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Cooper Discoverer STT Pro: Super Traction Tire

Posted in Product Reviews on November 15, 2015
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Cooper Tire & Rubber Company has a manufacturing history spanning just over a century and is currently the fifth largest tire company in North America. Cooper offers a handful of different SUV and light truck tires from very mild to all-terrain to mud-terrain. Each tire in the lineup is meant to provide competent highway handling but serve a second need in the dirt, with increasing levels of tread aggressiveness.

We recently had the opportunity to test out Cooper's newest mud-terrain offering, the Discoverer STT Pro. The tire is currently available in diameters up to 37 inches, with talk of going up to 40. Load ranges span C to E, with over 40 sizes available to fit 15- to 22-inch wheels.

The major tread portion utilizes a 3-2 inner tread pattern to break up tire noise harmonics while allowing for the adequate voiding needed on an open-tread mud tire. Along the side, Cooper adds side biters to aid with shoulder traction in terrain where your sidewall is under heavy surface contact.

The outside tire material is a chemically infused silica and carbon black tread compound. This optimized mixture increases wet-weather traction and treadwear life. Modern tire compound technology has progressed significantly in recent years, and the STT Pro design team sought a compound formula that met some lofty goals. They also used a tread feature design to minimize chip and tear of the tread under aggressive use. Additional tread surface features include mud-release dimples, side scallops, mud scoops, and variable-depth siping on some tread blocks.

On the side of the tire, Cooper's Armor-Tek3 sidewall construction is a full three-ply layering. The assembly starts with a typical two-ply stack, followed by the third protective ply that is placed at an angle to the first two. The angular stagger of the final ply improves sidewall damage resistance above traditional three-ply designs.

An annoying characteristic of some off-road treads is their habit of small stone and gravel retention in the tread area. Sometimes those trapped rocks leave the tread at high speed, raining on windshields on the highway. The STT Pro designers expended effort to reduce stone retention with the use of asymmetric draft angles and gravel ejection features in the thread. Simply put, they smartly engineered tread block angles and rubber bars in the void areas that prevent rocks from sticking, allowing them to exit the tire tread at low speeds or at a low pitch angle behind the moving tire. Keeping the tires free of stone entrapment also helps prevent stone drilling, which is tire carcass damage due to repetitive rock impacts.

Between the main tread area and the side biters is a molded flex groove. It is strategically placed in the buttress region to allow the tire a more compliant point of flex. This smoothes highway ride on rougher surfaces. Off-road, with lowered air pressure, this groove allows the tread and side biter areas to transition more smoothly over terrain edges. Cooper also adds a raised rubber area adjacent to the wheel to provide some protection against wheel scarring.

We tested road performance with the tires aired to 30-32 psi on Jeep JK Wranglers. If we didn’t already know, we wouldn't have guessed we were riding on a mud tire. Road feel and tracking were excellent for a tire of this type. Tire noise was pleasantly mild with only a slight hum. We left the highway to test off-road traction at about 18 psi under a variety of conditions, including mud, rock, and soft sand. Overall, we were highly impressed with the tire performance in the 35-inch size we were running.

Feedback consistently shows that a lot of off-road tire buyers are heavily influenced by tread appearance. If you want real performance, resist the urge to buy on cosmetics alone. Fortunately, we think the STT Pro satisfies the eye-candy desire, and it’s got the traction performance to stand behind it. We may have just found our next favorite mud-terrain for our vehicles that see dirt and highway use.

The new Cooper STT Pro is like, but unlike, the former Cooper STT. Both were designed as high-traction off-road tires with good highway manners. What the original could do the Pro can do better, especially in wet conditions. We found the road manners of the STT Pro to be excellent for a mud tire, with little noise and impressive tractability.

We had to try out the mud savvy of these tires so plopped them into some Colorado goo. Although the tires would load with thick gooey mud, they seemed to keep propelling us forward, pushing the mud through the tread without much spin or loss of traction.

Cooper incorporated a number of tread features to improve the tire's ability in the mud without having to resort to huge tread voiding that would have hurt highway performance. On various tread blocks, they added mud release dimples, scallops, and small mud scoops. Tread siping was also added in strategic areas.

Cooper redesigned the side biters as well, keeping them as an integral part of the tires ability to grab traction. We liked them on the original Cooper STTs, and we like these even better.

We climbed and descended a trail with slippery roots interspersed with loamy soil. With little spin, the tires were able to maintain traction and preserve forward momentum. Again, it seems the diverse tread features have the ability to provide additive traction for each terrain encountered.

Running the tires at 17-18 psi, we were surprised that they performed so well in soft sand going up and down hills. Even with the fairly aggressive tread design, they resisted the tendency to dig with reasonable throttle input.

Lateral traction held up well on the highway and during our dirt testing. The 3-2 inner tread pattern offers solid traction in all directions.

Between the main tread area and the side biters is a molded flex groove. It is designed as a foldover point in the tread to allow the tire increased flex to conform to terrain at low pressures. The tough sidewalls on these tires are also a benefit to those playing heavily in wooded areas or rocks, and the flex groove helps the tough sidewall flex as needed.

The Jeeps tackled some ledge climbs on Moab-like slickrock. On the steeper faces, or with jerky throttle play, we could get them to bark a little. With smooth throttle they gripped well seemingly due to the compound and the variety of tread structures in use, including the tread siping.


Cooper Tire
Findlay, OH 45840

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