So you've got a huge truck with big tires and truck nuts hangin' off the hitch. You figure it must be the bee's knees when it comes to off-roading. That tiny Suzuki Samurai's got nothing on your Goliath, right? Not so fast. Let's break down what really makes a car capable when the going gets tough.
Automakers love to tout their vehicles' off-road capabilities, often claiming that their newest vehicle is the most capable one yet. How do you know if they're joshin' ya? You need to have some sort of criteria by which to judge a vehicle's off-road capability. There are many different kinds of off-road driving, but here are fourteen basic off road vehicle attributes that can be the difference between getting stuck in the woods and having a successful excursion.
1). Ground Clearance
If you want to traverse harsh terrain, you're gonna want to make sure you've got plenty of space between the underside of your vehicle and the terrain. Without this ground clearance, you're likely to do serious underbody damage when climbing over rocks, boulders, and stumps. Not only that, but you'll probably find yourself high centering quite often as the underside of your vehicle gets caught on obstacles.
Photo Credit: Peltier Chevrolet
2). Skid Plates
Even if you have good ground clearance, off-roading can do a lot of damage to your vehicle, especially if you're rock crawling. That's why you want to protect the underside of your vehicle with steel skid plates. Rock sliders will protect your rocker panels, a gas tank skid plate will keep you from having to break out the fire extinguisher, and a transfer case skid plate may keep you from having to walk home. It's also always a good idea to protect your differentials, oil pan, and any other vulnerable components under your vehicle.
Photo Credit: Jarrod Lombardo
3). Approach, Departure, and Breakover Angles
A vehicle's approach angle represents the steepest hill it can climb without brushing or slamming its front bumper against the slope. The breakover angle is an indication of the steepest crest a vehicle can traverse without high centering. The departure angle represents the steepest grade a vehicle can descend without brushing or slamming its rear bumper against the slope. The approach and departure angles are a function of ground clearance as well as front and rear overhangs, respectively. If you have short overhangs and lots of ground clearance, you'll have very good approach and departure angles, allowing you to ascend and descend very steep slopes. The breakover angle is a function of ground clearance and wheelbase. A short wheelbase and good ground clearance will allow a vehicle to travel over very sharp crests.
4). Wheel Articulation
When it comes to going off-road, wheel travel is crucial. Keeping a tire on the surface means not only maximizing traction, but also maintaining vehicle stability. Many people in the off-road world prefer a solid axle to independent suspension, as it tends to provide more wheel articulation, especially when the sway bar is disconnected. For desert racing, however, independent suspension provides a smoother, more compliant ride thanks to less unsprung mass.
Photo Credit: Marek
5). Low End Torque
The engine you see above is AMC's 4 liter high output inline 6. Known for its great low end torque, the 4 liter is regarded as one of the best off-road engines of all time. Why is low end torque so important? Low end torque allows you to maintain momentum over obstacles without having to wring out your engine. With AMC's 4 liter inline 6, a soft touch of the pedal will get you over some impressive obstacles, especially if you have the proper gearing.
Low end torque is great, but gearing is what makes the difference. A low range transfer case multiplies engine torque and makes traversing obstacles that much easier. When coupled with a very low first gear (called a "granny gear"), a low range gear can allow you to traverse a steep grade without even touching the gas pedal. In addition to a low range transfer case, low gearing in the differentials is crucial, particularly if you have larger tires on the vehicle. An axle ratio is defined as the ratio of pinion gear teeth to ring gear teeth in the differential. A 4.10:1 rear axle ratio means that the ring gear has 4.10 times as many teeth as the pinion gear. In short, this means that the output halfshafts transmit 4.1 times as much torque as the input driveshaft.
Street tires suck off-road. If you want to hang with the big boys, get yourself some all terrain tires, or, better yet, mud terrain tires. The key is to have deep treads, tough sidewalls, and the right rubber compound for the terrain. The bigger the tires, the better. 33 inch and 35 inch tires are the most common among experienced off-road enthusiasts. Air your tires down when you get to the trail to grow your contact patch for maximum traction.
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8). Locking Differentials
Most vehicles have open differentials, meaning that the wheel with the least traction gets the most power. This is far from optimal when it comes to off-roading, where you want the wheel with the the most traction to receive the most power. There are numerous ways to go about getting power to the wheel with traction. There are viscous type coupling mechanisms wherein a dilatant fluid heats up when there is a difference in wheel speed, locking the two wheels or axles together. There are brake lock differentials, which trigger the caliper of the spinning wheel, sending power to the wheel with more traction. There are also pneumatic and solenoid-based lockers, called air lockers and electronic lockers, respectively. These two systems physically lock the two wheels on an axle together, ensuring that they spin at exactly the same rate and receive the same amount of torque.
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9). High Mounted Air Intake
Sucking water into your engine is bad. Very bad. As water is not compressible, getting it in your cylinder will probably damage your connecting rods (this is known as hydrolocking your engine), as the rods try to push the piston up to compress the liquid. We've seen before that hydrolocking can occur even if the vehicle travels through a shallow puddle. Therefore, you want to make sure that your air intake is elevated. Many off-road enthusiasts use snorkels, some of which can allow a vehicle to drive while completely submerged in water. While a high mounted air intake is important, driver technique is equally important when it comes to preventing hydrolock.
Photo Credit: Jurgen
One of the benefits of solid axles versus independent suspension is that solid axles are, by and large, more robust. Axles are basically just big metal tubes that house the differential and halfshafts. Independent suspension setups have CV joints with exposed rubber boots that can get torn or damaged. Control arms are also prone to bending and breaking, and halfshafts are exposed.
A good off-road vehicle can handle the high-stresses associated with off road driving. This means the vehicle has heavy duty axles, tie rods, shocks, motor mounts, and much more. A heavy duty radiator and transmission cooler are also key, as low speed driving does not provide much of a "ram air effect."
If you plan on travelling the Rubicon Trail, you're going to need spotters to make sure you're traveling the right lines. Otherwise, you may be walking home. For times when you don't have spotters, you'll have to rely on your vehicle's visibility to keep you and your vehicle safe. Visibility can be the difference between driving over something that damages your vehicle and successfully navigating a tough off road course.
12). Lightness and Smallness
The Suzuki Samurai is a great off-road vehicle. Its short wheel base, coupled with its high ground clearance and small overhangs, gives the Samurai incredible approach, departure, and breakover angles. In addition, the Samurai is light. Like 2000 lbs light. The low curb weight and small vehicle dimensions make the Samurai nimble off road and allows it to travel places a HMMWV can only imagine going. One thing to note, though, is that a short wheelbase makes gaining traction on inclines more difficult.
Photo Credit: Sebastian Krywult
13). Tow Points
When it comes to off-roading, getting stuck is almost inevitable. When you do get stuck, it's important that there is a safe and easy place from which to tow your vehicle. Tow hooks or trailer hitches are great tow points, as they connect to sturdy parts of the vehicle via high strength bolts. Even if you do have good tow points, for goodness sake, be careful when you tow your vehicle and use a nylon tow rope, not a chain.
Photo Credit: Ben Tesch
14). A Frame
If you look back at the original CJ Jeeps, Land Rover Defenders, Toyota FJ 40s, etc., they all sported body-on-frame construction. Lots of die hard off-road enthusiasts assert that a frame's capacity to twist to conform to terrain makes body-on-frame vehicles much tougher in off-road situations. Because unibody vehicles distribute stress throughout the entire vehicle, they are thought to "wear out" and develop cracks more easily. Eventually, doors become hard to close, and body panel fitment starts to become a problem.
The great thing about body-on-frame vehicles is that they allow for some flex between the frame and the body. The frame takes the majority of the stress, so the body will not wear out over time. In addition, body-on-frame vehicles are easier to modify (body lifts!) and are generally easier to repair after accidents on the trail.
The reality is that unibody vehicles can be as good off road as their body-on-frame counterparts. In fact, some of the greatest off-roaders of all time incorporated a unibody construction (think Jeep Cherokee and R50 Pathfinder). Their light weight and smaller size makes them more nimble. The weight savings also lessens stresses on many vehicle components. Only in extreme off-road situations would you notice the benefits of body on frame construction, and even then, you can use stiffeners to make your unibody vehicle handle the extreme loading.
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