A truck can be under the federal gross weight limit (80,000 lb) and still be illegal — because individual axle weights also have federal and state limits, and the cargo distribution must comply with the Federal Bridge Formula. Managing weight distribution is a daily skill for every driver hauling near-maximum loads.
Standard federal axle limits
Single axle (one tire on each side): 20,000 lb maximum. Tandem axle (two axles spaced 40 to 96 inches apart): 34,000 lb maximum. Tridem and other configurations: covered by the Federal Bridge Formula. Many states have lower limits or seasonal restrictions (frost laws, freeze-thaw periods).
Federal Bridge Formula
The Bridge Formula (W = 500 × (LN/(N-1) + 12N + 36)) limits the total weight on any group of consecutive axles based on their spacing. The longer the spacing, the higher the allowable weight, up to the federal gross of 80,000 lb. The formula is enforced at every weigh station and is the primary check against bridge damage from over-concentrated loads.
Sliding the fifth wheel
Most modern tractor fifth wheels can be slid forward or backward 4 to 12 inches. Sliding the fifth wheel forward shifts weight from the drive axles to the steer axle (and reduces drive-axle weight). This is useful when scaling reveals overweight drives but acceptable steer weight. Common shift: 1 inch of fifth-wheel slide changes drive-axle weight by approximately 250 to 400 lb.
Sliding the trailer tandems
Most 53-foot dry-van trailer tandems can be slid forward or backward over a range of 6 to 12 feet. Sliding tandems forward shifts weight from the drive axles to the trailer tandems. Sliding rearward shifts weight from the trailer tandems to the drive axles. Common shift: 1 inch of tandem slide changes weight distribution by approximately 400 to 600 lb between drives and tandems.
Scaling routine
After loading, scale the truck at a CAT scale (most major truck stops). Costs $13 to $14 per scale ticket. The ticket shows steer, drive, trailer, and gross weights. If any axle group is over the limit, return to the loading dock or shift loading; adjust fifth wheel and tandem positions; re-scale (re-weighs typically discounted to $3 to $5 within 24 hours).
Common scaling problems
Steer overweight: cargo loaded too far forward; slide fifth wheel rearward, slide tandems forward. Drives overweight: cargo balanced over drives; slide fifth wheel forward and slide tandems forward. Trailer tandems overweight: cargo loaded too far rearward; slide tandems rearward (toward the back of the trailer).
Bridge law variations
States have their own bridge laws that may be more restrictive than federal. California, Oregon, and Washington in particular have stricter rules for certain configurations. Always check the state-specific scaling rules for unfamiliar routes. Read our weigh stations guide.