Capítulo 3 de 327% del examen
Driving Combination Vehicles Safely
Combination vehicles are longer, heavier, and handle differently than smaller vehicles, and they can roll over or jackknife if driven carelessly. This chapter explains off-tracking, rollover prevention, trailer skids and jackknifing, and safe following distance. Understanding how the trailer follows the tractor helps you turn, brake, and steer safely.
Off-Tracking and Turning
When a combination vehicle turns, the rear trailer wheels follow a shorter path than the front wheels, called off-tracking. The longer the vehicle, the more the rear wheels cut inside the turn. You must plan turns so the trailer clears curbs, objects, and other vehicles.
Understand off-tracking
The rear trailer wheels track inside the path of the front wheels, so longer rigs need wider turns.
Turn wide at the end
Complete the turn wide while keeping the rear of the vehicle close to the curb, rather than swinging wide at the start.
Do not swing left first
Swinging wide to the left before a right turn invites drivers to pass on the right, where they can be crushed.
Plan lane changes
Allow extra room when merging or changing lanes so the trailing trailer clears other vehicles.
Preventing Rollovers
Most trailer rollovers happen because the driver took a curve or ramp too fast for a load with a high center of gravity. The driver often cannot feel the trailer beginning to tip until it is too late. Loading correctly and slowing down are the keys to prevention.
Slow before curves and ramps
Reduce speed before entering a curve or ramp, often below the posted advisory limit set for cars.
Keep the load low
Load cargo as low as possible to keep the center of gravity down and improve stability.
Balance the load
Distribute weight evenly so the trailer does not lean or tip more easily to one side.
Do not trust feel
A loaded trailer can roll over at a speed that feels safe, so rely on slowing down, not on how stable it feels.
Trailer Skids and Jackknifing
A trailer jackknife occurs when the trailer wheels lock and lose traction, causing the trailer to slide sideways toward the tractor. Hard braking, braking in a curve, or slippery roads can trigger it. Early recognition through your mirrors is essential because a full jackknife is hard to correct.
Know the cause
Locked trailer wheels lose traction and the trailer swings out, often from hard braking or braking in a turn.
Watch your mirrors
Check mirrors frequently to catch a trailer beginning to swing out before it becomes a full jackknife.
Release the brakes if braking-induced
If braking locked the trailer wheels, ease off the brakes so they regain traction and roll straight.
Avoid the trailer hand valve
Do not use the trailer hand valve to slow down normally, since applying only trailer brakes can cause a skid.
Braking and Following Distance
Because combination vehicles are heavy and long, they need much more distance and time to stop than cars. Smooth, early braking and a generous following distance keep the rig under control and reduce skid risk. Adjust for length, speed, and road conditions.
Use the length rule
Allow at least one second of following distance for every 10 feet of vehicle length at low speed.
Add for speed
Add one second above 40 mph; a 60-foot rig at highway speed needs about 7 seconds of space.
Brake early and smoothly
Brake gently while the vehicle is straight so the wheels keep traction and the trailer does not swing.
Adjust for conditions
Increase space and slow down further on wet, icy, or downhill roads to prevent skids.
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