At SEMA last fall, Cooper Tire invited me back into a secured room, slowly lifting a canvas veil from their latest creation, a new high-performance mud tire. The name had not been decided, and their engineering team was still refining the attributes, but this new tire certainly had promise. A few months later, the team at Cooper reached out to provide the first set of tires to our team, an opportunity for us to test the newly minted STT Pro before any media or consumers. We have experienced good success with other Cooper tires, so the thought of a trail performance oriented offering was intriguing, particularly because of the engineering focus the Discoverer line has received in recent years. Cooper is not just producing quality replacement tires, they are engineering best of breed solutions that are giving the original equipment producers a proper run for their rubber.
As with any new tire, sizing was limited, which resulted in a set of five 305/70 R18 STT Pros arriving via UPS and slated for installation on my daily driver, a Mercedes-Benz G-Wagen. Fortunately, my G-Klasse has a few modifications to accommodate a true 35-inch mud terrain, including a 40mm suspension lift. To this point, my boxy Merc has always run a 33-inch all-terrain tire, but the Germans always impress, and the 463-model Gelandewagens can magically stuff a 35 into the wheel wells. Taking the 70-pound monsters to our local Discount Tire, they mounted up to the factory wheels, requiring minimal weight per tire to achieve balance, an early testament to the quality of construction (concentricity). Once all five tires were installed on the G-Wagen, my initial impression was a positive one, at least visually. The tires were aggressive, but thoroughly modern, eschewing trendy or gimmicky design elements. I like the side lugs, and also appreciate the squared carcass profile. When mounted black wall out, the branding is subtle, but again, contemporary. Accelerating away from the tire store, I did note the increased rotating mass and the change to the effective gearing, but both were manageable- my G-Wagen was never fast to begin with. Having removed a set of all-terrains, I was expecting the STT Pro to be loud, but I measured less then one db increase in tire noise. There is a light hum, increasing slightly when in sweeping turns, but these really are quiet mud terrains. The low noise is primarily owed to the tread face design, the blocks alternating in size, angle and number to influence pitch and volume (i.e. noise canceling).
For road performance, I was impressed by a few factors, principally traceability, which is difficult for a mud terrain to achieve. The tires would hold a line at speed, requiring minimal correction at the wheel. Another notable attribute was limited wander on the highway, where cracks, grooves and surface irregularities imparted negligible directional shift; the result more consistent with an all-terrain than a high-void mud-terrain. The most apparent impact to the big Mercedes was the increased rotating and reciprocating mass of the 35″ diameter and the thick sidewalls. These tires are heavy, both from the three-ply radial construction, but also the deep tread. The STT Pro is 3-7 pounds heavier than most competing mud-terrains, an indication of their intended design- performance and durability on the trail.
It is difficult to find mud in Arizona in late spring but I managed to locate enough to validate a few unique design features of the STT Pro. It is certainly evident that Cooper has started to push the performance envelope and a few design elements come to the forefront in mud: 1. Mud release dimples are molded into the outer lugs, working much like the fuller of a sword, easing the evacuation of mud from the captive channel. 2. The outer lugs also employ mud scoops into the lug face, functioning like a paddle, and complimenting the alternating scalloped tread profile, which presents a sharp (and taller) edge to the mud surface, enhancing traction. While it is impossible for me to test each of these elements individually, I can say that the collective whole is effective in the mud, and the tires cleared easily and continued to pull with minimal wheel spin. I also noted the excellent directional stability, the benefit of a staggered lug profile and interrupted lateral grooves.
On the rocks, the STT Pro was again impressive, a testament to both the compound and the tread pattern. The compound is only average in durometer, but the ratio of carbon black and silica provide good adhesion. Traction on rock is also aided by the sharp lug profile and siping, both improving mechanical keying and micro-deformation. This tire will no-doubt be popular with the rock crawling segment because of the good adhesion, and lug keying attributes, but also the durable sidewall. Cooper tires are famous in Australia for their durability and reliability; much of this reputation is owed to the Armor-Tek3 sidewall which employs two (thick) primary plies with a third protective ply running at an angle to the radial layers. Lug integrity will also be excellent, with the dual-draft profile and connecting structures. The compound also includes anti-chip additives to improve durability at higher speeds on gravel roads and in lose, sharp rocks. Cooper also integrated a new technology for rock crawling (i.e. low pressure) performance, designing a location-specific flex groove into the sidewall to control the exact location the carcass will deform at low pressure. This buttress region manages how the side lugs interact with the obstacle, improving deformation and ensuring that maximum mechanical keying and traction is obtained.
Overall, the STT Pro is a high-quality performance tire for both the overland and recreational 4WD enthusiast, designed to be smooth and quiet on the highway (for a mud-terrain), yet afford the greatest possible tractive attributes in mud and on rock. This is not intended to be a mud-only tire, but an aggressive all surface solution that is easy to live with day to day. Look for a long term review on the STT Pro once we get a chance to test it in more mud, in deep sand in even some snow.
The STT Pro will be available for order June 1, 2015 with few weeks delivery time to the customer.