The tyre ranking – how are tyres tested?
- saturday, 9 september 2011
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The tyre ranking is made on the basis of tyre test results. How is tyre testing carried out? Which tyre performance parameters are measured, and how? Find out here.
Tests on dry and wet surfaces can be carried out at test tracks. Especially popular among motoring journalists is the Continental track.
Tests on snow and ice are carried out in high mountain regions or in Scandinavia, particularly when they take place before the start of the winter season. These tests are often performed after dusk, when the weather conditions have stabilized, in order to avoid changes in the consistency of the snow during testing. It is also important that in tests of adhesion in non-winter conditions, the temperature must not change by more than 10°C.
Which sizes are considered in tyre rankings?
ADAC can boast that for many years its rankings have been based on tyres that it has acquired for itself on the market. There is therefore theoretically no danger that a manufacturer might supply tyres that have been specially prepared for the tests. The sizes tested are those which are most popular – this is in the interests of the vast majority of drivers, since these are the tyres that they use, and thus they get a view of their own segment of the market. In the case of the ADAC ranking this year the sizes were 185/60 R14 and 205/55 R16, while in previous years they have been 195/65 R15, 155/70 R13 and 175/65 R14.
The most popular sizes are also usually selected for testing by journalists. When beginning to design their range, manufacturers pay most attention to these sizes, and it is these that have normally been refined the most – they form the core of the range, which is then subject to division into various dimensions. The manufacturers are aware that these tyres will be used in tests, and that the position of the whole range will depend on their results. Does this mean that other sizes in the range will be worse than these "pilot" tyres? It is possible that there may be differences, but they will tend to be minimal and not perceptible to the driver.
Are the tested tyres brand new?
he tyre ranking is based on tyres that have been "run in". Why?
The vulcanization process, particularly for winter tyres, is very demanding technologically. Tyre treads have widely varying shapes, and in order to avoid short casting of the lugs and sipes of the tread, and to facilitate removal of the tyres from the mould, they are painted with a special anti-adhesive fluid. The drawback of this is that a thin layer is left on the rubber surface, which may mean that a tyre just out of the factory will not perform to its full capacity. For this reason the ETRTO (European Tyre and Rim Technical Organisation) testing methodology indicates that tyres should be worn in over a distance of at least 100 km before measurements or tests commence.
Do the tests use vehicles with ABS and ESP?
Braking tests for tyre ranking are carried out on vehicles with ABS systems, in order to eliminate driver effects and ensure a stable, straight-line braking path. The results attained by particular tyres, and also those recorded for the same set of tyres on successive runs, should be free of distortions being a consequence of:
- braking technique
- driver errors
- differences resulting from the characteristics of a particular vehicle (e.g. fishtailing during braking)
The aim is to obtain objective results which are subject to as small an error as possible. Are such results reliable for drivers of vehicles that do not have ABS? Theoretically, a tyre which comes out best in a test using a vehicle with ABS ought also to obtain a good result on one without ABS, when the appropriate braking technique (pulsed braking) is used. If this technique is not used, the wheels will lock and the vehicle will skid.
Driving aids, ABS or ESP may be switched on or off during the tests depending on the instructions of the testing methodology (in particular types of tests, these systems may be either a help or a hindrance). Other devices are also used during the tests, such as high-precision GPS receivers, which record such parameters as speed, travel path, acceleration and run times. In some tests use is also made of computer systems that record such parameters as wheel revolutions, slide, overload, and – in noise and comfort measurements – the level of vibration and sound intensity.
What are objective and subjective tests?
The tyre ranking is drawn up based on two categories of tests:
Objective tests provide results in the form of parameters determined using measuring apparatus. During these tests the role of the driver is limited to following the procedures in as uniform manner as possible. Objective tests include:
- braking distance measurement
- attempts to accelerate on different surfaces, with measurement of the distance or time required to achieve a specified speed
- external and internal noise level (determined using measurement devices, not the driver’s evaluation of acoustic comfort)
- rolling friction (machine testing, expressed in kg/tonne)
- durability (machine testing or long-distance drive) – degree of tread wear, tread depth, evenness of wear, any anomalies or damage occurring
- snow traction – GPS is used to measure the maximum acceleration. This is multiplied by the mass to obtain the pull force value, expressed in daN
The subjective tests used for the ranking require that the driver have a high level of discipline and experience, since it is on these attributes that the results of such tests depend. Examples include time-trial driving on a track with a large number of turns, and evaluation of comfort. They must be conducted by a professional driver, with such experience and perfect mastery of the testing techniques that he himself has a negligible effect on the results and ensures a high degree of uniformity between runs. The training cycle of a professional driver testing tyres for a manufacturer lasts around three to four years. These drivers very often also have experience in competition.
What are reference tyres?
Reference tyres provide a point of reference for the results of tests on which a tyre ranking is based. They are a set of tyres which are not the subject of tests, used at the start and at the end of the testing process. The parameters of these tyres provide a baseline with which the results attained by other tyres are compared.
Reference tyres are also used during a test so that the test driver has a point of reference enabling him to identify possible changes in the conditions of the test.
How are particular parameters tested?
Below there are some typical tests carried out in order to determine individual parameters. They are divided into three typical surface conditions: dry, wet, and snow.
Usually, in order to obtain useful results, tests are carried out in relation to a set of reference tyres. In addition, so that the data will be statistically useful, the tests are carried out several times. In defining principles for the measurement of braking effectiveness on snow (adhesion) ETRTO recommends that the test be repeated at least eight times for a given set of tyres. It is also important that the standard deviation of the measurements should not be greater than 10% of the average result. For this reason, extreme measurement values are rejected.
- Acceleration on a dry surface
- Braking on a dry surface from 100km/h with ABS switched on
Behaviour – steerability
- Time-trial on test route
- Behaviour on turns (at the grip limit - lateral adhesion)
Comfort (tested in the same way for tests on dry and wet surface and on snow)
- Measurement of external noise
- Acoustic and mechanical comfort
Rolling friction (tested in the same way for tests on dry and wet surface and on snow)
- Machine tests
Durability (tested in the same way for tests on dry and wet surface and on snow)
- Machine tests
- Driving up on kerb etc
- Braking on a wet surface from 100 km/h with ABS switched on
- Acceleration on a wet surface
Behaviour – steerability
- Time-trial driving on a test route
- Driving in a circle (including driving on different surfaces)
- Longitudinal aquaplaning
- Lateral aquaplaning
- Braking on snow from 50 km/h with ABS switched on
- Acceleration on snow with ASR to 25 km/h
- Traction on snow – pull force
Behaviour – steerability
- Driving on snow-covered route