What is Bridge Formula B and when does it allow more weight than the tandem limit?

Bridge Formula B: W = 500 x [LN/(N-1) + 12N + 36]. For a standard 4-ft tandem it gives exactly 34,000 lbs. When axle spread exceeds 4 ft, the formula allows more. At 8-ft spread tandem: 38,000 lbs. Tridem at 12-ft: 45,000 lbs.

What happens when no valid CG position exists within axle limits?

If the CG Optimizer returns no valid position, the cargo weight exceeds one or both limits regardless of positioning. Solutions: obtain a state overweight permit, use more axles or spread configuration, or split the load if divisible.

How does a spread axle trailer increase permitted payload?

A spread axle at 8 ft between outer axles allows Bridge Formula B limit of 38,000 lbs on the rear group, versus 34,000 lbs for a standard 4-ft tandem. A 4,000-lb improvement that can eliminate the need for a special permit.

How does this calculator relate to the other heavy haul tools?

This tool completes the five-tool pre-dispatch checklist: WLL tie-down (securement), Angle Derating (strap WLL), Swept Path (intersection clearance), CG Rollover Threshold (stability), and this tool (axle weight compliance). All five should be run before any oversize move.

⚖️ Heavy Haul Tool 5 of 5

Dunnage Weight Distribution Calculator for Axle Load Compliance Planning

Q: What is dunnage and how does its placement affect axle weights?

Dunnage is wooden blocking placed between the trailer deck and cargo. Its position determines the cargo CG along the trailer, directly controlling front vs rear axle weight distribution through the lever beam principle. Moving dunnage 2 feet shifts thousands of pounds between axles.

Enter cargo weight, axle span, and CG position from the front axle. The calculator applies the lever beam model to compute front and rear axle loads, checks them against FHWA single, tandem, and Bridge Formula limits, and shows the full sensitivity curve so you can see exactly how far the cargo can shift before any limit is crossed.

Lever Beam Model FHWA 80K GVW Check Bridge Formula B Axle Sensitivity Chart CG Position Optimizer Free and No Signup

Trailer Axle Load Distribution and FHWA Compliance Calculator

Front load = cargo x (span – CG position) / span | Rear load = cargo x CG position / span | Bridge Formula W = 500[LN/(N-1) + 12N + 36]

Load and Geometry

Cargo Weight

lbs

Net weight of the cargo only. Do not include trailer tare weight here (enter that separately below).

Axle Span

Distance from the front trailer axle group centerline to the rear trailer axle group centerline (bolster-to-bolster for platform trailers).

Cargo CG Position from Front Axle

Distance from the front axle (or front bolster) to the cargo’s center of gravity along the trailer length. Must be between 0 and the axle span.

Axle Configuration

Front Axle Group Type

Rear Axle Group Type

Rear Axle Group Spacing (for Bridge Formula)

Distance between the outermost axles in the rear group. Standard tandem = 4 ft. Spread tandem = 8 ft. Leave at 0 to skip Bridge Formula check.

Trailer Tare (Optional)

Trailer Tare Weight

lbs

Empty trailer weight. Leave at 0 to compute cargo loads only (useful for axle-weight-by-cargo analysis).

Tare Front Axle Share

How much of the tare weight sits on the front axle group. Typical flatbed: 40-50% front. Lowboy: 30-40% front. Leave at 50% if unknown.

📍 CG Position Optimizer: Where Should My Cargo Sit?

Cargo Weight

lbs

Axle Span

Front Axle Limit

lbs

Rear Axle Limit

lbs

⚖️

Enter cargo weight, axle span, and CG position, then click Calculate.

Front and rear axle loads with progress bars, FHWA compliance checks, Bridge Formula result, and the full axle sensitivity curve appear here.

Why Cargo Positioning Controls Axle Weight More Than the Load Itself

A 60,000-pound load does not have a fixed weight distribution. It has a variable distribution that depends entirely on where that load sits on the trailer. Slide it 4 feet toward the rear axle and several thousand pounds transfer from the front axle to the rear. That transfer can be the difference between passing the highway scale and getting a citation that delays the move by hours and costs the carrier a four-figure fine.

The physics here are the same as a playground seesaw. The trailer’s axles are the fulcrum points, and the cargo is a weight sitting somewhere between them. Move the weight closer to one end and that end goes down, meaning more load on that axle. The lever principle makes the relationship perfectly proportional: a load sitting twice as close to the rear axle will put twice as much weight on the rear axle as a load sitting at midspan. This is why dunnage placement is not just blocking and bracing — it is weight management that directly affects compliance at weigh stations on the permit route.

The reason this matters for oversize and overweight loads specifically is that these loads are already at or near the limits on every axis: load weight, axle weights, GVW, and often Bridge Formula. A load that calculates as compliant with the CG at the center may fail rear axle limits if the cargo’s actual CG is 3 feet behind center. A load with a heavy bottom section and a light top may have its true CG a foot forward or rearward of the geometric center. Getting this right before the truck rolls prevents the worst outcome in heavy haul operations: being pulled from service at the scale, unable to move the load, with a pilot car and an escort waiting on the highway.

What Dunnage Has to Do With Weight Distribution

Dunnage is the wooden cribbing, blocks, and timber mats used to support and position cargo on a trailer deck. Most heavy haul operators think of dunnage purely as a safety device to prevent cargo from shifting or damaging the trailer deck. But dunnage placement also determines the cargo’s effective CG position along the trailer. If a piece of equipment is placed with its front edge 8 feet from the front bolster and its rear edge 22 feet from the front bolster, the geometric center of the load is at 15 feet. But if the equipment is heavier at the rear (as most track-type equipment is, with the undercarriage and final drives at the back), the actual CG may be 17 or 18 feet from the front — not 15.

This 2 to 3 foot discrepancy in CG position changes front and rear axle loads by several thousand pounds on a 50,000-pound cargo. The CG Optimizer tool in this calculator tells you the valid range of CG positions that keeps both axle groups within their limits, and the optimal center point that maximizes margin on both sides. That range is what the crew setting dunnage should be working within.

Field rule: When in doubt about a load’s exact CG position, use the CG Position Optimizer to determine the valid range for your specific cargo weight and axle configuration. If the valid range is 4 feet wide or more, you have comfortable operational flexibility. If it is 1 foot wide or less, the dunnage placement must be precisely controlled and the load should be weighed on a portable scale before departure.

How the Lever Principle Converts CG Position Into Axle Load Fractions

The two-point support beam model treats the trailer as a rigid beam resting on two support points: the front and rear axle groups. Any load placed on the beam distributes between the two supports in inverse proportion to the distance from each support. The further the cargo CG is from a support, the less weight that support carries.

The Core Formula and What Each Input Means

Front axle load from cargo = cargo_weight x (span – CG_position) / span. Rear axle load from cargo = cargo_weight x CG_position / span. Here, span is the distance between the two axle groups (front bolster to rear bolster in ft), and CG_position is the distance from the front axle to the cargo’s center of gravity. When CG_position equals zero, all cargo weight sits on the front axle. When CG_position equals the full span, all cargo weight sits on the rear axle. At midspan, the load splits exactly 50/50.

Adding Trailer Tare Weight

The trailer itself has weight that also distributes between the axles. Most trailers carry approximately 40 to 50 percent of their tare weight on the front axle group and 50 to 60 percent on the rear, depending on the trailer design and kingpin-to-axle geometry. When tare weight is entered, the calculator adds the tare front share to the cargo front load and the tare rear share to the cargo rear load to produce total axle loads. For compliance checks at a highway scale, it is the total axle load including tare that the scale measures.

The Axle Sensitivity Chart: Seeing the Full Picture

The chart plots front and rear axle loads on the vertical axis against CG position on the horizontal axis, from 0 (front axle) to the full axle span (rear axle). The front load line decreases linearly from left to right. The rear load line increases linearly. Their intersection is always at midspan at exactly half the cargo weight. Two horizontal dashed lines mark the front and rear axle limits. The region between these limit lines and the X-axis shows the permissible load for each axle. Where both limit lines cut across the curves defines the valid CG range.

Your specific CG position is marked as a triangle point on both lines simultaneously, giving you an immediate visual read on how close each axle is to its limit and how much room you have to move the cargo forward or backward before any limit is crossed. This chart shows, in a single view, what most carriers can only determine by running multiple calculations at different cargo positions.

Bridge Formula B and Why It Sometimes Matters More Than the Tandem Limit

Federal law (23 U.S.C. 127) limits not just individual axle groups but also groups of consecutive axles through Bridge Formula B: W = 500 x [LN/(N-1) + 12N + 36], where W is the maximum load in pounds on the axle group, L is the distance between the outer axles of the group in feet, and N is the number of axles. For a standard 4-foot tandem with N=2: W = 500 x (4×2/1 + 24 + 36) = 34,000 lbs, which matches the 34,000 lb tandem limit exactly. This formula becomes a binding constraint on spread axle configurations. A spread tandem with 8 feet between axle centers: W = 500 x (8×2/1 + 24 + 36) = 500 x 76 = 38,000 lbs. The spread configuration allows 4,000 lbs more than a standard tandem. Entering the rear axle group spacing in this calculator runs the Bridge Formula check so you can verify compliance before dispatch.

Federal Axle Load Limits: Single, Tandem, Tridem, and Bridge Formula Standards

The federal weight limits in 49 CFR Part 127 and 23 U.S.C. 127 apply to vehicles operating on the National Highway System. State highways may have different limits, and state overweight permits can allow loads above these federal limits on state-only routes. The federal standards are the baseline for route planning and weigh station compliance on interstates and federal aid highways.

Single axle: 20,000 lbs maximum. Applies to any single axle, including the tractor steer axle (which has a separate practical limit of 12,000 lbs on many routes) and trailer tag axles.
Tandem axle group: 34,000 lbs maximum. Two consecutive axles with centerlines 40 to 96 inches (3.33 to 8 feet) apart. Standard tractor drives and standard trailer rear axles are tandem.
Gross Vehicle Weight (GVW): 80,000 lbs maximum on the federal highway system without a special permit. Includes the weight of the tractor, trailer, and cargo.
Bridge Formula B: Controls any group of 2 or more consecutive axles based on axle count and spread distance. Governs spread axle and multi-axle platform configurations where the standard tandem limit is not directly applicable.

State overweight permits can authorize loads above these limits on state roads but not on federal interstates without Federal Highway Administration approval. Divisible loads (loads that can be broken into smaller shipments) are generally not eligible for overweight permits. Indivisible loads (single manufactured pieces that cannot be split) are eligible for oversize and overweight permits from state DOTs.

Three Commercial Moves Where Dunnage Placement Prevented an Overweight Citation

Example 1: Port of New Orleans to Baton Rouge Refinery

Steel Pressure Vessel, 52,000 lbs, Standard Flatbed Tandem-Tandem

A 52,000-pound steel pressure vessel is being transported on a 48-foot flatbed with tandem front and tandem rear axles (both limited at 34,000 lbs). The axle span is 32 feet. The vessel is 40 feet long and loaded with its geometric center at 18 feet from the front axle. However, the vessel has heavier end caps and flange hardware at the rear, shifting the actual CG to approximately 20 feet from the front axle.

At CG = 18 ft (assumed): front = 52,000 x (32 – 18) / 32 = 52,000 x 0.4375 = 22,750 lbs. Rear = 52,000 x 18 / 32 = 29,250 lbs. Both within tandem limits. But at the actual CG = 20 ft: front = 52,000 x (32-20)/32 = 52,000 x 0.375 = 19,500 lbs. Rear = 52,000 x 20/32 = 32,500 lbs. Still compliant, but the rear is now at 95.6% of the 34,000 lb tandem limit. Loaded with trailer tare of 14,000 lbs (50% split), total rear = 32,500 + 7,000 = 39,500 lbs — over the 34,000 lb limit by 5,500 lbs. The crew repositions the dunnage to move the vessel 4 feet forward, placing the CG at 16 feet, bringing total rear to 36,500 lbs — still over. Final position: CG at 14 feet, total rear = 52,000×14/32 + 7,000 = 22,750 + 7,000 = 29,750 lbs. Compliant.

CG moved from 20 ft to 14 ft: rear axle drops from 39,500 lbs to 29,750 lbs — compliant

Example 2: Pittsburgh to Cleveland, I-76 Permit Route

Industrial Compressor on Lowboy, Spread Axle Configuration

A 68,000-pound industrial compressor is on a lowboy trailer with a spread tandem rear axle configuration: the two rear axles are 8 feet apart instead of the standard 4 feet. The axle span is 28 feet. The cargo is relatively uniform in density, so the CG is approximately at the geometric center: 14 feet from the front axle. Trailer tare is 22,000 lbs with 40% on the front axle.

Cargo front = 68,000 x (28-14)/28 = 34,000 lbs. Cargo rear = 34,000 lbs. With tare front = 8,800 lbs and tare rear = 13,200 lbs: total front = 42,800 lbs. Total rear = 47,200 lbs. GVW = 90,000 lbs — over the 80,000 lb federal limit by 10,000 lbs. This load requires a state overweight permit. But the rear axle check: tandem limit at 34,000 lbs is irrelevant for a spread axle. Bridge Formula at N=2, L=8: W = 500 x (8×2/1 + 24 + 36) = 38,000 lbs. The rear axle cargo load of 34,000 lbs is within the Bridge Formula limit of 38,000 lbs. The front axle cargo load of 34,000 lbs exceeds the single axle limit (20,000 lbs). However, the lowboy front is a tandem — its limit is 34,000 lbs — exactly at the tandem limit for cargo alone, and over when tare is added. The carrier upgrades to a tridem front axle configuration, raising the front limit to 42,000 lbs (Bridge Formula at N=3, L=8). Total front of 42,800 lbs remains within the tridem Bridge Formula limit.

GVW permit required at 90,000 lbs. Bridge Formula resolves axle group compliance on spread config

Example 3: Stockton Port to Sacramento, California SR-4

Agricultural Harvester, CG Optimizer Used Before Loading

A farm equipment dealer is shipping a large combine harvester weighing 44,000 lbs on a 40-foot step deck trailer with a single front axle (20,000 lb limit) and tandem rear (34,000 lb limit). The axle span is 26 feet. Before the crew begins setting dunnage, the dispatcher runs the CG Optimizer to determine where the harvester’s CG must fall to keep both axles within limits.

CG Optimizer inputs: cargo = 44,000 lbs, span = 26 ft, front limit = 20,000 lbs, rear limit = 34,000 lbs. pMin = max(0, 26 x (1 – 20,000/44,000)) = max(0, 26 x 0.5455) = 14.18 ft. pMax = min(26, 34,000 x 26 / 44,000) = min(26, 20.09) = 20.09 ft. Valid CG range: 14.18 to 20.09 ft from the front axle. Optimal center: 17.14 ft. Range width: 5.91 ft.

The crew marks 14 ft and 20 ft from the front bolster on the trailer deck with chalk before the harvester is loaded. The combine is positioned so its estimated CG (slightly forward of center because the header attachment is heavier) falls at approximately 16 ft from the front. Post-load portable scale check: front = 18,200 lbs, rear = 25,800 lbs. Both within limits. The load clears the Caltrans agricultural station without issue.

CG range 5.9 ft wide: crew marks valid zone on deck before loading. Scale check confirms compliance

Six Practical Dunnage Placement Rules Proven by Heavy Haul Professionals

Mark the Valid CG Zone on the Trailer Deck Before Loading

Run the CG Optimizer before the cargo arrives at the staging yard, not after it is half-loaded. Mark the minimum and maximum valid CG positions on the trailer deck with chalk or tape. Give the crane operator a visual target zone. A 30-second calculation before loading prevents an hour of repositioning after the cargo is already on the deck and tied down.

Use Portable Axle Scales at the Staging Yard for Loads Near the Limits

When the CG Optimizer shows a valid range narrower than 3 feet, the load is sensitive to positioning. Any error in estimating the actual CG can push an axle over its limit. Use portable axle scales or drive over a certified platform scale at the staging yard before departure. Discovering an overweight condition at a roadside station costs time, potential fines, and may require on-road repositioning — all of which are avoided by a 15-minute scale check before dispatch.

Account for the Difference Between Geometric Center and Actual CG

Rectangular loads like fabricated modules, storage tanks, and structural sections tend to have CGs close to the geometric center. Equipment with drive components, engines, or heavy counterweights at one end can have CGs offset 2 to 6 feet from center. Before calculating, identify which end of the load is heavier and bias the CG estimate accordingly. When manufacturer specifications are available, use the published transport CG. When not available, a rule of thumb is to add 10 to 15 percent of the load length toward the heavier end as a CG offset correction.

Spread Axle Configurations Improve Bridge Formula Weight Allowances

When a standard tandem rear at 34,000 lbs is not enough, a spread axle configuration with 8 feet between axle centers allows up to 38,000 lbs under Bridge Formula B, a 4,000 lb improvement. At 12 feet of spread on a tridem, Bridge Formula allows up to 45,000 lbs on the rear group. Before deciding that a load requires a special permit, check whether upgrading to a spread axle or tridem configuration on the trailer eliminates the overweight condition within federal limits. Many carriers keep a spread axle slider on their platform trailers specifically for this reason.

Re-Check Axle Weights After Any Dunnage Restack or Load Reposition

Dunnage gets restacked during securement, and loads sometimes shift slightly from their initial placement during crane set-down. Any repositioning of the cargo, even by inches, changes the axle load distribution. After the final blocking and bracing is complete and before any tie-downs are tensioned, verify that the CG position has not shifted outside the valid range calculated by the optimizer. A scale re-check after blocking confirms the final position is consistent with pre-load calculations.

Bridge Formula Compliance Must Be Verified for the Entire Truck, Not Just the Trailer

Bridge Formula B applies to any group of consecutive axles on the vehicle, including axle groups that span both the tractor and the trailer (such as the tractor drives and the trailer front axle when they are close together). For longer trailers where the tractor drives and trailer front axle are far apart, this is rarely a concern. But on short-span platform configurations, the combined axle group can trigger Bridge Formula requirements that standard per-axle checks miss. The FHWA truck size and weight regulations cover the full group-by-group analysis for multi-axle configurations.

Axle Load Quick Reference: Permitted Weight by Configuration and Spread Distance

Bridge Formula B values use W = 500 x [LN/(N-1) + 12N + 36]. All values are federal maximums on the National Highway System without special permits.

Axle Configuration	Axle Count (N)	Outer Spread (L, ft)	Federal Limit	Bridge Formula Limit	Governing Limit
Single axle	1	N/A	20,000 lbs	N/A	20,000 lbs
Standard tandem (4 ft)	2	4	34,000 lbs	34,000 lbs	34,000 lbs
Spread tandem (6 ft)	2	6	34,000 lbs	36,000 lbs	34,000 lbs (governs)
Spread tandem (8 ft)	2	8	34,000 lbs	38,000 lbs	38,000 lbs (Bridge Formula)
Tridem 4+4 (8 ft total)	3	8	N/A (no tridem federal limit)	42,000 lbs	42,000 lbs (Bridge Formula)
Tridem 4+8 (12 ft total)	3	12	N/A	45,000 lbs	45,000 lbs (Bridge Formula)
Quad axle 4+4+4 (12 ft)	4	12	N/A	50,000 lbs	50,000 lbs (Bridge Formula)
Federal GVW (any config)	5-axle standard	N/A	80,000 lbs	N/A	80,000 lbs total

Where Bridge Formula allows more than the standard group limit, the Bridge Formula governs. State permit weights may exceed these federal standards on designated state routes.

Frequently Asked Questions About Axle Weight Distribution and Dunnage Placement

What is dunnage and how does its placement affect axle weights?

Dunnage is the wooden blocking, cribbing, or structural supports placed between the trailer deck and the cargo to protect the deck, distribute the load, and position the cargo for safe transport. Dunnage placement determines the cargo’s effective center-of-gravity position along the trailer length, which directly controls how much weight goes to the front versus rear axle group through the lever beam principle. Moving the dunnage stack 2 feet closer to the rear axle shifts thousands of pounds from the front axle to the rear — or the reverse if moved forward. For this reason, dunnage placement is a weight management decision as much as a securement decision.

What is the FHWA 80,000 lb gross vehicle weight limit and who does it apply to?

The 80,000 lb gross vehicle weight limit is established by 23 U.S.C. 127 and applies to all vehicles operating on the National Highway System, which includes the interstate highway network and other federally designated roads. It covers the combined weight of the tractor, trailer, and all cargo. State highways not on the National Highway System can have different weight limits set by state law, and state permits can authorize heavier loads on qualifying state routes. Loads that cannot be divided into sub-80,000 lb shipments (such as a single manufactured item like a transformer or pressure vessel) can apply for overweight permits. These permits are issued by state DOTs and typically specify the exact route, speed limits, permitted times of travel, and escort requirements.

How does the lever principle determine front versus rear axle loads?

The lever principle states that a load placed at distance D from one fulcrum and distance (span – D) from the other fulcrum exerts forces inversely proportional to those distances. In trailer terms: front axle load = cargo x (span – D) / span, and rear axle load = cargo x D / span, where D is the CG position measured from the front axle and span is the distance between the two axle groups. This means cargo placed right at the front axle (D=0) puts 100% of its weight on the front axle and nothing on the rear. Cargo at midspan splits exactly 50/50. Cargo at the rear axle (D=span) puts 100% on the rear and nothing on the front. The calculator uses Big.js precision arithmetic for all these calculations to avoid the floating-point rounding errors that standard JavaScript arithmetic produces with large weight values.

What is Bridge Formula B and when does it produce a higher limit than the tandem limit?

Bridge Formula B (23 U.S.C. 127) limits the weight on any consecutive group of 2 or more axles: W = 500 x [LN/(N-1) + 12N + 36], where L is the outer axle spread in feet and N is the number of axles. For a standard 4-foot tandem (N=2, L=4), Bridge Formula gives exactly 34,000 lbs, matching the standard tandem limit. As the axle spread increases beyond 4 feet, the Bridge Formula allows more weight than 34,000 lbs. At L=8 ft (spread tandem), it allows 38,000 lbs. At L=12 ft with N=3 (tridem), it allows 45,000 lbs. The Bridge Formula produces a higher limit than the standard tandem limit whenever L exceeds 4 feet. Carriers using spread axle or multi-axle configurations should always run the Bridge Formula check alongside the standard per-axle limit checks, because the Bridge Formula may be the binding constraint in some configurations and the liberating factor in others.

How does a spread axle trailer increase the permitted payload compared to a standard tandem?

A spread axle configuration moves the rear two axles further apart than the standard 4 feet, typically to 8 or 10 feet. This increases the Bridge Formula allowance from 34,000 lbs (standard tandem) to 38,000 or 39,500 lbs (8-ft spread) or more. Additionally, spread axles distribute the load over a longer wheelbase, which reduces the per-axle load impact on bridge structures, which is exactly what the Bridge Formula is designed to measure. For carriers moving loads in the 35,000 to 42,000 lb cargo range on the rear axle group, a spread axle trailer can be the difference between needing a special permit and operating within federal limits. The downside is that spread axle trailers are heavier (adding tare weight) and have larger swept paths at intersections, which matters for route planning.

What happens when no valid CG position exists within the axle limits?

When the CG Optimizer returns “no valid CG position,” it means the cargo weight is so large relative to one or both axle limits that no positioning on the trailer keeps both axles within limits simultaneously. This occurs when cargo weight alone exceeds at least one axle limit by a margin that cannot be compensated by repositioning. The solutions are: obtain a state overweight permit authorizing the excess axle weight on the specific route; use a different trailer with more axles or a wider axle spread to increase the applicable limits; split the cargo into smaller loads (if divisible); or use a multi-trailer system that shares the load weight across more axle groups. The CG Optimizer’s “no valid position” message is the trigger to move from operational planning to engineering and permitting.

What is the difference between axle span and bolster-to-bolster distance?

For standard flatbed trailers with visible front and rear axle groups, the axle span is the physical distance between the centerlines of the front and rear axle groups. For platform trailers and lowboys with removable bolsters (the support frames that the cargo rests on), the bolster-to-bolster distance is the distance between the front and rear bolster centerlines, which is where the load’s weight is transferred to the trailer. These two measurements are the same concept: they define the support point separation that governs the lever beam weight distribution. Use whichever measurement is most practical to determine in the field. For the CG position input, the corresponding reference is the front axle (or front bolster) centerline, not the trailer’s front edge.

How does the 5th wheel position on the tractor affect front axle loads on the trailer?

The 5th wheel position determines how much of the trailer’s load is transferred to the tractor, and specifically to the tractor’s drive axles. Moving the 5th wheel forward transfers more weight from the trailer front to the tractor drive axles. Moving it rearward shifts weight back to the trailer front axle. This calculator focuses on the trailer’s internal weight distribution between its own axle groups, which is the relevant constraint for trailer axle compliance. The 5th wheel interaction primarily affects the steer axle and drive axle loading on the tractor, which requires a separate calculation using the kingpin-to-drive-axle distance and the 5th wheel offset from the tractor drive axle centerline. For a complete 5-axle vehicle axle weight analysis, both the trailer internal distribution and the tractor-trailer kingpin load must be calculated.

Can state overweight permits authorize weights above the 80,000 lb GVW federal limit?

Yes, but only on state-designated routes, not on interstate highways without specific federal authorization. State DOTs issue overweight permits for indivisible loads that exceed federal limits. These permits specify the exact vehicle configuration, permitted weight, route, travel times, speed restrictions, and escort requirements. Some states issue routine annual overweight permits for specific vehicle types (logging trucks, agricultural vehicles) that regularly exceed federal limits on state roads. For very heavy loads exceeding 200,000 lbs or more, bridge engineering reviews may be required as part of the permit process. The process and fees vary significantly by state. Most states have an online permit portal; the FHWA maintains a directory at fhwa.dot.gov linking to each state’s permitting system.

How does cargo CG shift laterally affect axle loads?

This calculator handles longitudinal weight distribution (front-to-rear). Lateral CG offset (side-to-side) is a separate concern that affects axle side-loading and rollover threshold but does not change the total front or rear axle load calculated by the lever beam model. A laterally offset CG shifts more weight to the tires on one side of the trailer, which can cause unequal tire loading and, for very offset loads, contribute to rollover risk as analyzed in the CG Rollover Threshold Calculator. Weigh stations measure the total axle load (both sides combined), so lateral offset does not directly cause an axle weight violation. However, extreme lateral loading can exceed individual tire or wheel end ratings, which is a separate equipment limitation that tire and trailer specifications address.

What documentation should I carry for overweight permit compliance at roadside?

For overweight and oversize moves, the driver should carry: the state overweight permit (and any additional state permits if crossing state lines), the bill of lading or shipping manifest listing the cargo and its weight, the vehicle registration for both tractor and trailer, the carrier authority registration, and any route restrictions or special instructions issued with the permit. Some states require the permit to be printed rather than displayed on a mobile device. The PDF report from this calculator, showing the axle load calculations and compliance checks, is not a legal permit but provides useful documentation if a weight enforcement officer asks how the load was planned. Some carriers include the weight distribution analysis in the permit application package when applying for overweight authorizations that require engineering justification.

How do I estimate CG position for a load I haven’t weighed or measured precisely?

For symmetric, uniform-density loads (rectangular tanks, structural steel, bulk material in a box), use the geometric center: CG position = length of load / 2, placed at the mid-point of the cargo footprint on the trailer. For equipment with an obvious heavy end (engine room end of a generator, counterweight end of an excavator, drive end of a pump), shift the estimated CG 10 to 20 percent of the load length toward the heavy end. For loads with published engineering data, use the manufacturer’s specified CG in transport configuration. The CG Optimizer’s valid range output tells you how much error you can absorb: if the valid range is 6 feet wide and your CG estimate might be off by 2 feet, you have adequate margin. If the range is 2 feet wide and your estimate could be off by 2 feet, a portable scale check before departure is warranted.

What is the relationship between this calculator and the Bridge Formula at the full vehicle level?

This calculator applies Bridge Formula B to the rear trailer axle group as a compliance check. Bridge Formula B applies to ALL groups of consecutive axles on the vehicle, including the tractor steer axle alone, the tractor drives, the trailer front axle, and combinations that span the tractor-trailer junction. A complete full-vehicle Bridge Formula analysis requires knowing all axle spacings from the steer axle to the rear trailer axle. The FHWA provides a full-vehicle Bridge Formula table and calculator on their website that covers the complete vehicle, which is the appropriate tool for formal permit applications. This calculator’s Bridge Formula check is specifically for the trailer rear axle group as a planning-level compliance indicator, not a substitute for the full-vehicle analysis.

How do axle weight limits work on construction site access roads and industrial property?

Federal and state highway axle weight limits apply to public roads. Private roads, construction site access roads, and industrial property are governed by the road’s owner or the site operator. A construction site may post its own load limits based on the soil bearing capacity of the access road, the strength of temporary bridges or culverts, and the requirements of the general contractor’s site plan. These limits can be more or less restrictive than public road limits depending on the site conditions. For heavy haul moves that transition from public roads to private site access, the site-specific load limits must be obtained from the site engineer or general contractor. The geotechnical calculators on this site cover soil bearing capacity for ground-bearing axle load assessments on unpaved surfaces.

Does this calculator account for dynamic axle loads during acceleration and braking?

No. This calculator uses the static two-point beam model, which gives the steady-state axle loads when the vehicle is traveling at constant speed on a flat, level road. Dynamic effects during acceleration shift weight to the rear axles, and braking transfers weight forward to the front axles. These dynamic load transfers are typically modest for heavy haul loads (a few hundred to a few thousand pounds) because these vehicles operate at low speeds with smooth acceleration and braking profiles. Highway weigh stations measure static or low-speed rolling axle weights, so the static model is the appropriate one for compliance planning. If the vehicle must cross a weight-restricted bridge at any speed, the dynamic load at bridge crossing speed may require a more detailed analysis for bridge engineering approval.

How does this tool complement the other four calculators in the heavy haul hub?

The five heavy haul calculators together cover the full pre-dispatch checklist for an oversize/overweight move. The WLL Tie-Down Calculator ensures the cargo is secured adequately per 49 CFR 393. The Angle Derating Calculator corrects WLL for strap angles. The Swept Path Estimator verifies that route intersections are physically navigable. The CG Rollover Threshold Calculator confirms the load is stable at highway speeds. This Dunnage Weight Distribution Calculator closes the loop by confirming that the cargo’s positioned weight distribution complies with FHWA axle limits. Used together, these five tools address the securement, geometry, stability, and weight compliance dimensions of the move before the truck rolls.

Legal Disclaimer and Editorial Transparency

This calculator and educational content are provided for informational and planning purposes only. The two-point beam model used here assumes a rigid trailer and uniform support conditions. Actual axle loads may differ due to trailer frame deflection, dunnage compression, dynamic effects, and measurement error in CG estimation. This tool does not constitute engineering certification or legal compliance advice.

Federal axle weight limits are governed by 23 U.S.C. 127 and 49 CFR Part 127. State laws, permit conditions, and local regulations may differ. Always verify at a certified scale before travel on permit routes. Reference the FHWA truck size and weight regulations for authoritative federal standards. USCalculators.com is an independent educational platform not affiliated with the FHWA, FMCSA, or any state DOT. Content last reviewed August 2026.