Wet Grip Testing

Wet Grip Testing to UN ECE R117

UN ECE R117 sets the type-approval requirements for tire noise, rolling resistance, and traction on wet surfaces. Specific to wet grip:

• Annex 5 defines how the wet grip index (G) of a new tire is measured.
• Annex 9 extends that requirement to tires in an artificially worn state.

Together, these tests measure how well a tire design delivers dependable wet-road braking performance both off the production line and later in its service life.

Smithers runs both procedures on a purpose-built wet surface, supporting tire manufacturers and OEMs evaluating tires for regulatory, labeling, or independent verification of a design's wet traction performance. Through our on-site buffing services, we can buff tires to both regulatory and custom tread depths to validate performance.

Annex 5 and Annex 9, at a Glance

Annex 5: Wet Grip Index, New Condition

Annex 5 sets out the braking test used to establish the wet grip index (G) for Class C1 and C2 tires. A tire's wet braking performance is measured against a Standard Reference Test Tire (SRTT), producing a ratio that must meet the minimum G value set for its class before the tire can be approved.

Annex 9: Wet Grip Index, Worn Condition

Annex 9 applies the same comparative logic to tires buffed to a defined tread contour and reduced tread depth, close to the legal wear limit. Results are compared against a worn-state reference tire, so wet traction performance is verified across the tire's useful life, not only when new.

How Wet Grip Testing Is Conducted

Both annexes follow the same core braking-comparison method. What changes between Annex 5 and Annex 9 is the condition of the tires under test.

1. Track Preparation

Testing is performed on a wet asphalt or concrete surface engineered to a target friction band, with a continuous water film maintained across the test zone during every run.

2. Reference and Control Tires

A class-appropriate Standard Reference Test Tire (SRTT) is run alongside the candidate to normalize for track texture and weather drift on the day of testing. A control tire is used where a candidate size cannot share an axle configuration with the SRTT. For Annex 9, a different SRTT (F3611) with a reduced tread depth 16" tire is utilized. It maintains the same specifications, but the mold is shallower for the tread.

3. Braking Runs

The instrumented trailer is brought to speed and braked hard through the wet zone, engaging the brakes with a time to peak brake force between .02 and .05 seconds. Peak brake force coefficient (pbfc) is recorded, referenced to a defined speed window.

4. Repeat Runs and Data Reduction

Candidate and reference tires are each run multiple times, with outlying runs excluded, to arrive at a statistically valid average for each tire before the ratio is calculated.

5. Wet Grip Index

The wet grip index G is calculated as the candidate tire's performance divided by the SRTT's performance. For Annex 9, the same calculation is performed using the worn-state candidate and worn-state reference tire.  

Tire Classes and Sizes Covered

Testing is available across a range of tire sizes within each class defined by UN ECE R117, using the corresponding Standard Reference Test Tire for that class.
 

Class	Typical Application	Governing Standard	Annex 5 Test Method
C1	Passenger car tires	UN R30	Annex 5, Part A
C2	Light truck / commercial tires, load index ≤ 121	UN R54	Annex 5, Part B

Ideal Testing Surfaces. Over 100 Years Testing Tires.

Ready to test your tire's wet grip performance? Talk to our tire testing team about scheduling wet grip testing at Treadwell Research Park.