Doubles and Triples: Combination Handling

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

Sources: Newtonian mechanics, Passing time: t = (L_pass + L_truck + 2*gap) / (v_pass - v_truck), Power on a grade: P = m*g*sin(theta)*v + losses, Spray volume scales with tire contact patches and vehicle length, Static rollover threshold: a = g*t/(2h), TC 21-305-20 para 13-29

What you need to know

Off-tracking means your trailer wheels cut inside the path your steer tires take.

Giving the nose room toward the centerline is what keeps the trailer tires on the pavement. The trailer always takes the shorter path.

When driving a tractor/semitrailer, off-tracking must be allowed for. Therefore, when entering a right-hand curve, position the front of the vehicle as close to the centerline as possible

TC 21-305-20 para 13-29

The same geometry, mirrored.

In a left-hand curve the trailer cuts toward the centerline, so the tractor has to give it room by running wide to the right.

When driving into a left-hand curve, move as close as possible to the shoulder of the road (without running off the road) to keep the semitrailer tires from crossing the centerline.

TC 21-305-20 para 13-29

Braking in a curve spends the tire's grip on slowing when it is already spending it on turning.

On a combination that is how a trailer comes around. Get the speed off first.

Do not brake while in a curve. Brake before entering the curve.

TC 21-305-20 para 13-29

A steering input moves the tractor a little, the first trailer more, and the last trailer more again, because each hinge multiplies the swing of the one ahead of it.

That is why the rear trailer is the one that leaves the road first.

*Why:* A combination is a chain of hinged bodies. A lateral displacement applied at the tractor propagates through each coupling with a gain greater than 1 in the transient, so the amplitude at body n exceeds that at body n-1. With two couplings the rear trailer sees the largest excursion, which is the effect 383.115(c) calls oscillatory sway and drivers call crack-the-whip.

Two shorter trailers reach the same payload with the center of gravity higher than one long trailer would need.

Rollover begins when the sideways force at the center of gravity overcomes the weight acting across the track width.

*Why:* A vehicle rolls when lateral acceleration a exceeds g*t/(2h), where t is track width and h is center-of-gravity height. Track width is fixed by law at 102 in, so the only term the load changes is h. Stacking the same payload higher raises h and lowers the threshold directly.

The overtaking driver has to cover your length plus their own plus two safety gaps.

Doubling your length adds most of a second-trailer length to every pass, and all of it is spent on the wrong side of the road on a two-lane highway.

*Why:* Passing time t = (L_pass + L_truck + 2*gap) / (v_pass - v_truck). L_truck is the only term the combination changes, and it roughly doubles from a single to a double, so t rises with it at a fixed speed difference.

A grade costs power in proportion to weight.

Adding a second trailer adds weight without adding engine, so the speed the engine can hold on the hill falls. That is what puts you in the way of everybody behind you.

*Why:* Steady speed on a grade satisfies P = m*g*sin(theta)*v + losses, so for fixed engine power P the attainable v falls as mass m rises. A second trailer raises m without raising P, so v on the grade drops.

Every tire throws water, and a double has more of them spread over nearly twice the length.

In rain that is a longer curtain for a driver trying to pass you to see through, which is the traffic problem the rule has in mind.

*Why:* Spray volume scales with the number of tire contact patches lifting surface water and with the length over which they are distributed. A double roughly doubles both against a single, so the obscured region behind and beside the vehicle grows in the same proportion.

Check yourself

• Where do you place the front of your vehicle entering a right-hand curve?

• What keeps your trailer tires off the centerline in a left-hand curve?

• When do you do your braking for a curve?

• Your rear trailer swings further than your tractor. Why does that happen?

• Why does a loaded double roll over more easily than a loaded single?

• Why does passing you take another driver longer than passing a single truck?

• Why do you lose more speed than a single truck on the same upgrade?

• Why does your spray hide traffic more than a single truck's does?

Answers are in the practice test for this topic.