Ken Brubaker
Senior Editor, Four Wheeler
Cooper’s new Discoverer STT PRO is currently the most aggressive tire in the company’s light truck tire portfolio, and we recently had the chance to spend two days abusing the tire on- and off-road in the high country of Colorado. The design and construction of the STT PRO is remarkable, but is its performance as impressive? Read on.
Construction and Design
The STT PRO is the replacement for the popular Discoverer STT. The STT PRO uses Cooper’s proprietary three-ply carcass construction in the tread and sidewall, which the company says provides 50 percent more protection against impact damage than a standard two-ply tire. This system includes two normal plies that run at 90 degrees from the bead and a third Armor Tek3 Protective Ply that overlays the two normal radial plies at a unique angle to maximize carcass protection. And speaking of protection, the STT PRO also has a Rubber Rim Flange Protector that is, quite simply, a raised rubber ring on the lower sidewall area that helps to protect the wheel and tire from damage while off-road. The STT PRO is designed to excel in mud. The tire features a Mud-Flex Design (the back side is ramped to allow the tread block to flex, providing improved control and stability), Mud Release Dimples (small dimples along the shoulder tread block walls create mini air pockets that help prevent mud from sticking between the tread blocks, thus aiding self-cleaning), as well as Scallops and Mud Scoops (asymmetrical scallops on alternating lugs direct mud into the mud scoops to enhance traction). But since mud isn’t the only off-road obstacle, Cooper designed the tire with features that also help in sand, rocks, and loose soil. For example, there are Side Biter Cleats (large cleats of rubber on the shoulder area of the sidewall that are oriented in such a way as to promote traction on inclines to reduce tire side-slip) and a Flex Groove (it’s a small, circumferential groove in the buttress region of the tire that generates a designed flex point in the tire to help provide a smoother ride and allow the tread and Side Biter Cleats to better conform to obstacles when the tire is aired down off-road). Helping to evacuate mud and stones was also a priority in the design of the tire and there’s wide lateral grooves in the tread as well as strategically placed raised rubber ribs and pads at the bottom of the tread grooves. And speaking of tread, the STT PRO uses an alternating 3-2 pattern of inner tread ribs. Why is this important? Well, this pattern offers benefits including reduced road noise, increased stability and handling, great soft surface and mud traction, balanced wear, and reduced tendency for vehicle drift. Finally, last but certainly not least, the STT PRO has a silica-infused compound (also includes a carbon black mixture that helps provide improved on- and off-road wet traction, shorter stopping distances, and lower rolling resistance), variable depth siping (helps reduce cutting and chipping at the ends of the sipes by minimizing stress concentrations), and cut and chip compound additives (minimizes abrasions to the tread pattern caused by rocks and debris).
Testing
Cooper was clearly confident in the STT PRO and demonstrated that confidence by inviting us to flog its new tire over the course of two days in a variety of terrain. Base ops was Grand Junction, Colorado, and some of the places we accessed included the Long Slough Reservoir Road in Grand Mesa National Forest as well as Bangs Canyon. The STT PRO tires were mounted on a fleet of non-Rubicon Jeep Wrangler Unlimiteds. Heavy rain in the days preceding our arrival left much of the Grand Mesa and Long Slough areas waterlogged. The majority of the forest roads were slicker than snot and perfect for testing. The STT PRO had no problem navigating the slippery mud on Grand Mesa and the built-in Mud Release Dimples, Scallops, and Mud Scoops worked exactly as intended. As we tooled along at over 30 mph, the tires scooped, cleaned and repeated the procedure every rotation, offering a good bite into the mud that resulted in outstanding control. One of the toughest traction tests of a tire is to go from mud to slippery rocks and roots and we did that throughout the afternoon of day one. The trail was deep in the forest where the sun is filtered through trees and most of the terrain was damp or soaked with moisture. The STT PRO tires took a beating that day, with wet, muddy tread blocks grabbing at sharp rocks and the Side Biter Cleats latching onto pointy rocks and slippery tree roots. At one particularly slimy uphill section there was simply nothing for the tires to latch onto and we had to use forward momentum to make the climb. During the ascent, the sidewalls of the STT PRO tires were taking a beating from roots and branches, and it was then that the three-ply sidewalls showed their toughness, suffering no damage. At the end of the day, we saw no chunking of the tread blocks, which says a lot about the STT PRO’s ruggedness.
Day Two started with a hilly, lunar-type soft-terrain landscape near the Grand Junction airport. The STT PRO gripped this terrain with no problem, and showed amazing traction on extremely steep ascents and descents. The tread of the tires provided very impressive lateral traction as well, which is critically important when off-camber. The afternoon of day two was spent crawling sandstone ledges and exploring rock-strewn trails. Once again, the STT PRO just went about its job, gripping and pulling. It’s also worth noting that there were no tire failures during the two-day test. And since most of us have to drive our rigs on pavement the majority of the time, it’s important to note that the STT PRO had very good road manners. We actually spent more time off-road than on-road, but it was obvious that the STT PRO was quiet at speed for an aggressive tire, it didn’t feel “floaty,” it offered stable cornering, and tracked very well.
Bottom Line
In the end, we were impressed at Cooper’s new Discoverer STT PRO. It has the ruggedness and performance we want off-road, and it demonstrates civility on-road. We’re hoping to do a long-term test with the STT PRO soon that will include sand and snow, so stay tuned.
The STT PRO uses an alternating 3-2 pattern of inner tread ribs that, among other things, help to decrease road noise. In this photo you can also see the Mud Release Dimples, Scallops, Mud Scoops, and raised rubber ribs and pads at the bottom of the tread grooves.
Some of the testing took us deep into the woods on trails that were littered with slippery stones and tree roots. This is where the high-void tread of the STT PRO proved its worth as the beefy tread lugs hooked onto every available traction point and pulled the vehicle forward.
Deep water and slippery mud were no match for the STT PRO’s aggressive tread. The silica-infused tread compound seemed to be one of the key aspects to the tire working well in wet conditions.
In the dirt of the lunar-type landscape near the Grand Junction airport, the STT PRO stuck like glue.
The STT PRO did well on rock ledges, offering good traction for a mud-terrain tire.
Traction on descent is critical when traveling off-road to ensure smooth control of the vehicle. During testing, we found the STT PRO adhered well to the terrain as we navigated down rock ledges.
With the STT PRO tire pressure in the low 20s, the tire’s built-in Flex Groove helped the Side Biter Cleats get in contact with traction points.
One of the things we appreciated about this test was the access to Cooper’s team, including engineers. Lunchtime of Day One was a time when we could ask questions about the tire and get answers straight from the folks who were integral in the design and testing of the tire.
SPECIFICATIONS (as tested)
Tire: Cooper Discoverer STT PRO
Size: LT315/70R17
Type: Radial
Load range: E
Max load (lb): 3,195
Approved rim width (in): 8.0-11.0
Tread depth (in): 21/32
Tread width (in): 10.0
Section width (in): 12.6
Overall diameter (in): 34.57
Maximum psi: 65
Weight (lb): 66.6
Available Sizes
As of October 2015, the Cooper Discoverer STT PRO is available in 15 to 22-inch wheel diameters in a 29.5 to 37-inch overall diameter. Depending on size, the STT PRO will be either a C, D, or E load range.
