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Long-Term Tire Test: Cooper STT Pro

Posted in How To: Wheels Tires on July 6, 2018
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You’ve read hundreds of tire tests and reviews, many of them right here, but most are a snapshot of the tire over a matter of days or weeks of use. Those are perfectly legitimate and do yield valuable information. However, when possible we’re going to start giving you long-term tire tests, during which a select group of experienced Jp Magazine staff and contributors will put months’ worth of driving, on- and off-road, on a set of tires so we can tell your how they hold up over the long haul. These long-term tire tests will begin with a first impression of the tire’s performance, noise, physical attributes, specifications, and if possible details about other sizes of the tire in question being offered—that’s what you’re about to read. Short updates on the tires will follow every few months for as much as a year, depending upon the specific situation.

Cooper Tire is a brand that just about everyone has seen in advertisements, on the trails, and at the mall. They have been building top-quality tires since way back when Willys-Overland was building sedans. Old dogs can learn new tricks though. A few years ago, Cooper Tires opened its Global Technical Center in Finlay, Ohio, as headquarters for R&D. USA-designed and USA-made are nice things to see today.

Our trusty test lab is this 1979 CJ-7 powered by a 258ci inline-six with a 4.0L head swap, NV3550 transmission, Dana 300 transfer case with 4:1 gears, AMC 20 rear axle with Warn full-float axles and ARB Air Locker, narrowed Dana 44 front axle with ARB Air Locker, and a three-link front suspension. Weighing in at around 4,750 pounds loaded for bear, the Jeep has wheeled the mud- and moss-covered rocks around the mountains of Pennsylvania, and now spends time in the desert dirt and slickrock of Utah. It also sees snow in the harsh Utah winters, and lots of top-down, around-town, and highway driving too.

A set of 35x12.50R15 tires soon arrived all wrapped in paper, and I stared at them like Ralphie when he opened the Red Ryder BB gun—in my pajamas since they arrived at 8 a.m. The tire sidewall shows 35 and the spec sheet says the actual measurement is 34 1/2 inches, but the real, in the driveway, true, real-life, and actual measurement of these tires on my Jeep at max load pressure is 33 1/2 inches. We have seen this difference in a few tire brands, and it’s the same for a number of “35s.” The load range on this particular size is 113Q, which translates to: 113 = 2,535 pound max load (per tire) and Q = 99 mph maximum speed. There is no doubt we will never get near the limits of either of those specs with this old Jeep though. Weighing in at 67 pounds of rubber, this tire seems to be about average for the size. The tread depth is nice and deep (advertised and actual measurement) at 21/32 inch.

Overall, the early impressions of this tire are positive. This is a real-world, long-term test on these tires. We will be reporting back periodically for updates on our experiences with terrain performance, road manners, wear patterns, and robustness. They will indeed be put to the fire.

As soon as the 35x12.50R15 Cooper STT Pro tires were mounted on the Jeep, we were headed to Easter Jeep Safari 2018 to start testing the new rubber in Moab, Utah. New tires are stiff and need some flexing to break them in for the best traction. In this picture it seems like the tires are at street pressure, but they are actually aired down to around 12 psi. By the end of the first day on the trail the tires were wrapping around the rocks.
Since we have the convenience of selectable ARB Air Lockers in the CJ-7, we drove unlocked off-road as long as we possibly could so that the tires would show their worth unassisted. The tires met a variety of terrain from solid slickrock, larger boulders, loose rubble, and silt in Moab, and impressed us in all of these situations. The lockers did have to be engaged on some intricate obstacles a few times throughout the week, but in general, the Cooper Discoverer STT Pros met our expectations.
Once the tires “broke-in” they adapted to the environment well, grappling the obstacles as they came. Even the aggressive sidewall biters got in on the action here. The rubber compound was redesigned for this tire for better traction in wet conditions, but I think it’s safe to say that change provided traction benefits in most situations.
One of the more challenging areas in the general Moab area is the White Sand Dunes. Cruising along the beach is one thing—blasting around the dunes is another. Sand requires experienced driving technique as well as special adaptations to the tire. The STT Pros were aired down to 10 psi to spread the load over a larger footprint, and that made a big difference, but a little too much throttle and wheelspin on this climb and the lugs on the tires dug the Jeep in deep. Thankfully, the flotation characteristic of the tires allowed for easy reversal out of trouble.
A little momentum goes a long way in the sand, and with the right tire even a sand newbie can get by. The Coopers did their job of keeping themselves on the surface and paddling the Jeep forward.
The tread design has been focused around these key elements: traction, stability, wear, and road noise. Alternating large and small side lugs provide a paddlelike action in the mud, snow, and sand. It also reduces road noise when paired with the alternating 3-2 pattern center lugs. The vertical walls of the lugs have relief pockets molded into them to help chunks of mud release and prevent tread clogging. The voids are also specially designed to reduce the possibility of rocks being caught in the tread, which can be dangerous if dislodged at high speed. After the first 1,000 miles on these tires the road hum could not be heard over the sweet sounds of the motor, wind, and stereo.
Some trail damage did occur to one of the tires, and it happened on day two of Safari week. We noticed it at the end of the day, so we’re not entirely sure how or where it happened. The Crystal Geyser trail north of Moab is littered with particularly sharp, pointy rocks. This is the first sidewall slice we have experienced in years. Some may say that’s lucky. Since it happened on brand new, two-day-old tires though, I say that’s just my luck. I had a Cooper STT spare on the back so I was covered.
I did not change out the tire after the sidewall damage. It wasn’t leaking any air, so we wanted to see how tough these tires really were. I wheeled on the damaged tire a few more days making sure that I hit every bump, stump, and rock along the way. I was impressed that the tire never leaked any air the entire week. The Cooper Discoverer STT Pro tires are built with what the company calls Armor Tek3 technology. This is a triple-ply reinforced sidewall that has one offset-ply bias of 8 degrees to reduce gaps in the sidewall where obstructions can penetrate. Cooper advertises the sidewalls to be 66 percent stronger and two-and-a-half times more tear resistant than its previous STT.

A quick look at some popular tire size specifications of Cooper Tires Discoverer STT Pro tires:

Tire SizeService DescriptionUTQGLoad RangeSidewallApproved Rim WidthMeasured Rim WidthSection WidthOverall DiameterTread WidthTire WeightMax LoadTread Depth (1/32")
30X9.50R15LT104Q CRWL6.5 - 8.57.59.629.577.7244198519
31X10.50R15LT109Q CRWL7 - 98.510.730.758.548227020
32X11.50R15LT113Q CRWL8 - 10911.731.579.0954253520
33X12.50R15LT108Q CRWL8.5 - 111012.232.729.860220521
35X12.50R15LT113Q CRWL8.5 - 111013.134.5710.0867253521
37X12.50R17LT124Q DBLK8.5 - 11101336.7710.275352522
37X13.50R17LT121Q EBLK8.5 - 111113.336.7710.9884319522
40X13.50R17LT121Q CBLK8.5 - 111113.639.7610.9892319522

Sources

Cooper Tire
Findlay, OH 45840
800-854-6288
coopertire.com

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