Controlling Articulated Rigs

Preventing Rollovers and the Crack-the-Whip Effect

Combination vehicles are typically heavier and longer than single commercial vehicles. Because of these distinct physical traits, they require more driving skill and specialized knowledge than single commercial vehicles. Safety starts with understanding your vehicle's stability, especially since more than half of all truck driver deaths in crashes are the result of truck rollovers.

A major factor in rollovers is how freight is positioned. Piling cargo high inside a trailer raises the vehicle's center of gravity, and a higher center of gravity makes a commercial rig significantly easier to turn over. Keeping cargo as close to the ground as possible is essential for preventing rollovers, and this step is even more critical in combination vehicles than in straight trucks. Cargo must also be centered from side to side on the rig and spread out as much as possible. A load placed to one side can make the trailer lean and increase the risk of a rollover.

Fully loaded combination rigs are ten times more likely to roll over in a crash than empty rigs. Rollovers happen when a vehicle turns too fast. Drivers must slow down when traveling around corners, on-ramps, and off-ramps, always reducing their speed to a safe level before entering a turn. Driving slowly around turns is a primary method for preventing rollovers. In addition, quick lane changes should be avoided, particularly when a rig is fully loaded.

Rearward amplification causes what is known as the crack-the-whip effect in combination vehicles. When a driver makes a sudden lane change, this effect can overturn the trailer. In many crack-the-whip rollover crashes, only the trailer overturns while the tractor remains upright. A rearward amplification rating of 2.0 means the rear trailer is twice as likely to turn over as the tractor. In triple combinations, which have a rearward amplification factor of 3.5, the last trailer of the rig can roll over 3.5 times as easily as a five-axle tractor. To prevent trailers from tipping, steer smoothly and gently; sudden movements of the steering wheel can cause a trailer to tip over.

Stopping Distances, Light Loads, and Bobtails

Many new drivers assume that an empty vehicle stops faster than a full one, but combination vehicles behave differently. Large combination vehicles take longer to stop when empty than when fully loaded. This happens because stiff suspension springs and strong brakes give lightly loaded rigs poor traction. With poor traction on an empty or light rig, it becomes very easy to lock up the wheels. Once the wheels lock, a trailer can swing out and strike other vehicles, and a tractor can jackknife very rapidly when traction is lost.

A bobtail is a tractor operating without an attached semitrailer. Bobtail tractors are notoriously difficult to stop smoothly. In fact, bobtails take longer to stop than a tractor-semitrailer loaded to maximum gross weight. Drivers operating empty trailers or bobtails must exercise extra caution and allow plenty of room to bring the vehicle to a halt.

Maintaining a safe following distance helps avoid abrupt stops. Safe following distance requires at least one second for each 10 feet of vehicle length. When traveling at speeds over 40 mph, an additional second of following distance must be added. Looking far down the road helps drivers avoid being surprised and having to make a sudden lane change. At night, drive slowly enough to see obstacles with your headlights before it is too late to change lanes or stop gently.

Recognizing and Recovering from Trailer Skids

A trailer jackknife occurs when the trailer wheels lock up and swing out. Because traction is reduced when weight is missing, trailer skids occur more frequently when the trailer is empty or lightly loaded.

The earliest and best way to identify a trailer skid is by seeing it in the side mirrors. Because of this danger, a driver should check the mirrors whenever applying the brakes hard. Once a skidding trailer swings outside of its lane, preventing a jackknife is extremely difficult, making early detection vital.

If your trailer begins to skid, you must react properly to regain control. To stop a trailer skid, the driver must release the brakes to restore wheel traction. Once the trailer wheels regain grip on the road, the trailer will start to follow the tractor and straighten out. A driver should never use the trailer hand brake to straighten out a skidding trailer; do not use the trailer hand brake because the trailer wheel brakes caused the skid in the first place.

Off-Tracking, Low-Clearance Crossings, and Backing

Off-tracking, which is also called cheating, occurs when the rear wheels follow a different path than the front wheels during a turn. Longer commercial vehicles experience greater amounts of off-tracking. During any cornering maneuver, the rear wheels of a trailer off-track more than the rear wheels of the powered truck or tractor. Furthermore, in a multi-trailer combination, the rear wheels of the final trailer off-track the most. Drivers must steer the front of the vehicle wide enough to keep the rear wheels from hitting curbs or pedestrians.

Navigating right turns requires a precise path of travel. Drivers should keep the rear of the rig close to the curb during a right turn to stop other motorists from passing on the right. If you cannot complete a right turn without entering another traffic lane, turn wide as you complete the turn rather than swinging wide to the left before starting it. Swinging left before starting the turn can leave room for other drivers to pass you on the right.

Trailers with low underneath clearance can get hung up on raised railroad-highway crossings. Equipment types vulnerable to getting stuck include lowboy trailers, car carriers, moving vans, and possum-belly livestock trailers. A single-axle tractor pulling a long trailer with landing gear set for a tandem-axle tractor can also get hung up on a crossing. If your vehicle gets stuck on railroad tracks, get out of the vehicle and move away from the tracks immediately. Check signposts or signal housings for emergency notification information, call 911 or another emergency number, and give the crossing location using identifiable landmarks, especially the DOT number if posted.

Backing an articulated vehicle is significantly more complex than backing a straight vehicle. When backing a trailer, the steering wheel must initially be turned in the opposite direction of where the trailer needs to go. Once a trailer begins to turn while backing, the steering wheel must be turned the other way to follow the trailer. Drivers backing a trailer should position the vehicle to back in a straight line whenever possible. When backing on a curved path is unavoidable, the driver should back toward the driver's side to maintain visibility.