Oklahoma CDL Combination cheat sheet

A combination vehicle's high center of gravity and long wheelbase make it far less stable in turns than a single-unit truck; rollover threshold for a loaded tractor-trailer is roughly 0.35–0.45 g of lateral acceleration.

Rollover occurs when lateral acceleration exceeds the vehicle's roll threshold; speed in a curve is the dominant controllable factor because lateral acceleration rises with the square of speed and falls with increasing radius.

The trailer's rearward amplification effect means the trailer swings wider and faster than the tractor during abrupt steering, so a rapid lane change can produce trailer rollover even when the tractor stays upright.

Off-tracking and rollover are separate hazards: off-tracking is the rear wheels cutting inside the front wheels' path; rollover is lateral load transfer that lifts the inside tires.

Load height raises the center of gravity and lowers the roll threshold; a loaded trailer with a high, dense load rolls at lower lateral acceleration than the same trailer with a low, dense load.

Fifth wheel and trailer suspension condition affect stability: worn shocks, broken springs, or loose fifth wheel jaws allow trailer sway and amplify roll forces.

Steering inputs should be smooth and gradual; a combination vehicle requires roughly 20–40% longer to complete a lane change than a passenger car, so plan gaps accordingly.

A tractor-trailer's longer stopping distance and higher center of gravity mean curve entry speed must be set before the curve, not adjusted with braking inside it.

Inspect the fifth wheel before coupling: jaws open, locking mechanism free, mounting bolts tight, and the wheel plate greased; a closed jaw or debris prevents full lock.

Chock the trailer wheels before coupling or uncoupling; the trailer brakes must be set and the wheels chocked so the trailer cannot roll when the tractor is removed.

Back the tractor squarely under the trailer so the kingpin enters the throat centered; misalignment bends the kingpin and can cause the trailer to drop later.

After coupling, tug test: pull forward gently against the trailer brakes in low gear; a locked fifth wheel holds the trailer, a missed coupling lets the trailer slide or the tractor pull out.

Secure the tractor and trailer with the trailer parking brake, then visually confirm the fifth wheel jaws are closed around the kingpin and the release handle is fully in.

Connect the air lines: emergency (supply) line to the emergency glad hand, service line to the service glad hand; crossed lines cause trailer brakes to release or drag incorrectly.

After connecting, charge the trailer air system and check for leaks; the tractor protection valve should pop out and the trailer brakes should set when supply pressure drops.

Raise the trailer landing gear fully and secure the crank handle before moving; when uncoupling, lower the gear to firm ground contact before releasing the fifth wheel.

The pre-trip inspection covers three zones: tractor front and engine compartment, tractor rear and coupling area, and trailer body, suspension, brakes, and lights.

Check coupling security by confirming the fifth wheel is locked, the release handle is in, the safety latch is engaged, and the air lines are supported and not chafed.

Trailer air brake inspection includes checking slack adjusters (push rod travel within limit), brake chamber condition, and that the trailer brakes apply and release with the tractor control.

Tire inspection includes tread depth (minimum 4/32 inch on front steer tires, 2/32 inch on all other tires), sidewall condition, and matching dual tire diameters within 1/4 inch.

Lighting check covers clearance lights, turn signals, brake lights, and trailer marker lights; a helper or reflective surface is used to confirm trailer lights from the rear.

Landing gear inspection: fully raised, crank handle secured, no bent legs, and no cracks at the mounting brackets.

Check the trailer frame, crossmembers, and floor for cracks, sagging, or holes that could let cargo fall through; check the rear impact guard and mud flaps.

Air loss test: with the engine off and the system charged, apply the brakes and confirm leakage stays within acceptable limits; a rapid drop indicates a leak that must be fixed before dispatch.

A combination vehicle uses a dual-circuit air system; the tractor protection valve isolates the tractor's air supply from the trailer if trailer pressure drops below roughly 20–45 psi.

The trailer supply (emergency) line charges the trailer tanks; the service line carries the application signal; loss of supply pressure causes the trailer spring brakes to set automatically.

Total stopping distance equals perception distance plus reaction distance plus brake lag distance plus effective braking distance; air brake lag adds roughly 0.4–0.6 second at highway speed.

Normal operating pressure is about 100–125 psi; the low-pressure warning must activate before pressure falls below 60 psi, and the spring brakes should apply around 20–45 psi.

The governor cut-out is typically near 125–130 psi and cut-in near 100 psi; a compressor that fails to build pressure from 85 to 100 psi within about 45 seconds at fast idle indicates a problem.

Slack adjusters should be checked with brakes released and applied; push rod travel beyond the manufacturer's limit means the brakes are out of adjustment and the vehicle is out of service.

Brake fade on long downgrades results from heat; use engine braking and a low gear, and apply brakes firmly enough to feel, then release, rather than riding them lightly.

The tractor protection valve and trailer spring brakes are the fail-safe: a breakaway or line separation sets the trailer brakes, so never move a trailer with a known air leak.

ABS prevents wheel lockup by modulating brake pressure at each wheel or axle; it does not shorten stopping distance on all surfaces but preserves steering control during hard braking.

A combination vehicle typically has ABS on the tractor and on the trailer; the trailer ABS is powered through the electrical line, so a damaged cord can disable trailer ABS.

The amber ABS warning lamp should illuminate at startup and go out above roughly 4 mph; a lamp that stays on means ABS is disabled and the vehicle still has normal (non-ABS) brakes.

With ABS active, hold firm steady pressure on the brake pedal and do not pump the brakes; pumping defeats the system.

On a tractor-trailer, ABS may activate on only some wheels; the driver should continue braking and steer around the hazard rather than releasing the pedal.

Trailer ABS malfunction is indicated by a trailer-mounted amber lamp; the vehicle may still be driven but with reduced skid control, and the fault must be repaired.

ABS does not compensate for excessive speed, following distance, or load; it cannot overcome the physical limits of tire traction.

When ABS is working, a pulsating pedal and mechanical noise are normal; a hard pedal with no pulsation during a panic stop may indicate the system did not engage.

Off-tracking is the difference between the path of the front wheels and the path of the rear wheels; a tractor-trailer's rear wheels cut inside the turn, and the longer the wheelbase the greater the cut.

A rule of thumb: a combination vehicle can off-track by roughly 1 foot for every 10 feet of wheelbase in a 90-degree turn, so a 50-foot wheelbase can cut about 5 feet inside.

To compensate, begin the turn later and swing wider than the lane; watch the trailer's rear wheels in the mirrors and keep them clear of curbs and poles.

Right turns are the highest off-tracking risk because the trailer cuts toward the curb; left turns risk the trailer cutting into oncoming lanes.

The pivot point of a tractor-trailer is the rear axle of the tractor, not the fifth wheel; the trailer pivots around the tractor's rear wheels, so the trailer's rear swings outward on tight turns.

Lane control requires checking mirrors frequently and maintaining a space cushion; a combination vehicle needs more room to change lanes and cannot stop or swerve as quickly as a car.

When turning, do not cross the center line or curb line; if the turn cannot be completed within the lane, stop and re-evaluate rather than forcing the trailer over the line.

At intersections, use the whole lane and turn from the correct lane; a wide swing into an adjacent lane can strike vehicles or pedestrians.

Back a tractor-trailer only when necessary; get out and check the area behind and around the trailer before backing, and use a helper or spotter when available.

Back toward the driver's side (left) when possible; the driver's side mirror gives a better view of the trailer's path and the blind side is reduced.

Use the mirrors continuously while backing and keep the trailer visible; if the trailer disappears from the mirror, stop and re-establish position.

Steering while backing is reversed relative to the trailer's movement: turn the wheel toward the direction you want the trailer's rear to go.

Back slowly and use the lowest gear; a combination vehicle's rearward visibility is limited, and a spotter or GOAL (get out and look) prevents backing collisions.

When parking, set the parking brake, place the transmission in the lowest gear or park, chock the wheels on a grade, and turn the wheels toward the curb on a downhill.

Parking on a hill: set the parking brake, chock the wheels, and turn the front wheels so the vehicle rolls into the curb if it moves; never rely on air pressure alone.

Pull forward to straighten the trailer before backing into a dock or space; a jackknifed angle makes backing harder and increases the risk of hitting the trailer with the tractor.

A jackknife occurs when the tractor's rear wheels lock and the trailer pushes the tractor around, or when the drive wheels lose traction and the tractor folds toward the trailer.

Drive-wheel skids are corrected by releasing the brake or throttle and steering in the direction the rear of the tractor is sliding; do not steer opposite the skid.

Trailer skids: if the trailer's rear swings out, steer in the direction of the trailer's slide to straighten the combination.

To prevent jackknifing, brake smoothly, avoid locking the drive wheels, and use ABS; a loaded trailer can jackknife a lightly loaded tractor quickly.

In an emergency, brake firmly and steer around the hazard only if there is room; a combination vehicle cannot stop as short as a car, so the escape route must be chosen early.

If a blowout occurs, hold the steering wheel firmly, avoid hard braking, and slow gradually while keeping the vehicle straight.

Off-road recovery: do not jerk the wheel to return to the pavement; slow down, then steer gently back onto the road when the tires are ready.

Skid recovery requires looking and steering where you want to go, not where the vehicle is sliding; overcorrecting causes a secondary skid.

Federal weight limits: 20,000 lb on any single axle, 34,000 lb on a tandem axle group, and 80,000 lb gross vehicle weight on the Interstate System without an overweight permit.

The bridge formula limits the weight on any group of axles based on the number of axles and the distance between them; it prevents overloading short spans.

Cargo must be secured with tiedowns whose working load limit and number are sufficient; the aggregate working load limit must be at least half the cargo weight, and the cargo must not shift.

Load distribution matters: keep the center of gravity low and centered, place heavy freight over the tractor's drive axles or forward on the trailer, and avoid concentrating weight at the trailer's rear.

Axle weight limits are checked at weigh stations; a legal gross weight can still be illegal if one axle or axle group is overloaded.

Cargo must be secured against forward, rearward, and lateral movement; headboards, blocking, and tiedowns must be inspected before and during the trip.

Overloading raises stopping distance, lowers the roll threshold, and can overheat brakes and tires; underloading the drive axles reduces traction and can cause wheel spin.

Loose or shifting cargo can cause a rollover or jackknife; check tiedowns after the first few miles and periodically during the trip.

Following distance for a combination vehicle should be at least one second per 10 feet of vehicle length at speeds under 40 mph, plus one second; a 60-foot rig needs about 7 seconds.

At highway speed, a loaded tractor-trailer needs roughly 300 feet or more to stop; the total stopping distance includes perception, reaction, brake lag, and braking distance.

Space management means keeping a cushion ahead, behind, and to the sides; avoid traveling in other vehicles' blind spots and leave room for lane changes.

Speed control on downgrades: select a gear that holds the desired speed without braking, and use intermittent firm braking rather than continuous light braking to avoid fade.

Hazard perception requires scanning 12–15 seconds ahead, checking mirrors every 5–8 seconds, and watching for brake lights, merge lanes, and road debris.

In rain, snow, or ice, reduce speed and increase following distance; a combination vehicle's trailer can hydroplane or swing out before the tractor loses traction.

At railroad crossings and intersections, slow down and be prepared to stop; a combination vehicle cannot clear a crossing as quickly as a car.

Night driving and fog reduce visibility; reduce speed to a level where you can stop within the distance you can see, and use low beams in fog.