Tyre Industry preliminary worn tyre definition proposal
Annex 8 (R117)
Worn tyre preparation
1.1 This guide outlines the preparation of artificially worn passenger car tyres by tread rubber removal (For example cutting, grinding, …) for subsequent wet grip index testing. The purpose is to permit the preparation of test tyres with a uniformly reduced tread groove depth and tread geometry that will yield repeatable test results while avoiding the time-consuming and costly over-the-road natural wearing of tyres.
1.2 The values stated in SI units are to be regarded as the standard. The values given in parentheses are for information only.
1.3 This standard does not pur port to address all the safety concerns, if any, associated with its use. It is the responsibility of the user of this standard to establish appropriate safety and health practices and determine the applicability of regulatory limitations prior to use.
2. Referenced Documents
2.1 ASTM Standards: 2
F421 Test Method for Measuring Groove and Void Depth in Passenger Car Tyres
F762 Test Method for Determining Change in Groove (or Void) Depth With Distance Traveled for Passenger Car Tyres
2.2 ISO Standards:
ISO 4287:1997 - Geometrical Product Specifications – Surface texture: Profile method – Terms, definitions and surface texture parameters
3. Terminology - Definitions
3.1 groove: void volume between two adjacent ribs or blocks in the tread pattern 2
3.2 groove depth: a measurement of the perpendicular distance from a real or calculated reference plane defined by edges of two adjacent ribs to the lowest point in the groove.
3.3 groove average depth: the average of all tyre groove depth measurements in a single groove.
3.4 reference tread width : see figure 1 
3.5 treadwear indicator: means the projections within the tread grooves designed to give a visual indication of the degree of wear of the tread  .
3.6 Central part of the tread : 3/4 (75%) of the reference tread width (C)
3.7 main grooves : grooves in the central part of the tread where tread-wear indicators are located
3.8 void , n —a volume (in the tread band) defined by the lack of rubber; the depth dimension of this volume may vary from point to point in on the tread band
3.9 position: groove or void
3.10 lateral position : position along the tyre width
3.11 centerline: line in the center of the tread width. To be determined by measuring tire width S G and placing the centerline in the middle at ½ S G .
3.12 Mold parting line : border circumference in which mold tread pattern segments connects with mold sidewall plates
3.13 Tread pattern limit points Li and Le: points located on the tyre profile between mold parting line and hypothetical point up to 15 mm on the tyre profile towards center line (see figure 2)
3.14 Worn tyre theoretical target profile :
Worn tyre profile curve is the theoretical target profile for the preparation of the tyre. This curve is built in two parts (see figure 3) :
- Central part of the tread: curve built on a circle with its center located on the radial axle passing through the center line and its radius built on a fit on all the points located at 2 mm height on all the control points as defined in 7.1 . Alternatively, depending of the specificity of the tread pattern geometry, the fitting curve can be the offset of the original tyre profile.
The external part of the tread: edges of the artificial worn profile in the central part of the tread are connected with Le and Li points. Regularity of the whole artificial worn tyre profile (on the 75% of the center part to the shoulders) shall be assured (for example by an arc of circumference or another curve).
4. Summary of Guide
This guide outlines a method for groove depth measurement, for the removal of a predetermined amount of tread rubber and for final surface finish.
5. Significance and Use
This procedure is aimed at preparing an artificial worn tyre. For the purpose of this standard, the tyre is considered in the worn state when the remaining tread depth is at the height of the tread wear indicator as defined in the UN R.30 (1.6 + 0.6 mm – 0 mm) 1
6.1 Tread Depth Gauge, meeting the requirements of Test Method F421 .
6.2 Tyre Tread Removal Machine, with equipment to remove tread rubber in a predetermined manner. Specifically, the equipment shall grant a buffing accuracy and precision on the “average final depth” as required in the paragraph 7.3 and 7.4
7.1. Choice of the control measurement points of the reference tread width
To control the conformity of the preparation process (see section 7.3), choose n measurement points in the center zone , in the lateral direction (see figure 4)
- The number of points n has to be ≥ 4 (compatibly with pattern design)
- 1 measurement point at each main groove having tread wear indicator
The other measurement points have to be chosen in grooves:
- At the maximum tread depth in the corresponding groove/zone
- In order to have the most regular distribution of the n points
Note: Measurement locations in the main grooves must be positioned at locations with full tread depth, that is, avoiding rubber ridges, tie bars, treadwear indicators, and so forth. Measurements are done in accordance with Test Method F762.].
For each of these transversal locations, choose 4 circumferential positions equally spaced.
This process defines 4 x n measurement points on the tyre, and 4n depths (h i ) to be controlled.
7.2. Worn tyre preparation .
Inspect the tyre to determine that there are no tread defects that would affect the finished tyre. If such conditions are noted, do not use the tyre for this procedure.
Depending on the worn tyre preparation processing technique, the removing of the rubber can be managed by targeting directly the worn tyre target profile, or by a manual regular controlling of the rubber removal, or other means.
orn tyre profile accuracy
Regularity of the whole worn tyre profile (from center to shoulders) shall be assured
At the end of the preparation process, measure and document in a report the depths in the 4 x n measurement locations defined in 7.1.
The acceptance rules for a properly prepared worn tyre are the following (see figure 6) .
For all the measurement points defined in section 7.1. ,
- The individual final tread depths have to be 2 mm ± 0.4 mm
- The average tread depth has to be 2 mm ± 0.2 mm
Verification of the shoulder areas (outside the 75% zone)
- The final tread depth in the shoulder area shall not be higher than 2mm.
If one of the above conditions is not met another tyre candidate must be prepared.
7.5 W orn tyre surface state
The arithmetic average roughness of the final surface “Ra” shall not exceed
Measurements are done in accordance with Test Method ISO 4287:1997 . We may need to further specify the exact procedure
This Ra is defined as the average of 3 lateral measurements approximately equally spaced on the remaining tread surface, at 4 equally spaced azimuths.
If the above condition is not met another tyre candidate must be prepared.
Tyre description (size, name, brand) , measurement locations, final tread depths for all measurement points, and roughness measurements shall be recorded in the report..
 ETRTO Manual – DESIGN GUIDE - STATIC ENVELOPE CONTOUR FOR PASSENGER CAR TYRES
 UN R.30