Tread Pattern Anatomy
As the part of a tire that rolls in contact with the road, tread patterns incorporate a variety of features molded into their rubber compound that support the vehicle’s load while resisting heat and wear. Tread patterns are often classified by the design of their ribs, blocks, lugs and/or grooves that have allowed engineers to tune the tire’s traction, handling and noise.
Tread pattern: includes multiple ribs, blocks, lugs or a combination of all three. The most common passenger and light truck tires feature 5-rib designs, where 5 tread ribs are separated by 4 circumferential grooves. The ribs include the outboard shoulder, outboard intermediate, center, inboard intermediate and inboard shoulder.
Outboard Shoulder: the outer edge of the tread between the tire’s footprint and its sidewall. The outboard shoulder encounters the highest lateral stresses as it holds a vehicle in a corner.
Outboard Intermediate: the tread rib adjacent to the outboard shoulder. The outboard edge of outboard intermediate ribs experience the second highest lateral stresses and wear when cornering.
Center: contributes to straight-line acceleration and braking traction, as well as highway cruising stability and tracking.
Inboard Shoulder: the inward edge of the tread between the tire’s footprint and its sidewall. The inboard shoulder encounters the lowest stresses in a corner. Prone to faster wear if the vehicle’s alignment specifiesnegative camber.
Rib: a continuous strip of tread rubber around a tire’s circumference. Multiple side-by-side ribs are typically molded across a tire’s tread.
Lug: extra large, freestanding, independent lugs positioned around the tire’s circumference. Commonly used to describe design elements of light truck tire tread patterns.
Circumferential Grooves: major grooves molded around the tire.
Circumferential grooves represent a significant portion of the tire’s void ratio (groove area vs. contact area) that contribute to wet traction by permitting water to flow directly through the tread design. Circumferential grooves have the greatest influence on the tire’s hydroplaning resistance when driving through standing water. Circumferential grooves also provide lateral biting edges that enhance cornering traction on loose surfaces.
Circumferential grooves are the deepest grooves molded in a tread pattern and remain visible throughout the tire’s life. Tread depths are measured from the bottom of the circumferential grooves to the tread surface.
Lateral Grooves: major grooves molded across the tire.
Lateral grooves contribute to the tire’s void ratio (groove area vs. contact area) to aid wet traction and increase the number of biting edges that enhance acceleration and braking traction on loose surfaces.
Lateral grooves intersect their adjacent outboard and inboard circumferential grooves and result in the formation of independent tread blocks. A lateral groove that reaches only one circumferential groove (or the tire’s shoulder) is referred to as a lateral notch.
In order to provide more stability and handling, lateral grooves often feature less tread depth (often about 2/32” less) than their adjacent circumferential grooves. This results in lateral grooves visually disappearing completely as the tire wear approaches 2/32” of remaining tread depth.
Tie Bars: short rubber links molded across lateral grooves between adjacent tread blocks. Often used between independent shoulder tread blocks, tie bars connect tread elements circumferentially to reduce tread squirm when rolling, as well as during acceleration and braking. By opposing block movement, tie bars promote stability that help resist irregular wear. However as the tire wears down to the tie bars, the tie bar portion of the lateral groove will disappear and it will be reduced into a lateral notch.
Sipes: typically featured in all-season, all-terrain and winter tires, sipes are thin slits molded into an otherwise smooth tread surface to improve traction in wet, snowy or icy road conditions. Sipes essentially subdivide tread blocks into smaller elements to provide additional biting edges for foul-weather traction.
Straight sipes simplify the manufacturing process and make it easier to remove a tire from its mold, however many of today’s sipes feature zigzag shapes or 3-dimensional designs that help lock the elements of the tread block together to enhance handling while increasing foul-weather traction.
The quantity, location and design of sipes are carefully engineered into a tire’s tread design to balance traction, handling and wear. Engineers strategically locate sipes in tread blocks to reduce the possibility of squirmy handling in dry, hot conditions.
Sipes can be molded with a v-shaped blade of less depth than the tread design’s circumferential grooves. While its overall depth will remain the same, sipes designed this way will shorten in length as the tire wears. Additionally some sipes will often virtually disappear as the tire wear nears/reaches the treadwear indicators.
This evolution in a tread design as the tire wears typically sacrifices the tire’s wet and wintertime traction, reducing the vehicle’s ability to accelerate, brake and corner in foul-weather conditions. Fortunately this evolution in tread design is visually apparent as the design elements disappear and the tire approaches a slick, unadorned tread pattern.
Treadwear Indicators: 2/32” (1.6mm) tall raised bars molded across the bottom of the each circumferential tread groove from shoulder to shoulder. Treadwear indicators will appear as a solid strip across the tread design to enable a person visually inspecting a tire to easily determine whether any of the circumferential grooves have worn to 2/32” (1.6mm) of remaining tread depth. Tire warranties expire when the remaining tread depth matches the height of the treadwear indicator and tires are considered worn out.
NOTE: Tires with 12-inch or smaller rim diameters are required to have a minimum of three sets of treadwear indicators.
Tire tread depth is never measured at the treadwear indicators.