Method and Equipment for Continuous Measuring of the Coefficient of Friction at Incipient Skid GOSTA K U L L B E R G , National Swedish Road Research Institute, Stockholm, Sweden

The coefficients of friction were experimentally determined for rubber tires on road surfaces in relation to slip when a vbeel is braked. The method and equipment are described. The slip can be varied from 0 to 50 percent.

When measuring with this method, no brake is used. Hie test wheel is connected to the driving wheels of a truck by means of transmission imits. A variable transmission makes it possible to change continu­ously the gear ratio between the test wheel and the driving wheels. Thus i t is possible to measure the coefficient of friction continuously on longer distances along the road.

The paper discusses the difference between the coefficient of f r i c ­tion at incipient skid and at locked wheel. E:Q>erimental results are presented. This method is also used to measure the behavior of winter tires.

• THE NATIONAL Swedish Road Research Institute has studied the coefficient of friction between tire and road surfaces. The major items in this research of friction properties or skid resistance at the Institute have been (a) to develop equipment and testing methods, (b) to make exhaustive studies of skid resistance of many different surface types and test conditions, (c) to provide the backgroimd e^erience in developing standard pro­cedures for making skid resistance measurements, (d) to provide the basic information concerning factors that cause slippery road conditions, and (e) to determine construc­tion methods or corrective measures that should be a<te.pted to prevent slippery condi­tion.

The last item—corrective measures—is very important, especially in winter time when roads are covered with ice and snow. Because the winter is long in Sweden, the Institute has been able to spend a lot of time in studies of the friction problem on snow-and ice-covered roads and runways.

M E T H O D OF M E A S U R E M E N T

General considerations have led to the method of measuring the friction on one test wheel. The Institute has designed and constructed test equipment with a test wheel mounted on a lorry or car and with a test wheel mounted on a trailer towed by a lorry.

The basic principle of all these test vehicles is to measure the coefficient of friction at fully controlled slip ratio. Figure 1 shows the variation of the coefficient of friction, as a tire is braked from free rolling, with percentage slip vAiich is defined as

W , - 0 1

S = t • 100 in which

S = slip ratio in percent; o), = angular velocity of a wheel when free rotation is allowed at a certain

speed; and (A = angular velocity of that wheel when the brakes are applied at the same

speed (speed means the speed of the vehicle). 18

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SO PERCENTAGE SLIP

100 I

F R E E ROTATION LOCKED WHEEL

Figure 1. Characteristic curve of coef­f i c i e n t of f r i c t i o n and s l i p .

As a tire is braked the friction increases with slip ratio to a maximum at incipient skid and then decreases to a smaller value at fu l l skid (locked wheel).

The basic principle of the test wheel device often used by the Institute is shown in Figure 2. It is clear from the figure that the brake force F causes no effect at al l on the wheel load N if the trailing arm Is arranged parallel to the road surface. S the trailing arm is long, the effect by F on N of small angular variations may also be neglected. The brake force Fcan be measured either by applying a towing force transducer at the front end of the trailing arm or by applying a torque trans­ducer at the wheel axle. In the latter case, knowledge of the rolling radius of the wheel is necessary. The coefficient of friction is then easily calculated by dividing the brake force by the wheel load N.

The f i rs t designed equipment, friction test vehicle 1 (Fig. 3) had the friction-meas­uring device mounted on a truck. This equipment came into use in 1949. On this ve­hicle, the test wheel was connected to the driving wheels of the truck by means of trans­mission units. A variable V-belt transmission made it possible to change continuously the gear ratio between the test wheel and the driving wheels of the truck, hi this way it was possible to get the slip ratio fully controlled. As the truck was moved forward, the test wheel was forced to operate with positive slip, and thus it developed a braking force which was measured. With this device i t was possible to measure the friction coefficient at any slip ratio. Thus, it was easy to find the slip ratio at which the maxi­mum coefficient of friction occurred. From preliminary tests it was found that the maximum coefficient of friction occurred at a slip ratlb of 0.15 to 0.20.

By means of the transmission units the brake torque was transmitted from the test wheel to the driving wheels and developed a driving force. Because the braking force on the test wheel and the driving force work in opposite directions, the required towing force of the lorry is only the dif­ference between these forces, which is only a fraction of the force developed by the braking wheel.

With the tire continuously rotating at small slip ratios, the entire circumference goes through the contact area in short time, thus reducing the heat flux to each tread element. Furthermore, there is little sliding of the tread on the surface and hence little wear so continuous opera­tion in a braked condition is possible.

Hie method of transferring torque from braking wheel to driving wheel or wheels is a great advantage, hi this way it is possible to maintain a constant speed also on surfaces with high coefficient of friction.

FRICTION TEST VEHICLE 5 Friction test vehicle 5, shown in Fig­

ure 4, has the friction test wheel mounted Figure 2. Arrangement vice.

of test wheel de-

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Figure 3. Test v e h i c l e 1.

Figure it. Test v e h i c l e 5. ( l ) Test wheel. (2) Wheel load s p r i n g . (3) Measuring wheel. (it-) Watering tank.

on a lorry (the same basic principle as in test vehicle l ) . This test wheel has a con­trolled slip ratio between 0 and 50 percent and also can be totally locked. The friction force between the wheel and the road is measured with electrical devices, and the coef­ficient of friction can be calculated because the wheel load is known.

Mechanical Equipment of Vehicle The friction-measuring device is mounted on a lorry, which has a 120-hp petrol en­

gine. The highest driving speed during measurement is about 90 km per hr (55 mph). The maximum speed of the lorry is 105 km per hr.

The arrangement of the mechanical equipment is shown in Figure 5. The mechani­cal equipment of the test vehicle consists of the following groups: (a) test wheel with

V A R I A B L E V - B E L T T R A N S M I S S I O N - A N G L E D R I V E - \ C L U T C H

T A C H O M E T E R G E N E R A T O R S

E N G I N E

D I S C B R A K E - ' W A T E R P U M P - /

W A T E R N O Z Z L E T O R Q U E T R A N S D U C E R

T E S T W H E E L

- M E A S U R I N G W H E E L

Figure 5. Arrangement of mechanical equipment, t e s t v e h i c l e 5.

CO to

B A T T E R Y

D I S T A N C E M E A S U R I N G W H E E L

R E C T I F I E R R E L A Y 12 V D C

C O N V E R T E R I N S T R U M E N T P A N E L

Dtm/h l/mirt

V A R I A C

A L T E R N A T O R 2 2 0 V A C

A M P L F I L T E R A M P L E N G I N E

R E C O R D E R V A R I A B L E G E A R

P I L O T L A M P S

C L U T C H D I S C . B R A K E

T O R Q U E T R A N S D U C E R B R A K E C O N T R O L

P A N E L B R A K E F L U I D

S U P P L Y

M A S f r e r ^ALVE D E T E C T O R ON T H E R O A D W A T E R S U P P L Y

F L O W M E T E R

T - T A C H O M E T E R G E N E R A T O R

K - L I M I T S W I T C H

W H E E L L O A D D Y N A M O M E T E R

U t E S T W H E E L

W A T E R N O Z Z L E

R E C E I V E R

Figure 6. Principal c i r c u i t of e l e c t r i c a l equipment, test vehicle 5-

T R A N S M I T T E R

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suspension device, load spring, and vibration absorber, (b) variable V-belt transmis­sion, (c) disc brake, (d) measuring wheel with suspension device and load spring, (e) watering system, and (f) control system.

Test Wheel. —The test wheel is normally equipped with a tire of the size 5.90 - 15. It can also be equipped with a 7.50 - 14, 4 PR pavement standard t ire. The test wheel is carried by a trailing link. The wheel is located beside the frame, between the front axle and the rear axle of the lorry. A spring used for loading the test wheel is fitted be­tween a lever on the trailing link and the frame of the lorry. This spring can be stretched by means of a screw. Normally, a wheel load of about 300 kg (662 lb) is used. The stiffness of the spring corresponds to a spring constant of 0.6 kg per mm at the test wheel, and the natural frequency of the wheel device is 10 cps. Near the test wheel, a vibration absorber is attached to the trailing link. This is a dynamic vibration ab­sorber, which consists of a spring-suspended piston in a cylinder. The natural f r e ­quency of the piston combined with the spring is equal to the natural frequency of the test wheel. Thus, the vibrations of the wheel are damped.

Transmission. —The test wheel is connected to the driving wheels of the lorry by means of a number of transmission units. One of them is a variable V-belt transmis­sion, which makes it possible to change continuously the gear ratio between the test wheel and the driving wheels. The slip of the test wheel can be varied from 0 to 50 percent.

Disc Brake. — A disc brake is fitted in the shafting of the test wheel. This brake makes it possible to lock the test wheel. When the brake is to be used, the test wheel is disconnected from the driving wheels by a multiple disc clutch.

Measuring Wheel. —To render possible accurate measurements of the vehicle speed and of the slip of the test wheel, the lorry is equipped with a measuring wheel located behind the rear axle. The measuring wheel is carried by a trailing link and is pressed down by a spring against the road. As the measuring wheel has to overcome only small resistances, it may be considered to ro l l practically without slip.

Watering System. —During friction measurements, the road surface in front of the test wheel is watered by a watering system in the test vehicle. This system includes a water tank of 1.7-cu m (450-gal) capacity, a water pump, and a nozzle. The pump is driven by a power take-off in the transmission of the test wheel. The water f i lm on the road has a mean thickness of 0.3 mm independent of the driving speed.

Control System. —Some of the movable parts of the test vehicle are operated by means of a hydraulic control system, for which pressure is obtained from a pump driven by the truck engine. Control valves in the cab distribute the pressure oi l to the hy­draulic cylinders. Thus, the test wheel and the measuring wheel can be raised and lowered, the variable V-belt transmission can be adjusted, the disc brake and the disc clutch in the transmission of the test wheel, as well as the water pump, can be engaged or disengaged.

Electrical Equipment of Vehicle A diagram of the electrical equipment is shown in Figure 6. Hie equipment consists

of the following parts: (a) power supply, (b) torque-measuring equipment, (c) load-measuring equipment, (d) recorder, (e) speed- and slip-measuring equipment, (f) V-belt transmission gear ratio indicator, (g) water flow indicator, (h) indicating equip­ment, (i) brake programer and (j) remote control of brake program. Figure 7 shows the instrumentation in the cab.

Power Supply. —The electrical equipment of the test vehicle requires for its opera­tion both 12 volts dc and 220 volts ac, 50 cps. A three-phase alternator driven by the engine of the lorry is used to supply current. The alternator has a power output of 1 kw. The alternating current is rectified and the rectified voltage is fed to a voltage regulator. From this regulator a voltage of 12 volts dc Is delivered both to the ordin­ary electrical system of the test vehicle and to the other electrical equipment. The al­ternating voltage necessary for a part of the electrical equipment is obtained from a rotary converter, which has a power output of 250 w. The converter is equipped with manual frequency control. A magnetic power stabilizer is used to maintain a constant voltage.

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Figure 7- Control panels f o r e l e c t r i c a l equipm'ent, t e s t v e h i c l e 5.

Torque-Measuring Equipment. —The friction force between the test wheel and the road is converted to an electrical quantity by a transducer in the shafting of the test wheel. This transducer (Vibro-Meter AG, type TG/50) which has an inductive meas­uring system is linear within 0.5 to 1 percent up to the rated torque. A Vibro-Meter AG, type 8-ATR l /S oscillator-amplifier feeds a measuring bridge and amplifies the signal. The signal is then carried from the amplifier to a recorder through a filter. The purpose of the filter is to attenuate oscillations of higher frequency than 1 cps. The evaluation of the friction records is therefore considerably facilitated. The signal is also carried from the amplifier to a pointer instrument (upper instrument in Fig 7), graduated in terms of the coefficient of friction. Thus, an approximate value of the coefficient of friction is quickly obtained.

Load-Measuring Equipment. —The spring loading of the test wheel is equipped at one end with a force transducer (load cell) for measuring the actual load on the test wheel. This transducer (Vibro-Meter AG, type ZTIOOO) has a measuring range of 0 to 1,000 kg. The transducer consists of a steel ring and an inductive measuring system. When sub­jected to a load, the ring changes its shape, and the measuring system converts this change to an electrical quantity. The transducer is connected with a measuring bridge (Vibro-Meter AG, type 005/A) fed with 220 volts ac, 50 cps. The actual load on the wheel is indicated by a pointer instrument in the bridge imit. This instrument is gradu­ated in kilograms. Besides, it is possible to record the load on the wheel by the re­corder.

Recorder. —The recorder (Elema, type Mingograf 230) has three channels. Two of these channels have built-in amplifiers, and are used for recording the friction force and load. The third channel is used for recording the rotary speeds of the test wheel

25

and of the measuring wheel. The chart paper is fed with a speed of 20 mm per s. Speed- and Slip-Measuring Equipment. —Both the test wheel and the measuring wheel

are equipped with a tachometer generator. The voltages of these two generators are carried to a crossed coil instrument, which shows the slip of the test wheel. This in­strument is graduated in percent from -10 to +30. The voltage of the tachometer gen­erator of the measuring wheel is also carried to a moving coil instrument, which shows the driving speed in kilometers per hour. In both these tachometer generators breaker points are mounted, which give 1 pulse per revolution. These pulses have nega­tive and positive polarity, respectively, i , and are recorded by the recorder. Cal­culation of the slip of the test wheel and of the driving speed is made possible by this recording.

Transmission Gear Ratio Indicator. — Each of the two shafts of the variable transmission is also equipped with a ta­chometer generator. The voltages of these generators are carried to another crossed coil instrument graduated in per­cent from -25 to +25.

Water Flow Indicator. —A flow meter transducer is mounted in the pipe of the watering nozzle. The voltage of this transducer is carried to an instrument graduated from 0 to 100 1 per min. This instrument indicates the magnitude of the water flow.

Indicating Equipment. —A number of limit-switches are mounted at the mov­able, mechanical parts. These switches actuate some pilot lamps, which show the positions of the previously mentioned parts to the operator.

Brake Programer. —The test wheel can be locked manually as well as by the auto­matic brake programer. This programer facilitates the work of the operator and also improves the reproducibility of re­peated tests at the same place. The dis­tance over which the test wheel is locked (10 to 120 m at intervals of 10 m) and the distance between two consecutive lockings (10 to 120 m at intervals of 10 m) as well as the number of lockings are preset on the brake programer. A set of breaker points on the measuring wheel governs the programer, which transmits 12 volts dc to a three-way magnet valve, and the oil supply to the braking system is regu­lated by this valve. Figure 8 shows the instrumentation of the programer in the vehicle.

Remote Control of Brake Program. — To obtain comparable test results when making repeated tests on the same road section, the test vehicle is equipped with a small radio system for remote control

STARTAUTOMAriK

Figure 8. Automatic braking program s t a r t ­ing device, t e s t v e h i c l e 5. Receiver and co n t r o l panel, ( l ) Receiver f o r remote w i r e l e s s s t a r t i n g s i g n a l s of braking pro­gram. (2) Switch f o r magnet valve of brake system. (3) S e l e c t o r switch f o r number of lo c k i n g s . (U) S e l e c t o r switch f o r d i s t a n c e over which t e s t wheel i s looked. (5) Se­l e c t o r switch f o r distance between two consecutive l o c k i n g s . (6) S e l e c t o r switch and push buttons f o r remote or manual s t a r t i n g of braking program. (7) Switch and push button f o r i n d i v i d u a l manual

braking.

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Figure 9. Automatic braking program s t a r t i n g device, t e s t v e h i c l e road and t r a n s m i t t e r .

Detector on the

of the brake program. The receiver in the test vehicle gets the signal to start the brake program from a portable system of a road detector and a battery-powered transmitter, placed on the road at the beginning of the test section, where the test vehicle actuates the road detector. The receiver is provided with selective circuits so that the test ve­hicle is the only vehicle that can start a brake program. The radio system has three channels, which allows up to three different test sections on the same road. Figure 9 shows the automatic brake program starting device.

Calibration of Test Equipment The friction-measuring equipment of the test vehicle is calibrated by two different

methods: (a) calibration in the laboratory, and (b) calibration in the field. Both these methods give the correlation between the tangential force acting on the test wheel and the reading of the chart paper.

Record Chart Figure 10 shows a typical oscillogram obtained in a braking test on a pavement. The

chart speed is 20 mm per sec. The length of the test section is 90 m (300 ft). The pulses from the measuring wheel and from the test wheel for calculating the slip ratio are recorded. The average value of the friction force is easy to estimate because the variation is very small (about 2.1 mm in Fig. 10).

Frequency Response of Measuring Circuit The mechanical parts of the test wheel have a natural frequency of about 10 cps. To

obtain clear and easy-to-read recordings from the torque transducer, the influence of this frequency is reduced with the aid of an electrical low-pass filter.

Figure 11 shows a recording of a square wave of 0.4 cps impressed on the input of the filter. As can be seen from the recording, the signal will reach a steady-state value in about 0.75 sec if the cut-off frequency is 1 cps which is normally used.

The electrical equipment makes it possible to switch over to other cut-off frequencies.

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fclM fclfc M fci fc

M e a s u r i n g Iffheel M a r k i n g s

jgbf^ B r a k e F o r c e

T e s t Whee l M a r k i n g s

Figure 10. Recordings on chart paper. Necessary measurements for determining length of test section^ s l i p , and coeff icient of f r i c t i o n marked.

% A1 a* "»/

ER l i .

•

.4.; 1 jl li-1

r> 9^ 1' ' 'i l 1 , . , 1' ' 'i

I r ^.

I •

,, ! ' I .L ,\ ••1 i . '

1, Vl t

J i ! 11 t , • • III J'

' \ i Itti •

i ' rV W l i t 11 K i f,k'l{ Hi V Q

Figure 11. Response at different f i l t e r cut-off frequencies; impressed square wave f re ­quency O.k cps.

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F igu re 11 a l so shows the inf luence of the c u t - o f f f r equenc ie s 2 and 10 cps on the r e ­sponse.

A c c u r a c y o f Measurements at Constant Sl ip

F i g u r e 12 shows a number of tes ts at d i f f e r e n t percentage s l i p at a constant speed of 20 k m p e r h r f o r a r i b b e d s tandard t i r e on wet asphal t ic conc re t e . A l l tes ts a re made on the same sec t ion of the r o a d . F i g u r e s 13, 14, and 15 show the r e s u l t s at 40 , 60, and 80 k m p e r h r , r e s p e c t i v e l y . In F i g u r e 16, the m a x i m u m values and the locked-whee l values o f the c o e f f i c i e n t of f r i c t i o n a re p lo t t ed against the speed.

The s l i p r a t i o and not the s l i p v e l o c i t y ( i . e . , the d i f f e r e n c e of the v e l o c i t i e s of the f r e e l y r o l l i n g whee l and o f the b r a k e d wheel) i s used as abscissa ax is i n the d i ag rams because the tes t s show that the s l i p v e l o c i t y at m a x i m u m f r i c t i o n c o e f f i c i e n t i s a lmos t p r o p o r t i o n a l t o the t r a n s l a t o r y v e l o c i t y . T h i s means that the m a x i m u m f r i c t i o n coe f ­f i c i e n t g e n e r a l l y occu r s at the same s l i p r a t i o . F o r t h i s r eason , i t i s advantageous to use the s l i p r a t i o , i n do ing w h i c h the shape of the cu rves does not change much w i t h the v e l o c i t y .

As a r e s u l t of a s tudy w i t h extensive tes ts i t has been ind ica ted tha t i t i s a l lowed to c a r r y out rou t ine measurements on a l l types of su r f aces f r o m ice to d r y concrete at the same percentage s l i p w i t h r a t h e r good accuracy , p r o v i d e d that the same type of tes t t i r e and the same i n f l a t i o n p r e s s u r e a re used . I f another s ize o r type of tes t t i r e i s to be used, i t i s necessary t o check at w h i c h s l i p r a t i o the m a x i m u m c o e f f i c i e n t of f r i c t i o n o c c u r s .

F o r r ou t i ne measurements , a c e r t a i n tes t p rocedure has been es tab l i shed . Meas ­u rements a re c a r r i e d out at 20, 40 , 60, and 80 k m p e r h r and at a constant s l i p of 15 to 20 p e r c e n t . Measurements a re a lso c a r r i e d out at 100 percen t s l i p except on d r y ,

1.0

0,9

0,8

H 0,6 U -

L i _

: 0,5 5E U J

t 0,4

0,3

0,2

0,1

0

/ / / •

Speed 20 k i / h

10 20 30 40 50

PERCENTAGE SLIP

100

Figure 12. Coefficient of f r i c t i o n and s l ip at 20 km per hr for a ribbed standard t i r e on wet asphaltic concrete.

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1,0

0,9

0,8

CD

5 0,6 U _

U -

° 0,5 U J

S 0,4 U J <=>

0.3

0,2

0,1

0

>

/ / <

—w—

Speed 40 k i /h

10 2 0 30 40 50

PERCENTAGE SLIP

100

Figure 13. Coefficient of f r i c t i on and s l ip at 1̂0 km per hr for a ribbed standard t i r e on wet asphaltic concrete.

1,0

0,9

0,8

0,7

0,6

0,5

0,4

0.3

0,2

0.1

0

•

Speed 60 ka/h

10 20 30 40 50

PERCENTAGE SLIP

IDS

Figure Ik. Coefficient of f r i c t i o n and s l ip at 60 km per hr for a ribbed standard t i r e on wet asphaltic concrete.

30

1.0

0,9

0,8

0,7

0,6

0,5

0,4

0,3

0,2

0.1

0

1

(

Speed 80 kio/h

10 20 30 40 50

PERCENTAGE SLIP

100

Figure 15. Coeff icient of f r i c t i on and s l ip at 80 km per hr for a ribbed standard t i r e on wet asphaltic concrete.

1.0

0.9

0,8

0,7

I 0,6 ae

^ 0,5

a= LU ^ 0,4 u.

" 0,3

0,2

0.1

0

Kaiii 0 2 -

ua val 17 pe

lies r cent s l ip)

a

Locked iheel values .^^^ (100 per cent s l ip)

10 20 30 40 50 60 70 80 90 SPEEO kn/h

Figure 16. Coefficient of f r i c t i on and speed for a ribbed standard t i r e on wet asphal­t i c concrete.

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rough surfaces which w i l l cause rapid and uneven wear of the test t i r e .

O T H E R I N S T I T U T E F R I C T I O N T E S T V E H I C L E S

The Institute has also designed some other frict ion test vehicles of which three are br ie f ly described.

In fr ict ion test vehicle 2 (Figure 17), a more simple mechanical sys tem i s used with a fixed velocity ratio which is obtained by coupling two different wheel s i zes together. T h i s equipment was constructed for the National Swedish Board of C i v i l Aviation and has been used for measuring the coefficient of fr ict ion on i c e - and snow-covered runways.

Tes t vehicle 2 is principal ly a t r a i l e r that can be towed by any standard motor truclc. Towing speed i s only l imited by the maximum speed of the truck . In the B r o m m a A i r ­port 50 mph i s reached with a 135 hp Scania Vabis truck. The t r a i l e r consists of a heavy frame supported by two independently sprung wheels with t ire s ize 11.00 -20. A test wheel with a plain c ircumferent ia l ribbed t i re , s ize 7.50 - 20, i s arranged be­tween the supporting wheels . By means of propeller shafts with universa l joints, the test wheel i s connected to the supporting wheels . A l l the three wheels are c a r r i e d on trai l ing a r m s . The frame is supported on two coi l springs and the load is distributed by means of balance a r m s to the wheels . The load on the test wheel i s kept constant. The suspension is completed by hydraulic shock absorbers . The t ra i l er is very stable in operation even at high speed. When no brake action is required, the connection between the three wheels i s broken by disengaging fr ict ion clutches in the outer wheels .

Fr ic t ion test vehicle 3 ( F i g s . 18 and 19) is also principal ly a t r a i l e r and uti l izes a l ­most the same simple mechanical system as test vehicle 2 but is equipped with two test wheels and three s l ip rat ios . T h i s vehicle has especial ly been used for investigation of winter t i r e s . With two test wheels it i s possible to compare two t i res at the same time and test condition.

The latest fr ict ion test vehicle (No. 6) i s very s i m i l a r to test vehicle 2 and consists of a heavy frame supported by two independently sprung wheels, t ire s ize 7.50 - 16. A test wheel, t ire s ize 5.90 - 15, i s placed between the supporting wheels . By means of shafts with un iversa l joints, the test wheel i s connected to the supporting wheels .

The arrangement with large and s m a l l wheels connected together means that when the t r a i l e r i s moved, the angular velocity of the test wheel w i l l not correspond to the t r a n s -latory speed of the t r a i l e r ; i . e . the test wheel i s braked and w i l l s l ip on the ground. Torque is transmitted f r o m the test wheel to the supporting wheels . When the t r a i l e r i s

Figure 17. Te s t v e h i c l e 2.

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Figure 18. Te s t v e h i c l e 3.

Figur e 19. Arrangement of mechanical equipment, t e s t v e h i c l e 3.

towed, the test wheel i s forced to operate with positive s l ip and thus develops a braking force while the other wheels operate at negative sl ip (spin) and thus develop a d r i v i i ^ force . The load on the supporting wheels (the "driving" wheels) i s twice the load on the test wheel, which means that the negative sl ip is very low. Because the frict ion forces on the test wheel and on the supporting wheels work in opposite directions, the towing vehicle i s required only to supply the difference between these forces which is only a fract ion of the force developed by the braking wheel.

S U M M A R Y O F R E S U L T S F R O M S O M E F R I C T I O N M E A S U R E M E N T S

T e s t s on Snow-Covered and Icy Roads

F i e l d tests of sand and salt treatments on snow-covered and icy roads have been c a r r i e d out by the Institute since the winter of 1953. At such tests fr ict ion measure­ments at incipient skid are to be pre ferred to fr ict ion measurements at fu l l skid be ­cause of the destructive effect on the sand layer f rom a skidding locked wheel which makes repeated tests on the same road section difficult. Further winter driving with­out locking the wheels when braking is to be pre ferred . The tests show that sand with­out addition of salt i s useful on a snow-covered carriageway, but i s not of e n o u ^ d u r a ­bility on a hard i ce - course . Sand with addition of salt proved to give greater frict ion as we l l as better durability than sanding without salt on a hard icy or snow-covered course . "Fix ing" the sand by freezing water has given the most interesting test resu l t s . The freezing of the sand to the surface resulted in a substantial increase in frict ion and in a decrease of the number of sandings needed and thus also of the maintenance cost. The prerequisite condition for the usability of the method is a frost period.

33

The c o e f f i c i e n t of f r i c t i o n (at inc ip ien t sk id) on im t r ea t ed ice v a r i e d f r o m 0 .08 to 0 .20 , and the c o e f f i c i e n t of f r i c t i o n on sand- t rea ted ice v a r i e d f r o m 0 .20 to 0 .32 when no veh ic les had passed o v e r the tes t s ec t ion . On a tes t sec t ion where the sand had been f i x e d by f r e e z i n g w a t e r the c o e f f i c i e n t of f r i c t i o n reached as much as the f i g u r e of 0 . 7 .

F r i c t i o n P r o p e r t i e s of W i n t e r T i r e s and A n t i - S k i d Devices

Since the w i n t e r of 1952-53 the Ins t i tu te has undertaken inves t iga t ions of w i n t e r t i r e s and a n t i - s k i d devices f o r m o t o r c a r s . The purpose of the inves t iga t ions has been to de­t e r m i n e the g r i p on the r o a d of v a r i o u s w i n t e r t i r e s and a n t i - s k i d devices c o m p a r e d w i t h the r i b b e d - p a t t e r n t r e a d of s tandard t i r e s on roads cove red w i t h ice and snow. T w o tes t veh ic les (Nos . 1 and 3) were used f o r these inves t iga t ions .

The t e s t s w e r e c a r r i e d out o n i c e - and snow-cove red roads i n d i f f e r e n t cond i t i ons . These roads w e r e d i v i d e d in to the f o l l o w i n g m a i n groups : i c y roads , roads w i t h newly f a l l e n snow on i c e , roads w i t h compacted snow, as w e l l as such roads w i t h newly f a l l e n snow on t op . W i t h i n these m a i n groups t he re were roads w i t h qui te d i f f e r e n t f r i c t i o n c h a r a c t e r i s t i c s , but ob jec t ive methods to descr ibe i n m o r e d e t a i l the v a r i o u s roads d i d not e x i s t . The inves t iga t ions showed that the d r i v i n g speed on ly had an i n s i g n i f i c a n t e f ­f e c t on the g r i p on the r o a d , whereas the e f f e c t o f the s l i p was d i f f e r e n t f o r the v a r i o u s types of t i r e s . A t a h i ^ e r f r i c t i o n l e v e l ( c o e f f i c i e n t of f r i c t i o n , 0 .3 to 0 .4 ) the f r i c t i o n c o e f f i c i e n t reached i t s m a x i m u m at 10 to 20 percent s l i p f o r a s tandard t i r e and at 30 to 40 percen t s l i p f o r a w i n t e r t i r e w i t h heavy p a t t e r n t r e a d ( F i g . 20) . A t a l ow f r i c t i o n l e v e l ( c o e f f i c i e n t o f f r i c t i o n , 0 . 1 to 0 .2 ) the f r i c t i o n reached i t s m a x i m u m at about 10 pe rcen t s l i p f o r a s t anda rd t i r e and at 40 percent f o r a w i n t e r t i r e w i t h heavy p a t t e r n t r e a d ( F i g . 21) .

A c o m p a r i s o n of the g r i p on the r o a d of d i f f e r e n t w i n t e r t i r e s and a n t i - s k i d devices i n r e l a t i o n to r i b b e d s tandard t i r e s gave d i f f e r e n t r e s u l t s on the va r i ous types of r o a d s . On i c y roads a l l types of t i r e s had a lmos t as bad a g r i p on the r o a d . W i t h t r a n s v e r s e d -grooved t i r e s and t i r e s w i t h s t ee l w i r e in l ays o r s tuds, the f r i c t i o n was inc reased to some extent . H i e inves t iga t ions have only been c a r r i e d out w i t h comple t e ly new t i r e s and s tuds . On the o the r hand, snow chains gave a m a r k e d Improvemen t ; i . e . a coe f ­f i c i e n t tha t was a p p r o x i m a t e l y th ree t i m e s h ighe r than tha t of the s tandard t i r e . On i c y roads w i t h newly f a l l e n snow, a s m a l l but i n s i g n i f i c a n t i m p r o v e m e n t i n the g r i p on the road was obtained f r o m d i f f e r e n t types of w i n t e r t i r e s . W i t h f r i c t i o n c o e f f i c i e n t s as low as 0 .07 these roads were e x t r e m e l y s l i p p e r y . On roads w i t h compacted snow o r on such roads w i t h newly f a l l e n snow on top, w i n t e r t i r e s p r o v e d to have a m a r k e d l y be t t e r g r i p on the r o a d than the s tandard t i r e . The d i f f e r e n t w i n t e r t i r e s when compared w i t h each o the r showed themselves to be i n the m a i n equal ly good.

0,5

§ 0,4 >— U

£ 0,3

£ 0,2

0

— ^ — ^ 1 p l l n t e r tyre ( + )

1 H Z

// - s tender 1 tyre ( • )

10 20 30 40 50

PERCENTAGE SLIP

100

Figure 20. Coefficient of f r i c t i o n and s l i p at llO km per hr for a standard t i r e and a winter t i r e on compacted snow.

34

0,5

I 0,4 I — U

E 0,3 U -CD

S 0,2

I 0,1

' stand and typ e ( • :

•HIntBP tyre ( + ) 1 1

10 20 30 40 50

PERCENTAGE SLIP

100

Figure 21. Coeff icient of f r i c t i o n and s l i p at Uo km per hr for a standard t i r e and a winter t i r e on dry I c e .

1,0

0,9

1 0.7 OS U J O -

g 0,6

i 0,5 t—

- 0,4 o

S 0,3

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o

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o

o 0

< >

0 0,1 0,2 0,3 0,4 0,5 0,6 0,7 0,8 0,9 1,0

COEFFICIENT OF FRICTION AT 17 PER CENT SLIP

Figure 22, Coeff icient of f r i c t i o n at 100 and 1? percent s l i p for a ribbed standard t i r e on different bituminous pavements in wet condition.

F r i c t i o n P r o p e r t i e s of Concrete Roads

The Ins t i tu te has a lso c a r r i e d out f r i c t i o n inves t iga t ions o f concre te roads . T o f o r m an idea of the f r i c t i o n c o e f f i c i e n t l e v e l r e l a t e d t o tha t of c o m m o n b i t uminous pavements , f r i c t i o n measurements were i n t h i s case a l so made on such pavements . The m e a s u r e ­ments w e r e c a r r i e d out w i t h tes t veh ic le 1 . D u r i n g the t es t s , the tes t whee l of the v e ­h i c l e was equipped w i t h a s t andard t i r e , s ize 5 .90 - 15.

The f r i c t i o n p r o p e r t i e s o f d i f f e r e n t concre te and asphal t sec t ions w e r e s tudied at 17

35

1,0

0.9

a. _ j CO

0,8

h -

L U U 0.7 oc

0.6

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L U 0.3 U

COEF

F

0,2

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0

•

•• •

0 0,1 0,2 0,3 0,4 0,5 0.6 0,7 0,8 0,9 1,0

COEFFICIENT OF FRICTION AT 17 PER CENT SLIP

Figure 23. Coefficient of f r i c t i o n at 100 and 17 percent s l i p for a ribbed standard t i r e on different concrete pavements in wet condition.

and 100 percent s l i p ( locked wheel) as w e l l as at 40 and 80 k m p e r h r . F o r c o m p a r i n g the r e s u l t s obta ined f r o m the d i f f e r e n t sect ions i n a s i m p l e way , mean

c o e f f i c i e n t s of f r i c t i o n r e f e r r e d to the speed i n t e r v a l 90 to 0 k m per h r were ca l cu la t ed . Thus each sec t ion was r ep resen ted only by two coe f f i c i en t s of f r i c t i o n , one appl icable to 17 percent s l i p and the o the r to 100 percen t s l i p .

The concre te sec t ions i n themse lves showed r e l a t i v e l y h igh c o e f f i c i e n t s o f f r i c t i o n both at 17 and 100 pe rcen t s l i p . The mean values were 0.73 and 0 .46 r e s p e c t i v e l y . T h e d i s p e r s i o n was f a i r l y s m a l l ; t h i s type o f pavement seems t o be v e r y homogeneous f r o m the f r i c t i o n po in t of v i e w . F u r t h e r m o r e , the tes t r e s u l t s ind ica ted that new c o n ­c re te roads have a v e r y h i g h c o e f f i c i e n t of f r i c t i o n .

The asphalt sec t ions showed s l i g h t l y h i ^ e r coe f f i c i en t s of f r i c t i o n . The mean values w e r e 0 .76 and 0.53 at 17 and 100 percent s l i p , r e s p e c t i v e l y . However , the c o e f f i c i e n t s f o r asphalt v a r i e d cons iderab ly more than those f o r conc re t e .

Both on concrete and asphal t sect ions the sca t te r d i ag rams ( F i g s . 22 and 23) seemed to indicate a m a r k e d l i n e a r t r e n d i n the r e l a t i onsh ip between the coe f f i c i en t s of f r i c t i o n at 17 and 100 percen t s l i p . A n e s t i m a t i o n of the r e g r e s s i o n i n t h i s respec t was c a r r i e d out to t h r o w t h i s r e l a t i o n s h i p in to a m a t h e m a t i c a l f o r m .

The f r i c t i o n va lues w e r e a lso s tudied w i t h respect to such v a r i a b l e s as pavement age, t r a f f i c f l o w , and pavement c o m p o s i t i o n . The c o e f f i c i e n t of f r i c t i o n was f o u n d t o decrease w i t h increase i n age. T h i s decrease was sl lg^i t f o r concre te and more s i g n i f i c a n t f o r asphal t . However , i t i s not l i k e l y that t h i s decrease depends only o r at a l l on age f a c ­t o r s . Surface p o l i s h i n g caused by t r a f f i c p robab ly p lays an i m p o r t a n t r o l e i n t h i s c o n ­nec t ion . The re was no ind i ca t i on of a r e l a t i o n s h i p between the c o e f f i c i e n t of f r i c t i o n and the method of pavement cons t ruc t i on o r the pavement c o m p o s i t i o n . Such r e l a t i o n ­ships may of course e x i s t but i n v i e w of the s l i g h t d i spe r s ion i n the f r i c t i o n c o e f f i c i e n t s they cannot be of g rea t i m p o r t a n c e . T h i s i s p a r t i c u l a r l y t r u e of concre te .