When I first became interested in getting an SUV or truck that could go off-road, I spent a lot of time just figuring out basic information. The lexicon of off-roading and overlanding vehicles – drive trains, suspensions, wheels, and tires (a.k.a. the stuff that makes your vehicle go) – can be daunting.
Old hands will bombard you with the lingo: four-wheel drive, all-wheel drive, differentials, axle lockers, transfer cases, suspension lifts, coil-over springs, sway bars, traction control, and so on and so on. There is a lot to process.
To help make this learning curve a little less arduous, in this article I will provide a basic overview of the core characteristics and components of highly capable off-road vehicles and what makes them different from regular cars and trucks.
Instead of just throwing a bunch of terms at you and explaining them separately, I’m going to tell you the (helpful, I hope) story of how and why I went from driving a modestly capable Honda Element to driving a Jeep Wrangler JK Rubicon (which I nicknamed the Ordeal Mobile), one of the most off-road capable stock vehicles you can buy.
By the time you finish reading this, you should have a basic understanding of how all essential parts of a vehicle fit together to make it capable of tackling challenging off-road terrain.
End of the Road (or Why I Bought a 4×4)
We’ll start at the end. Or at least the end of the road on a day 8 years ago. I’d recently moved to San Diego for work and discovered the many backcountry roads and trails in the region, which I took to exploring in my Honda Element. On the day in question, I was exploring a truck trail in the nearby mountains.
At first, the road was covered in gravel and pitted with just a few potholes, but as I climbed the mountain, the road turned to dirt and the holes got more frequent and deeper. I started hearing scraping as the Element bottomed out on especially deep holes. The road was muddy in places causing my wheels to slip.
Despite all this, I was determined to get to the top of the ridge. When I did, I could see the road continued down the other side of the mountain and further into the wilderness. It was clear the Element wasn’t up to the job and that it was time to turn back.
But the seed had been planted. In the following months, I would obsess over one question:
What kind of vehicle did I need to keep going?
After much research, I ended up buying a Jeep to replace my Element. There are many other off-road capable trucks and SUVs out there, but Jeeps are highly capable and can be endlessly modified, which appealed to me. I learned a lot in the process of buying my Wrangler and since, from improving it over time.
Below, I’ll give you a brain dump of what I know, in hopes that it’s helpful for anyone in the process of choosing or upgrading an off-road capable vehicle.
Characteristics of Off-Road Vehicles
I’m no engineer, so take this with a grain of salt, but I like to think of capable off-road vehicles as having five key characteristics:
- Excellent traction
- High clearance
- Low-range gearing
- Superior stability
- Durable components
These factors are all closely intertwined and great off-road vehicles are engineered to approach them holistically.
In the Four Wheeler’s Bible, author Jim Allen writes that traction is the “transformation of engine torque into vehicular motion.” He identifies traction as one of two key factors in off-road capability – the other being clearance.
Traction is the tires’ ability to apply enough friction to the ground to push the vehicle forward. Good traction allows a vehicle to accelerate, brake, and turn effectively, while poor traction can lead to sliding, spinning wheels, and an inability to surmount obstacles.
It’s helpful to think of three things that contribute to traction in off-roading, namely the tires, the suspension, and the drivetrain. I’ll get into each of these in more detail below.
Next to traction, vehicle clearance is of critical importance in off-road travel. Clearance refers to a vehicle’s ability to travel over obstacles without parts of the vehicle – wheel’s aside – hitting the obstacles.
Typically, when people talk about clearance in the context of off-road travel, they are referring to the axles and chassis underneath the vehicle. The higher these are from the ground, the less likely they are to hit rocks or bottom out in soft ground, such as snow, mud, and sand.
Large tires are a very common and effective way to improve clearance, as they raise the axles off the ground. My 2005 Element had relatively small wheels and just 6.9 inches of ground clearance. My stock 2015 Jeep Wrangler Unlimited Rubicon had 10 inches of ground clearance, with tires that were 32.1 inches tall. By adding a suspension lift, I was able to use larger, 35-inch tires that brought the axles up another inch and a half.
In addition to letting you run bigger tires, a suspension lift also raises the chassis of a vehicle further above the ground. This creates more clearance for drive shafts, oil pan, exhaust, and other components mounted to the underside of the vehicle.
Clearance is also affected by your vehicle’s approach, breakover, and departure angles, which I’ve covered in another article. Approach, breakover, and departure angles are relevant for climbing up or down steep inclines while off-roading, including climbing over large rocks or boulders.
There is a saying about off-roading that goes “as fast as necessary and as slow as possible.” Or something like that. The gist is that it may take some momentum to get up a steep hill or across some soft ground, but driving cautiously – sometimes meaning “slowly” – is often the most prudent path forward.
Low-range gearing helps you move slowly across terrain while maintaining the necessary wheel torque. Low-range gearing refers to how the power from the engine is geared down to control the speed and torque with which the wheels turn. In fact, in many cases, gearing is more important for off-road capability than the horsepower produced by the engine.
Many four-wheel drive vehicles have four-wheel high and four-wheel low options, which improve traction and control when traveling off-road. This gearing occurs within the transfer case. Four-wheel high is good for driving off-road at higher speeds on smoother terrain. Four-wheel low is better for technical terrain where you need to go slow.
Another place where low-range gearing comes into place is in the axles. Above, I explained how larger tires can improve traction and clearance. They are also heavier and require more force to rotate. It is sometimes necessary to regear a vehicle’s axles to a lower gear range so that the engine can apply more torque to the wheels at slower speeds.
When I put 35-inch tires on my Jeep, the axle gearing was adequate, because I have the Rubicon model, which is geared lower than other models off the lot. If I were to put larger tires, say 38 or 40 inches in diameter, I’d need to regear my axles (among other things).
Stability is a factor in off-road performance often overshadowed by the more exciting topics of big tires, suspension lifts, and powerful engines.
A stable vehicle is one that can progress over the terrain without tipping over and without compromising the passengers’ safety and (relative) comfort. It’s not uncommon to see someone put an extreme lift on a vehicle that raises the center of gravity so high that it becomes prone to tipping over. The ideal is to have enough clearance to travel over obstacles, but not so much as to become tippy.
Suspension articulation also plays a key role in stability. Vehicles with limited suspension travel will have trouble keeping their wheels on the ground on terrain that is highly uneven. This often causes the body to lean alarmingly and can lead to a tip.
Factors that can improve suspension travel and provide better stability are longer control arms, taller suspension springs, and long-travel shocks. While solid axles tend to provide better articulation in highly uneven terrain at slower speeds (e.g. rock crawling), independent suspensions, particularly in the front, are preferred for high-speed desert travel, as the ability to compress separately helps smooth the ride.
For overlanding, where you may be carrying heavier loads for multi-day camping, it’s important to keep in mind how much cargo your vehicle can carry safely. If you are overloaded or top-heavy, it can dramatically impair your vehicle’s stability. For a deep dive into this topic, check out this article on vehicle cargo capacity.
A good rule of thumb is to raise the center of gravity of your vehicle only as much as you need to handle the terrain you plan to explore. Putting a 5-inch suspension lift on an overlanding rig that will travel mostly on pavement and dirt roads is probably unnecessary and raises the risk of a rollover. But a 5-inch lift could be just the thing for a crawler around the extreme terrain of the Moab desert.
One general characteristic of off-road capable rigs is that they tend to be built with highly durable components to take more abuse than your typical car parts. This means they are made of more durable materials and often designed to be thicker and heavier so they are rated to handle greater forces. For example, off-road buffs will replace stock drive shafts with stronger aftermarket drive shafts or swap out stock axles for beefer axles.
There is often a tradeoff between weight and durability. When parts are too heavy, they can be overkill and the added weight puts an unnecessary strain on other components. So, it’s important to think about how much punishment your vehicle will take and try to find the sweet spot for the components you’ll use.
There’s also a trade-off between performance and cost. It’s possible to buy very lightweight AND durable components, but you’ll pay a pretty price for them. For instance, I am trying to make my Jeep as light as possible and have been looking into aluminum bumpers – but they are about twice the price of an equivalent steel bumper. Half the weight, twice the price.
Another feature of many off-road rigs is that they are armored. My Jeep JK came off the lot with some armor such as rock rails and skid plates under the chassis. Armor like this helps protect the body and mechanics of your vehicle.
Components of Off-Road Vehicles
Above we introduced the characteristics of vehicles that hold their own while traveling off-road. Now, we’ll dive a bit deeper into the various components that give them these terrain-conquering superpowers. First, here are a few rules of thumb:
- Bigger tires with knobby treads improve traction and provide greater clearance on soft ground and rocks
- Lifted suspensions with specialized springs, shocks, and other components improve traction, clearance, stability, and ride quality
- Special transfer cases allow for low-range gearing that improves technical off-roading performance
- Axle differential lockers improve traction in situations where one or more wheels lose grip on the ground
- Traction control technologies utilize anti-lock braking systems to improve the vehicle’s handling
While these axioms are helpful for getting oriented, in reality, there are many nuances and trade-offs in outfitting a vehicle for off-road travel.
Tires are where the rubber meets the road…or rocks, mud, sand, and snow. Off-road tires have treads with deep grooves and blocks that help grip soft and uneven ground. These are typically classified as all-terrain tires or, even more aggressive, mud terrain tires.
Larger tires provide the ability to roll over obstructions more easily. They also provide more surface area to connect with the ground, thus increasing traction.
Another way to increase the footprint of the tire on the ground is to lower the tire air pressure, something many drivers will do before hitting the trail. On soft ground (sand, snow, mud), increasing the footprint will cause the tire to “float” on the surface more, preventing it from sinking into the soft ground and getting stuck.
There is a compromise to be made between off-road performance and on-pavement performance when choosing tires. The make and design of tires designed specifically for off-road travel are not ideal for driving on pavement. All-terrain tires are a good middle ground, providing decent performance both on and off road. Mud terrain tires are great off-road, but don’t grip pavement well and tend to be noisy. Also, mud terrains tend to wear quickly if driven on the pavement a lot and don’t help your gas efficiency one bit.
My Element had decent all-terrain tires on it, but they were relatively small (28.7″ tall x 8.5″ wide). This meant they had more difficulty rolling over obstacles and had a smaller footprint, thus providing less tire flotation. So the tires fell into even small potholes and sunk into the mud. I’ll get into it more below, but small tires also limit a vehicle’s clearance.
My Jeep Wrangler has large all-terrain tires (35” tall x 12.5” wide), allowing the tires to bridge larger obstacles. The aggressive tread and large tire/ground footprint provide a great deal of traction, particularly when I air down the tires. I had to install a suspension lift so that I could use these larger tires (see below).
The suspension connects the frame of the vehicle to the axles with a system of springs, shock absorbers, and control arms. The suspension plays a large role in its ability to maintain traction. Think about it this way: the more moving wheels on the ground at any given time, the more traction the tires get and the more motive force they provide.
The suspension allows the wheels and axles to move up and down and (to a lesser extent) forward and backward to maintain contact with the ground. This movement is called “articulation,” and is critical to maintaining traction when traveling over uneven ground.
The suspension also dampens shocks and vibrations caused by traveling over rough terrain, which greatly improves handling and reduces wear and tear on both the vehicle and the passengers.
Off-road capable suspensions consist of several components, including:
- Springs: These are typically leaf or coil springs that provide the main support for the vehicle’s weight. They compress and expand to absorb shocks and vibrations.
- Shock Absorbers (“Shocks”): These are dampers that are designed to absorb the energy from the springs and control the rebound of the springs.
- Air Suspension: This is an alternative to the traditional spring and shock absorber system. It uses airbags or air springs that are filled with compressed air to provide support for the vehicle’s weight.
- Stabilizer Bars: These are also called anti-sway bars (or often, counterintuitively, “sway bars”) and are used to reduce the amount of body roll that occurs when a vehicle goes around corners.
- Sway Bar Disconnects: On technical terrain, being able to disconnect the sway bars from the axle can improve articulation and stability. Some vehicles incorporate sway bar disconnects to make this possible.
- Control Arms: These connect the axle to the vehicle frame and direct and limit the movement of the axle and wheels as they move over the terrain.
In addition to improving traction, these components work together to provide a smooth and stable ride by absorbing road shocks and vibrations and keeping the wheels in contact with the road.
Independent Suspension vs Solid Axles Suspensions
There are two types of suspension setups common among vehicles used for off-roading and overlanding: solid axle suspensions and independent suspensions. Both can work well off-road and many trucks have a mix of both, with a solid axle in the rear and independent suspension on the front.
A solid axle suspension (sometimes called a “live axle”) single axle housing crosses the entire width of the vehicle and connects the wheels on either side. Thus when one wheel articulates, the other moves as well. An independent suspension, sometimes called a “wishbone” suspension, uses separate articulation and dampening systems for each wheel, allowing them to move independently of each other.
The debate over which is better for off-roading is heated and an in-depth exploration warrants an article of its own. As a general rule of thumb, independent suspensions are considered better for faster off-road travel, while solid axles are superior for slow, technical rock crawling.
In theory, independent suspensions can be built to be superior to solid axles for rock crawling, which is why many off-road racing rigs use them. But such outfitting can be very expensive. In practice, many non-pro off-road enthusiasts who are into rock crawling prefer solid axles as they can be built for less cost. That said, there are plenty of very capable vehicles out there with both types of suspension. Also, I might be a bit biased as my Jeep has solid front and rear axles.
Speaking of building an off-road rig, many people will install a suspension lift on their vehicle to improve both traction and clearance. A good lift kit can improve your vehicle’s articulation, which can help a great deal with traction and stability.
My Element had independent front and rear suspensions which provided some articulation, but not nearly as much as the stock suspension that came with my Jeep’s front and rear solid axles. I improved further on the Ordeal Mobile’s articulation with an aftermarket 3.5-inch aftermarket suspension lift. The lift also puts more room between the body and axles which allows me to run larger tires.
The drive train is the third major system that plays a significant role in off-road performance. Put simply the drivetrain powers the wheels to move the vehicle. The engine provides energy that is transferred to the transmission which, through a system of gears, allows the driver to control how much energy is transmitted further down the drive train. From the transmission, the spinning force is passed through the drive shaft to the axle and, finally, to the wheels.
The drive trains on modern vehicles can be very complex. So as to not get bogged into the many different technologies, I’ll focus on the core components that differentiate off-road vehicles from your average car or truck.
Four Wheel Drive vs All-Wheel Drive
When I bought my Element, I was impressed that it had something called all-wheel drive – and indeed it gave my box on wheels an edge over cars without all-wheel drive on wet and snowy roads. But don’t confuse all-wheel drive for four-wheel drive.
All-wheel-drive vehicles, like the Element, use a system of sensors in the wheels and steering to provide information about the road conditions to the car’s computer. An algorithm in the computer then controls the speed of the wheels to optimize the vehicle’s traction for different types of road conditions. All-wheel-drive typically remains on constantly and is common in SUVs and found in some trucks.
All-wheel drives aren’t considered true four-wheel drive as the wheels aren’t all re
A typical four-wheel drive vehicle directs power from the engine to all four wheels, which generally speaking provides twice the traction provided by a two-wheel drive. The drive shaft connects to a device called a transfer case that conveys the power to two additional drive shafts, one powering the front axle and the other the rear axle.
The transfer case allows the driver to toggle between two-wheel drive when driving on dry pavement and four-wheel drive when driving on slick pavement or off-road. It can also let the driver choose different gear ranges of four-wheel drive high range for faster speeds and low range for slow rock crawling speeds.
So which is better? It depends. Sophisticated all-wheel drive systems can be great if you’ll be mostly driving on road and doing some mellow off-road travel. My Element did great on snowy mountain roads and handled better on pavement than my Jeep does.
That said, all-wheel drive has its limitations. The systems aren’t designed for truly rugged off-road trails, so they will not do the best job of maintaining traction. Also, they don’t have the option to lock the axles, something we’ll get into in the next section.
All in all, true four-wheel drive is better for challenging terrain and thus is the preferred option for off-roading enthusiasts. If you are planning to overland mostly and will only be traveling on relatively mellow off-road terrain such as maintained forest service roads, all-wheel drive might be all you ever need.
Differentials and axle lockers
The last drive train components I’ll mention are differentials and axle lockers.
When a vehicle is turning, the wheels on the outside of the turn travel a longer distance than the wheels on the inside. Thus, an outside wheel must spin faster than an inside wheel over the course of the turn, even though they are on the same axle.
Differentials, found on all vehicles, are part of the axle that allows the wheels on either end of an axle to spin at separate speeds. This avoids overstressing the axle, wheels, and tires and all manner of handling issues.
But there is a drawback. Differentials direct power to the wheels that get the most traction, which can cause some wheels to spin and others to not spin – even if four-wheel drive is engaged. This can cause loss of traction on soft ground or on slick rocks, where having power distributed equally to all wheels would be more effective.
You’ve probably seen a vehicle stuck in mud, snow, or sand because one overpowered wheel was spinning and the others providing no help.
AWD systems can deal with this by using the traction control systems mentioned above. The solution for a four-wheel drive is a differential locker, also called an axle locker. These prevent the differentials from allowing the wheels to spin separately, so both wheels on an axle spin at the same time, with the same amount of force.
If you’re in the market for an off-road vehicle or looking to modify your existing rig, keep in mind the characteristics I covered above. Excellent traction, high clearance, low-range gearing, durable components, and superior stability are all important factors to consider.
Also, be aware that there are tradeoffs between some of these characteristics – stability and clearance, for instance– so it’s important to understand how you will be using your vehicle so you can optimize it for your needs.
While your vehicle plays a critical role in your ability to navigate terrain, your driving abilities are also key. Our intro to basic off-road driving covers everything from choosing the right vehicle to basic tips and techniques for navigating adverse terrain.
I also highly recommend joining an off-road or overlanding club where you can learn from other enthusiasts and possibly even get some formal training. Lastly, check out our article on choosing an overland vehicle, if you are looking for something that balances off-road chops with cargo capacity and other overland-specific characteristics.