Cooper STT Pro MudTire
Tire testing an oldfangled way
You’ve read hundreds of tire tests and reviews, many of them right here, but most are a snap-
shot 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 you 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 Findlay, Ohio, as headquarters for R&D. USA-designed and USA-made are nice things to see today.
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 341⁄2 inches, but the real, in the driveway, true, reallife, and actual measurement for this set of tires on my Jeep at max load pressure is 331⁄2 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.
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 axlesand 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 mudand 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, aroundtown, and highwaydriving too.
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.
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.
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.