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Cooper Tires Discoverer S/T MAXX

by Moses Ludel

Cooper Tires Discoverer® S/T MAXX™

Our tests confirm that Cooper Tires Discoverer® S/T MAXX™ tires deliver superior highway and off-pavement traction. Testing included interstate, city and a variety of off-pavement uses. During the tests, the magazine’s Jeep® XJ Cherokee plied Nevada’s remote high desert and wild horse country!

Test and Evaluation: On- and Off-Road Performance

Jeep® XJ Cherokees now serve a large segment of off-pavement users. Our tire test vehicle has served well for fifteen years, benefiting from a 6-inch long arm suspension system with quality gas charged shock absorbers and 33″ diameter tires. Wheelbase and off-pavement performance compare with a Jeep® TJ/LJ Wrangler. Today, XJ Cherokees are found in every kind of urban and four-wheel drive venue.

The Test Vehicle: 1999 Jeep® XJ Cherokee 4×4 Sport 4-Door

Our Jeep® XJ Cherokee has been an excellent test bed for tire testing. The goal is consistent, accurate tire comparisons. This Jeep® has an optimal off-road suspension package and chassis height. We routinely check front wheel alignment and tire pressures. Recommended tire rotations and balance checks are always part of our long term tests.

Tire size often dictates the degree of suspension lift. We set up the XJ Cherokee for safe highway use, good ride quality and stable, easy handling—without sacrificing off-pavement ability. To get 33″ diameter tires beneath stock XJ Cherokee wheel wells and fender flares has required a 6-inch lift.

A long-arm lift helps assure the proper “arc of radius” as the four-link and coil front suspension works. The front end rises and compresses; proper caster degrees are critical for stable straight line steering. Caster helps tires steer straight as the vehicle comes out of corners. This Full-Traction Suspension 6-inch long arm lift installation has worked well for over 90,000 miles now.

6-inches of lift requires a “long arm” (long radius arms) kit. Short arm lift kits work to a maximum of 3″-3.5″ lift on an XJ Cherokee. Beyond this level of lift, a short arm suspension system will create front end geometry problems that contribute to the notorious “death wobble”.

Death wobble resembles a traditional “kingpin shimmy” at the steering knuckles. With short link arms, this can be caused by a poor arc of radius and abnormal caster angle changes over that arc. Axle misalignment, a weak steering damper, poor wheel alignment, loose knuckle ball joints and worn tie-rod ends can also encourage a front wheel shimmy.

Four doors and a hatchback make SUVs more versatile. While the rare 2-door XJ Cherokee with a 4.0L six and manual transmission is now prized, the 4-door XJ Cherokee’s success was overwhelming. Between 1984 and the 2001 end of production, the 101.4″ wheelbase Cherokee sold over 2.8-million units. Discovered by savvy, cost-conscious four-wheelers, the XJ Cherokee has become a popular on- and off-road 4×4. Today, this easy, inexpensive to service model has large support from the aftermarket.

The 6-inch Full-Traction Suspension lift kit has delivered nearly 100,000 miles of trouble-free service! Routinely inspected to assure safe and precise steering and suspension, this lift kit includes a dropped track bar and sway bar links plus a dropped pitman arm for proper geometry. Toe set and wheel alignment are performed at our shop/studio (see coverage). This provides consistent tire test results.
We test tires suitable to our vehicle’s intended use…Our pick for this long range on- and off-pavement tire test is the Cooper Tires Discoverer® S/T MAXX™ LT 255/85R16.

The Tires: Cooper Tires Discoverer® S/T MAXX™ LT255/85R16 LRE

16″ x 8″ American Racing Baja ATX wheels have been our baseline wheel rims for tire testing. This rim size is popular for the Jeep® YJ and TJ Wrangler as well. We use Load Range E tires for maximum stamina and load carrying capacity when aired down. A tire diameter of 33″ has been consistent; however, we have used 285/75R16 tires in previous tests with competitive brands. For the Cooper Tires Discoverer® S/T MAXX™ test and evaluation, we picked the LT255/85R16 size. This size works slightly better with our XJ Cherokee body and the 6-inch long arm lift.

The tire type and size complement the suspension layout. These tires rate a 123/120Q Service Description in Load Range E (LRE). A true “Light Truck” tire, they have a no-nonsense black sidewall and fit a 6.5″-8″ width rim; optimal rim size for this tire is seven inches. The alloy American Racing wheels fit the maximum range with an 8-inch width.

Tire sectional width is 10.2″, the limit for our fender wells, flares and bumper clearance with the beam front and rear axle articulation. A tire diameter of 32.8″ is close enough to 33″ and also maintains speedometer accuracy. There is no need for a speedometer driven gear tooth count change.

Tire weight of 58 pounds plus the rim weight of 25 pounds brings each assembled tire/wheel to 83 pounds plus wheel nuts. This is plenty of unsprung weight mass, handled by the higher spring rates of the FTS lift kit and its gas-charged shock damping.

These tires can each support 3,415 pounds at full inflation pressure. The XJ Cherokee’s curb weight with aftermarket bumpers, armor, a winch and oversized tires tips the scale near 3,800 pounds. Light truck (LT) tires fit higher gross vehicle weight (GVWR) trucks. On a lighter XJ Cherokee, we run tire pressures at 24-26 PSI on-highway. Thick sidewalls and LRE rating allow airing down to 14-18 PSI for loose sand and slick mud.

Note: For stock size and load rating tires, we never lower pressure below 18 PSI, even under severe off-pavement use. Lower pressures drop a tire’s load carrying capacity and compromise the sidewall stamina. Oversized tires like LT255/85R16 LRE on a lightweight XJ Cherokee are way stiffer and far more resilient than the OEM size/load range tires.

These tires feature Armor-Tek3® Carcass Construction for durability, resilience and longevity.

Cooper tires have a reputation for long tire life. Armor-Tek3® is the latest technology for tire durability. The construction of the Discoverer® S/T MAXX™ targets our kind of multipurpose use. A combined goal of “tough terrain, including rocks and gravel, with on-road stability and performance” is the desirable benchmark we want in our 4×4 tires.

An impressive 18.5/32-inch tread depth assures long mileage when wheel alignment and tire air pressure stay within specification. We rotate tires at regular intervals and check balance at the same time. This is now easier with our Gaither equipment.

Wheel Alignment Note: We follow OEM wheel alignment specifications, adding slightly more positive caster when possible. As mileage on the steering linkage adds up, slightly more toe-in than spec improves tracking on-highway while maintaining normal tire wear. Beam axle camber should always be on spec; cross-camber should be noted, too. If necessary, camber and caster can be changed or fine adjusted with Specialty Products Company (SPC) offset steering knuckle ball joints.

We were drawn to several of the Discoverer® S/T MAXX™ tread design attributes. From experience, a diagonal tread block with normal void space provides the balance for lateral stability and good grip on- or off-pavement. Armor Tek3® Carcass Construction adds to durability while the tougher tread compound resists cuts and chips. As expected, these tires are Mud & Snow (M+S) rated, optimal for our all-season interstate and secondary highways at northwest Nevada and the High Sierra region.

These tires spec out well on paper. LRE tires are long life workhorses on our lightweight 4x4s. Traditionally described as “10-ply rating”, in the radial tire era we rate these tires as LT (light truck) with high load carrying capacity: Load Range E. These tires can carry a load both on- and off-highway—including camping gear over two-track dirt trails.

Mounting and Balancing: Prepping Tires for Testing

Accurate tire testing and comparisons require a well set up vehicle and properly mounted and balanced tires. We have traditionally used Discount Tires for mounting and balancing. Friends at the Sparks/Los Altos store do exceptional tire balancing work. Accuracy has been the target with our test tires. Unfortunately, the store is a forty mile drive each way from our shop/studio.

For convenience and precise control of tire balancing, we recently equipped our shop for in-house mounting and balancing. The Gaither Tool Company GT12-1US commercial computer tire balancer was our pick for accuracy. These Discoverer® S/T MAXX™ tires became the first full set of tires balanced on our new equipment.

We mounted these tires by hand with a European Gaither 12772 manual tire changer and two tire irons. Experience came from commercial tire mounting decades ago. Overlanders can benefit from this method. The Gaither mounting stand was modified for field or remote trail use behind a 4×4. For insight into manual tire changing—anywhere—see the 46-minute video and article!

Manual tire mounting assured a safe fit and can be done anywhere. We broke a good sweat mounting LRE Discoverer® S/T MAXX™ tires. The follow-up was the purchase of a new Weaver W-898XS tire changer with the optional (“Combo”) W-PL240 Assist Arm. For a light truck/4×4 shop, an active 4×4 club or the serious DIY enthusiast, this rim clamp style automatic tire changing machine will handle 49″ diameter tires and a 15″ maximum wheel width.

There were insights gained when mounting and balancing our own tires. After working in the tire service industry and teaching tire service at the adult education level, there were two takeaways from using the GT12-1US balancer on these Cooper Discoverer® S/T MAXX™ tires: 1) these tires have well constructed, sturdy carcasses and 2) aftermarket wheels pose real challenges for tire service work and balancing!

The new Gaither GT12-1US computer tire balancer is a manually spun, highly accurate computer balancer. Designed for large commercial truck tires, the machine’s weight limit is 330 pounds! Our large 37″x12.5″x17″ Cooper Discoverer® AT3 XLT tires on the Ram 3500 4×4 weigh 102 pounds per wheel/tire assembly. This portable machine’s spindle can be raised and lowered to lift heavy wheel/tire assemblies from the floor to work level.

Balancing, ideally, should be dynamic method when possible. Aftermarket alloy wheels, however, usually do not provide a place for mounting weights at the faces of the rims. This places weights inboard at the rim flange. On most rims, the inner flange weights can only fit near the centerline.

Tire stores attempt a dynamic or two-plane balance with tape weights staggered inside the alloy aftermarket rim. With weights placed at the inner rim edge and close to the rim’s centerline, a true dynamic balance is not possible. The tire/wheel assembly becomes an imbalanced centerline or single plane balance rather than a dynamic balance.

This is why many big tires and wide aftermarket alloy wheels have excessive staggered weight applied or wind up centerline/single-plane balanced. Without a provision for placing weights at the inner and outer flanges of the wheel (next to the tire beads like with steel wheels), dynamic balancing becomes difficult.

Unfortunately, when tire balancing becomes a problem, the tire manufacturers and tire stores get blamed. Often, the tire design or construction is not at fault. The problem is an aftermarket wheel design that will not allow a true dynamic balance.

This 58-pound tire with 25-pound wheel (an 83 pound assembly) took less than five ounces of centerline/single plane weight to balance. A dynamic balance was not possible for this wheel/tire, the machine called for counter-weighting the wheel—which should never be done. One of these Cooper Tires Discoverer® S/T MAXX™ wheel/tire assemblies took a mere two ounces of weight to accurately centerline (static or single plane) balance!

The Gaither GT12-1US balancer’s accuracy and computer software algorithm quickly identified the root cause of the balance issue. When dynamic balance is not possible, this machine will show the need to counter-weight or counter-balance the tire: placing offset weight at the opposite clock position from where new weights have been installed. When the wheel called for counter-weighting, we removed all weights and switched the balance method to a “static”* or centerline/single-plane balance method.

*Note: The term “static balancing” gets confusing with spin wheel/tire balancing machines. The balancer machine’s spindle is not static. The computer simulates a classic “static” (bubble) type balance. Without the ability to mount weights at the same clock position on the rim flanges (old bubble balancer fashion), alloy aftermarket rims require placing the weight on the centerline of the inner wheel rim. This is actually a “centerline” or single-plane balance. Ideally, clip-on weights should go at the inner and outer flanges of the rim like with steel OEM rims. This would allow a true dynamic balance…Many OEM alloy rims do allow clip-on weight placement at the rim flanges. Aftermarket alloy rims, largely for “looks” and certainly not function, do not have that provision. The magazine’s HD video on balancing these tires illustrates how aftermarket alloy wheels affect balancing results.

Ultimately, with centerline/single-plane balancing, we discovered how little weight these 58 pound tires required. None of the tires had runout, either. Each tire ran true. These are examples of quality tire construction. Quality alloy wheels usually run true as well. This is easy to check with the wheel or assembly mounted on the wheel balancer spindle.

Results were clear with the first drive on Interstate 80. Balancing, both dynamic and static with this set of tires, was optimal. In all steering modes, tire angles and speeds, these tires run smoothly, without any sign of imbalance, tramping or roughness. This reflects Cooper Tires’ quality construction, the accuracy of the Gaither GT12-1US balancer and choosing static over dynamic balancing when necessary.

The Tire Test Environment

The first part of our test was typical urban driving that we all experience during commutes and open highway driving. We racked up numerous trips on I-80 from our base at Fernley, Nevada to Reno/Sparks. Stretches have become congested, much like urban freeways with big rigs and commuters. U.S.A. Parkway Industrial Park exit is now home to the Tesla/Panasonic battery plant, an Apple facility, Switch, Walmart’s largest warehouse, Chewy and many other large businesses.

At times, there is stop-and-go for construction or accident zones, braking and 70 mph lane changes along the curvy Truckee River roadway sections. Buffeting wind and 18-wheeler traffic, winter snow and black ice provide a real world setting for steering and maneuvers tests. City and freeway traffic at Reno/Sparks, now typical of any urban driving, rounded out our asphalt tests. The Discoverer® S/T MAXX™ tires provide a stable, predictable and safe feel under all of these driving conditions. They are also substantially quieter and ride much smoother than more aggressive, wide void “mud” or “Baja” racing tires.

Safe and responsive steering with the ability to track and hold a lane were clear assets. The 4.5″ backspacing on the American Racing 16″ x 8″ wheels gives the XJ Cherokee additional track width to help offset the 6-inch chassis lift’s higher center of gravity. The spring rates and gas shocks have always provided steady handling and easy tracking.

Recently, we rebuilt the OEM steering gear with a reseal and precise worm bearing and backlash adjustments. On pavement, the XJ Cherokee tracks like a slot car with these well-engineered Discoverer® S/T MAXX™ tires.

Tires that work in the back country also need to perform well on the highway. These tires accept TSMI #15 studs for icy winter driving. (See factory stud holes at inner and outer edges of the tread.) To extend tire life, we opt for prudent use of 4WD during inclement weather. Our tires stay on year round for both on- and off-pavement driving. The vehicle serves as a daily driver, commuter and back country access 4×4.
For our off-pavement tire tests, we use a consistent, predictable 4×4 route. The XJ Cherokee has ARB Air Lockers front and rear; however, for these tests, we run open differential mode and highway tire pressures to shift full traction to the tires. 4WD mode, both high and low range, will be used appropriately.

The second phase of tire testing is off-pavement. Our typical route includes a combination of climbs, some moderate rock and sandy, gravel washes. Tires must grip, get bite in mud or sand and resist spinning. All of our tests are low environmental impact, and tires get judged for their footprint. Impact is partly driving technique and partly tire performance. We stay consistent. A tire’s tread gets credit for its ability to clean out mud or slush.

This is Nevada wild horse country. We have a commitment and responsibility to protect and share this open range. That includes horses, sagebrush and other wildlife. A past Tread Lightly board member and 4WD trail guide, my emphasis has always been staying on the trail and not disturbing sensitive flora and fauna.
Keep eyes peeled for signs of wild horses. They seldom pose a threat unless you encroach upon their space or threaten them. Here is a sign, an unshod hoof print in a vital late summer rivulet of spring water. At the high desert, horses will range miles to access these sparse water sources.
This is the water source, a hillside spring. The horses have pawed at the mud during hotter, dry weather. This is a challenging environment. Numerous small bands and harems compete for browse and water. Try not to disturb these settings.
This mature stallion has the scars from many territorial struggles and fights with other stallions. His determination and tenacity keep a harem and band alive. Horses vie with other animals, fend off predators and battle elements for scarce resources on this range.
An established spring and water resource on BLM grazing land serves as a resource for the wild horses. Water determines territories for these horses. They cover great distances when necessary for grazing and water.
This watering spring has been a riding destination for our enduro or “open desert” dirt bikes as well. A fifty mile view begins with the nearby hills where horses graze. Make a point of not scaring or running the horses. An injury can be deadly for a wild horse on the range. Birthing mares and foals are at high risk and vulnerable. Protect these animals, they have enough challenges. (Photo courtesy of James Langan)
A healthy yearling finds browse and water. On this high range, horses survive minus-20 F winters plus a chill factor from winds raking over the summits at 80-plus mph. This is tough country for equally tough horses!
Sure traction leaves the least imprint in this vital rivulet of water. Note that there is zero tire spin with these Cooper Tires Discoverer® S/T MAXX™ tires.
Clay soil is like glue when wet. Any tire will pick up mud and gravel mix in this soil composition. The mark of a good off-pavement tire is its ability to clean out afterward…
These tires do an exceptional job of cleaning out. Note the sharp tread pattern in the wet gravel. Traction on side angles is stable, a highly desirable trait for slippery four-wheeling.
This is not a wide-void “mud tire” yet the S/T MAXX™ can clean out quickly. A few revolutions after collecting mud and debris, this tire has already cleaned out. For a multipurpose, on- and off-pavement tire, the Cooper Tires Discoverer® S/T MAXX™ tread pattern is as good as it gets!
We rank the Cooper Tires Discoverer® S/T MAXX™ among the best tires available in this class. For our multipurpose 4×4 use, the tread pattern is a wise choice. Driving at a four-season climate with winter highways plus off-pavement driving needs, a 4×4, SUV or 4WD light truck owner should consider this tire!
Leaving their print, we leave ours. Minimize your impact on the land. Cooper Discoverer® S/T MAXX™ LT 255/85R16 LRE tires and the right driving techniques can make a difference.

Our travels are often remote two-tracks, sometimes a hundred miles at a stretch. Destinations include historic ghost towns, landmarks, streams for fly fishing, bird hunting or exploring paleo sites and petroglyphs. Sometimes we tote the motorcycle trailer loaded with a pair of dirt bikes for single tracking and open desert events.

Always short of abuse, we have done some hard trail wheeling with this XJ Cherokee. Trails have included Gold Lake Trail and several runs at Moab. Cooper Tires Discoverer® S/T MAXX™ tires would serve well at any of these venues. A properly equipped 4×4 could do the Rubicon Trail on correctly sized Discoverer® S/T MAXX™ LRE tires. These are safe, reliable tires capable of getting you to your destination and home again!

Considering our multipurpose driving, we rank the Cooper Tires Discoverer® S/T MAXX™ tires among the best tires in this market. If you need tires for all-season highway driving and four-wheeling on weekends or even lengthy overlanding ventures, visit the Cooper Tires site to see the lengthy list of popular sizes available. Cooper Tires offers an excellent warranty and even a trial period to introduce the Discoverer® S/T MAXX™ tires:

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