Vehículos combinados — examen de práctica 2026

IncorrectoRespuesta correcta: B — Empty or only lightly loaded

The less weight pressing the trailer tires onto the road, the less braking they can take before they stop turning, and a wheel that has stopped turning cannot hold the trailer behind you. That is why the empty return leg deserves more braking room, not less.

Assuming a heavy trailer is the dangerous one in every respect. Heavy loads drive rollover risk; light loads drive skid and jackknife risk.

IncorrectoRespuesta correcta: C — Sealing the tractor's own air in if the trailer separates or springs a serious leak

Without it, a torn trailer hose would empty the whole system and leave the tractor with no service brakes at all. It seals the tractor's air in, and in doing so it vents the trailer emergency line, which sets the trailer's emergency brakes.

Thinking the valve protects the trailer. It protects the powered unit's air supply, and the trailer's emergency brakes coming on is a consequence of that, not its purpose.

IncorrectoRespuesta correcta: C — With so little weight pressing the tires down, the brakes can overpower what grip is left and stop the wheels turning instead of slowing the rig

Brakes only slow a vehicle while the tires still grip. Empty, the suspension keeps little weight on the tires and the brakes overpower that grip, so the wheels stop turning and the rig slides rather than stops.

The intuition that lighter always means shorter stopping distance. That holds only while the tires can still use the braking force; a light combination loses grip before it runs out of brake.

IncorrectoRespuesta correcta: C — Brake early and gently, instead of making a panic stop that can lock the wheels and swing the trailer

Space is what turns an emergency into a routine slowdown. A gradual application keeps the wheels turning and the trailer behind you, while the hard stab that space would have prevented is what starts most jackknifes.

Treating following distance as a paperwork rule rather than the thing that decides whether your next stop is gradual or violent.

IncorrectoRespuesta correcta: C — Locked trailer wheels started the skid, and the hand valve does nothing but hold those same wheels locked

The hand valve is a trailer-brake control and nothing else. Adding trailer braking to a trailer that is sliding on locked wheels reinforces the exact condition that caused the swing, which is why the correction is to release rather than apply.

Picturing the hand valve as a way to pull the trailer straight, like tightening a rope. It cannot steer the trailer; it can only stop its wheels from turning.

IncorrectoRespuesta correcta: B — In that maneuver the rear trailer runs about double the tractor's chance of going over

Rearward amplification compares how violently the rear unit reacts with how violently the tractor reacts, so 2.0 means double the roll tendency at the back. It is the reason triples, rated far higher, demand the gentlest steering of all.

Confusing rearward amplification with off-tracking. One describes how easily the rear unit rolls in a fast swerve; the other describes the path the rear wheels take through a slow corner.

IncorrectoRespuesta correcta: B — Release the brakes so the trailer wheels can roll again and get their grip back

A trailer swings because its wheels have stopped turning and are sliding. Only rolling tires can hold the trailer in line, so letting the brakes off is what ends the skid, while any added braking keeps the wheels locked and the slide going.

The instinct that a vehicle going somewhere wrong needs more brake. In a skid the brakes are the cause, and releasing them is the correction.

IncorrectoRespuesta correcta: C — The coupling failed: the kingpin is most likely sitting on top of jaws that were already closed, and the trailer is barely held

A correctly coupled trailer rests flat on the fifth wheel with no daylight between the plates. A gap means the kingpin never dropped into the jaws, and the only thing holding the trailer is friction that will let go on the first bump.

Explaining a gap away as settling or as something that closes under load. There is no amount of gap that is acceptable, and a rig in this state must not be driven.

IncorrectoRespuesta correcta: C — Get down to a safe speed before you enter the curve, then hold that speed steady through it

Braking inside a curve adds a second force to a rig that is already leaning, and a loaded trailer has very little lean left to give. Doing the slowing before the curve leaves the tires to handle one job at a time.

Trusting a posted ramp speed to be safe for a high, heavy trailer. That number is an advisory, not a guarantee for a rig whose center of gravity is far above a car's.

IncorrectoRespuesta correcta: A — May need more room to stop than the loaded combination, and does not slow down evenly

Testing has found the bobtail needs more room than a tractor-semitrailer at maximum gross weight and will not come down evenly, because the drive tires carry almost no weight and lock easily. Drivers who assume a bobtail is nimble get caught out on the trip back empty.

Reading bobtail as the easy, light-duty configuration. It removes weight from the very axles doing the braking, which is a handling problem, not a bonus.

IncorrectoRespuesta correcta: A — A lower stack holds the center of gravity down, so the rig resists tipping when you turn

A turn pushes the mass of the trailer sideways, and the higher that mass sits, the more leverage it has to lift the inside wheels. Lowering the stack shortens that lever arm, which is why load height decides so many rollovers.

Believing rollover is settled by how much you are hauling rather than how high it rides. Weight matters, but the height of the center of gravity is what turns a firm corner into a rollover.

IncorrectoRespuesta correcta: B — Roughly ten times more likely to roll

Loading a trailer raises the mass that a crash or a hard turn has to swing, and the loaded rig rolls far more readily than the empty one. Rollovers account for more than half of the drivers killed in truck crashes, so the loaded trip is the one that demands the extra caution.

The idea that a heavy load anchors a truck to the road. Extra weight up in the box does the opposite: it feeds the roll rather than resisting it.

IncorrectoRespuesta correcta: A — Keep the load low and take turns slowly

Rollover comes from the mass sitting high and from cornering force building too fast. Lowering the load attacks the first, and slowing before turns attacks the second, which is why these two together do more than any other habit.

Reaching for a device or a gear change when the answer is load placement and speed. Nothing on the dash offsets a high load taken through a corner too fast.

IncorrectoRespuesta correcta: C — The driver's side, so you can watch the trailer directly out of your own window

Curving to the driver's side keeps the whole trailer in view through your window and mirror instead of hiding it behind the rig. Blind-side backing forces you to guess at exactly the moment corrections have to be small and early.

Choosing the side by where the open space looks bigger. Visibility matters more than clearance, because you cannot correct a drift you never saw.

IncorrectoRespuesta correcta: A — The rear wheels of the last trailer

Every unit behind the steer axle cuts inside the one ahead of it, and the effect stacks up down the length of the rig. The final trailer's rear wheels therefore travel the tightest line of anything on the vehicle, and that is what climbs the curb.

Watching only the tractor's drive wheels in the mirror. They off-track, but the last trailer cuts in considerably more, and that is where the damage happens.

IncorrectoRespuesta correcta: C — If the landing gear folds or sinks into the surface, the frame catches the nose instead of letting it hit the ground

The moment the trailer's weight transfers to its legs is when soft ground or damaged gear shows itself. Leaving the frame underneath means a failure ends with the nose resting on steel rather than falling to the ground.

Treating the pull-out as one continuous move because the coupling is already released. The pause exists precisely for the seconds when the landing gear is being trusted for the first time.

IncorrectoRespuesta correcta: C — Tilted down toward the rear of the tractor, jaws open, and the safety unlocking handle set so it locks automatically

The rearward tilt lets the trailer's plate ride up onto the wheel instead of catching its edge, and open jaws are what the kingpin drives into. If the jaws are already closed, the kingpin cannot enter and can end up sitting on top of them.

Believing closed jaws mean the coupling is ready and secure. Closed jaws before coupling are the setup for the most dangerous failure there is, a trailer resting on top of the fifth wheel.

IncorrectoRespuesta correcta: A — It denies other drivers the gap on your right, where you would otherwise turn into them

The space you leave between the trailer and the curb is exactly the space a car will occupy, and it is the space the trailer sweeps through as the turn closes. Closing it before you commit is how you keep anyone from getting there.

Thinking curb clearance is only about protecting the trailer's tires and skirts. The real purpose is denying a smaller vehicle a place to sit alongside you.

IncorrectoRespuesta correcta: C — A load pushed to one side makes the trailer much easier to roll in a turn, so it should be re-centered before you drive

A trailer that already leans has spent part of the lean it can survive before the inside wheels lift. Add a corner or a ramp and the remaining margin disappears, so the fix is to center the load and spread it out, not to drive it as it sits.

Treating legal weight as proof the load is safe. Scale tickets say nothing about where the weight sits inside the box, and side-to-side placement is what decides how much lean a turn can add.

IncorrectoRespuesta correcta: A — The shank of the kingpin

The shank is the narrow section the jaws are shaped to grip; the head is the wider flange below it. Jaws closed on the head look locked from a distance but can pull free, so this check is made by eye from underneath.

Accepting jaws that appear closed without identifying what they closed on. Jaws around the head is a classic false lock that passes a casual glance.

IncorrectoRespuesta correcta: B — 20 to 45 psi

That range is where the tractor protection valve closes automatically, sealing off the tractor and setting the trailer emergency brakes. Testing it means charging the system, shutting the engine down and fanning the pedal until the control pops.

Mixing this range up with the low air pressure warning, which comes on much earlier. By the time the knob pops out, the trailer's emergency brakes are applying and control may be affected.

IncorrectoRespuesta correcta: C — How hard you are pressing the brake pedal, or the trailer hand valve

The service line is a signal, not the working supply. It tells the relay valves how much of the trailer's stored air to send to the trailer brakes, so the pressure in it rises and falls with your foot.

Believing the service line is what actually feeds the trailer brakes. The braking air comes from the trailer's tanks; the service line only says how much of it to use.

IncorrectoRespuesta correcta: A — Open the whole length of the rig, with only the pair at the very back of the final trailer shut

Air has to travel the full length of the rig for every set of brakes to work, so every valve along the way stays open. The pair at the very back has nothing to feed, so closing them is what keeps the system sealed instead of venting to the air.

Leaving the rear valves open because open looks like the safe default. Left open, the whole system bleeds out the back and no trailer brakes work properly.

IncorrectoRespuesta correcta: C — Air pressure alone is holding those brakes, and once it bleeds down a trailer with no spring brakes has nothing left

The hand valve holds the trailer brakes with air pressure alone, and air left standing will eventually bleed off. Parking brakes hold mechanically with springs, and where a trailer has none, wheel chocks are what keep it from rolling.

Treating any brake that feels firm right now as a parking brake. Air-applied brakes hold only while the air lasts; parking brakes hold when the air is gone.

IncorrectoRespuesta correcta: A — A red knob with eight sides: in to charge the trailer, out to cut the air off

The shape and color are standardized so you can find and identify it without looking, and the direction is consistent: pushed in means the trailer is charged, pulled out means the air is cut and the trailer emergency brakes are on.

Confusing it with the yellow diamond parking brake control, which sets the tractor's own spring brakes. Two different knobs, two different jobs.