Trucks 101

When You Should Use 4WD

What all those buttons, levers, and knobs actually do in your 4x4

A Land Rover plugs some mud. (Martin Bridgen)

Understanding how the four-wheel drive, low-range transfer case, and locking differentials in your truck work is key to getting the most of it off-road and driving safely anywhere.

Let’s break it down to make this easy for any driver.

Four-Wheel Drive

On pavement, where you have a lot of traction, your wheels need to spin at different speeds when you go around a corner. Your outside wheels must travel a greater distance around a corner than your inside ones, and the relative speed of the front and rear wheels will also vary. So there’s a differential (which allows different speeds, get it?) between both your front and rear axles and between your left and right wheels on both axles.

This works great when you have good traction, but because torque will follow the path of least resistance within your drive system, it allows the wheel with the least traction to spin freely in slippery conditions. If your truck nominally functions in two-wheel drive, this will send available torque to the rear wheel with the least traction. If you have full-time four-wheel drive (functionally similar to all-wheel drive), torque will go to whichever of your four wheels has the least traction. I’m sure you can see why that would be a problem if you’re trying to get unstuck—if one of your wheels is spinning freely while the others stay still, you’re going nowhere.

Four-wheel drive locks the speed of the front and rear axles together, meaning that the wheel with the least traction can spin only as fast as its counterpart on the other axle. This doubles your traction.

Because the front and rear axles need to spin at different speeds on the road, you should activate 4WD only when your tires leave the pavement. That might be turning onto a dirt road or entering consistently snowy conditions, where the road surface is completely covered. Four-wheel drive doubles traction in slippery conditions, but it also requires slippery conditions to work safely. Employ it on dry pavement and your vehicle will become dangerously unstable while cornering—and you’ll be damaging your transmission with every mile.

I like to enter 4WD (sometimes designated “4 High” or “4H”), whenever I leave pavement. Even on simple dirt roads, locking the speed of the front and rear axles together will help you climb hills most effectively and descend them safely (the speed of the axles is locked while braking, too) and make crossing obstacles as easy on your vehicle as possible. Just make sure you switch out of it before returning to pavement.

Some older vehicles may require you to manually engage the front wheel hubs before shifting into 4WD. If this is the case, you can freely shift into and out of 4WD with them engaged, and it is safe to drive on the road in 2WD with them engaged, but doing so while amplify wear on them. So, you can leave the hubs engaged for a short section of pavement, but disengage them before hitting the highway to drive home.

Low-Range Gears

Gears multiply the force your engine is able to exert on your wheels. You see this every time you drive—your truck accelerates faster in first gear than it does in fifth. Low-range gears exaggerate this effect by multiplying torque to a far greater degree. But as you know from using your transmission on the road, the lower the gear, the lower its maximum speed. So, going into an even lower ratio limits you to very low speeds.

There are two scenarios in which you want to employ your low-range gears: going up or down steep obstacles. Going up, the low gears give you enough torque to climb stuff you’d never make it up otherwise. Going down, the low gears also multiply the effects of engine braking. Using your low gears instead of the brakes enhances control of your vehicle’s speed and doesn’t cause the front suspension to dive, which could be unsafe on very steep descents.

Typically, you can only select low range (“4 Low” or “4L”) while in four-wheel drive. If you’re able to select low range in two-wheel drive (“2 Low” or “2L”), then that’s for situations where you need to climb or descend something steep, with good control, while on pavement—a boat ramp is about the only scenario I can think of where you’d do this.

Switch into and out of low range as you need it, because it will dramatically limit your speed. Off-road, it is an exceptionally good idea to be in low range whenever you’re going down a steep hill, as it will keep your brakes from overheating and give you better control of your vehicle.

Note that you can still shift gears in low range. In fact, it’s a good idea to. If you drive a truck with an automatic transmission, it will shift like normal in low range, but you can also manually take control of gear selection to gain further control of your vehicle’s speed. The steeper the obstacle, the lower the gear you should use.

Locking Differentials

Four-wheel drive locks the speed of the front and rear axles together but cannot lock the speed of the wheels on that axle together. So, since torque follows the path of least resistance, it causes the wheels with the least traction on both axles to spin. To maximize traction, you’ll want to lock the speed of all four wheels together.

As we identified previously, the inside and outside wheels travel at different speeds while cornering. So, locking their speeds together will cause you to break traction in corners. This has two effects: At low speeds, it will become very difficult to turn the steering wheel, and at high speeds, your vehicle will turn by sliding.

For these reasons, you should employ your locking diffs sparingly. Switch on one (rear lockers are much more common) or both if you identify an obstacle that requires maximum traction to cross—deep sand, mud, or similar—then switch them off after you’ve completed that obstacle.

Because it’s always easier to avoid getting stuck than it is to get unstuck, switching on your lockers before entering that sand or mud is a good idea—if you can visually identify it before you hit it. If you can’t, and you get stuck with your lockers off, switching them on should be the first thing you try to get out. Never engage your locking diffs on pavement.

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