Basic Vehicle Control: Practice Questions & Explanations

Backing a commercial vehicle is dangerous because you cannot see everything behind the trailer. The safest approach is to use a helper whenever possible and to back slowly. A helper should stand near the back of the vehicle in a position where the driver can see them in the side mirror, and driver and helper should agree on hand signals before backing begins, including a clear signal to stop. The driver should back slowly so any developing problem can be corrected with minimal damage. Quick backing reduces reaction time and increases jackknife risk. Mirrors alone are insufficient because of large blind spots directly behind a tractor-trailer. Backing in a straight line is preferred where possible because turning while reversing rapidly increases the chance of jackknifing.

Off-tracking is the natural geometric tendency of the rear wheels of a tractor-trailer to follow a tighter path through a turn than the front wheels. The longer the wheelbase, the more pronounced the off-track. This is why a tractor-trailer making a right turn must swing wide to the left before turning, to give the trailer's rear wheels room to clear the curb on the inside of the turn. Failing to swing wide enough causes the trailer's rear wheels to climb the curb or run over a pedestrian or vehicle in the right turn lane. New drivers often turn too sharply because they steer like a car. Always plan the turn so that the trailer ends up in the correct lane, even if the tractor has to take an unusual line to get there.

When making a right turn in a tractor-trailer, position yourself in the rightmost lane, begin the turn slightly later (further into the intersection) than a car would, and keep the rear of the trailer close to the curb to prevent vehicles from squeezing into the gap on your right. Swinging wide to the left before turning right is one of the most dangerous habits a CDL driver can develop because it invites smaller vehicles to dart into the right-side gap and creates 'right hook' collisions. The correct technique uses the natural off-tracking of the trailer to bring it through the turn smoothly. If you absolutely must occupy two lanes to complete the turn safely on a tight street, signal early and use both lanes from the start so no driver tries to pass on either side.

When backing a tractor-trailer in a straight line, steer in the direction you want the rear of the trailer to go — a counterintuitive technique for drivers used to backing cars. As the trailer begins to drift to one side, gently turn the steering wheel in the direction of the drift to bring the trailer back into line. Use small corrections; large steering inputs amplify trailer movement and cause oscillation that is hard to recover from. Move slowly so corrections happen at low speed and any error can be undone before it becomes serious. The use of a helper outside the vehicle is strongly recommended for any backing maneuver where blind spots are unavoidable. Practice in empty lots until straight-line backing becomes second nature; it is a fundamental CDL skill tested on the road exam.

OFF-TRACKING (also called CHEATING) is the geometric difference between the path of the front (steering) wheels and the path of the rear wheels when a vehicle makes a turn. For a standard car, off-tracking is small and mostly unnoticeable. For a 53-foot tractor-trailer, it is dramatic and dangerous. HOW IT WORKS: The front wheels follow an arc on the outside of the turn. The rear wheels, attached much farther back, follow a SHORTER arc on the INSIDE of the turn. The degree of off-tracking increases with: (1) Longer vehicle (more distance between steer axle and rear axle); (2) Tighter turn radius; (3) More articulations (a double 53-footer has worse off-tracking than a single). REAL-WORLD CONSEQUENCE: On a right turn from a two-lane road, a tractor-trailer's rear wheels may track over the curb, into a bicycle lane, or over pedestrians in the crosswalk — even if the driver began the turn from the correct position. CORRECT TECHNIQUE: 'Button hook' or swing wide before the turn — steer slightly left before turning right to create space for the rear to track; watch the mirrors to monitor where the rear wheels are going; be aware that pedestrians standing on the corner are in your rear path even if they're clear of the front; never turn from the wrong lane.

The TRAILER HAND VALVE (also called the TROLLEY VALVE or INDEPENDENT TRAILER BRAKE) is located on the steering column or dashboard and applies ONLY the trailer brakes when activated. CORRECT USE — PRE-TRIP INSPECTION ONLY: The hand valve is properly used to TEST trailer brakes during the pre-trip inspection (pull gently to ensure it creates resistance, then release). WHAT IT SHOULD NOT BE USED FOR: (1) Parking: Using the hand valve as a parking brake is dangerous — it doesn't lock (spring brakes do); releasing the valve allows the trailer to roll; (2) Driving on slippery roads: Applying only trailer brakes on a slippery surface while the tractor brakes are not applied can fold the trailer at the fifth wheel — a textbook jackknife cause; (3) Regular service braking: The foot service brake applies all brakes simultaneously with proportional force — this is the correct brake for all stops; (4) Helping stop on grades: The instinct to 'grab the trailer brake' when descending is dangerous — use engine braking and foot brake. WHY HAND VALVE CAUSES JACKKNIFE: When you apply only trailer brakes, the trailer decelerates faster than the tractor (which is still rolling freely forward without brake force). The articulation point (fifth wheel/kingpin) becomes the pivot point for the trailer to swing sideways relative to the tractor.