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OSOWloads

FBF Solver

Federal Bridge Formula solver for brokers. Enter your load weight, tune the max overall length, and see approximate rig setups (tractor, jeep, trailer, booster) that clear FBF. Useful for scoping what carrier configuration a load actually needs.

lbs
⚠ Overweight — FBF ✓

Tandem-drive tractor + 2-axle jeep + 2-ax 35-ton mech detach RGN + 2-axle (tandem) booster

jeep gap 8' · trailer gap 29' · booster gap 5'

9
Axles
112k
GVW
52k
Tare
12.6k
/Axle
79'
OB
12345678919'4'6"4'6"4'6"4'6"tractorjeeptrailerbooster8'29'5'
Tandem-drive tractor 21.0k2-axle jeep 7.2k2-ax 35-ton mech detach RGN 16.0k2-axle (tandem) booster 8.3kLoad 60.0k

All FBF-passing configs (7)

How the FBF solver works

The Federal Bridge Formula (23 CFR § 658.17) caps the gross weight on any group of consecutive axles based on the spacing between the outermost axles of that group: W = 500 × ((L × N) / (N − 1) + 12N + 36). For a rig to be legal, every consecutive subset of axles has to pass, not just the outer bridge. Every internal group too.

The solver iterates physically realistic equipment combinations (with hardware-anchored spacing ranges per piece) and returns the most compact setup per total axle count. The lowest-axle-count config is tagged recommended.

What this is not: a routing tool, a permit cost calculator, or a substitute for state DOT engineering review on superloads. Per 23 CFR § 658.17(h), states can waive FBF for nondivisible loads under permit and do their own bridge-by-bridge structural analysis. Use this to scope equipment, not to commit a quote.

Got a specific axle layout already? Run it through the Axle Weight Calculator to check per-state weight limits, GVW caps, and the lowest permit class that clears your load across all 48 states.

Equipment the solver knows about

The catalog covers the heavy-haul building blocks: tandem-drive and tri-drive tractors, 2-axle and 3-axle jeep dollies, 35-ton through 5-axle 80-ton hyd detach RGN trailers, 1- to 3-axle boosters, and the Talbert 6-axle steer dolly for superload-class moves above 120,000 lb. Each piece carries hardware-realistic axle spacings sourced from manufacturer spec sheets: Talbert and Fontaine 55-ton tridems at 54 inches, Fontaine STRETCH variants at 60 inches, hyd RGN trailing-axle slide to 8'4" for federal spread tandem advantage. Tare weights are anchored to current Aspen, Talbert, and Fontaine published specs where possible, marked medium confidence where derived from owner-reported scale weights, and flagged low confidence where the entry is estimated.

The solver does not model self-propelled modular transporters (SPMT), dual-lane configurations, or specialty modular equipment like Goldhofer PST/SL or Nelson beam transports. Those operate on engineering-reviewed superload permits where FBF is explicitly waived and route-specific bridge analysis governs.

When the solver returns no result

Heavy loads above roughly 90,000 to 100,000 lb often have no FBF-passing configuration in standard heavy-haul equipment. The honest answer is that the load needs an overweight permit that waives FBF, plus state DOT engineering review on superload-class routes. The solver shows No FBF-passing config in those cases rather than fake-passing with implausible geometry. If you see that banner, you are in permit territory: the load can still move, it just needs a different toolset (state-by-state permit review, route survey, possibly a modular transporter) that this page does not cover.

Frequently asked questions

What is the Federal Bridge Formula?
The Federal Bridge Formula caps the maximum legal weight on any group of consecutive axles based on the spacing between them. The formula is W = 500 × ((L × N) / (N − 1) + 12N + 36). W is the maximum allowed group weight in pounds, L is the wheelbase of the axle group (first axle to last axle, in feet), and N is the number of axles in the group. The rule exists because short, heavy axle groups damage bridges. Every consecutive subgroup of axles on a rig has to clear the formula independently, not just the outermost group.
How do I calculate the Federal Bridge Formula?
For a single axle group, plug L (axle group wheelbase in feet) and N (number of axles) into W = 500 × ((L × N) / (N − 1) + 12N + 36). Round the result down to the nearest 500 pounds. For an entire rig, you have to run the formula on every consecutive axle subset, not just the outer bridge. A typical 13-axle setup has roughly 78 consecutive axle pairs and groups that all need to pass. This solver does the full subset check automatically and returns rig configurations where every internal group clears.
Do overweight permits waive the Federal Bridge Formula?
Per 23 CFR § 658.17(h), states have authority to waive the Federal Bridge Formula for nondivisible loads operating under permit. In practice, most states still use FBF as the baseline and run their own bridge-by-bridge structural analysis on top. Divisible loads almost never get an FBF waiver. For a broker scoping a load, the safest assumption is that the rig configuration should clear FBF whether or not a permit is in play. If FBF cannot be cleared, the load needs state-engineering review and route survey, which adds significant cost and lead time.
How many axles do I need for a 100,000 lb load?
A 100,000 lb load on standard heavy-haul equipment typically needs 8 to 10 total axles, depending on the rig configuration. A tandem-drive tractor plus a 3-axle 55-ton hydraulic detach RGN gives 6 axles, which clears federal legal weight (80,000 GVW) but starts to bind on FBF as load weight climbs. Adding a jeep dolly or a tandem booster brings the count to 8 to 9. For loads above 120,000, a tri-drive tractor plus a 5-axle hyd RGN plus a tandem or tridem booster (up to 13 axles) is more typical. The solver returns the lowest axle count that clears FBF for your specific load weight and overall-length ceiling.
What is the difference between a tandem-drive and tri-drive tractor?
A tandem-drive tractor has two drive axles behind the steer axle (3 total axles, ~21,000 lb tare). A tri-drive has three drive axles (4 total axles, ~28,000 lb tare). Tri-drives carry more weight on the drive group and pass FBF on the drive tandem group at higher per-axle loads, which matters once you climb past about 50,000 lb of payload. The trade-off is the extra ~7,000 lb of tare and the higher capital cost. Most heavy-haul fleets keep a mix and match the tractor to the load.
When do I need a jeep dolly or booster?
Jeep dollies sit between the tractor and the trailer and add axles ahead of the trailer kingpin. Boosters trail behind the trailer and add axles at the rear. Both exist for the same reason: spreading a heavy load across more axles to clear the Federal Bridge Formula and the per-axle weight cap. As a rough rule, you start needing a jeep or booster around 90,000 to 110,000 lb of load weight on standard equipment. The solver picks the right combination automatically based on the load you enter.
What is a stretch RGN?
A stretch RGN is a hydraulic detachable removable-gooseneck trailer with an extending deck. Standard heavy-haul RGNs run 28 to 32 feet of well length; stretch versions extend to 40 to 50 feet for longer loads, with the trailer axles sliding back to maintain wheelbase geometry. Fontaine, Talbert, and XL Specialized all offer stretch variants. The longer outer bridge helps the rig clear FBF on heavier loads, which is why stretch trailers show up on permit moves above ~100,000 lb.