1. IWG Worn tyres Tyre Industry work status July 17th 2019
Document WT-02-02
Agenda items: 5, 6 and 7
IWG Worn tyres Tyre Industry work status July 17th 2019
European Tyre and Rim Technical Organisation

2. Industry assessment on: Definition and preparation of a worn tyre
SUMMARY
Industry assessment on:
Definition and preparation of a worn tyre
Test procedure for the evaluation of wet grip performance of a worn tyre
Industry proposal for test campaign
European Tyre and Rim Technical Organisation

3. European Tyre and Rim Technical Organisation
1. Definition and preparation of worn tyre
Industry is of the opinion that before having any test campaign,
the starting point is to agree on a definition of the worn tyre.
ETRTO Task force discussed and worked on the following parameters to define a tyre in the worn state. The status as of today is the following:
Tread depth
From the previous IWG meeting, the proposal was to consider, for the central part of the tread, a tread depth of (1.6 + α) mm, being α the buffing tolerance.
High level of precision and accuracy is required for the tread depth, as it has significant impact on the WGI performance in the worn state. For this reason the α value is proposed to be 0.4 mm. This proposal will require a buffing machine able to consider also the radial runout of the tyre.  Proposal that the target for the remaining tread depth of the worn tyre is ( )mm = 2.0mm
European Tyre and Rim Technical Organisation

4. 1. Definition and preparation of worn tyre
Tread depth (continued)
Industry considers that measurement points have to be taken at least in all the principal grooves (identified by the location of tread wear indicator) and at least at 4 azimuths positions
In the workplan, it will be assessed whether and which of the following requirements are achievable and can be included in the standard:
any individual tread depth shall not be outside the range of 2.0 +/-0.4 mm (1.6 – 2.4)mm or accept that a small percentage of individual measurement (not statistically significant) is outside this range
the average tread depth and/or the average principal groove depth have also to be controlled , for example, respectively at 2.0 +/- 0.2 mm and 2.0 +/- 0.3 mm
See illustration next slide

5. Average principal groove
Illustration
2,3,4 = principal grooves
a,b,c,d = equally spaced azimuths 2 3 4 a * b c d
Average principal groove **
***
*For each individual measure  2 mm +/- 0.4 mm or accept a small percentage outside this range
To be confirmed whether requirements on accuracy can and shall be set for each principal groove average** and for the average of all measurements***

6. 1. Definition of worn tyre
Wear profile
Definition of remaining tread depth Industry propose to define a center zone tread width as 75% (as in EU directive 89/459/EEC) of the reference tread width (as defined in ETRTO EDI booklet)
Principal grooves
Industry confirms the existence of tyres with different tread-depth in the principal grooves
Two scenario’s still under discussion concerning the final tread depth (see illustration in the next slide) :
Scenario 1 – FLAT profile. Same remaining tread depth in the principal grooves of the tyre
Scenario 2 – OFFSET of the new profile. Shallowest principal groove is the reference for the buffing thickness

7. Illustration 7.6 mm 7.6 mm 8 mm 2mm 2 mm 2 mm 7.6 mm 7.6 mm 8 mm 2mm
Scenario 1 :FLAT 2 mm as target for all points
2mm
2 mm
2 mm
7.6 mm
7.6 mm
8 mm
Scenario 2 : OFFSET OF NEW PROFILE - shallowest groove as reference
2mm
2.4 mm
2mm

8. Rationale for the proposed scenarios for the worn tyre profile
Industry proposes to represent the uniform / regular wear for the worn tyre profile;
Hence it is assumed that
- Tyre is properly designed in terms of wear performance (no irregular wear)
- Tyre is properly used during its life (proper IP, vehicle fitment, driving style…)
- Average of wear profiles of natural worn tyre (field) are quite centered on regular wear
It is still under evaluation which of the 2 scenarios can best match the needs for the artificial worn tyre definition and preparation
Rationale / Assumption / considerations for scenario 1 = FLAT
Rationale / Assumption / considerations for scenario 2 = OFFSET NEW
The same min tread depth is reached simultaneously in all principal grooves at the end of tyre life
During its wear life, tyre keep its original profile in the center part (uniform tread thickness reduction)
Additional considerations for Scenario 1
Additional considerations for Scenario 2
Possibly need to add other grooves definition and additional measurements in those grooves in the central part to assure that all the 75% area is covered (e.g. manage case of only 1 principal groove).
Tread depth measurements in the principal grooves are enough
The worn profile in the worn conditions prior buffing is defined through the 2mm in the grooves independly from the new profile.
the new profile is taken as reference for buffing

9. 1. Definition of worn tyre
Shoulder area
Regularity of the whole artificial worn tyre profile (from center to shoulders) shall be assured.
Tyre industry proposes that the shoulder profile starts tangent at the edge of the central profile at 75% of the reference tread width (see figure left).
Definition of a connection point (see figure right) . Options to identify the connection point are still under industry investigation (for example by a k-factor = drop vs Overall Diameter, single k-factor for all tyres, …)
A regular connection from central part of the tread to Mold parting line could be for example constituted by a circumference arc passing through and tangent at edge point of the center profile and passing through the connection point. Still under assessment

10. 1. Definition and preparation of worn tyre
c. Ageing
Proposal is to consider ageing impact assessment during the test campaign
d. Buffing roughness
Expected impact of buffing roughness is considered by Industry as less significant.
But industry request to confirm this in the proposed workplan and Industry proposes to assess the impact (or not) of the buffing roughness during the Step 1 of the proposed workplan to define what the roughness of a buffed tyre should be.
European Tyre and Rim Technical Organisation

11. 2. Test procedure for the evaluation of wet grip performance
of a worn tyre
ETRTO has been asked by Tüv whether the specifications of the draft
revised ISO test method is available and can be shared within the IWG.
The ISO rules are clear with regards to the sharing of the revised ISO During the ISO TC31 Plenary meeting in Japan begin of June, it was agreed to launch the DIS ballot by mid of July, meaning that at this step the new ISO test method can be shared within the IWG.
ETRTO can anyhow support Tuv request by showing a presentation which summarizes the logic and the principles of the new wet grip procedure

12. During the preparation of Industry rough workplan proposal ETRTO
considered that a 2 steps approach as following is the best way to go ahead:
step 1:
To assess the parameters necessary to define the worn tyre and the preparation of the worn tyre (see previous item 1).
To assess parameters to define the test methodology (example: water depth, …)
Step 2:
Round Robin Test to validate all the parameters of a worn tyre wet grip performance test procedure:
Definition of the worn tyre
Test method
Threshold definition
Content and method of the round robin test to be defined after completion of step 1.
European Tyre and Rim Technical Organisation

13. 3. Workplan proposal (step 1)
Parameters necessary to define the worn tyre and the preparation of the worn tyre
Tread target remaining tread depth (Nominal = 2mm)*
Natural worn (field) vs artificial worn tyres
Roughness (2 levels: um)
Variation of the profile due to the variation of tread depth distribution within limits (e.g. obliquities toward center wear or shoulder wear)
Parameters to define the test methodology
water depth (0.5, 1.0, 1.5 mm)
SRTT evolution (after buffing and after a certain number of repeats)
Vehicle vs Trailer
*Open point
Decision how to include the combination of water depth and remaining tread depth (Nominal, min and max)
IWG to consider a resources allocation proposal.
See Appendix for details

14. Thank you
European Tyre and Rim Technical Organisation

15. Appendix. Workplan proposal (step 1)
Decision how to include this combination to be further defined
European Tyre and Rim Technical Organisation