Tires seem relatively simple at first glance, but the engineering and manufacturing expertise that is poured into creating them is pretty impressive. We spoke with Ken Reuille, Lead SUV & RLT Tire Engineer for Cooper Tire & Rubber Company, about what goes into designing and making off-road tires today and got answers relevant to how we choose and use tires off-road. Here's what Reuille shared.
"New rubber compounding strategies and new materials like higher tensile polyester body ply and steel belt materials have allowed for added tire design flexibility and reduced tire weight while maintaining or improving overall tire strength. Historically, large off road tires such as a 40x13.50R20 were limited to bias tire constructions, as it took a while for the newer radial construction technology to be implemented in the large sizes. Bias tire constructions do not provide for the same level of on-road drivability or off-road conformability (when aired down) as do tires with a radial construction. Radial tires have a ply cord path that goes directly from one bead to the bead on the opposite side of the tire—in other words, it extends “radially” across the tread pattern. Radial construction takes the least length of ply to go across from bead to bead. Bias construction means the ply cord goes “off radial” or at an angle from bead to bead. Bias constructed tires have the advantage of increased puncture resistance as compared to radial tires. However, traditionally the on-road handling and tread wear in bias construction tires was not as good as radial tire construction. Recent technology enhancements, such as Cooper’s 3-ply Armor-Tek3 technology sidewall construction, provide a lot of the puncture resistance benefits of a bias construction tire while retaining all of the advantages of radial tire construction-- ultimately giving consumers the best of both worlds. In addition, the incorporation of silica in the tread compound is a relatively recent light truck tire development in radial tires. The added silica increases the tire’s wet traction performance without sacrificing other characteristics required of the tread compound. As an example, Cooper’s recently launched Discoverer STT Pro off-road tires use these advanced belt and ply materials, a silica enhanced tread compound, and our Armor-Tek3 technology."
Although not very common in DOT approved form, what is considered a directional tire and when are they advantageous (or not) to use?
A directional tire is one whose tread blocks are oriented to form a “V”-shaped groove pattern in the tread, with the tip of the “V” typically located at the center of the tread. If maximum tire traction is critical, directional tires have some advantages, but they are not without trade-offs. Directional tires can provide enhanced traction on soft surfaces off-road, and improved water evacuation by providing a path for water trapped at the center of the tire’s contact patch to be displaced out and over the shoulder of the tread. Other tread design features can be optimized for tread block stability in directional tires since the forces exerted on the blocks are limited to specific directions when the tire is used. By their nature, a disadvantage of directional tires is that they should not be rotated in both directions on a vehicle. You will not get the optimal level of traction performance if you rotate them in both directions, as the tread was not designed for this type of rotation. Consistent tire rotation in one direction can lead to uneven tread wear across individual tread blocks, where the leading edge of a block wears more quickly than the trailing edge. Over time, this type of wear can cause the noise generated by the tire to become increasingly louder. This is especially noticeable in deep non-skid, higher-void tread patterns that might be used in off-road applications
What are the roles of silica and carbon black in rubber composition and why are they important?
The roles of both silica and carbon black in a rubber compound are to reinforce the rubber to make it stiffer and tougher. This additional stiffness provides the tire with better handling capabilities. When silica is paired with a coupling agent, it further improves the wet traction and rolling resistance of a tire compared to carbon black alone.
In general, is there a correlation between overall tire life mileage for a street-driven vehicle and the traction and performance one can expect in the dirt?
In general, tire mileage on the street is impacted by the tire’s tread compound, tread depth, and amount of tread volume available to wear, which all combine to form the overall tread wear characteristics of the tire. It’s also important to note that driving habits, driving conditions, and tire and vehicle maintenance all play a part in the tread life of a tire and differs with each purchaser. Tires with deeper tread depths and lower tread pattern void or “openness” will wear longer than shallower patterns with more void, given the same tread compound characteristics. Tread compounds that are formulated for longer wear are typically “harder” and do not offer as much cut and chip resistance. Cutting and chipping can occur along tread block edges or adjacent to the siping in the tread blocks when tires are used in gravel or rocky terrain. “Harder” tread compounds cannot strain as readily as softer ones and therefore are generally more susceptible to tearing. Traction in dirt is primarily a function of the tire’s tread pattern and contact pressure, since in dirt the surface is loose and shears easily when the tire applies torque to it in the contact patch. On harder surfaces like rock, the tire’s tread compound becomes more important since the surface is stable. Additionally, on irregular rock surfaces, the contact patch area is a critical factor in allowing the tire to develop traction. This is why some off-road enthusiasts choose to air-down their tires, in order to achieve a maximum contact area which allows the tire to envelop irregular obstacles. Lower tire pressure also minimizes the potential for tire damage since any impact energy in the tread region is distributed over a greater area of the underlying tire structure. On a more micro-level, individual tread block surfaces can achieve greater contact area with irregular rock surfaces in tires with a softer tread compound. Lower temperatures also make the tire’s tread compound harder, which is why decreased tire traction can sometimes be observed at lower temperatures. Siping of the tread blocks can also provide a geometrical means of increasing individual tread block conformability when a softer tread stock cannot be employed due to other design considerations. This is why tires designed for winter traction typically contain a high degree of siping of the tread blocks.
Can you comment on off-road tire “break-in” and how tire performance improves afterward?
Although a tire’s contact patch may increase slightly after a “break-in” period, it should not significantly alter the tire’s performance, although this technique is sometimes used in extreme applications like rock climbing competitions where maximum traction is an imperative.