Doubles and Triples: Multiple Trailer Handling

29 questions in the bank cover this topic. Everything below is drawn from the sources those questions cite.

Sources: 49 CFR 393.42, 49 CFR 393.43, 49 CFR 393.48, 49 CFR 393.70, 49 CFR 393.71, 49 CFR 571.121, Rearward amplification and load placement, Rearward amplification in a multi-unit combination, TC 21-305-20 para 13-15, TM 9-2330-203-14&P para 2-11, TM 9-2330-203-14&P para 2-12, TM 9-2330-203-14&P para 2-13, Weight transfer and the position of a full-mounted load

What you need to know

393.70(a) sets 3 inches.

In a double or triple the error compounds at every connection, so a tracking fault that looks minor at the first trailer is amplified at the last.

the path of the towed vehicle will not deviate more than 3 inches to either side of the path of the vehicle that tows it

49 CFR 393.70

Each articulation point responds to the one ahead with a delay and an overshoot, so the sideways motion grows down the combination.

The last trailer can reach several times the tractor's lateral acceleration, which is why a sudden swerve can roll the rear unit while the cab feels fine. Each hinge amplifies the movement of the one before it.

*Why:* Treat each unit as responding to its tower's yaw with a lag and a gain greater than one. The lateral acceleration ratio from unit n to unit n+1 exceeds unity for abrupt inputs, so the overall gain from tractor to last trailer is the product of those per-hinge gains and grows with the number of units. This is rearward amplification. It rises with steering input frequency, so a quick swerve amplifies far more than a gradual lane change, and it is the reason the rearmost trailer rolls first.

The rearmost unit experiences the largest amplified sideways motion, so putting the heavier, more stable load in front and the lighter one at the back reduces the risk of the rear trailer rolling.

Putting the heavy trailer last gives the biggest force to the position that moves most.

*Why:* Rollover threshold scales roughly with track width divided by twice the center-of-gravity height, so a heavier high load has a lower threshold. Rearward amplification means lateral acceleration is highest at the last unit. Placing the low-threshold trailer where acceleration is highest multiplies the two worst factors together, so the heavier trailer goes forward where amplification is least.

The manual's warning that control must be regained immediately or not at all applies with more force the more articulation points there are.

Each hinge is another place the combination can fold, and the rear unit is both the most amplified and the hardest to see. With more hinges in the combination there is more to go wrong before you can react.

Control must be regained immediately if it is to be regained at all.

TC 21-305-20 para 13-15

393.42(b)(5) excepts the steering axle of a three-axle dolly steered by a co-driver, and 393.48(c)(4) repeats the exception for the brakes-operative rule.

It is a narrow case, not a general dolly exemption.

Brakes are not required on the steering axle of a three-axle dolly which is steered by a co-driver.

49 CFR 393.42

393.48(c)(3) names unladen converter dollies of 1,361 kg, which is 3,000 pounds, or less, built before 1 March 1998.

Both the weight and the date must apply. Both the weight and the date must apply.

Unladen converter dollies with a gross weight of 1,361 kg (3,000 lbs) or less, and manufactured prior to March 1, 1998

49 CFR 393.48

393.43(d) applies to every trailer required to have brakes, which in a double or triple is each unit.

They must apply automatically and immediately and stay applied at least 15 minutes.

Every trailer required to be equipped with brakes shall have brakes which apply automatically and immediately upon breakaway from the towing vehicle.

49 CFR 393.43

393.43(d) sets 15 minutes minimum.

A separated trailer on a grade that released sooner could start moving before anyone secured it. None of the figures offered is the one the rule gives.

The brakes must remain in the applied position for at least 15 minutes.

49 CFR 393.43

FMVSS 121 S5.2.3.2 requires a trailer designed to tow other air-braked trailers to relay their ABS malfunction signals forward.

The chain of relays is what gets a fault from the rear unit to the cab. To the vehicle towing it, for each trailer designed to tow another.

In addition, each trailer manufactured on or after March 1, 2001, that is designed to tow other air-brake equipped trailers shall be capable of transmitting a malfunction signal from the antilock brake systems of additional trailers it tows to the vehicle towing it.

49 CFR 571.121

FMVSS 121 S5.1.6.2(b) requires a lamp separate from the one required for the towing vehicle's own ABS, so the driver can tell a trailer fault from a tractor fault.

Separate from the tractor's own ABS lamp.

Each truck tractor manufactured on or after March 1, 2001, and each single unit vehicle manufactured on or after March 1, 2001, that is equipped to tow another air-braked vehicle, shall be equipped with an electrical circuit that is capable of transmitting a malfunction signal from the antilock brake system(s) on one or more towed vehicle(s) (e.g., trailer(s) and dolly(ies)) to the trailer ABS malfunction lamp in the cab of the towing vehicle

49 CFR 571.121

393.71(a)(3) requires brakes on all but the final towed vehicle.

The last unit is the exception, which pairs with 393.42(b)(2) excusing it from operative brakes.

When motor vehicles are towed by means of triple saddle-mounts, all but the final towed vehicle must have brakes acting on all wheels in contact with the roadway.

49 CFR 393.71

393.71(a)(2) permits only one.

Each additional articulating coupling adds a hinge the combination can fold at, and the rule limits how many are allowed.

No more than one tow-bar or ball-and-socket type coupling device may be used in any combination.

49 CFR 393.71

393.71(d) prohibits bumper-dependent tow-bars above 5,000 pounds.

A bumper is not a structural member and is not designed to carry towing loads. None of the figures offered is the one the rule gives.

Tow-bars of the type which depend upon the bumpers as a means of transmitting forces between the vehicles shall not be used to tow a motor vehicle weighing more than 5,000 pounds.

49 CFR 393.71

393.71(h)(2) requires free motion in both planes, so the vehicles can turn and ride over uneven ground without transmitting stresses to each other except along the tongue's longitudinal axis.

The tow-bar shall be so constructed as to freely permit motion in both horizontal and vertical planes between the towed and towing vehicles.

49 CFR 393.71

393.71(h)(4) allows no more than 0.25 inch of sag in any direction, at any length setting.

A sloppy adjustment mechanism is how a tow-bar starts to bend.

With the tow-bar supported rigidly at both ends and with a load of 50 pounds at the center, the sag, measured at the center, in any direction shall not exceed 0.25 inch under any condition of adjustment as to length.

49 CFR 393.71

393.71(e) ties the requirement to projection beyond the widest part of either vehicle.

An unrestrained wheel that swings wide becomes an unseen obstacle at the side of the combination. Turning them is what would cause the projection.

A motor vehicle towed by means of a saddle-mount shall have the motion of the front wheels restrained if under any condition of turning of such wheels they will project beyond the widest part of either the towed or towing vehicle.

49 CFR 393.71

393.71(f) requires the front end mounted, so the steered wheels are off the ground.

The only alternative is locking the steering straight ahead.

Unless the steering mechanism is adequately locked in a straight-forward position, all motor vehicles towed by means of a saddle-mount shall be towed with the front end mounted on the towing vehicle.

49 CFR 393.71

393.71(b)(3) makes weight distribution the organizing principle.

The same reasoning drives fifth wheel placement in 393.70(b)(3): where weight sits determines whether the vehicle steers and stops. Otherwise there is undue interference with steering, braking or maneuvering.

Saddle-mounted vehicles must be arranged such that the gross weight of the vehicles is properly distributed to prevent undue interference with the steering, braking, or maneuvering of the combination of vehicles.

49 CFR 393.71

393.71(c)(4) removes any brake exemption once a vehicle is carrying another.

The extra mass has to be stopped by the unit carrying it. The requirement holds at all times while the vehicle is so loaded.

If a motor vehicle towed by means of a saddle-mount has any vehicle full-mounted on it, the saddle-mounted vehicle must at all times while so loaded have effective brakes acting on all wheels in contact with the roadway.

49 CFR 393.71

Think of the rear axle as a fulcrum.

Load behind it pushes the back down and lifts the front, taking weight off the wheels that steer and off the coupling ahead. Keeping the center of gravity forward of that line keeps the front loaded and the combination controllable. A center of gravity behind the rear axle unloads the front axle.

*Why:* Treat the saddle-mounted vehicle as a beam on two supports, the front coupling and the rear axle. A load at distance d behind the rear axle produces a moment W*d about that axle, which transfers a reaction of W*d/L off the front support, where L is the distance between supports. As d grows positive the front load falls and can reach zero, at which point the steered wheels carry nothing and both steering and braking at the front are lost. Requiring the center of gravity forward of the rear axle keeps d negative and the front support loaded.

Chocks first.

You are about to back a dolly into a free-standing trailer, and anything that pushes it away or lets it roll turns a routine coupling into a chase.

Position chock blocks behind semitrailer wheels.

TM 9-2330-203-14&P para 2-11

Open before backing, and they lock themselves on engagement.

Backing into closed jaws at best wastes the maneuver and at worst damages the kingpin or the fifth wheel.

Ensure that coupler jaws (2) on dolly fifth wheel (1) are open. Align center of fifth wheel with semitrailer kingpin. Slowly back dolly under semitrailer. When coupler jaws engage kingpin, jaws will automatically lock.

TM 9-2330-203-14&P para 2-11

The hoses.

The procedure says that if more distance is needed, disconnect them, and only after confirming the semitrailer brakes are set. Pulling against connected hoses tears them. Unless you disconnect them, having first set the trailer brakes.

Do not drive towing vehicle forward beyond limits of air hose length, unless air hoses have been disconnected.

TM 9-2330-203-14&P para 2-11

Connected is not the same as working.

A cable can seat and still have a broken conductor or a corroded pin, and the only way to know is to switch the lights on and walk back. Connected is not the same as working, so walk back and look.

Operate semitrailer lights from

towing vehicle and ensure that all lights operate properly

TM 9-2330-203-14&P para 2-11

They set automatically.

Loss of air in the emergency line is what applies spring brakes, which is the same principle as the breakaway requirement in 393.43(d). It is a safety feature, not a fault.

Semitrailer brakes will set automatically when emergency air hose is uncoupled.

TM 9-2330-203-14&P para 2-13

Both sides.

Note the difference from coupling, where chocks go behind only: once uncoupled the trailer stands alone and could roll either way depending on the ground.

Position chock blocks in front of and behind semitrailer wheels.

TM 9-2330-203-14&P para 2-13

The lever goes fully forward.

Partway may feel like it has released while the jaws are still partly engaged, which is how a trailer drops as the tractor pulls out.

Pull control lever (2) as far forward as possible to release semitrailer kingpin from jaws (3) of fifth wheel (1).

TM 9-2330-203-14&P para 2-13

Every hinge is another place the combination can fold and another lag between your steering input and the last unit's response.

That is also the mechanism behind rearward amplification.

Turning and backing operations will be affected because towing vehicle, dolly, and trailer act as hinged unit.

TM 9-2330-203-14&P para 2-12

Both.

And if parked on a hill, chock the wheels as well. Relying on one unit's brakes to hold a multi-unit combination leaves everything behind it depending on a single system. And chock the wheels if parked on a hill.

When parking for extended periods, set handbrakes on towing vehicle and trailer.

TM 9-2330-203-14&P para 2-12

Check yourself

• How closely must your towed vehicle track behind its tower on a straight, level, smooth paved surface?

• You steer around an obstacle in a set of triples. Compared with the tractor, how does the last trailer move?

• In a set of doubles, where should the heavier trailer normally be placed and why?

• Why is your trailer skid in a multiple-trailer combination less recoverable than in a single?

• Which dolly axle is excepted from the brake requirement, and on what condition?

• Which converter dollies are excepted from the requirement that all brakes be operative?

• What is required if any trailer in a double or triple breaks away?

• For how long must breakaway brakes hold?

• In a triple, how does an ABS fault on the last trailer reach the driver?

• What indicates a towed unit's ABS fault inside the cab?

• In a triple saddle-mount combination, which vehicles must have brakes on all wheels touching the road?

• How many tow-bars or ball-and-socket couplings may a driveaway-towaway combination use?

• Above what weight may a bumper-type tow-bar not be used?

• What movement must your tow-bar permit?

• What sag limit applies to an adjustable-length tow-bar under a 50 pound center load?

• When must the front wheels of a saddle-mounted vehicle be restrained?

• Which end of a saddle-mounted vehicle normally goes on your towing vehicle?

• What is the rule governing how saddle-mounted vehicles must be arranged?

• A saddle-mounted vehicle carries another vehicle full-mounted on it. What brake requirement applies?

• Why does the rule require the center of gravity of a full-mounted vehicle to lie FORWARD of the rear axle center line of the saddle-mounted vehicle below it?

• What is the FIRST step when coupling your semitrailer to your dolly?

• What must be true of the dolly's coupler jaws before you back under the semitrailer?

• A coupling attempt failed and you need to pull forward and retry. What limits how far you can go?

• After connecting the electrical cable, what must you do?

• What is the effect on the semitrailer brakes when you disconnect the emergency air hose?

• When uncoupling your semitrailer from your dolly, where do the chocks go?

• How is your kingpin released from the dolly's fifth wheel?

• Why do turning and backing change with your dolly in your combination?

• What does the procedure require when parking your combination for an extended period?

Answers are in the practice test for this topic.