ANNEX V

Testing method for measuring the wet grip index (G) of C1 tyres

1. MANDATORY STANDARDS

The following documents listed apply.

(1) ASTM E 303-93 (Reapproved 2008), Standard Test Method for Measuring Surface Frictional Properties Using the British Pendulum Tester;

(2) ASTM E 501-08, Standard Specification for Standard Rib Tire for Pavement Skid-Resistance Tests;

(3) ASTM E 965-96 (Reapproved 2006), Standard Test Method for Measuring Pavement Macrotexture Depth Using a Volumetric Technique;

(4) ASTM E 1136-93 (Reapproved 2003), Standard Specification for a Radial Standard Reference Test Tire (SRTT14″);

(5) ASTM F 2493-08, Standard Specification for a Radial Standard Reference Test Tire (SRTT16″).

2. DEFINITIONS

For the purposes of testing wet grip of C1 tyres, the following definitions apply:

(1) “test run” means a single pass of a loaded tyre over a given test track surface;

(2) “test tyre(s)” means a candidate tyre, a reference tyre or a control tyre or tyre set that is used in a test run;

(3) “candidate tyre(s) (T)” means a tyre or a tyre set that is tested for the purpose of calculating its wet grip index;

(4) “reference tyre(s) (R)” means a tyre or a tyre set that has the characteristics indicated in ASTM F 2493-08 and referred to as Standard Reference Test Tyre 16 inches (SRTT16″);

(5) “control tyre(s) (C)” means an intermediate tyre or a set of intermediate tyres which is used when the candidate tyre and the reference tyre cannot be directly compared on the same vehicle;

(6) “braking force of a tyre” means the longitudinal force, expressed in newton, resulting from braking torque application;

(7) “braking force coefficient of a tyre (BFC)” means the ratio of the braking force to the vertical load;

(8) “peak braking force coefficient of a tyre” means the maximum value of a tyre braking force coefficient that occurs prior to wheel lockup as the braking torque is progressively increased;

(9) “lockup of a wheel” means the condition of a wheel in which its rotational velocity about the wheel spin axis is zero and it is prevented from rotating in the presence of applied wheel torque;

(10) “vertical load” means the load in newton imposed on the tyre perpendicular to the road surface;

(11) “tyre test vehicle” means a dedicated special purpose vehicle which has instruments to measure the vertical and the longitudinal forces on one test tyre during braking.

3. GENERAL TEST CONDITIONS

3.1 Track characteristics

The test track shall have the following characteristics:

(1) The surface shall have a dense asphalt surface with a uniform gradient of not more than 2 % and shall not deviate more than 6 mm when tested with a 3 m straight edge.

(2) The surface shall have a pavement of uniform age, composition, and wear. The test surface shall be free of loose material and foreign deposits.

(3) The maximum chipping size shall be 10 mm (tolerances permitted from 8 mm to 13 mm).

(4) The texture depth as measured by a sand patch shall be 0,7 ± 0,3 mm. It shall be measured in accordance with ASTM E 965-96 (Reapproved 2006).

(5) The wetted frictional properties of the surface shall be measured with either method (a) or (b) in section 3.2.

3.2 Methods to measure the wetted frictional properties of the surface

(a) British Pendulum Number (BPN) method

The British Pendulum Number method shall be as defined in ASTM E 303-93 (Reapproved in 2008).

Pad rubber component formulation and physical properties shall be as specified in ASTM E 501-08.

The averaged British Pendulum Number (BPN) shall be between 42 and 60 BPN after temperature correction as follows.

BPN shall be corrected by the wetted road surface temperature. Unless temperature correction recommendations are indicated by the British pendulum manufacturer, the following formula is used:

BPN = BPN (measured value) + temperature correction

temperature correction = – 0,0018 t 2 + 0,34 t – 6,1

where t is the wetted road surface temperature in degrees Celsius.

Effects of slider pad wear: The pad shall be removed for maximum wear when the wear on the striking edge of the slider reaches 3,2 mm in the plane of the slider or 1,6 mm vertical to it in accordance with section 5.2.2 and Figure 3 of ASTM E 303-93 (Reapproved 2008).

For the purpose of checking track surface BPN consistency for the measurement of wet grip on an instrumented passenger car: the BPN values of the test track should not vary over the entire stopping distance so as to decrease the dispersion of test results. The wetted frictional properties of the surface shall be measured five times at each point of the BPN measurement every 10 meters and the coefficient of variation of the averaged BPN shall not exceed 10 %.

(b) ASTM E 1136 Standard Reference Test Tyre (SRTT14″) method

By derogation with point (4) of section 2, this method uses the reference tyre that has the characteristics indicated in ASTM E 1136-93 (Reapproved 2003) and referred to as SRTT14″ (1).

The average peak braking force coefficient (μpeak,ave ) of the SRTT14″ shall be 0,7 ± 0,1 at 65 km/h.

The average peak braking force coefficient (μpeak,ave ) of the SRTT14″ shall be corrected by the wetted road surface temperature as follows:

peak braking force coefficient (μpeak,ave ) = peak braking force coefficient (measured) + temperature correction

temperature correction = 0,0035 × (t – 20)

where t is the wetted road surface temperature in degrees Celsius.

3.3 Atmospheric conditions

The wind conditions shall not interfere with wetting of the surface (wind-shields are allowed).

Both the wetted surface temperature and the ambient temperature shall be between 2 °C and 20 °C for snow tyres and 5 °C and 35 °C for normal tyres.

The wetted surface temperature shall not vary during the test by more than 10 °C.

The ambient temperature must remain close to the wetted surface temperature; the difference between the ambient and the wetted surface temperatures must be less than 10 °C.

4. TESTING METHODS FOR MEASURING WET GRIP

For the calculation of the wet grip index (G) of a candidate tyre, the wet grip braking performance of the candidate tyre is compared to the wet grip braking performance of the reference tyre on a vehicle travelling straight ahead on a wet, paved surface. It is measured with one of the following methods:

— vehicle method consisting of testing a set of tyres mounted on an instrumented passenger car,

— testing method using a trailer towed by a vehicle or a tyre test vehicle, equipped with the test tyre(s).

4.1 Testing method using an instrumented passenger car

4.1.1 Principle

The testing method covers a procedure for measuring the deceleration performance of C1 tyres during braking, using an instrumented passenger car equipped with an Antilock Braking System (ABS), where “instrumented passenger car” means a passenger car that is fitted with the measuring equipment listed in section 4.1.2.2 for the purpose of this testing method. Starting with a defined initial speed, the brakes are applied hard enough on four wheels at the same time to activate the ABS. The average deceleration is calculated between two pre-defined speeds.

4.1.2 Equipment

4.1.2.1 Vehicle

Permitted modifications on the passenger car are as follows:

— those allowing the number of tyre sizes that can be mounted on the vehicle to be increased,

— those permitting automatic activation of the braking device to be installed.

Any other modification of the braking system is prohibited.

4.1.2.2 Measuring equipment

The vehicle shall be fitted with a sensor suitable for measuring speed on a wet surface and distance covered between two speeds.

To measure vehicle speed, a fifth wheel or non-contact speed-measuring system shall be used.

4.1.3 Conditioning of the test track and wetting condition

The test track surface shall be watered at least half an hour prior to testing in order to equalise the surface temperature and water temperature. External watering should be supplied continuously throughout testing. For the whole testing area, the water depth shall be 1,0 ± 0,5 mm, measured from the peak of the pavement.

The test track should then be conditioned by conducting at least 10 test runs with tyres not involved in the test programme at 90 km/h.

4.1.4 Tyres and rims

4.1.4.1 Tyre preparation and break-in

The test tyres shall be trimmed to remove all protuberances on the tread surface caused by mould air vents or flashes at mould junctions.

The test tyres shall be mounted on the test rim declared by the tyre manufacturer.

A proper bead seat should be achieved by the use of a suitable lubricant. Excessive use of lubricant should be avoided to prevent slipping of the tyre on the wheel rim.

The test tyres/rim assemblies shall be stored in a location for a minimum of two hours such that they all have the same ambient temperature prior to testing. They should be shielded from the sun to avoid excessive heating by solar radiation.

For tyre break-in, two braking runs shall be performed.

4.1.4.2 Tyre load

The static load on each axle tyre shall lie between 60 % and 90 % of the tested tyre load capacity. Tyre loads on the same axle should not differ by more than 10 %.

4.1.4.3 Tyre inflation pressure

On the front and rear axles, the inflation pressures shall be 220 kPa (for standard- and extra-load tyres). The tyre pressure should be checked just prior to testing at ambient temperature and adjusted if required.

4.1.5 Procedure

4.1.5.1 Test run

The following test procedure applies for each test run:

(1) The passenger car is driven in a straight line up to 85 ± 2 km/h.

(2) Once the passenger car has reached 85 ± 2 km/h, the brakes are always activated at the same place on the test track referred to as “braking starting point”, with a longitudinal tolerance of 5 m and a transverse tolerance of 0,5 m.

(3) The brakes are activated either automatically or manually. (i) The automatic activation of the brakes is performed by means of a detection system made of two parts, one indexed to the test track and one on board the passenger car. (ii) The manual activation of the brakes depends on the type of transmission as follows. In both cases, a minimum of 600 N pedal efforts is required. For manual transmission, the driver should release the clutch and depress the brake pedal sharply, holding it down as long as necessary to perform the measurement. For automatic transmission, the driver should select neutral gear and then depress the brake pedal sharply, holding it down as long as necessary to perform the measurement.

(4) The average deceleration is calculated between 80 km/h and 20 km/h.

If any of the specifications listed above (including speed tolerance, longitudinal and transverse tolerance for the braking starting point, and braking time) are not met when a test run is made, the measurement is discarded and a new test run is made.

4.1.5.2 Test cycle

A number of test runs are made in order to measure the wet grip index of a set of candidate tyres (T) according to the following procedure, whereby each test run shall be made in the same direction and up to three different sets of candidate tyres may be measured within the same test cycle:

(1) First, the set of reference tyres are mounted on the instrumented passenger car.

(2) After at least three valid measurements have been made in accordance with section 4.1.5.1, the set of reference tyres is replaced by a set of candidate tyres.

(3) After six valid measurements of the candidate tyres are performed, two more sets of candidate tyres may be measured.

(4) The test cycle is closed by three more valid measurements of the same set of reference tyres as at the beginning of the test cycle.

EXAMPLES:

— The run order for a test cycle of three sets of candidate tyres (T1 to T3) plus a set of reference tyres (R) would be the following: R-T1-T2-T3-R

— The run order for a test cycle of five sets of candidate tyres (T1 to T5) plus a set of reference tyres (R) would be the following: R-T1-T2-T3-R-T4-T5-R

4.1.6 Processing of measurement results

4.1.6.1 Calculation of the average deceleration (AD)

The average deceleration (AD) is calculated for each valid test run in m·s– 2 as follows:

where:

Sf is the final speed in m·s– 1; Sf = 20 km/h = 5,556 m·s– 1

Si is the initial speed in m·s– 1; Si = 80 km/h = 22,222 m·s– 1

d is the distance covered in m between Si and Sf .

4.1.6.2 Validation of results

The AD coefficient of variation is calculated as follows:

(Standard Deviation / Average) × 100

For the reference tyres (R): If the AD coefficient of variation of any two consecutive groups of three test runs of the reference tyre set is higher than 3 %, all data should be discarded and the test repeated for all test tyres (the candidate tyres and the reference tyres).

For the candidate tyres (T): The AD coefficients of variation are calculated for each candidate tyre set. If one coefficient of variation is higher than 3 %, the data should be discarded and the test repeated for that candidate tyre set.

4.1.6.3 Calculation of adjusted average deceleration (Ra)

The average deceleration (AD) of the reference tyre set used for the calculation of its braking force coefficient is adjusted according to the positioning of each candidate tyre set in a given test cycle.

This adjusted AD of the reference tyre (Ra) is calculated in m·s–2 in accordance with table 1 where R1 is the average of the AD values in the first test of the reference tyre set (R) and R2 is the average of the AD values in the second test of the same reference tyre set (R).

Table 1

Number of sets of candidate tyres within one test cycle Set of candidate tyres Ra 1 (R1 -T1-R2 ) T1 Ra = 1/2 (R1 + R2 ) 2 (R1 -T1-T2-R2 ) T1 Ra = 2/3 R1 + 1/3 R2 T2 Ra = 1/3 R1 + 2/3 R2 3 (R1 -T1-T2-T3-R2 ) T1 Ra = 3/4 R1 + 1/4 R2 T2 Ra = 1/2 (R 1 + R2 ) T3 Ra = 1/4 R1 + 3/4 R2

4.1.6.4 Calculation of the braking force coefficient (BFC)

The braking force coefficient (BFC) is calculated for a braking on the two axles according to Table 2 where Ta (a = 1, 2 or 3) is the average of the AD values for each candidate tyre (T) set that is part of a test cycle.

Table 2

Test Tyre Braking force coefficient Reference tyre BFC(R) = |Ra/g| Candidate tyre BFC(T) = |Ta/g| g is the acceleration due to gravity, g = 9,81 m·s-2

4.1.6.5 Calculation of the wet grip index of the candidate tyre

The wet grip index of the candidate tyre (G(T)) is calculated as follows:

where:

— t is the measured wet surface temperature in degree Celsius when the candidate tyre (T) is tested

— t0 is the wet surface reference temperature condition, t0 =20 °C for normal tyres and t0 =10 °C for snow tyres

— BFC(R0) is the braking force coefficient for the reference tyre in the reference conditions, BFC(R0) = 0,68

— a = – 0,4232 and b = – 8,297 for normal tyres, a = 0,7721 and b = 31,18 for snow tyres

4.1.7 Wet grip performance comparison between a candidate tyre and a reference tyre using a control tyre

4.1.7.1 General

Where the candidate tyre size is significantly different from that of the reference tyre, a direct comparison on the same instrumented passenger car may not be possible. This testing method uses an intermediate tyre, hereinafter called the control tyre as defined in point 5 of section 2.

4.1.7.2 Principle of the approach

The principle is the use of a control tyre set and two different instrumented passenger cars for the test cycle of a candidate tyre set in comparison with a reference tyre set.

One instrumented passenger car is fitted with the reference tyre set followed by the control tyre set, the other with the control tyre set followed by the candidate tyre set.

The specifications listed in sections 4.1.2 to 4.1.4 apply.

The first test cycle is a comparison between the control tyre set and the reference tyre set.

The second test cycle is a comparison between the candidate tyre set and the control tyre set. It is done on the same test track and during the same day as the first test cycle. The wetted surface temperature shall be within ± 5 °C of the temperature of the first test cycle. The same control tyre set shall be used for the first and the second test cycles.

The wet grip index of the candidate tyre (G(T)) is calculated as follows:

G(T) = G1 × G2

where:

— G1 is the relative wet grip index of the control tyre (C) compared to the reference tyre (R) calculated as follows:

— G2 is the relative wet grip index of the candidate tyre (T) compared to the control tyre (C) calculated as follows:

4.1.7.3 Storage and preservation

It is necessary that all the tyres of a control tyre set have been stored in the same conditions. As soon as the control tyre set has been tested in comparison with the reference tyre, the specific storage conditions defined in ASTM E 1136-93 (Reapproved 2003) shall be applied.

4.1.7.4 Replacement of reference tyres and control tyres

When irregular wear or damage results from tests, or when wear influences the test results, the use of the tyre shall be discontinued.

4.2 Testing method using a trailer towed by a vehicle or a tyre test vehicle

4.2.1 Principle

The measurements are conducted on test tyres mounted on a trailer towed by a vehicle (hereafter referred to as tow vehicle) or on a tyre test vehicle. The brake in the test position is applied firmly until sufficient braking torque is generated to produce the maximum braking force that will occur prior to wheel lockup at a test speed of 65 km/h.

4.2.2 Equipment

4.2.2.1 Tow vehicle and trailer or tyre test vehicle

— The tow vehicle or the tyre test vehicle shall have the capability of maintaining the specified speed of 65 ± 2 km/h even under the maximum braking forces.

— The trailer or the tyre test vehicle shall be equipped with one place where the tyre can be fitted for measurement purposes hereafter called “test position” and the following accessories: (i) equipment to activate brakes in the test position; (ii) a water tank to store sufficient water to supply the road surface wetting system, unless external watering is used; (iii) recording equipment to record signals from transducers installed at the test position and to monitor water application rate if the self-watering option is used.

— The maximum variation of toe-settings and camber angle for the test position shall be within ± 0,5 ° with maximum vertical load. Suspension arms and bushings shall have sufficient rigidity necessary to minimise free play and ensure compliance under application of maximum braking forces. The suspension system shall provide adequate load-carrying capacity and be of such a design as to isolate suspension resonance.

— The test position shall be equipped with a typical or special automotive brake system which can apply sufficient braking torque to produce the maximum value of braking test wheel longitudinal force at the conditions specified.

— The brake application system shall be able to control the time interval between initial brake application and peak longitudinal force as specified in section 4.2.7.1.

— The trailer or the tyre test vehicle shall be designed to accommodate the range of candidate tyre sizes to be tested.

— The trailer or the tyre test vehicle shall have provisions for adjustment of vertical load as specified in section 4.2.5.2.

4.2.2.2 Measuring equipment

— The test wheel position on the trailer or the tyre test vehicle shall be equipped with a rotational wheel velocity measuring system and with transducers to measure the braking force and vertical load at the test wheel.

— General requirements for measurement system: The instrumentation system shall conform to the following overall requirements at ambient temperatures between 0 °C and 45 °C: (i) overall system accuracy, force: ± 1,5 % of the full scale of the vertical load or braking force; (ii) overall system accuracy, speed: ± 1,5 % of speed or ± 1,0 km/h, whichever is greater.

— Vehicle speed: To measure vehicle speed, a fifth wheel or non-contact precision speed-measuring system should be used.

— Braking forces: The braking force-measuring transducers shall measure longitudinal force generated at the tyre–road interface as a result of brake application within a range from 0 % to at least 125 % of the applied vertical load. The transducer design and location shall minimise inertial effects and vibration-induced mechanical resonance.

— Vertical load: The vertical load-measuring transducer shall measure the vertical load at the test position during brake application. The transducer shall have the same specifications as described previously.

— Signal conditioning and recording system: All signal conditioning and recording equipment shall provide linear output with necessary gain and data reading resolution to meet the specified previous requirements. In addition, the following requirements apply: (i) The minimum frequency response shall be flat from 0 Hz to 50 Hz (100 Hz) within ± 1 % full scale. (ii) The signal-to-noise ratio shall be at least 20/1. (iii) The gain shall be sufficient to permit full-scale display for full-scale input signal level. (iv) The input impedance shall be at least 10 times larger than the output impedance of the signal source. (v) The equipment shall be insensitive to vibrations, acceleration, and changes in ambient temperature.

4.2.3 Conditioning of the test track

The test track should be conditioned by conducting at least ten test runs with tyres not involved in the test program at 65 ± 2 km/h.

4.2.4 Wetting conditions

The tow vehicle and trailer or the tyre test vehicle may be optionally equipped with a pavement-wetting system, less the storage tank, which, in the case of the trailer, is mounted on the tow vehicle. The water being applied to the pavement ahead of the test tyres shall be supplied by a nozzle suitably designed to ensure that the water layer encountered by the test tyre has a uniform cross section at the test speed with a minimum splash and overspray.

The nozzle configuration and position shall ensure that the water jets are directed towards the test tyre and pointed towards the pavement at an angle of 20° to 30°.

The water shall strike the pavement 0,25 m to 0,45 m ahead of the centre of tyre contact. The nozzle shall be located 25 mm above the pavement or at the minimum height required to clear obstacles which the tester is expected to encounter, but in no case more than 100 mm above the pavement.

The water layer shall be at least 25 mm wider than the test tyre tread and applied so the tyre is centrally located between the edges. Water delivery rate shall ensure a water depth of 1,0 ± 0,5 mm and shall be consistent throughout the test to within ± 10 per cent. The volume of water per unit of wetted width shall be directly proportional to the test speed. The quantity of water applied at 65 km/h shall be 18 l·s-1 per meter of width of wetted surface in case of a water depth of 1,0 mm.

4.2.5 Tyres and rims

4.2.5.1 Tyre preparation and break-in

The test tyres shall be trimmed to remove all protuberances on the tread surface caused by mould air vents or flashes at mould junctions.

The test tyre shall be mounted on the test rim declared by the tyre manufacturer.

A proper bead seat should be achieved by the use of a suitable lubricant. Excessive use of lubricant should be avoided to prevent slipping of the tyre on the wheel rim.

The test tyres/rim assemblies shall be stored in a location for a minimum of two hours such that they all have the same ambient temperature prior to testing. They should be shielded from the sun to avoid excessive heating by solar radiation.

For tyre break-in, two braking runs shall be performed under the load, pressure and speed as specified in 4.2.5.2, 4.2.5.3 and 4.2.7.1 respectively.

4.2.5.2 Tyre load

The test load on the test tyre is 75 ± 5 % of the test tyre load capacity.

4.2.5.3 Tyre inflation pressure

The test tyre cold inflation pressure shall be 180 kPa for standard-load tyres. For extra-load tyres, the cold inflation pressure shall be 220 kPa.

The tyre pressure should be checked just prior to testing at ambient temperature and adjusted if required.

4.2.6 Preparation of the tow vehicle and trailer or the tyre test vehicle

4.2.6.1 Trailer

For one axle trailers, the hitch height and transverse position shall be adjusted once the test tyre has been loaded to the specified test load in order to avoid any disturbance of the measuring results. The longitudinal distance from the centre line of the articulation point of the coupling to the transverse centre line of the axle of the trailer shall be at least 10 times the “hitch height” or the “coupling (hitch) height”.

4.2.6.2 Instrumentation and equipment

Install the fifth wheel, when used, in accordance with the manufacturer’s specifications and locate it as near as possible to the mid-track position of the tow trailer or the tyre test vehicle.

4.2.7 Procedure

4.2.7.1 Test run

The following procedure applies for each test run:

(1) The tow vehicle or the tyre test vehicle is driven onto the test track in a straight line at the specified test speed 65 ± 2 km/h.

(2) The recording system is launched.

(3) Water is delivered to the pavement ahead of the test tyre approximately 0,5 s prior to brake application (for internal watering system).

(4) The trailer brakes are activated within 2 metres of a measurement point of the wetted frictional properties of the surface and sand depth in accordance with points 4 and 5 of section 3.1. The rate of braking application shall be such that the time interval between initial application of force and peak longitudinal force is in the range 0,2 s to 0,5 s.

(5) The recording system is stopped.

4.2.7.2 Test cycle

A number of test runs are made in order to measure the wet grip index of the candidate tyre (T) according to the following procedure, whereby each test run shall be made at the same spot on the test track and in the same direction. Up to three candidate tyres may be measured within the same test cycle, provided that the tests are completed within one day.

(1) First, the reference tyre is tested.

(2) After at least six valid measurements are performed in accordance with section 4.2.7.1, the reference tyre is replaced by the candidate tyre.

(3) After six valid measurements of the candidate tyre are performed, two more candidate tyres may be measured.

(4) The test cycle is closed by six more valid measurements of the same reference tyre as at the beginning of the test cycle.

EXAMPLES:

— The run order for a test cycle of three candidate tyres (T1 to T3) plus the reference tyre (R) would be the following: R-T1-T2-T3-R

— The run order for a test cycle of five candidate tyres (T1 to T5) plus the reference tyre R would be the following: R-T1-T2-T3-R-T4-T5-R

4.2.8 Processing of measurement results

4.2.8.1 Calculation of the peak braking force coefficient

The tyre peak braking force coefficient (μpeak) is the highest value of μ(t) before lockup occurs calculated as follows for each test run. Analogue signals should be filtered to remove noise. Digitally recorded signals must be filtered using a moving average technique.

where:

μ(t) is the dynamic tyre braking force coefficient in real time;

fh(t) is the dynamic braking force in real time, in N;

fv(t) is the dynamic vertical load in real time, in N.

4.2.8.2 Validation of results

The μpeak coefficient of variation is calculated as follows:

(Standard Deviation / Average) x 100

For the reference tyre (R): If the coefficient of variation of the peak braking force coefficient (μpeak) of the reference tyre is higher than 5 %, all data should be discarded and the test repeated for all test tyres (the candidate tyre(s) and the reference tyre).

For the candidate tyre(s) (T): The coefficient of variation of the peak braking force coefficient (μpeak) is calculated for each candidate tyre. If one coefficient of variation is higher than 5 %, the data should be discarded and the test repeated for this candidate tyre.

4.2.8.3 Calculation of the adjusted average peak braking force coefficient

The average peak braking force coefficient of the reference tyre used for the calculation of its braking force coefficient is adjusted according to the positioning of each candidate tyre in a given test cycle.

This adjusted average peak braking force coefficient of the reference tyre (Ra) is calculated in accordance with table 3 where R1 is the average peak tyre braking coefficient in the first test of the reference tyre (R) and R2 is the average peak tyre braking coefficient in the second test of the same reference tyre (R).

Table 3

Number of candidate tyre(s) within one test cycle Candidate tyre Ra 1 (R1 -T1-R2 ) T1 Ra = 1/2 (R1 + R2 ) 2 (R1 -T1-T2-R2 ) T1 Ra = 2/3 R1 + 1/3 R2 T2 Ra = 1/3 R1 + 2/3 R2 3 (R1 -T1-T2-T3-R2 ) T1 Ra = 3/4 R1 + 1/4 R2 T2 Ra = 1/2 (R1 + R2 ) T3 Ra = 1/4 R1 + 3/4 R2

4.2.8.4 Calculation of the average peak braking force coefficient (μpeak,ave )

The average value of the peak braking force coefficients (μpeak,ave ) is calculated according to table 4 whereby Ta (a = 1, 2 or 3) is the average of the peak braking force coefficients measured for one candidate tyre within one test cycle.

Table 4

Test tyre μpeak,ave Reference tyre μpeak,ave(R) = Ra as per Table 3 Candidate tyre μpeak,ave (T) = Ta

4.2.8.5 Calculation of the wet grip index of the candidate tyre

The wet grip index of the candidate tyre (G(T)) is calculated as follows:

where:

— t is the measured wet surface temperature in degree Celsius when the candidate tyre (T) is tested

— t0 is the wet surface reference temperature condition

— t0 = 20 °C for normal tyres t0 =10 °C for snow tyres

— μpeak,ave(R0) = 0,85 is the peak braking force coefficient for the reference tyre in the reference conditions