Continuous Friction Measuring Equipment (CFME) requires standardised testing tires. In the fixed slip friction testers, the testing tire is slipping on the surface at all times when measuring. The friction is measured between two surfaces, the testing tire and surface where friction value is required. The both surface characteristics impact on the friction coefficient value (µ) measured.
When we wish to achieve constant results and measure the slipperiness of the runway or highway surface, the other component, the tire in this case, should be as constant as possible. This way we minimise the deviation on the results. Therefore it’s critical to use only standardised measuring tires which are in perfect condition. All the measurement tires have special rubber construction and the rubber formula imitates real-world scenarios as closely as possible.
The tire condition is rated with following criteria:
- Enough compound on the surface
- Age of the tire
- How the tire has been stored throughout its lifetime
- Tire inflation pressure
All these criteria are important and will affect the results.
Different tires are used in different measurement cases. The correct tire in each measurement case is always based on standards and regulations. If you are not sure which tire to use, please contact us for further information. In general the following tires are used:
Unitester T-520: this is a grooved tire which is generally used at airports for runway friction measurements in dry or winter conditions, such as ice, snow, frost, slush, etc. It’s used without any watering system spraying water in front of the measurement tire. It still can be used on naturally wet surfaces, for example in case of heavy rain. These measurements without watering system are called operational measurements, which tells the current state at the surface but the state can quickly change, when new measurement is needed.
ASTM E1551: The tire name comes from the ASTM E1551 standard. It’s a generally used name for the slick measurement tire. The tire is used with a watering system spraying a standardised amount of water (usually 1.0mm) in front of the measurement wheel. This measurement type is called maintenance measurement and it’s done periodically within a predetermined schedule. This type of measurement tells the trend of the runway condition over a longer period of time, in terms of surface polishing and texture.
T-49: This tire is a special grooved tire which is closer to vehicle tire compound. It has special grooving pattern and less surface depth. This tire is used for road measurements with different road friction measurement standards, such as CENTS 15901-12.
A friction tester usually also has reference wheels, that make the fixed slip on the measurement wheel possible. The purpose of the reference wheels is to spin at the same speed as the vehicle is moving forward. They should not slip to keep the measurement wheel slipping at constant ratio. The condition of these tires is also critical to get accurate measurements.