Practice questions written from the official CDL manual. Correct answers and explanations are shown below each question. (Turn on JavaScript for the interactive practice and exam-simulation modes.)
1. In a set of doubles or triples, which trailer is most likely to turn over?
- AThe front trailer, because it carries the most weight
- BThe converter dolly, because it has only one axle
- CThe last trailer, because of the crack-the-whip effectCorrect
- DAll trailers are equally likely to turn over
Why: Sudden steering movements grow stronger as they pass down the line of trailers. This is the crack-the-whip effect, so the rear trailer gets whipped around hardest and is the one most likely to roll over.
Based on: CDL Manual §7 — Pulling Double/Triple Trailers Safely
2. Because doubles and triples are prone to the crack-the-whip effect, how should you steer and take turns?
- ASteer gently and smoothly, and slow down before curves and turnsCorrect
- BSteer quickly so the trailers straighten out sooner
- CSpeed up slightly through the turn to keep the trailers in line
- DBrake hard in the middle of the turn to settle the rear trailer
Why: Quick, hard steering makes the crack-the-whip effect worse and can flip the rear trailer. You should steer gently and gradually and slow down before you reach a curve or turn, not while in it.
Based on: CDL Manual §7 — Pulling Double/Triple Trailers Safely
3. The tendency of sudden steering movements to become stronger on each trailer toward the rear of the rig is called:
- AOff-tracking
- BTrailer skid
- CLoad surge
- DRearward amplificationCorrect
Why: Rearward amplification, also known as the crack-the-whip effect, means a small steering movement up front becomes a much larger, more violent movement at the back trailer. That is why the last trailer is the one most likely to tip over.
Based on: CDL Manual §7 — Pulling Double/Triple Trailers Safely
4. Compared with driving other combination vehicles, following distance and space management with doubles and triples should be:
- AThe same, since all trucks handle alike
- BGreater, because these long rigs need more room to stop and maneuverCorrect
- CShorter, because the extra trailers add braking power
- DOnly a concern on wet roads
Why: Doubles and triples are longer, need more room to change lanes and turn, and cannot stop as quickly as you might expect. Look far ahead and keep plenty of space so you can slow down gradually instead of steering or braking suddenly.
Based on: CDL Manual §7 — Pulling Double/Triple Trailers Safely
5. On trailers used to tow other trailers, where must the trailer air supply (service and emergency) shut-off valves be set?
- AClosed on every trailer to hold air pressure
- BOpen only on the last trailer
- COpen on all trailers except the last, where they must be closedCorrect
- DOpen on the last trailer only, closed on all others
Why: The shut-off valves at the rear of a trailer feed air to the next trailer behind it. They must be OPEN on every trailer that has another trailer behind it, and CLOSED on the last trailer so air is not lost out the back.
Based on: CDL Manual §7 — Coupling and Uncoupling
6. What can happen if the shut-off valves at the rear of the last trailer are left OPEN?
- AAir will leak out the back and the trailers may not get enough air to work the brakesCorrect
- BThe trailer brakes will lock up and drag
- CThe rig will get better fuel mileage
- DNothing; open or closed makes no difference on the last trailer
Why: Open valves at the very back of the rig are an open path for the air to escape. That can rob the whole system of the air pressure the brakes need, so the last trailer's rear shut-off valves must be closed.
Based on: CDL Manual §7 — Coupling and Uncoupling
7. After hooking up the trailers, how do you check that air actually reaches the brakes on the rearmost trailer?
- AWatch the dash gauge for a pressure rise
- BPump the brake pedal ten times and listen at the cab
- CRoll the rig forward and apply the brakes hard
- DOpen the emergency line shut-off at the rear of the last trailer to check for air pressure, then close itCorrect
Why: With the system charged, briefly open the shut-off valve at the back of the last trailer. If air is present there, you know the air made it all the way through every trailer. Then close that valve again so the last trailer is set correctly.
Based on: CDL Manual §7 — Coupling and Uncoupling
8. A converter dolly (A-dolly) is coupled to the front trailer and held in place by:
- AThe fifth wheel on the tractor
- BThe pintle hook and eye, secured with safety chainsCorrect
- CTwo glad hands locked together
- DThe trailer landing gear
Why: The dolly's drawbar eye drops into the pintle hook on the rear of the trailer ahead of it. You lock the pintle hook closed and hook up the safety chains as a backup so the dolly cannot come loose.
Based on: CDL Manual §7 — Coupling and Uncoupling
9. When coupling a converter dolly, in what order should the steps generally happen?
- ABack the second trailer over the dolly first, then attach the dolly to the front trailer
- BRaise the dolly wheels off the ground before connecting anything
- CCouple the dolly to the rear of the front trailer and lock the pintle hook, then back it under the rear trailerCorrect
- DConnect the air lines first, then hook the pintle, then position the dolly
Why: You first move the dolly to the rear of the first trailer, connect its eye to the pintle hook and lock it, and hook the safety chains. Then you push or back the dolly under the front of the second trailer so its kingpin locks in the dolly's fifth wheel.
Based on: CDL Manual §7 — Coupling and Uncoupling
10. Before backing the converter dolly under the rear trailer, the trailer height should be:
- ASlightly lower than the center of the fifth wheel so the trailer is raised a little as the dolly goes underCorrect
- BMuch higher than the fifth wheel so the dolly slides freely
- CExactly level with the ground
- DHeight does not matter as long as the kingpin locks
Why: If the trailer sits slightly lower than the center of the dolly's fifth wheel, the trailer gets lifted a little as the dolly is pushed under it. That helps the kingpin seat and lock properly instead of catching on the edge of the fifth wheel.
Based on: CDL Manual §7 — Coupling and Uncoupling
11. What is the purpose of the safety chains on a converter dolly hookup?
- ATo carry electrical power to the rear trailer
- BTo keep the trailer air lines from dragging
- CTo hold the dolly level while parked
- DTo keep the dolly attached to the trailer if the pintle hook comes openCorrect
Why: The safety chains are a backup connection. If the pintle hook accidentally opens or the drawbar eye slips out, the chains keep the dolly and rear trailer tied to the rig so they don't break away from the truck.
Based on: CDL Manual §7 — Coupling and Uncoupling
12. When connecting the air lines on a doubles or triples rig, the glad hands must be matched so that:
- ABoth service and emergency lines are crossed for backup
- BThe service (blue) line connects to the service coupler and the emergency (red) line to the emergency couplerCorrect
- COnly the emergency line needs to be connected
- DAny glad hand can go on any coupler as long as it seals
Why: Glad hands must connect the correct lines together. The service line goes to the service side and the emergency (supply) line goes to the emergency side. Crossing them means the brakes will not work right, which is dangerous.
Based on: CDL Manual §7 — Coupling and Uncoupling
13. While inspecting a converter dolly, you should check that:
- AThe fifth wheel is greased on the tractor only
- BThe dolly has no brakes since it only has one axle
- CThe tow bar eye, safety chains/hooks, and pintle hook are secure and not worn or damagedCorrect
- DThe dolly air tank petcock is left open at all times
Why: During your inspection, look over the dolly's tow bar and eye, the safety chains and their hooks, and the pintle hook for wear, cracks, or damage. These parts hold the rear trailer to the rig, so they must be in good shape and properly secured.
Based on: CDL Manual §7 — Inspecting Doubles and Triples
14. When you inspect the additional trailers and dolly on a doubles/triples rig, one thing you must confirm about the air system is that:
- AAir pressure reaches all trailers, including the last one, so all the brakes workCorrect
- BThe air lines are disconnected until you reach the highway
- COnly the first trailer needs air pressure
- DThe dolly air tank petcock stays open to bleed off water
Why: Every trailer's brakes need air. Part of the inspection and hookup check is making sure the trailer air supply flows all the way to the rearmost trailer, which you verify by checking for air at the back of the last trailer.
Based on: CDL Manual §7 — Inspecting Doubles and Triples
15. After hooking up a converter dolly, how can you make sure the pintle hook connection is secure?
- ABounce the dolly up and down by hand
- BRev the engine and release the clutch quickly
- COnly look at it; a visual check is enough
- DPull gently against the pintle hook while the dolly brakes or wheels are locked to confirm it is latchedCorrect
Why: A visual check alone can miss a hook that looks closed but is not fully latched. Applying a gentle pulling test against a locked dolly confirms the pintle hook is truly holding before you trust it on the road.
Based on: CDL Manual §7 — Coupling and Uncoupling
16. A conventional set of doubles has a rearward amplification of about 2.0. This means the rear trailer is:
- AHalf as likely to roll over as a single trailer
- BAbout twice as likely to turn over as a five-axle tractor-semitrailerCorrect
- CExactly as stable as a single trailer
- DUnable to roll over at highway speed
Why: A rearward amplification of 2.0 means a steering movement is roughly doubled by the time it reaches the rear trailer, making that trailer about twice as likely to tip over compared with a standard tractor-semitrailer. That is why smooth, gentle steering matters so much.
Based on: CDL Manual §7 — Pulling Double/Triple Trailers Safely
17. When you uncouple the rear trailer of a set of doubles, you should first:
- APull the tractor and front trailer away immediately
- BDisconnect the safety chains before anything else
- CLower the dolly landing gear and secure the rear trailer, then disconnect the air and electrical lines and unlock the dolly's fifth wheelCorrect
- DOpen the shut-off valves on the last trailer
Why: To uncouple safely you support the rear trailer by lowering the dolly landing gear, chock or brake the wheels, then disconnect the lines and release the fifth wheel and pintle hook in order. Rushing or skipping steps can drop a trailer or let it roll.
Based on: CDL Manual §7 — Coupling and Uncoupling
18. If your doubles or triples rig is equipped with antilock brakes (ABS) on the trailers or dolly, you will usually know it by:
- AA yellow ABS malfunction lamp on the trailer or dolly, and often an ABS label near the wheelsCorrect
- BA red triangle reflector on the rear door
- CThe absence of any warning lamps
- DA green light on the dash that stays on while driving
Why: Trailers and dollies with ABS have a yellow malfunction lamp so you can tell the system is present and whether it is working. Many also carry an ABS label on the frame near the wheels. ABS helps you keep steering control during hard braking.
Based on: CDL Manual §7 — Antilock Braking Systems
19. What does the antilock braking system (ABS) on a trailer or dolly actually do for you?
- AIt lets you brake much harder and stop in a shorter distance every time
- BIt replaces the need to keep a safe following distance
- CIt automatically steers the trailer around obstacles
- DIt keeps the wheels from locking up so you can keep steering control during hard brakingCorrect
Why: ABS keeps the wheels from locking up during hard or slippery braking, which helps you steer and stay in control. It does not shorten your stopping distance or let you drive faster, so you still need good following distance and gentle inputs.
Based on: CDL Manual §7 — Antilock Braking Systems
20. When loading a set of doubles or triples, where should the more heavily loaded trailer generally be placed?
- AIt does not matter where the heavy trailer goes
- BToward the front, with lighter trailers behind, for better handling and to reduce the whip on the rearCorrect
- CAlways at the very rear so the tractor pulls easier
- DOnly on the converter dolly
Why: A lighter trailer at the rear is whipped around more easily and rolls over more readily. Putting the more heavily loaded trailer toward the front and keeping lighter loads behind improves stability and reduces the crack-the-whip risk on the last trailer.
Based on: CDL Manual §7 — Pulling Double/Triple Trailers Safely