Bridge Formula Minimum Axle Spread
The Federal Bridge Formula solved for the minimum outer-to-outer axle spread that legally carries a target group weight.
Example
You enter
- Target group weight (lb) 80000
- Number of axles in the group 5
You get
- Minimum outer-to-outer spread 51.2 ft
Details, formula, and sources
L = ((W/500) - 12 N - 36)(N-1)/N. A 5-axle group at 80,000 lb needs at least 51.2 ft; a 2-axle group at 34,000 lb needs 4 ft. If the spread comes out zero the axles already satisfy the formula bunched together. The 20,000 lb single / 34,000 lb tandem / 80,000 lb interstate caps apply independently, and over 80,000 lb needs a permit. The state DOT governs.
Federal Bridge Formula B solved for the spread: L = ((W / 500) - 12 N - 36)(N - 1) / N, where W is the target group weight (lb) on N consecutive axles spaced L feet outer-to-outer.
23 CFR 658.17 by section (Federal-aid Highway Act of 1956 / FHWA Bridge Formula), rearranged for the axle spread.
Free at ecfr.gov.
Math aid for personal verification. The ELD on the truck and the carrier tariff are the legal record. State limits may be lower than federal.
Field names used by the API: target_weight_lb, num_axles, min_spacing_ft
- Spread inverse L = ((W/500) - 12 N - 36)(N-1)/N; zero when the axles satisfy the formula bunched together23 CFR 658.17
- Independent caps the 20,000 lb single / 34,000 lb tandem / 80,000 lb gross limits still apply; over 80,000 lb needs a permit23 CFR 658.17