Cooper Discoverer STT PRO Review

Dec. 18, 2015 By Josh Burns, Photos by Josh Burns and CJ'Z Photography

Just a few months ago, Cooper Tire unveiled its most aggressive off-road tire to date, the Discoverer STT PRO. This new tire, which offers larger voids and a more aggressive tread design than the company’s S/T MAXX, is the company’s first true mud-terrain and is rated for both mud and snow travel.

We were fortunate enough to get our hands on a set of the new STT PRO to see how they perform on a wide variety of terrain. Read along for our impression of this new tire.

Technical Info
Up to this point, the most off-road savvy tire in Cooper’s lineup was its S/T Maxx, which also features an aggressive tread design but falls under the all-terrain category more so than a mud tire. Long-time contributor Jaime Hernandez reviewed Cooper’s S/T Maxx tire back in 2011 when it was introduced, and overall his impression of the tire was positive – and he put them through tens of thousands of miles after that report. His only real gripe along the way was that the largest size available initially was a 33-inch version, though since then Cooper has expanded to a 35-inch offering.

For the new STT PRO, Cooper looked to create a tire that could tackle the most challenging terrain. As they put it, the STT PRO is its “most advanced, extreme tire to date.” So the STT Pro needed to offer superior traction in harsh terrain while also being tough enough that it wouldn’t fail under the demands of serious abuse.

Featuring Cooper’s proprietary rubber compound that blends silica and a carbon black mixture, the STT PRO features a three-ply carcass construction in the tread and sidewall, which the company says adds up to 50 percent more protection against impact damage compared to a conventional two-ply tire. This third layer for the carcass is what Cooper calls ArmorTek3, and this more durable radial compound is laid over two normal radial plies to help give the tire added durability. Cooper also constructed the tire with a raised rubber edge that sits right near the bead contact at the wheel to help protect the wheel and tire from trail debris.
As shown here, Cooper employs an alternating tread pattern of three-two for the inner tread blocks, which Cooper says provided ideal traction during testing and low road noise. 

The tread design for the STT PRO is what Cooper calls a hybrid layout that uses an alternating pattern of three tread blocks to two tread blocks for the inner tread pattern. During testing, Cooper found that this design provided superior traction stability while also reducing road noise.

This illustration of the STT PRO tread shows the scallops and mud scoops (shown in green) on the out tread blocks and also the small dimples on the outer tread (shown in orange) to help keep mud from sticking to the tire.

The outer tread blocks feature the largest voids of any light truck tire from Cooper to offer superior traction in soft terrain such as mud and snow. Each outer tread block is constructed with small mud-release dimples that actually create small air pockets to help prevent the mud from sticking to the tread. For additional traction in soft terrain, Cooper also employs alternating scallops and mud scoops on the outer tread blocks.

Being that the SST PRO is Cooper’s most aggressive tire to date, it’s no surprise that it also features its most aggressive sidewall design, taking the aforementioned S/T MAXX’s aggressive sidewall design a step (or two) further. Larger side-biter cleats are employed on the shoulder and extend well onto the sidewall itself, separated only by what Cooper calls a “flex groove,” which is a small groove line between the tread and the sidewall to provide a smoother ride on all terrain – especially with the tires aired down for the trail. Cooper says the sider-biter cleats of the STT PRO are orientated in such a way as to prevent tire side-slip on the trail. Anti-stone retention blocks and wide lateral grooves are used in the STT PRO’s tread design to resist stone drilling, reduce stone retention and promote self-cleaning of the tread.

With over 5,000 miles on the STT PROs to date, we're impressed with its performance both on and off the highway.

Impressions from the Trail and Highway
For our testing of the Discoverer STT PRO, we’d be outfitting a well-traveled Ford F-150 4x4 that spends most of its life in the mountain town of Mammoth Lakes, California. It’s not be the shiniest truck in town, but it has put in its fair share of time on the trails and has been a reliable vehicle over the years. All it needs a new set of dependable tires that could tackle the wide variety of terrain the mountain town has to offer: dirt, rocks, mud, and, thanks to El Nino, plenty of snow. Coincidentally, the tires that the STT PROs would be replacing on the truck are Cooper’s Discoverer A/T3 all-terrain tire, which saw plenty of use in their lifespan and held up well in spite of the punishing they took on the trails around Mammoth.

We took the previous tires off and mounted and balanced the 305/70R16 STT PROs we’d be installing on the truck. Our first positive note came during this process, as we were happy to see that no wheel required more than 5 ounces to balance (most of them were 3 to 4 ounces). We were impressed (and honestly, a little surprised) to see the tires balance so well for being such an aggressive tire.

The first impressions of the tire on the highway were positive, and that’s an impression that hasn’t changed over 5,000 miles. Moving from an all-terrain to a mud-terrain tire, some additional road noise is expected with the STT PRO, but we were impressed that the road noise really isn’t too bad. In terms of handling, we were pleasantly surprised how well the tire driving around town and on the freeway. The STT PRO has been well-mannered on the highway for such an aggressive tire, providing great traction and a smooth ride overall. Looks can be deceiving to an extent, because the aggressive look of the STT PROs don’t appear as though they would be good as a daily driver, but, all things considered, they’ve been great in this regard.

The grip of the STT PRO is both dirt and mud is excellent and where this tire is as its best.

Off the highway is where the STT PRO really shines. Having put tens of thousands of miles on the previous Cooper all-terrains, the amount of traction and grip was a night and day difference once we switched over. The grip of the STT PRO is tremendous in soft terrain.

The STT PRO provides great traction in snow as well.

We actually drove the truck into trails covered in fresh snow that was about 1 to 1.5 feet deep while keeping the truck in 2WD, and the tires helped pull the truck through and never got stuck. Even when the going gets rougher and 4WD is necessary, the STT PRO has performed well in every situation we’ve encountered – post-rain mud, rocky slopes, ice-and-snow-covered trails, etc.

Cooper's self-cleaning tread design of the STT PRO clearly works, as we didn't notice any major buildup or sticking or snow or mud, which means the tire retains its great traction when needed most.

The features Cooper engineered into the tread design to eject snow and mud clearly do their job, as the tread blocks have stayed free from buildup and keep consistent traction in soft terrain. Mud and snow buildup equate to a loss in traction, and during our testing we noticed no such problems. This truck sees a lot of snow during the winter months, so the fact that it doesn’t retain ice in the tread like some mud-terrains affirms Cooper’s claim of the STT PRO’s snow capability.

The tires now have just over 5,000 miles under their belts and are due for a rotation, but there are no signs of uneven wear. In spite of tackling some plenty of rough terrain (rocks, trees, snow and mud), there is no chunking from the tire (large chunks missing from the tread). In spite of the punishing they’ve been through, there are only minor signs of wear.

Regardless of the terrain, the Cooper STT PRO has been a great tire during testing overall.

Final Verdict
Cooper aimed to create an extreme off-road tire, and we think they hit the mark dead on. The STT PRO has the look of an aggressive tire and its performance backs up that appearance, but we’re most impressed that it still features good road manners, confident handling, and a well-designed tread compound that isn’t showing major signs of wear despite being punished during its first 5,000 miles. 

We’re also happy to see Cooper launched the Discoverer STT PRO in a wide variety of sizes. The STT PRO will be available in up to 37-inch tire, available for 15- to 22-inch wheels, 41 sizes in total. For truck and SUV owners who need a tire that will perform in just about any terrain, the Discoverer STT PRO is a great option for the outdoor explorer. Newsletter
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