Doubles and triples
Questions and answers
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How stable are doubles and triples compared with other commercial vehicles?
Less stable
More can go wrong with two or three trailers, and doubles and triples are less stable than other commercial vehicles.
Section 7 · 7.1 – Pulling Double/Triple Trailers ↗PDF, 41 MB -
How do you prevent the trailers of a double or triple from rolling over?
Steer gently and go slowly around corners, ramps and curves
Gentle steering and low speed in corners, on ramps and on curves keep the trailers upright.
Section 7 · 7.1.1 – Prevent Trailer from Rolling Over ↗PDF, 41 MB -
A speed is safe on a curve for a single trailer combination. Is it safe for doubles?
It may be too fast for doubles or triples
Doubles and triples need a lower speed on curves than straight trucks or single trailer rigs.
Section 7 · 7.1.1 – Prevent Trailer from Rolling Over ↗PDF, 41 MB -
Why are doubles and triples more likely to turn over than other combination vehicles?
Because of the crack-the-whip effect
The crack-the-whip effect makes multi-trailer rigs roll over more easily, so steer gently.
Section 7 · 7.1.2 – Beware of the Crack-the-whip Effect ↗PDF, 41 MB -
Why must you steer especially gently when pulling triples?
The last trailer is the most likely to turn over
The crack-the-whip effect is strongest at the end of the combination, so the last trailer rolls most easily.
Section 7 · 7.1.2 – Beware of the Crack-the-whip Effect ↗PDF, 41 MB -
Why does inspecting doubles and triples take more work?
There are more critical parts to check
Two or three trailers bring more critical parts, and every one must be checked.
Section 7 · 7.1.3 – Inspect Completely ↗PDF, 41 MB -
Why should you look far ahead when pulling doubles or triples?
So you can slow down or change lanes gradually
Doubles and triples must be driven very smoothly, so plan early and make changes gradually.
Section 7 · 7.1.4 – Look Far Ahead ↗PDF, 41 MB -
Why must doubles and triples be driven very smoothly?
To avoid rollover or jackknife
Smooth driving prevents the rollovers and jackknifes these rigs are prone to.
Section 7 · 7.1.4 – Look Far Ahead ↗PDF, 41 MB -
Besides being longer, why do doubles and triples need more space?
They cannot be turned or stopped suddenly
Because they cannot turn or stop suddenly, doubles and triples need extra room around them.
Section 7 · 7.1.5 – Manage Space ↗PDF, 41 MB -
How should following distance for doubles and triples compare with other commercial vehicles?
It should be greater
Allow more following distance when pulling doubles or triples.
Section 7 · 7.1.5 – Manage Space ↗PDF, 41 MB -
What must you make sure of before entering or crossing traffic with doubles?
That the gap is large enough
A long rig needs a large gap to enter or cross traffic.
Section 7 · 7.1.5 – Manage Space ↗PDF, 41 MB -
Before changing lanes with doubles or triples, what must you be certain of?
That you are clear at the sides
Check that the whole length of the rig is clear at the sides before changing lanes.
Section 7 · 7.1.5 – Manage Space ↗PDF, 41 MB -
Why is there more chance of skids with doubles and triples in bad weather?
Greater length and more dead axles to pull with the drive axles
More length and more dead axles pulled by the drive axles mean more chance of skids and loss of traction.
Section 7 · 7.1.6 – Adverse Conditions ↗PDF, 41 MB -
In bad weather, slippery conditions and mountain driving, what is more likely with doubles and triples?
Skids and loss of traction
Be especially careful in adverse conditions, because skids and loss of traction are more likely.
Section 7 · 7.1.6 – Adverse Conditions ↗PDF, 41 MB -
When parking doubles or triples, what kind of spot should you avoid?
A spot you cannot pull straight through
Know how the lot is laid out so you are not stuck backing out of a tight spot.
Section 7 · 7.1.7 – Parking the Vehicle ↗PDF, 41 MB -
Why should you be aware of how a parking lot is arranged when driving doubles?
To avoid a long and difficult escape
Knowing the layout keeps you out of spots that are hard to get out of.
Section 7 · 7.1.7 – Parking the Vehicle ↗PDF, 41 MB -
Which converter dollies must have antilock brakes?
Those built on or after March 1, 1998
Converter dollies built on or after March 1, 1998 are required to have ABS.
Section 7 · 7.1.8 – Antilock Braking Systems on Converter Dollies ↗PDF, 41 MB -
Where is the ABS lamp on a converter dolly that has antilock brakes?
A yellow lamp on the left side of the dolly
Dollies with required ABS carry a yellow lamp on their left side.
Section 7 · 7.1.8 – Antilock Braking Systems on Converter Dollies ↗PDF, 41 MB -
The second trailer has no spring brakes. How can you set its emergency brakes before coupling?
Connect the emergency line, charge the trailer air tank, then disconnect the emergency line
Charging the tank and then disconnecting the emergency line sets the trailer emergency brakes.
Section 7 · 7.2.1 – Coupling Twin Trailers ↗PDF, 41 MB -
Setting the second trailer's emergency brakes by disconnecting the emergency line works only if what is true?
The slack adjusters are correctly adjusted
The brakes will hold only if the slack adjusters are correctly adjusted.
Section 7 · 7.2.1 – Coupling Twin Trailers ↗PDF, 41 MB -
You have doubts about the second trailer's brakes before coupling. What should you do?
Chock the wheels
If there is any doubt about the brakes, chock the wheels.
Section 7 · 7.2.1 – Coupling Twin Trailers ↗PDF, 41 MB -
For the safest handling, where should the more heavily loaded semitrailer be?
In first position behind the tractor
Put the heavier trailer right behind the tractor and the lighter one at the rear.
Section 7 · 7.2.1 – Coupling Twin Trailers ↗PDF, 41 MB -
How many axles does the converter gear on a dolly have?
One or two
A converter gear is a coupling device of one or two axles and a fifth wheel.
Section 7 · 7.2.1 – Coupling Twin Trailers ↗PDF, 41 MB -
What kind of rig does a converter dolly make when it couples a semitrailer behind a tractor-trailer?
A double bottom rig
The dolly lets a second semitrailer be coupled behind the first, forming a double bottom rig.
Section 7 · 7.2.1 – Coupling Twin Trailers ↗PDF, 41 MB -
How do you release the brakes of a converter dolly that has no spring brakes?
Open the air tank petcock
Opening the dolly air tank petcock releases its brakes. A dolly with spring brakes uses the dolly parking brake control.
Section 7 · 7.2.1 – Coupling Twin Trailers ↗PDF, 41 MB -
If the distance is not too great, how can you move the converter dolly into position?
Wheel it by hand so it lines up with the kingpin
A short distance can be covered by hand, lining the dolly up with the kingpin.
Section 7 · 7.2.1 – Coupling Twin Trailers ↗PDF, 41 MB -
After hooking the dolly to the first semitrailer, how do you secure the dolly support?
In the raised position
Lock the pintle hook and raise the dolly support before pulling the dolly into position.
Section 7 · 7.2.1 – Coupling Twin Trailers ↗PDF, 41 MB -
Before backing the dolly under the rear trailer, how high should the trailer be?
Slightly lower than the center of the dolly fifth wheel
The trailer should be raised slightly as the dolly is pushed under it.
Section 7 · 7.2.1 – Coupling Twin Trailers ↗PDF, 41 MB -
After backing the dolly under the rear trailer, why do you raise the landing gear slightly off the ground?
To prevent damage if the trailer moves
Lifting the gear slightly keeps it from being damaged if the trailer shifts.
Section 7 · 7.2.1 – Coupling Twin Trailers ↗PDF, 41 MB -
How do you test the coupling between the dolly and the second semitrailer?
Pull against the pin of the second semitrailer
A pull against the kingpin of the second trailer tests that the dolly fifth wheel has locked.
Section 7 · 7.2.1 – Coupling Twin Trailers ↗PDF, 41 MB -
After coupling the second trailer, what do you connect?
Safety chains, air hoses and light cords
Connect the safety chains, the air hoses and the light cords.
Section 7 · 7.2.1 – Coupling Twin Trailers ↗PDF, 41 MB -
After coupling doubles, what do you do with the converter dolly air tank petcock?
Close it
Close the dolly air tank petcock and the shut-off valves at the rear of the second trailer.
Section 7 · 7.2.1 – Coupling Twin Trailers ↗PDF, 41 MB -
After coupling doubles, which shut-off valves do you open?
Those at the rear of the first trailer, and on the dolly if equipped
Air must reach the second trailer, so the valves at the rear of the first trailer are opened.
Section 7 · 7.2.1 – Coupling Twin Trailers ↗PDF, 41 MB -
You charge the trailer brakes and find no air at the rear of the second trailer. What does it mean?
Something is wrong and the brakes will not work
Air must reach the rear of the last trailer. If it does not, the brakes will not work.
Section 7 · 7.2.1 – Coupling Twin Trailers ↗PDF, 41 MB -
Where should you park doubles before uncoupling the rear trailer?
In a straight line on firm level ground
Park the rig straight on firm level ground and set the parking brakes.
Section 7 · 7.2.2 – Uncoupling Twin Trailers ↗PDF, 41 MB -
When uncoupling doubles, when do you chock the wheels of the second trailer?
If it does not have spring brakes
A second trailer without spring brakes must be chocked.
Section 7 · 7.2.2 – Uncoupling Twin Trailers ↗PDF, 41 MB -
Why do you lower the landing gear of the second semitrailer before pulling the dolly out?
To remove some weight from the dolly
Lowering the gear enough takes some weight off the dolly.
Section 7 · 7.2.2 – Uncoupling Twin Trailers ↗PDF, 41 MB -
When uncoupling doubles, which air shut-offs do you close?
Those at the rear of the first semitrailer, and on the dolly if equipped
Close the shut-offs at the rear of the first semitrailer before disconnecting the dolly lines.
Section 7 · 7.2.2 – Uncoupling Twin Trailers ↗PDF, 41 MB -
How do you get the dolly out from under the rear semitrailer?
Release the dolly fifth wheel latch and slowly pull the rig forward
With the dolly brakes and fifth wheel latch released, slowly pull tractor, first trailer and dolly forward.
Section 7 · 7.2.2 – Uncoupling Twin Trailers ↗PDF, 41 MB -
When uncoupling the converter dolly, how do you keep it from rolling?
Apply the converter gear spring brakes or chock the wheels
After lowering the dolly landing gear and removing the chains, set its spring brakes or chock it.
Section 7 · 7.2.2 – Uncoupling Twin Trailers ↗PDF, 41 MB -
Which step must never be done while the converter dolly is still under the rear trailer?
Unlocking the pintle hook
Unlocking the pintle hook with the dolly under the trailer can make the tow bar fly up, causing injury and making re-coupling hard.
Section 7 · 7.2.2 – Uncoupling Twin Trailers ↗PDF, 41 MB -
When coupling triples, how do you couple the tractor to the first trailer?
By the method for coupling tractor-semitrailers
The first trailer is coupled like any tractor-semitrailer, then the others are added with dollies.
Section 7 · 7.2.3 – Coupling and Uncoupling Triple Trailers ↗PDF, 41 MB -
When uncoupling a triple-trailer rig, which trailer do you uncouple first?
The third trailer
Pull the dolly out from under the third trailer and unhitch it, then uncouple the rest like a double.
Section 7 · 7.2.3 – Coupling and Uncoupling Triple Trailers ↗PDF, 41 MB -
How do you learn to couple a combination not covered in the handbook?
From the manufacturer and owner specifications
There are too many types to cover, so learn the correct way for your vehicle from its manufacturer or owner specifications.
Section 7 · 7.2.4 – Coupling and Uncoupling Other ↗PDF, 41 MB -
Where are the shut-off valves on doubles and triples?
At the rear of the trailers, in the service and emergency lines
Shut-off valves sit at the rear of each trailer in both the service and the emergency lines.
Section 7 · 7.3.1 – Additional Checks ↗PDF, 41 MB -
During the inspection, in what position should the converter dolly air tank drain valve be?
Closed
The dolly air tank drain valve must be closed so the dolly holds air.
Section 7 · 7.3.1 – Additional Checks ↗PDF, 41 MB -
A spare tire is carried on the converter dolly. What do you check?
That it is secured
A spare tire on the dolly must be secured.
Section 7 · 7.3.1 – Additional Checks ↗PDF, 41 MB -
What do you check about the dolly's pintle-eye during inspection?
That it is in place in the pintle hook of the trailer
The pintle-eye must sit in the pintle hook, and the hook must be latched.
Section 7 · 7.3.1 – Additional Checks ↗PDF, 41 MB -
During the inspection of doubles, what must be true of the pintle hook?
It is latched
An unlatched pintle hook could let the dolly separate from the trailer.
Section 7 · 7.3.1 – Additional Checks ↗PDF, 41 MB -
What do you check about the light cords on doubles and triples?
That they are firmly in their sockets on the trailers
Light cords must be seated firmly in the trailer sockets, and safety chains secured to the trailers.
Section 7 · 7.3.1 – Additional Checks ↗PDF, 41 MB -
How do you check the brakes on a double or triple trailer?
As you would on any combination vehicle, plus added checks
Use the combination vehicle brake check and add the checks for doubles and triples.
Section 7 · 7.4 – Doubles/Triples Air Brake Check ↗PDF, 41 MB -
In the air flow check, how do you hold the vehicle in place?
Use the tractor parking brake and/or chock the wheels
Set the tractor parking brake or chock the wheels before charging the system.
Section 7 · 7.4.1 – Additional Air Brake Checks ↗PDF, 41 MB -
In the air flow check, how do you provide air to the service line?
Use the trailer handbrake
Pushing in the air supply knob charges the supply lines, and the trailer handbrake puts air in the service line.
Section 7 · 7.4.1 – Additional Air Brake Checks ↗PDF, 41 MB -
In the air flow check, what does pushing in the red trailer air supply knob do?
Supplies air to the emergency (supply) lines
The air supply knob charges the emergency, or supply, lines.
Section 7 · 7.4.1 – Additional Air Brake Checks ↗PDF, 41 MB -
In the air flow check, you hear no air from the lines at the rear. What do you check?
That the shut-off valves on the trailers and dollies are open
Closed shut-off valves stop the air, and you must have air all the way back for all the brakes to work.
Section 7 · 7.4.1 – Additional Air Brake Checks ↗PDF, 41 MB -
In the tractor protection valve test, how do you lower the air pressure in the tanks?
With the engine off, step on and off the brake pedal several times
Fanning the brake pedal with the engine off drops the pressure until the control should pop out.
Section 7 · 7.4.1 – Additional Air Brake Checks ↗PDF, 41 MB -
In the trailer service brake test, what tells you the trailer brakes are connected and working?
You feel the brakes come on when you use the hand control
Move forward slowly and apply the trailer brakes with the hand control: you should feel them come on.
Section 7 · 7.4.1 – Additional Air Brake Checks ↗PDF, 41 MB
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