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How Off-road Vehicles Work


The right vehicle, sporting the right equipment, can handle almost any obstacle. See more off-roading pictures.
Steve Allen/Getty Images

In recent years, millions of Americans have purchased four-wheel- and all-wheel-drive vehicles. Many of these folks never change the factory design of their truck or sport utility vehicle (SUV) and never tackle a driving challenge more severe than commuting to work through snow or rain. But a few feel the inexorable pull of nature and the road less traveled. These intrepid explorers spend countless hours and a few extra dollars to modify their factory trucks and SUVs into off-road vehicles capable of chewing up dirt roads, crawling over rocks and climbing near-vertical hills. A few even purchase vehicles -- all-terrain vehicles (ATVs) and utility-type vehicles (UTVs) -- designed exclusively for cross-country adventures.

If you're hearing the call of the wild and feeling the need to explore nature, then you might be ready to become an off-roader. But before you head for the hills, take some time to understand what off-roading is all about and what it takes to transform a factory vehicle into an off-road vehicle. Going unprepared into the wilderness can have serious consequences. You can get stranded. You can get lost. And, in a worst-case scenario, you can get hurt. In fact, the U.S. Consumer Product Safety Commission voted in October 2009 to write mandatory rules to regulate UTVs, following 116 deaths and 150 injuries since 2003. Many of these accidents were caused by vehicle rollovers, which occurred because drivers failed to follow the manufacturer's safety instructions [source: CBS/AP].

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So, take it slow and easy. Understand how the various systems of your vehicle work. Then understand how to modify those systems to ensure that your vehicle functions effectively and safely in off-road situations. This article will provide a good first step. In it, we're going to cover everything you need to know about off-road vehicles, from the ground up. And speaking of the ground, let's start with off-road tires, which increase your vehicle's clearance and give it traction in dirt, mud, snow and sand.

Off-road Tires

For a 4x4 vehicle equipped with mud-terrain tires, a drive through mud and water is a walk in the park.
For a 4x4 vehicle equipped with mud-terrain tires, a drive through mud and water is a walk in the park.
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The primary function of tires is to provide traction. For normal vehicles, this means gripping a relatively smooth paved or gravel surface. For off-road vehicles, this means gripping a variety of surfaces, from rocks and boulders to sand and dirt to snow and mud. As a result, tires are one of the most important purchases you'll make for your off-road vehicle.

Before you head to the local tire shop, think about the kind of driving you'll be doing. It's possible to buy smooth-riding tires (or street tires), but this implies you'll rarely, if ever, be driving off regular roads. As this is an article about off-road vehicles, let's set aside any discussion of smooth-riding tires and focus on the three major types of off-road tires available for four-wheel-drive vehicles:

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  • If your vehicle will be splitting its time between highway and off-highway use, it will need all-terrain tires. These tires feature a tread design with interlocking tread elements that provide good traction in dry snow, ice and mud, as well as on paved surfaces. They are relatively inexpensive and have good longevity.
  • If you spend more time driving off-road, then you should consider mud-terrain tires. With aggressive tread patterns and larger lugs, these tires offer extraordinary gripping power in all kinds of off-highway terrain, especially wet snow and mud. They are also extremely durable, with super-tough sidewalls to absorb the impact of rocks and other off-road hazards. They will, however, set you back a few more dollars than all-terrain tires and will lose their tread faster.
  • You might want dedicated snow tires if you drive in areas that get lots of the white stuff. Snow tires stay soft and pliant even in subzero temperatures. They also have a number of grooves and channels, known as sipes, in the tire tread. Sipes bite into wet snow and ice to improve traction in slippery conditions.

Because ATVs and UTVs are off-road vehicles by definition, they generally come standard with mud-terrain or all-terrain tires. Sand tires are also available for ATVs that hit the dunes. Sand tires feature either horizontal or V-angled paddles, or extrusions, that churn sand and deliver superior traction.

Once you've chosen a tire, you can decide on a wheel. Up next, we'll talk about the types of wheels available for your off-road vehicle.

Off-road Wheels

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You should consider wheels only after you've chosen tires. Generally, larger is better for off-road driving, but it's the weight of your vehicle that will determine the tire and wheel sizes for your vehicle.

After you settle on a wheel size, you need to think about composition. There are two basic choices: alloy wheels and steel wheels. In alloy wheels, aluminum is the predominant metal, which makes for a lightweight wheel. The reduced weight helps your vehicle get better mileage and puts less strain on the bearings and other parts. But alloy wheels aren't as sturdy. A good collision with a rock can break the wheel, which may be difficult to replace when you're in the middle of nowhere. Steel wheels offer greater strength because they feature large amounts of iron mixed with carbon and other elements. The added strength of steel wheels will cost you more in terms of mileage and wear and tear, but they are more practical in off-road conditions.

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You might also consider a bead-lock wheel if you're going to air down your tires a significant amount. To understand why, it helps to understand how wheels and tires work together. On a normal wheel, air pressure pushes against your tire's bead, a ring of steel wire shaped to wrap around the rim of the wheel. As pressure increases on the bead, it presses against the lips of the wheel, keeping the tire firmly attached. As pressure decreases, so does the grip of the bead. Air down too much, and your tires can spin on the wheel or, worse, peel completely off.

That's where external bead-lock systems come in. External bead locks feature a ring that clamps the outer bead of the tire to the wheel with a series of bolts. Some bead-locks use eight bolts, while others use 32, 36, even 40 bolts. The number is significant because you need to check bead-lock wheels regularly to make sure the bolts are tight. If you have a 32-bolt system and five tires (including the spare), you'll be checking 160 bolts!

As you can see, you must put a lot of thought into the wheels and tires you buy for an off-road vehicle, but other big decisions still await you. On the next page, we'll look at the many options available for off-road suspensions.

Off-road Suspensions

Off-road suspensions must perform well on the trail and the highway.
Off-road suspensions must perform well on the trail and the highway.
Sami Sarkis/Getty Images

Vehicle suspension has three main jobs: smooth out bumps, keep tires touching the road and control vehicle stability. You can read more about the basics in How Car Suspensions Work, but let's do a quick review. The spring absorbs the energy of forces acting on your wheels. The two most common types of springs are coil springs and leaf springs. Both types absorb energy well, but require shock absorbers to decrease or dampen their vibration after hitting a bump.

Each vehicle wheel gets a spring and a shock absorber, but the front suspension isn't the same as the rear suspension. That's because the front two wheels must turn right and left. The front suspension also absorbs more braking torque. Accordingly, front suspensions tend to be more complex.

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The front suspensions of off-road vehicles tend to be even more complicated for two reasons: The wheels are usually bigger, and the terrain offers nastier bumps. The front suspension systems of four-wheel-drive vehicles and SUVs most often feature a solid axle with leaf springs. Such a suspension is called a dependent system because the wheels are connected laterally so that they move together as a unit. Leaf springs attach to the frame of the vehicle and then, via U-bolts, to the axle housing. A sway bar mounted to each side of the axle controls body roll.

Other 4x4/SUV front suspensions include the following:

  • Solid axle with coil springs, another dependent system that replaces multiple-leaf springs with coil springs. Many off-road drivers prefer coil springs because they have a compact design and deliver a quieter, smoother ride.
  • Independent front suspension (IFS), a type of suspension in which the two front wheels move independently. In an IFS system, an upper and a lower control arm attach to the wheel on one side, to the frame on the other side. Springing is accomplished either with torsion bars, which act like straightened-out coil springs, or coil struts, which combine a coil spring and a shock absorber into a single unit.
  • Twin-traction beam (TTB), or twin I-beam, a suspension designed by Ford to combine the best of dependent and independent systems. A TTB system has two beams at the front of the vehicle. Each beam mounts on a pivot on one end, on the wheel on the other. The beams overlap quite a bit, so they act in essence like long control arms. A U-joint in the center allows for independent movement of both beams.

On the rear of most 4x4 vehicles, you'll almost always find a solid axle with leaf or coil springs. Off-road drivers often mount shock absorbers on opposing sides of the axle -- one in front and one behind the axle -- to reduce axle tramp, a rapid up-and-down motion of the rear axle caused by sudden acceleration.

Off-road Kits

Some 4x4 owners never modify the stock suspension on their vehicles, but most do. The primary goal of a suspension modification is to raise the body and frame of the vehicle to create additional ground clearance. This also provides room for larger tires and increases suspension travel, which refers to the amount of vertical wheel movement allowed by the suspension.

Several manufacturers make lift kits, or lift systems, specifically for this purpose. Before you invest in a lift kit, think about your off-road driving habits. High-speed desert running requires a much different suspension modification than rock crawling. And don't forget to think about the on-road driving you'll have to do. If you have a dedicated off-road vehicle -- one that will be towed to and from the trail -- you might consider an extreme kit. Most people aren't that lucky, so they choose a kit that provides good performance on both street and trail.

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There are two basic types of lift kits. To understand the difference, it helps to visualize the foundation of an off-road vehicle. Most trucks and SUVs feature body-over-frame construction. In this design, a steel frame acts as the foundation onto which the body and all major parts are attached. Now let's tackle suspension lift kits. These kits raise a vehicle by suspending the frame, body, engine and power train above the wheels. In other words, the distance between the axles and the chassis increases. Such a kit typically costs more and requires more installation expertise to handle any braking, steering and axle modifications that must be made to the vehicle.

Body lift kits, the second type, work a little differently. They use a system of blocks, or spacers, to raise the body above the frame. In other words, only the distance between the body and the frame increases, which means the geometry of the power train and the steering is not affected. You still enjoy more clearance for your tires without having to modify any of the core suspension components. As a result, body lift kits cost less and require less installation expertise.

Lift kits make your vehicle more functional, but they don't necessarily make it a smooth ride. In the next section, we'll discuss the equipment you'll need for cushioning that bumpy ride.

Custom Off-road Seats

Rough road? Make sure you have off-road seats to stay safe and comfortable.
Rough road? Make sure you have off-road seats to stay safe and comfortable.
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You may be tempted to make your butt a footnote when it comes to outfitting your off-road vehicle. Don't do it. A good seat can keep you comfortable and safe through even the roughest rides. These days, serious truck, SUV and buggy drivers rely on suspension seats, which suspend the driver in a web of elastic cords. The webbing lies beneath the seat cover and acts to reduce the force applied to the driver as the vehicle slams down or wrenches to the side. This keeps the driver focused on driving instead of trying to stay upright.

As with any piece of off-road equipment, the devil is in the details. Everything begins with the frame. It should be made of mild steel tubing, which gives the seat a strong, lightweight foundation. Next, check the quality of the suspension system. The webbing should be fabricated with military-grade cords, with secondary rubber straps added for extra support in the tailbone area. A high-quality liner will cover the suspension in one seamless piece and tie to the frame with extra-strong parachute cord. The foam that attaches to the liner offers the best choice for customization. Some seat manufacturers will custom-fit the foam cushion to your body contour and driving position. Finally, the seat cover should be made of vinyl-coated fabric for extra durability and include slots for 3, 4 or 5-point harness systems.

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When it comes to ATVs and UTVs, your decisions about seats will focus as much on configuration as composition. The traditional ATV form features a motorcycle-type seat for a single rider. Newer ATVs, however, come in two-up models that seat two riders comfortably. And some switch from one-up "work" mode to two-up "play" mode using a quick-switch process in which the passenger seat folds up and out of the rear dump box.

A UTV may be a better choice if you routinely carry two or more passengers. Utility type vehicles come standard with side-by-side seating for two, and a few models offer seating for four. You can usually choose between bench and bucket seats, with the latter affording a bit more support and protection on rough terrain.

Off-road Lights

Auxiliary lights can brighten your off-road adventures.
Auxiliary lights can brighten your off-road adventures.

When night falls in the wilderness, far from the array of lamps lining city streets and interstates, you'll welcome the glow of your vehicle's lights. Most 4x4 vehicles roll off the factory floor with some off-road lighting. For example, fog lamps -- high-efficiency round lamps capable of producing a low, wide beam -- come standard on many trucks and SUVs. ATVs sport headlights, as well, with some featuring a tri-beam system that allows the driver to run the low beams, the low and upper high beams together or turn them all off.

Most off-road drivers choose to supplement their vehicle's factory lights with auxiliary lights. If you do, here are some things to consider:

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  • Round vs. rectangular. Off-road lights come in round or rectangular designs and sizes. Round lights range between 5 and 9 inches (13 and 23 centimeters) in diameter, while rectangular lights typically come 5 by 7 inches or 6 by 9 inches (13 by 18 centimeters or 15 by 23 centimeters, respectively). Many off-road drivers outfit their vehicles with both round and rectangular lights.
  • Housing construction. Off-road lights must be durable enough to withstand abuse. Look for a sturdy housing and a shatterproof polycarbonate lens. Some manufacturers offer mesh covers to help protect the lamp.
  • Lighting efficiency. There are two things to consider here: the shape of the lens and the bulb technology. Many off-road enthusiasts prefer round lights because they feature parabolic lenses with the filament located at the focal point. This design throws a more efficient light pattern than a lens with a square or rectangular shape. Xenon bulbs also increase light efficiency over halogen bulbs. A xenon bulb provides more light, but consumes far less power. Not only that, the light it produces is whiter and more similar to sunlight (see sidebar).
  • Beam patterns. Many lamps are available in fog or driving designs, a designation indicating the kind of beam produced by the lamp. Fog lights throw a low, wide beam that doesn't extend very far in front of the vehicle. Driving lights send a narrower beam a much greater distance. Some lamps produce a "pencil" beam, a long stream of light with a range of 2,500 feet (762 meters). And still others can have their beam pattern adjusted.

Of course, selecting off-road lights is just the beginning. You also have to mount them. Light kits, which typically come with lamps, wiring harness, relay and switch, can make installing lights much easier. Lastly, a light bar, mounted to the bumper or roof, can provide the extra surface you need to hold all of your auxiliary lights.

Off-road Winches

A gnarly bumper, such as this one on a Hummer off-road vehicle, makes a winch much more practical.
A gnarly bumper, such as this one on a Hummer off-road vehicle, makes a winch much more practical.
Scott Olson/Getty Images

If you drive off the beaten trail long enough, you'll eventually have a stuck vehicle that needs to get unstuck. This is known as "recovery" in off-road vernacular, and a winch is the most common type of recovery tool. A winch is a hauling or lifting device consisting of a rope, cable or chain winding around a horizontal rotating drum. A power source, usually a motor, turns the drum to spool up the line. On most off-road vehicles, the winch mounts to a bumper, usually the front.

Those are the basics. Next, you need to think about the kind of winch to install based on the challenges you anticipate facing. Hydraulic and power-take-off (PTO) winches require a running vehicle to operate. The former works via fluid pressure coming from a hydraulic pump driven by the main engine. The latter is driven by a gearbox mounted on the rear of the transfer case. Both deliver good power, so they are ideal for heavy lifting. And they can run continuously without draining the battery or overheating. However, they can't operate if the motor is dead, although you can hand-crank some PTO winches in this situation.

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The most common types of off-road winch are electric winches, which draw power directly from the vehicle's battery. For this reason, they can run with the engine off, but they won't work indefinitely. They also generate less power than hydraulic or PTO winches, though in most recovery situations, they function well.

You can use either steel cable or synthetic rope in an off-road winch. Some enthusiasts argue that steel cable is superior, but some synthetic rope boasts a breaking strength of almost 20,000 pounds (9,072 kilograms) -- more than twice as strong as steel. More important, rope is much lighter, which makes the entire winch package lighter, which in turn puts less strain on the vehicle's suspension. Either way, you might want to think about upgrading your springs to handle a winch's extra weight. And don't forget to upgrade your factory bumper, which likely won't be strong enough to handle the demands of self-recovery winching. Look for an aftermarket bumper that supports extreme loads without intruding on your vehicle's crumple zone.

If you're still on the trail for more information, keep driving to the next page. We have a full list of links and articles about off-road driving.

Related HowStuffWorks Articles

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