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Mired In Tires: Tire Tech For The Dirt

Posted in How To: Wheels Tires on January 10, 2017
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Photographers: Trenton McGee

Often, the first modification any off-road fan makes to his or her 4x4 is to add tires and wheels. This is for good reason. New tires and wheels can totally change the look and feel of a rig making it more of a reflection of the owner all while honing the utility of the 4x4 to suit the owner’s needs. Tires on a 4x4 perform many unenviable jobs, and while differences in construction tread design, shape, air pressure, and more, and their effect on performance are usually subject to opinion, you can trust us to help you learn about what tires are right for you.

We here at Four Wheeler have years of experience using tires in a wide variety of environments, and while we, like all humans, have opinions about tires and how they perform, you can bet our opinions are rooted in off-road experience and reality. Having said that, tires are also part of your vehicles safety equipment, and nothing about vehicle safety should be taken lightly.

Tire Size

If you’ve ever been off-road, then you’ve probably been stuck. If you haven’t then, you aren’t trying hard enough. One of the more common reasons a 4x4 gets stuck off-road has to do with the undercarriage hitting the trail (dirt or rocks) or getting mired in mud, silt, or sand. One way to combat this is to add a larger-diameter tire—one that increases the clearance between the trail and the undercarriage. The idea is simple—simple, that is, if there is space for more tire. Generally speaking, going up a tire size or two is possible on most 4x4s. More than that and chances are you are going to need a suspension lift, fender trimming, more custom stuff, or all of the above. The best formula for adding bigger tires is to either run what fits without hitting or, if your wallet and drivetrain can handle even bigger tires, to add as little lift as possible. Bigger isn’t always better, and you can do lots of four-wheeling on stock-size tires.

Just about any 4x4 looks better with larger than stock tires, but increasing tire size isn’t always a good or simple proposition. Increasing tire size is akin to changing your stock trucks overall gearing. This can hurt your trucks performance, fuel economy, and more.

Mo’ Tire, Mo’ Problems
Changing tire size also changes your 4x4s overall drive ratio. Bigger tires have the effect of using a higher gear, like going from First to Second, or Second to Third, giving you that higher-speed pull. Smaller tires gear a 4x4 down, like going from Second to First, or Third to Second, giving you more grunt, but who wants to do that? You’d lose precious ground clearance. When we tire size-up for more under-4x4 clearance, we also (unintentionally) gear up. That can be bad, making your 4x4 feel more sluggish from a stop and adversely changing fuel economy in town and on the highway. How bad is that and how big of a change will you experience? That depends on the percentage of tire size change you’ve made. Also larger tires require more clearance. That means you need suspension or body lift, or custom parts. On top of that, larger tires are harder on your vehicles axles, brakes, suspension, and more. If you don’t want to make a number of changes to your 4x4, then you may want to stay with stock-sized tires, or just up-size slightly.

Tire Size and Electronics
Bigger tires can throw things way out of whack, and the newer the vehicle, the more can change. New 4x4s have lots of technology that will be off with larger size tires. Many newer 4x4s will require the use of an electronic tuner to adjust the tire-size parameters for the truck to drive, shift, and function as intended. We’ve even heard rumors of some newer SUVs and trucks rendered inoperable when tire size was changed due to ground-sensing sensors.

Tire Size and Axle Gears
The only way to reclaim lost power and torque is to adjust the gear ratio of the drivetrain of your 4x4. The easiest way to do this is to regear axles. There are many sources that can help you figure out what gear ratio will work best with your new tire size. Generally, the best approach is to figure out the percentage of tire change and adjust gear ratios accordingly. Since gear ratios are fixed, you may have to go up or down slightly to meet the closest ratio for your tire size and axles.

Tire Size Math
(New tire diameter/old tire diameter) x original gear ratio = new gear ratio Example: (32/28) x 3.73 = 4.2629 (since that doesn’t exist, you have to move up to 4.30:1 gears or down to 4.10:1 gears).

Newer 4x4s have gobs and gobs of electronics. If you change tire size on most trucks built in the past 10 years or so you will probably have to tell the 4x4 what’s going on. Otherwise it may freak out or transmission shift points could be so off that the transmission could get damaged. Best-case scenario, the speedometer will be off a few miles per hour. Some vehicles can have their computers adjusted with a simple tuner; others need to have the computer adjusted by someone with advanced software and knowledge of what changes need to be made. We sent the PCM from our ’05 Chevy Tahoe to Tilden Motorsports to have info on tire size and axle ratios modified, as well as to have info that would allow our 2WD computer to understand what’s happening when the truck is in low range.
Regearing your 4x4s axles is the easiest way to compensate for larger tires, and getting this done properly can get costly, especially because it requires specialized knowledge and tools. At this point, it might be a good idea to add lockers. It’s also a good time to replace all of the bearings associated with the axle’s ring-and-pinions since there may be wear and damage to these seldom-serviced parts. And yes, that means in a 4x4, both front and rear axles must be regeared. The parts and their associated costs racks up quickly.

Tire Construction and Design

A tire is more than a doughnut-shaped balloon for your wheels. They are more of a composite of materials, like fiberglass, designed to bend and flex without damage. Made of carefully calculated rubber compounds layered with varying types of structural fabrics and steel wire, a tire has more anatomy than just shaped rubber. And while tires have evolved a lot since the time of Ford’s Model T, many of the basic tire-carcass designs carry very old technology. There are two types of tires that are both named for the orientation of the tires plies, another name for the fabric and metal structures inside a tire.

Radial tires have a carcass made of plies or cords that run diagonally from one bead (where the tire mounts on the wheel) of the tire to the other perpendicular to the circumference of the tire. Radials also have a steel belt or belts that run radially around the circumference of the tire just under the tread. This helps stabilize the tread of the tire and allows for less material on the sidewall of the tire. That makes for a smoother ride and long-lasting tread wear. Unfortunately for off-roaders, less material on the sidewall means they are more prone to damage from sticks, rocks, and other trail debris. Despite this, modern quality radials will have multiple sidewall plies that help radials resist sidewall damage much like a bias-ply tire. If your 4x4 gets driven on-road a fair amount, then a high-quality radial tire is what you need.

Bias-ply tires have a carcass made of plies or cords that run from one bead of the tire to the other at an angle, or at a bias relative to the circumference of the tire with multiple plies in a crisscross pattern. This makes the tires have a rounder profile and since several layers of plies are necessary the sidewalls of a bias-ply are stiffer (and generally much more puncture resistant off-road) than radials. Bias-ply tires are great off-road, but the tread wears out faster than radials, they “flat spot” when parked for more than a few hours, and they tend to follow ruts or surface changes on the road. Dedicated off-road rigs that see little on-road driving benefit from the brick-outhouse strength of a bias-ply

It’s pretty amazing what kind of abuse modern radial tires will take and still run down the road at speed without any issues. Sure, the heavy-duty radials are heavy like bias-ply tires and lightweight radials generally have weaker sidewalls than bias-ply, but mud-terrain radials are much more behaved on-road than any bias-ply mud tire. These huge 42-inch Goodyear MT/Rs have Kevlar in the sidewalls to help prevent punctures.
Our pal, Shane, loves running old-school bias-ply tires on wide (10-12-inch) wheels. It’s a method of getting traction and a look that works for him, and he doesn’t get many flats despite years of playing in the sharp rocks of Arizona and California. At the same time, we’ve never seen or heard of Shane driving his awesome CJ-5 on the road.

Any true off-roader will gravitate towards a mud-terrain tread design over any other tire type. Mud-terrain tires work well off-road over a wide variety of surfaces (not just mud). The wider spaces between beefy tread lugs act like fingers grabbing at surface irregularities, digging for something solid to hold onto. Mud-terrain tires are also generally built on a stronger carcass, better resisting punctures and damage. This is because the tire company knows what the mud-terrain tire buyer is after, namely durability at the expense of treadwear, noise, and on-road manners. Having said that, if your 4x4 is used mostly on-road, mud-terrain tires are probably not for you. As we’ve hinted, they can be noisier than other tread types, wear quicker, and can reduce on-road performance (like cornering, fuel economy, inclement weather traction, and more).

In our opinion, there are two types of tread design tires for your 4x4: mud-terrain and all-terrain. If you own a 4x4 that will never go off-road, then you need to look elsewhere for tire advice—we don’t understand you. All-terrains are a great compromise for anyone who uses their 4x4 on-road a lot. They are quiet, great for on-road use during inclement weather, and they provide longer treadwear. Also, with ever-evolving tire compounds, many all-terrain tires work very well on the trail, some so well that we’ve even considered abandoning mud-terrains for those more well-mannered tires.

Tall Skinnies vs. Short Fatties
We’re not talking about the looks of who you’d rather date but rather the shape of your tires. Arguments have been made for either tall skinny tires or short wide tires being better or worse in many different environments. The basic debate is as follows: skinny tires will sink to the bottom of a loose surface where there is traction, while wide tires will “float” on top. We can see benefits and minuses to both sides of these arguments. Some surfaces don’t have bottoms, and 4x4s are heavy and won’t always “float.” The question generally boils down to variables that are impossible to control and difficult to interpret. Skinny tires would, in theory, provide a smaller contact patch with more pressure per square inch, while wider tires allow for a larger contact patch with less pressure per square inch. When you go out four-wheeling, your tires are going to see multiple different surfaces so your tires will have to work everywhere, and how you drive, what lines you take, and what works or doesn’t, will be up to personal preference. The same can be said about general tire shapes with tire pressure, tread design, compound, and more making a greater difference than general tire shape.

Tire Pressure
Fine-tuning your tire’s air pressure makes a huge difference off-road. Knowing how and when to air down is critical to making it farther down the trail, and carrying the equipment to air back up at the end of the trail is also critical to a safe and fun time in the dirt. Airing down is a fine balance between gaining more traction and having the tire slip off the bead and go flat. Different environments and different vehicles require different tire pressures. We’ve run tire pressures on heavy trucks in the low 20 psi range and seen small 4x4s run 0 psi on the trail effectively. On most compact and 1/2-ton–rated 4x4s without bead locks, you won’t want to drop tire pressure much below 15 psi, and if you are driving fast on hard-packed surfaces, you may want more pressure than that to ensure a bead doesn’t pop at the wrong moment. With a full-bodied, fullsize SUV or a 3/4-ton truck, you may want to stay above 17-18 psi. Any lower than that and you’d better be prepared and equipped to change a tire or reseat the bead.

Different tires can carry different loads and require varying amounts of air pressure to do this. Understanding what your vehicle weighs and what the tires you want are capable of is important. A tire’s sidewall will tell you what amount of weight that tire can support at a maximum air pressure, and you should always ensure that replacement tires can support the weight of your vehicle and what it can hold.

There are basically two types of tread design useful for off-road use. Most dedicated off-roaders overlook the on-road noise and lower treadwear ratings of mud-terrains for the improved traction and durability on the trail. Having said that, modern all-terrain tires work very well off-road thanks to beefy sidewalls and modern rubber compounds. The larger tread blocks and voids of mud-terrain tires are vital when rockcrawling on jagged surfaces where tires can grab the rock edges and in mud where smaller voids easily get packed with mud turning your tires into slicks.
Airing down can be done using many different tools. We’ve aired down thousands of times, often using a key or a small rock to let air out of the tire when nothing else is available. We’ve also used valve core tools to remove the valve core to let air out. All of these work, with the latter being much faster, but it also includes the danger of letting too much air out or losing the valve core. We also like deflators like this one from ARB. Once you’ve figured out how to use them, they are a great tool, release air fast, and capture the valve core so it can’t launch like a tiny missile. There are also automatic deflators, balance hoses, and special valves that require special holes in your wheels, depending on how fancy you’d like to get.
Same tire, same rock, but very different air pressures. One has 35 psi (the maximum)(top image) and the other about 6 psi (bottom image)—a good pressure for rockcrawling with beadlock wheels. This demonstrates how airing down increases the contact surface of a tire while off-road. The image with 6 psi shows the tire enveloping the rock while the image with 35 psi shows the tire barely balanced on top of the rock. Airing down too low can be dangerous since tires can slip over the wheels safety bead and rapidly deflate. Even on-road, it’s probably not necessary to run the maximum tire pressure unless you are approaching the maximum load rating with your vehicle when fully loaded.
A tire’s sidewall carries lots of information including size, air pressure maximum, age, sometimes country of origin, and load ratings. Load rating is an important yet often overlooked aspect of running aftermarket tires. You always want to maintain your vehicles tire load rating so that tires don’t get overloaded. Overloading tires causes heat that will lead to damage and could cause an accident. Most light truck tires are rated C, D, or E.

Used Tires

Tires can be an expensive investment—or divestment. As a result, many folks want to buy used tires for their 4x4s, and this can be a great way to save money. We’ve bought and sold a few sets of used tires but only when the tires meet a few strict criteria. Again, this is a safety concern. Any tires with dry rot, cracking, chunks missing, and/or damage should be avoided.

Date Codes
Since 2000, all tires sold in the U.S. have information on when they were built. The last four digits of the DOT number indicate the week and year the tire was made. Most, if not all, reputable tire mounting facilities will refuse to mount a tire more than 10 years old. This is for a good reason and not just an arbitrary amount of time that should force you to buy new tires. As a rule of thumb, we advise against buying any used tires that are more than eight years old—assuming they look great and are not dry and cracked. As tires age, they “breath” and release chemicals and oils that they need to stay flexible and remain in one piece. Even those stored inside can go bad and come apart when least expected. That can kill you. If you need tires for a trail-only rig or something to roll a project around on, you may be able to stretch these rules.

Flats happen, and holes in tires don’t have to be the proverbial nail in the coffin. A lot depends on what punctured the tire and where it was damaged. Generally anything in the center of the tread can be repaired or plugged with pretty good luck. Damage to the side of the tread or sidewall is fairly fatal. Now, different people have different ideas about what is acceptable, and we’ll say that lots of on-trail tire damage can be repaired to hold enough air to get a rig back out from the backwoods. Running down the highway with any hole that requires more than a plug or two can quickly cause a problem. Plugs don’t like speed, and tires that go flat quickly cause cars to behave erratically. Rarely, holes in tires can be vulcanized and repaired, but don’t count on it, and reserve any tire that has large repairs as an emergency trail spare only. When it comes to buying used tires stick to ones that don’t have plugs or any repairs.

Your tires sidewall also tells you when it was built. Any tire more than eight years old is suspect when it comes to hauling oats down the highway. Keep those old tires on roll-around projects or on trailers to the trailhead. Yes, this tire is from 2001. It’s not safe for road use and probably should get recycled.

Vintage Tires
Any tire more than eight years old, regardless of how cool it is, can be a liability on a vehicle that sees regular road driving. Luckily there are a few companies who still make retro-style off-road tires. Honestly, we think this is an underserved market and would love to see retro-looking off-road tires on radial carcasses. Until then, resist the temptation to run old tires unless your rig is off-road only and you are prepared to deal with the consequences of a cool looking, yet brittle, tire breaking apart.

Small holes in the center of the tread are generally repairable. Holes on the side of the tread or on the sidewall are fatal to the tire. We’ve cut tires and filled large holes with plugs on the trail to get back to the trailhead. Driving down the road at speed with a large tire plug is a bad idea because the damage can get worse and the plug can easily get dislodged, which will cause the tire to go flat fast. It’s not a good idea for a 4x4 that will see road driving and highway speeds.
We’ve even dabbled in having damaged tires repaired and vulcanized. While we can’t recommend this for any tire that sees use on the road, repairing and reusing an otherwise-good tire for a trail spare is a great plan. Notice how this newish tire is smooth in the center of the image and missing a letter from the name? That’s because a rather large hole was patched there. If you do this and someone gets hurt, don’t blame us—it’s your fault and you should have known better.
Our pal, Mike, has kept this cool retro Armstron Tru-Trac 11-15LT tire. It won’t travel down the road at speed ever again, but it makes for a nice nostalgia piece. We’d love to see more retro-style tires like this on radial carcasses.


Renton, WA 98057

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