Because each trailer amplifies the motion of the one in front of it, doubles and triples roll over and jackknife more easily than other commercial vehicles. Drive them smoothly and look far ahead so you can ease off or move over gradually.
Controlled braking means braking as hard as you can without locking the wheels, while stab braking means braking fully, releasing when the wheels lock, and reapplying once they roll; stab braking is only for vehicles without antilock brakes. Both keep the rig straight, whereas holding the pedal to the floor or braking with the trailers alone can lead to a skid or jackknife.
A standard double bends in three places: at the tractor's fifth wheel, at the pintle hook joining the first trailer to the converter dolly, and at the dolly's fifth wheel beneath the second trailer.
Doubles and triples are long and cannot be stopped or turned quickly, so they need more room ahead than smaller vehicles. Their length and extra unpowered axles also raise the risk of skids and lost traction, so slippery roads, bad weather and mountain driving call for special caution.
A B-train has no converter dolly: the second trailer couples to a fifth wheel mounted at the back of the lead trailer. That leaves only two articulation points, the tractor's fifth wheel and the lead trailer's fifth wheel.
The crack-the-whip effect magnifies a sudden steering move toward the back of a double or triple, so the last trailer can tip over; steer smoothly and avoid braking while you swerve.
Pressing the brake pedal sends air to the service brakes on the tractor and all trailers simultaneously. The trailer hand valve exists solely to test the trailer brakes, while the red knob supplies or shuts off air to the trailers; neither is for ordinary stops.
For the safest handling, the most heavily loaded trailer goes right behind the tractor and the lightest goes at the rear. The rear trailer feels the strongest crack-the-whip effect and is the one most likely to overturn after a sudden lane change.
Multiple trailers are harder to control on slick roads and steep grades and need more room to stop, so you must be especially careful in bad weather and mountain driving and also allow extra following distance. Both habits are needed, not just one.
The federal training rules for longer combination vehicles require at least six months of experience in a tractor-semitrailer or a tractor-semitrailer-trailer combination before a driver can start triples training. Time in a straight truck or bus does not count, and a longer period is not required.
Begin with one second for each 10 feet of vehicle length, which comes to 10 seconds for this rig, and then add a second because you are going faster than 40 mph, for a total of 11.
The crack-the-whip effect magnifies a quick steering move at each joint, so every trailer swings harder than the one ahead of it. In a triples combination the last trailer swings the most and is the one most prone to rolling over, which is why you must steer gently.
Backing up a double or triple combination is very hard, so avoid any spot you cannot leave by driving forward. Knowing how a lot is laid out before you pull in helps you avoid a long, awkward exit.
The crack-the-whip effect makes doubles and triples less stable than single-trailer combinations, so they overturn more readily. The farther back a trailer sits in the set, the greater its rollover risk.
Controlled or stab braking slows the rig while keeping it straight and letting you steer. Holding the pedal to the floor locks the wheels and invites a skid or jackknife, and using only the trailer brakes can make the trailers skid.
Below 40 mph, the rule is one second of gap per 10 feet of rig length, so this 100-foot combination needs 10 seconds; the additional second applies only when going faster than 40 mph.
Doubles and triples must be driven very smoothly, because abrupt steering or braking can roll them over or make them jackknife. Looking far ahead gives you time to slow down or change lanes gradually instead of suddenly.
The crack-the-whip effect magnifies each steering input as it travels back through the combination, so an abrupt turn of the wheel can flip the last trailer. Off-tracking has nothing to do with shorter stops, and the whip effect makes the rear trailer more likely to overturn, not less.
Each joint that lets one unit swing independently of another is a pivot point. This rig has five: the tractor's fifth wheel under the first trailer, plus a pintle hook and a dolly fifth wheel at each of the two dolly connections.
When the trailer wheels lose their grip, the trailer can swing out to the side and fold the rig into a jackknife. Check your mirrors often so you notice a trailer skid early; once the trailer slides completely out of your lane, you may be unable to recover.
Doubles and triples cannot be stopped or turned suddenly, so they need extra following distance. The heavier trailer belongs right behind the tractor, with the lighter one at the rear.
Hook the heaviest trailer up first, directly behind the tractor, and put the lightest one at the rear; a heavy trailer at the back makes the combination less stable and more likely to overturn.
Locked trailer wheels lose their grip, and the trailer tends to swing around to the side, folding toward the vehicle pulling it. Releasing the brakes lets those wheels roll and grip again so the trailer can fall back in line.
Of these three setups, only the Rocky Mountain double mixes trailer lengths, pairing a long lead semitrailer with one 28.5-foot pup. A standard STAA double uses two short pups, and a turnpike double uses two long trailers.
Every question in this set has its own page with the correct answer and an explanation.
