Weight Distribution, Load Limits, and Safe Operation
Weight Distribution, Load Limits, and Safe Operation
This domain tests how weight is spread across the axles of a double or triple and why axle limits matter even when gross weight is legal. Expect questions on axle limits, bridge formula, load placement, and securement.
Legal limits
- Typical highway limits are 20,000 pounds on a single axle, 34,000 pounds on a tandem axle group, and 80,000 pounds gross vehicle weight.
- The bridge formula limits the weight carried between axles according to the number of axles and the distance between them, so a short combination can be legal on gross weight and still illegal on the bridge formula.
- Tire ratings and the vehicle's registered weight can be lower than the statutory limits.
Load placement
- Put the heaviest part of the load over the axles and keep the center of gravity low.
- Loading the last trailer's rear lightens the steer axle, which hurts steering and increases sway.
- Too little weight on the drive axles reduces traction; too much on a trailer tandem overloads that group.
- Secure the load so the combined working load limit of the tiedowns is at least half the cargo weight, and recheck after the first few miles.
Worked example
A triple scales 33,000 pounds on the rear tandem. The driver adds 2,500 pounds of pallets directly over that tandem. New tandem weight is 35,500 pounds, over the 34,000-pound limit, even though gross weight remains under 80,000. Reasoning: axle limits are independent of gross weight. The fix is to move the pallets forward over an earlier trailer's axles and re-scale.
Traps
- Checking only gross weight and ignoring axle group limits.
- Forgetting that rear loading lifts weight off the steer axle.
- Assuming a load that was legal at the shipper is still legal after a partial unload or a load shift.