Don’t know a bead-lock wheel from a locking differential? Start here to learn more about them.
Off-roading terminology can be needlessly confusing and overly technical. In this glossary, you will find many of the more common off-road terms simplified with explanations for the beginning off-road driver. This glossary is a good starting point for starting a deeper understanding of the various equipment and ideas every off-road driver should know to be safe on the trails. If you’re shopping for a new off-road SUV or truck, use this guide to help you wade through the many technical specs you’ll find as you research your vehicle.
This term applies to how much vertical movement the axle and wheel have. It is often also referred to as wheel travel. Quite simply, the more articulation the vehicle has, the better it can climb over obstacles like rocks because the wheels have more up and down motion.
Before any off-road excursion, owners should air down their tires to a lower pressure (measured in PSI) than what is stated in the owner’s manual or the door-jamb sticker for road use. This can be done by using the back end of a tire pressure gauge, or with an off-road-specific air-down kit. The goal is to air down all four tires to a matching PSI that is lower than the recommended level. It improves traction and can reduce wear on the suspension, including the shocks.
There is no hard rule of thumb for how low you can go. It is determined by the tire, the load on the vehicle, and the wheel type (beadlocks can be aired down lower than stock wheels; see below for more on them).
A bead lock is a tire/wheel setup where the bead of the tire has been secured to the wheel. This is often done at a tire shop with a special mechanical tool. Bead lock wheels are easy to distinguish by the ring of bolts around the circumference of the wheel that keeps the tire in place. The benefits for doing this is you are able to air down the tire to a much lower pressure without the tire slipping off the rim, which maximizes traction.
In scientific terms, the crawl ratio is the ratio of torque at the wheel to the torque at the engine’s flywheel. It is how many times the engine torque is multiplied before the actual propulsion occurs and is factored by three components: the transmission, the transfer case, and the differential. That number also shows how many times the final driveshaft will rotate per rotation of the engine’s crankshaft.
Crawl ratio is often represented by a number, like 73.1:1 for the manual-transmission Jeep Wrangler. The idea is to show how much torque is at the tire when driving in the lowest gear and in the lowest transfer-case range. Vehicles with high crawl ratios are better for a variety of situations like pulling large loads, climbing steep inclines, and driving over large obstacles, although they’ll travel very slowly in the crawl ratio.
The heart of a four-wheel-drive or all-wheel-drive system is the transfer case. It is often a box that simply transfers power from the transmission to the front and rear axles through the drive shafts when engaged. Essentially, when you put a vehicle into 4x4 or AWD, it is the transfer case you are utilizing to shift power to all four wheels. It’s also used to select low range, allowing drivers to access the vehicle’s crawl ratio when off-roading.
Lifting a vehicle is typically done through a kit which changes out various suspension parts and/or puts blocks near the wheels to raise the vehicle upward. This can be done to improve ground clearance or simply to achieve a certain look on a vehicle.
Generally, solid-axle pickups and body-on-frame SUVs are lifted since they have solid steel components to work with. Unibody SUVs and vehicles with independent front suspension can be lifted, yet it requires much more work to achieve the same results with their extra parts and complexity.
This is maximum angle of clearance a vehicle has when climbing a hill or approaching another obstacle. It is defined as the angle between the ground and the line drawn between the front tire and the lowest-hanging part of the vehicle (often the bumper). Exceeding the approach angle will cause damage to the front end.
Like approach angle, this is maximum angle of clearance on a vehicle when exiting an obstacle or descending a hill. It is defined as the angle between the ground and the line drawn between the rear tire and the lowest-hanging part of a vehicle (often the bumper).
This is the maximum angle or degree of apex a vehicle can drive over with one forward wheel and one rear wheel touching the ground without getting high-centered (stuck) on an obstacle.
Another important item when off-roading is the amount of ground clearance, or the ride height of a vehicle. This is defined as the amount of space between lowest part of the vehicle and the ground. Vehicles with higher ground clearance are less likely to get hung up on obstacles like rocks. Lifting a vehicle (see above) can improve ground clearance, as can fitting larger tires.
Tires come in a variety of different types, including passenger, truck, all-terrain, and mud/snow. Each of these tires is for a specific purpose and will perform differently. Most cars have passenger tires and heavy-duty trucks have truck tires, which are distinguished by load range. SUVs and consumer pickups often have all-terrain tires with tread meant to handle a variety of elements like rain, sleet, highway, and moderate off-road terrain. Mud/snow tires are often found on serious off-road vehicles and can have large pieces of rubber sticking out for better traction. These tires offer a trade-off between traction and ride comfort; their knobbly design is also loud on pavement.
Off-road vehicles can come with a variety of different locking differentials either from the factory or purchased through an aftermarket parts supplier. These differentials all help improve traction by preventing excess wheel spin. They go by a variety of names like locking differential, differential lock, diff lock, or locker. Differentials come in five main types: open, selectable, automatic, limited slip, and spool.
An open differential is common in most road-going cars and does not restrict how much more one wheel can spin than the other.
A selectable differential is a locker activated by a button or a switch inside the vehicle. It is also often known as an E-Locker. This is one of the more preferred differential options because it allows the driver manual control over the lockers. It also is a costly option to choose and adds to the price of stock off-road vehicles offered by many manufacturers. It’s found in vehicles like a Chevy Colorado ZR2.
An automatic differential locks down the axle when it senses slippage and lets go under normal use, like highway driving. It is beneficial to drivers who don’t want to deal with remembering to engage the differential, and is also helpful as an aid to a driver who doesn’t know when to turn on the differential or underestimates how slippery the terrain is. The downside of an automatic is you lose manual control over the differential and it could possibly engage when you don’t want it to.
A limited-slip differential is a type that limits the slip between the wheels, but does not fully lock them. This is a great system for mild off-road adventures and everyday driving since the system engages automatically and it isn’t a fully engaged locker. When it operates, it still allows the driver to turn effectively, whereas a full locker hampers turning (and thus should only be used off road). The downside is that it isn’t a true locker and can’t fully engage to lock the axle; some slip is still possible.
A spool differential is a simple, one-piece unit that keeps the axle always locked. It is useful for vehicles meant to go in a straight line, like drag cars, and isn’t a real option for most off-road vehicles that need to maneuver on trails.
These are metal hooks attached to either the front or rear of a vehicle and generally anchored to the frame. Better tow hooks extend beyond the bumper to make hooking them up with a tow strap or chain easier than crawling underneath the vehicle. They are extremely useful for freeing or towing off-road vehicles that are prone to getting stuck, and many owners upgrade the size of them for easier use.
When off-roading, the type of suspension you have can make a big difference in how well your vehicle can tackle obstacles – and in ride comfort. There are three main types of suspensions used for off-roaders: solid axle, four-link, and independent front suspension.
Solid axle suspension typically uses either leaf springs or coil springs to handle various loads applied to it, with solid axles delivering power to the wheels. It is generally thought to be the best suspension for modification and for serious off-road oriented vehicles.
Four-link suspension uses four “links” – metal bars – to handle the ride load and articulation of the vehicle. Essentially, two top links connect the frame to the top center of the rear axle, while two lower links connect to outer ends of the axle tubes (axle shafts). These four links work in harmony to handle the various loads placed on the vehicle, including side-to-side loads, to provide a smooth ride quality.
Independent front suspension uses control arms to handle the loads and road vibrations instead of a solid axle. It can be found on a variety of off-road vehicles and all GMC and Chevy trucks and SUVs, including heavy-duty models. Generally, it is considered IFS vehicles have a better ride quality than a solid axle vehicle, yet they sacrifice wheel articulation.