n

. ORGANIZATION NAME AND ADDRESS 10. Pro ject/Task/Work Unit No. NATIONAL BUREAU OF STANDARDS

DEPARTMENT OF COMMERCE

WASHINGTON, D.C. 20234

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

15. SUPPLEMENTARY NOTES

13. Type of Report & Period Covered

final

14. Sponsoring Agency Code

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

bibliography or literature survey, mention it here.)

The Consumer Product Safety Commission has expressed

interest in the development of a riding test method for

evaluating the braking performances of bicycles in wet weather.

In this report three different testing approaches for caliperbraked bicycles are examined using kinetic analyses, a review

of the literature, and an evaluation of available domestic and

foreign test results. On the basis of the findings it is recommended that the riding test include the intentional wetting of

both the bicycle brakes and the test pavement; the former to

obtain meaningful results and the latter to enhance the repeatability of the test results. A tentative pass-fail criterion is

also offered, based on a maximum wet stopping distance which, at

this time, appears to be generally attainable only with bicycle

wheels having aluminum-alloy rims. Error analyses of the test

methods are presented.

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 semicolons) Bicycles; brakes, bicycle; braking, wet; error

analysis; friction, brake; friction, tire/pavement; kinetics, bicycle;

measurements, bicycle braking; road tests; standards, bicycle safety;

test m ethods, bicycle; w et braking

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