Michigan CDL Doubles/Triples flashcards

32 cards. Read the term and the definition here. The deck at the top of this page is optional.

Tug test

After backing under a trailer, pull forward gently in low gear with the trailer brakes set to confirm the fifth-wheel jaws are locked around the kingpin.

Fifth-wheel jaw gap

A visible space between the closed jaw and the kingpin means the coupling is not fully seated; do not drive until the jaws lock with no gap.

Converter dolly

A separate axle-and-frame unit with its own brakes and kingpin that couples a second trailer to the rear of the first trailer in a doubles or triples set.

Uncoupling sequence

Set parking brakes, lower landing gear or dolly legs to firm ground, disconnect the electrical cord and air lines, release the fifth-wheel handle, then pull the tractor slowly forward.

Heaviest trailer placement

In a doubles or triples combination, load the heaviest trailer directly behind the tractor and place lighter trailers behind it to keep the combination stable.

Damaged upper coupler

A bent, cracked, or worn upper coupler plate or apron can let the kingpin pull out under load; never couple to a trailer with this damage.

Dolly safety chains

Cross the chains under the dolly drawbar and attach them to the towing trailer so the dolly stays connected if the coupling fails.

Combination leak rate

With the engine off and the system charged, air loss greater than 4 psi per minute in a combination, or 3 psi in a single vehicle, indicates a leak.

Tractor protection valve test

Disconnect the trailer supply line; the valve should pop out and stop air loss from the tractor. A valve that stays open makes the tractor unsafe to drive.

Rear-most trailer check

Inspect the last trailer's lights, reflective tape, and mud flaps, because that unit is what following traffic sees first.

Per-unit inspection

Inspect every trailer and every converter dolly separately; each added unit has its own brakes, lights, tires, and coupling hardware that must be checked.

Dolly brake function

Confirm the converter dolly's own brake system is connected and working; a dolly with dragging or dead brakes destabilizes the whole combination.

Air line colors

The service or control line is typically blue and the emergency or supply line is red; swapping them prevents the trailer brakes from releasing.

Gladhand seals

Rubber seals inside each gladhand prevent air leaks at the coupling; a missing or flattened seal leaks air and can keep trailer brakes applied.

Seven-pin connector

The electrical cord uses a seven-pin plug; confirm it reaches the tractor receptacle without stretching during turns and is secured against dragging.

Shut-off valve position

Open the shut-off valves at the trailer and dolly before driving so air reaches the trailer brakes; close them before uncoupling to avoid releasing spring brakes.

Gladhand leak check

Listen at each gladhand after opening the valves; a steady hiss means a bad seal, a loose coupling, or a cracked line that must be fixed.

Line routing

Route air lines and cords clear of the catwalk, dolly drawbar, and landing gear; chafing through an air line causes sudden loss of trailer brakes.

Light circuit walk-around

Turn on headlights, four-way flashers, and turn signals, then walk to the rear-most trailer to confirm each function works at every connection point.

Off-tracking

The rear trailer of a combination cuts inside the path of the tractor in a turn; the effect grows with each added unit, so swing wide.

Articulation points

Each coupling between tractor, trailers, and dolly adds a pivot that amplifies sway; reduce speed before curves, downgrades, and lane changes.

Hard-braking risk

A hard stop can let the rear trailer push the lead trailer, causing a jackknife or a fold at the dolly; brake earlier and more gently.

Downgrade gear choice

Select a gear low enough to hold speed with engine braking and use trailer brakes sparingly to prevent overheating and brake fade.

Backing limitation

A converter dolly makes conventional backing of doubles and triples impractical; plan routes to avoid backing rather than attempting it.

Crosswind handling

Reduce speed and keep steady steering input in strong crosswinds; the large sail area of multiple trailers makes the combination prone to lateral push.

Trailer hand valve use

Use the trailer hand valve only to test brakes, not for routine stopping; holding it while driving can cause the trailers to swing out.

Doubles and triples length

Federal limits allow two 28-foot trailers as doubles or three 28-foot trailers as triples on designated highways, subject to state permits and routes.

Dolly registration

A converter dolly is a separate vehicle with its own registration and brakes; it must be inspected and maintained like a trailer, not treated as an accessory.

Axle weight limits

Load each trailer so no axle exceeds its rating, and check total gross combination weight against the tractor's capacity and the state bridge formula.

Following distance rule

Allow at least one second per 10 feet of vehicle length below 40 mph, and add one second when speed is above 40 mph.

Warning devices placement

If any unit in a triple breaks down, place warning devices behind the rear-most trailer so approaching traffic sees them in time to stop.

Dolly pretrip

Inspect the dolly separately during pretrip; a dragging brake or flat tire on the dolly can make the whole combination pull to one side.