Cargo Securement and Weight Distribution for Combination Vehicles
This domain covers legal weight limits, how to shift weight between axles without creating a new overload, and the securement rules that apply to combination vehicles.
Weight limits
Federal limits on the Interstate System: 80,000 lb gross, 20,000 lb on a single axle, 34,000 lb on a tandem group, all subject to the bridge formula. The steer axle is usually capped by its tire and axle rating, often around 12,000 lb. Being under 80,000 lb gross does not make you legal - any overloaded axle is a violation.
Shifting weight
- Slide the trailer tandem rearward to move weight onto the drive axles; forward to take weight off them.
- Move the tractor's fifth wheel rearward to add weight to the drive axles and lighten the steer axle; forward for the opposite.
- A light steer axle makes steering vague and cuts front traction; too much on the steer axle overloads tires and hurts braking.
- Keep the center of gravity low and centered; a high load lowers the rollover threshold.
Securement
Cargo must be immobilized against braking and turning. The combined working load limit of the tiedowns must be at least half the cargo weight, and you need at least one tiedown for every 10 feet of article length, with a minimum of two for articles longer than 5 feet or heavier than 1,100 lb.
Worked example
After loading, the steer axle scales at 11,800 lb and the drive tandem at 35,200 lb. Moving the trailer tandem forward a notch transfers load from the drives to the trailer axles, and because the tractor pivots about the steer axle, steer weight rises slightly - to roughly 12,100 lb. Both axles are now legal; re-scale to confirm.
Traps
- Loading heavy freight at the nose and driving off without re-weighing.
- Sliding tandems and never checking the new axle weights.
- Counting tiedowns without checking their working load limit.