Federal Weight Limits and Axle Configuration
Federal law (23 USC 127) establishes maximum weight limits for commercial vehicles operating on the Interstate Highway System. Exceeding these limits causes accelerated pavement damage (pavement damage increases as approximately the fourth power of axle load — doubling axle weight causes roughly 16 times the pavement damage) and can compromise bridge structural integrity. The federal weight limits: Gross Vehicle Weight (GVW) — maximum 80,000 pounds for a standard 5-axle tractor-trailer combination. Single axle — maximum 20,000 pounds. Tandem axle group (two axles spaced 40–96 inches apart) — maximum 34,000 pounds (17,000 lbs per axle). Tridem axle group (three axles) — calculated differently under the Bridge Formula. These per-axle limits apply regardless of GVW — a combination can be under 80,000 lbs GVW but still be overweight on an individual axle. A standard 5-axle (18-wheel) tractor-trailer distributes weight across: the steer axle (front of tractor, single axle — max 12,000 lbs in most states, though federal limit is 20,000 lbs); the drive tandem (two rear tractor axles — max 34,000 lbs); and the trailer tandem (two rear trailer axles — max 34,000 lbs). Total at limits: 12,000 + 34,000 + 34,000 = 80,000 lbs. Proper weight distribution is the driver's practical concern: the fifth wheel position controls weight on the steer and drive axles; the trailer axle slider controls weight on the drive and trailer axles. Moving the fifth wheel forward increases steer axle weight and decreases drive axle weight. Moving the fifth wheel rearward decreases steer axle weight and increases drive axle weight. Moving the trailer axles rearward shifts weight from trailer to drive axles; forward shifts weight from drive to trailer axles.