Axle Weight Limits for Heavy Haul: What Shippers Need to Know

When shippers think about heavy haul weight limits, the 80,000-pound gross vehicle weight figure tends to get most of the attention. But the 80,000-pound GVW limit is only one of several weight constraints a heavy haul move must satisfy simultaneously. Axle weight limits — the maximum weight allowed on each axle or axle group — are often the binding constraint that determines what trailer a carrier needs, how many axles must be deployed, and what a superload permit must specifically authorize.

R&RM LLC has been moving overweight loads and oversize loads since 2011. This guide explains how axle weight limits work in practice — not just what the numbers are, but how they interact with the bridge formula, state permit rules, and trailer selection decisions.

Federal Axle Weight Limits: The Baseline

Federal law (23 U.S.C. § 127) sets the maximum axle weights on the Interstate Highway System. These limits apply to any vehicle combination operating without an overweight permit:

• Single axle: 20,000 pounds
• Tandem axle (two axles spaced 40 to 96 inches apart): 34,000 pounds
• Gross vehicle weight: 80,000 pounds

The tandem axle limit of 34,000 pounds — 17,000 pounds per axle in a tandem pair — is significantly lower than the 20,000-pound single axle limit. This reflects the engineering reality that axles spaced close together stress bridge spans differently than widely spaced axles. The federal Bridge Formula (see below) governs how closely spaced axles interact for bridge loading purposes.

It is important to understand that these federal limits are floors, not ceilings. States may set lower limits on state and local roads within their boundaries. Many states impose seasonal weight restrictions in spring — called spring thaw or frost law restrictions — that reduce allowable axle weights on state roads temporarily to protect road surfaces that have been weakened by freeze-thaw cycles. These restrictions add another layer of axle weight constraints on top of the year-round limits during the spring season.

State Axle Weight Limits: Where It Gets Complicated

While the federal limits apply on the Interstate System, each state has its own authority to set limits on state and local roads. Some states are more permissive than the federal baseline in certain respects; many are more restrictive. A few key examples:

States with Higher Permit Axle Allowances

Several states allow higher per-axle weights under overweight permits than the federal baseline, provided the carrier applies for and obtains the appropriate permit. Texas, for example, permits tandem axle groups above 34,000 pounds under specific overweight permit categories, with the exact limit depending on the number of axles in the group and the axle spacing. This flexibility makes Texas permit axle calculations more favorable than in some other states for heavy equipment transport — a relevant consideration given how much heavy equipment moves through Texas to and from oilfield operations.

States with Spring Weight Restrictions

Northern states — including Minnesota, Wisconsin, Michigan, North Dakota, South Dakota, Montana, and others — impose seasonal spring weight restrictions that can reduce allowable axle weights by 20 to 40 percent for periods ranging from a few weeks to several months, depending on annual conditions. A load that is legal under a standard overweight permit during summer or fall may require either waiting until spring restrictions lift or obtaining a separate seasonal permit with restricted routing. Carriers experienced with northern routes factor seasonal weight restriction dates into delivery scheduling as a standard part of route planning.

State-Specific Limits on Non-Interstate Roads

A load routed on state highways or county roads — because a permit-restricted bridge prohibits the Interstate route, or because the origin or destination is off the Interstate network — may encounter lower axle weight limits than the Interstate limits. Some states set lower weight limits on specific road classifications that must be checked route-by-route, not assumed to match the Interstate limits.

The Bridge Formula: Why Per-Axle Weight Alone Isn't Enough

The federal Bridge Formula, codified at 23 U.S.C. § 127, establishes a maximum allowable weight for any set of consecutive axles based on two factors: the number of axles in the group and the distance between the outermost axles in the group. The formula is:

The practical effect of the Bridge Formula is that axles spaced closely together are limited to lower combined weights than the same number of axles spaced farther apart. A tandem axle group (two axles spaced 4 feet apart) is limited to 34,000 lbs. But if those same two axles are spread to 10 feet apart, the Bridge Formula allows up to approximately 40,500 lbs — which is why some heavy haul trailers use spread-axle configurations rather than tight tandem groupings.

For heavy haul shippers, the key implication is this: a load may appear to be within its overweight permit's GVW authorization, while still violating the Bridge Formula at specific bridges. Permit engineers and carrier dispatchers check Bridge Formula compliance bridge-by-bridge on the permitted route, not just at the aggregate GVW level. An axle configuration that passes the Bridge Formula on the Interstate may still fail at a lower-rated bridge on the approach road to a job site.

Our guide on the bridge formula and weight limits covers the full formula and common scenarios in more detail.

Overweight Permits and Axle Weight Authorization

When a load exceeds legal axle weight limits, the carrier must obtain an overweight permit from each state the load traverses. Critically, overweight permits do not simply authorize the load's gross weight — they authorize specific axle weights for a specific trailer configuration on a specific route. The permit document specifies:

• The authorized gross vehicle weight
• The authorized weight per axle or axle group (steering axle, drive axles, trailer axle groups)
• The number and configuration of trailer axles required
• The permitted route (which bridges have been analyzed and cleared for the specified axle loading)
• Any route-specific restrictions (bridge approach speeds, weight monitoring requirements at specific crossings)

This axle-specific authorization is why carriers cannot simply load a piece of equipment and drive it under a general overweight permit. If a shipper provides weight information for a different configuration than what the carrier used in the permit application, the permit may not actually cover the axle weights the loaded trailer produces on the road — even if the gross weight is within the permit's GVW limit.

How Many Axles Does a Heavy Haul Load Need?

The minimum number of axles required for a heavy haul move is determined by dividing the total transport weight by the highest per-axle weight the permit will authorize at the most restrictive bridge on the route. In practice, carriers calculate this iteratively — start with a plausible axle count, calculate per-axle weights, check Bridge Formula compliance at every bridge, and add axles where needed.

Example: An 80,000-Pound Excavator

A standard 80,000-pound excavator on a 5-axle RGN (one tractor steering axle, two drive axles, two trailer axles) distributes weight roughly as follows:

• Steering axle: ~12,000 lbs
• Drive axle tandem: ~34,000 lbs
• Trailer tandem: ~34,000 lbs
• Total: ~80,000 lbs

This configuration is right at the legal limit on a standard 5-axle rig — no overweight permit required if weight distribution can be managed within these limits. Adding a booster axle to the trailer, creating a three-axle rear group at 51,000 lbs total (17,000 per axle), gives the same load a comfortable buffer below the per-axle limit.

Example: A 140,000-Pound Bulldozer

A 140,000-pound bulldozer like a Cat D10 or D11 requires an overweight permit regardless of axle configuration. At 140,000 lbs total, even distributing the load across 10 trailer axles produces 14,000 lbs per trailer axle — within the permitted per-axle range in most states. But the tractor drive axles and any fixed configurations on the trailer also contribute to total axle loads, and Bridge Formula compliance must be verified for the specific axle spacing of the trailer selected. Most Cat D10/D11 transports use 8- to 10-axle lowboy or RGN configurations, with bridge analysis performed for each permitted route.

Example: A 300,000-Pound Superload

At 300,000 pounds, a load requires a hydraulic multi-axle platform trailer with 14 to 24 axle lines, depending on the per-axle weight the permitting states will authorize. If a state's superload permit caps axle weight at 18,000 lbs per axle line, a 300,000-pound load requires at least 17 axle lines on the transport trailer — plus the tractor axles, which add their own weight to the calculation. See our guide on superload permits for more on the process for loads in this weight class.

Why Axle Weight Matters More Than Gross Weight for Route Approval

Many shippers are surprised to learn that a route may be denied or modified not because the load's gross weight exceeds the permit limit, but because the axle loads the specific trailer configuration produces cannot be cleared at a specific bridge on the requested route. Bridge ratings are expressed in terms of the load pattern they can handle — not just a single gross weight number — and an adverse axle arrangement (axles too closely spaced, or too few axles for the weight) can stress a bridge in ways that a higher-GVW but better-distributed load does not.

This is why experienced heavy haul carriers do not simply say "80,000 pounds, we can handle that" or "we'll get a permit for 100,000 pounds." The question carriers and permit engineers actually work through is: for this specific load, on this specific trailer configuration, with these specific axle spacings, will the Bridge Formula-derived loading at each bridge on the route fall within that bridge's rated capacity? If the answer at any bridge is no, the route changes, the axle configuration changes, or additional axles are added to change the load distribution.

Permit Axle Limits vs. Legal Axle Limits: The Difference

A standard overweight permit typically allows per-axle weights above the legal limits — that is the point of the permit. But permit axle limits are not unlimited. Most states cap the per-axle weight that a single-trip overweight permit will authorize at some maximum, above which either a different permit category (superload, special permit) or additional axles are required. Common permit axle limits in major heavy haul states:

• Georgia: Standard overweight permits authorize up to 20,000 lbs per single axle; tandem groups up to 40,000 lbs. Loads requiring higher axle weights need special permits reviewed by the Georgia DOT.
• Texas: TxDOT's standard overweight permit schedule allows graduated per-axle limits based on axle type and spacing, with specific limits by road classification. TxDOT uses a complex but flexible permit schedule that experienced carriers navigate using TxDOT's online permit system.
• Colorado: CDOT allows standard overweight permits for axle weights up to specific limits per axle group; loads above those thresholds require a CDOT engineering review and are processed as superloads.
• Pennsylvania: PennDOT's Special Hauling Permit (SHP) system sets axle weight limits that vary by route class. Pennsylvania is notably more restrictive than many other states regarding overweight permits on non-Interstate routes.

Because axle weight permit limits differ by state, a carrier planning a multi-state move must use the most restrictive state's limit as the binding constraint when selecting trailer axle configuration. A configuration acceptable in Georgia and Texas may require additional axles to satisfy Pennsylvania or Colorado's permit rules on the same trip.

Steer Axle Weight Limits

One axle weight limit that catches some operators off guard: the steer axle limit. Federal law caps the tractor's front (steering) axle at 20,000 lbs — the standard single-axle limit — even when the load's rear axles are operating under an overweight permit for much higher weights. Steer axle overloading is a common violation that can result in a permit being voided, because the permit authorizes specific axle groups and cannot override the statutory steer axle limit without a specific permit authorization for the steer axle itself (rare in most states).

Overloading the steer axle typically happens when a load is positioned too far forward on the trailer, shifting weight to the tractor's fifth wheel and thence to the steer axle. Proper load placement — positioning the load's center of gravity slightly rearward on the trailer to keep the steer axle within limits — is an essential step during loading. Carriers with experience in overweight load transport check axle weights after loading at a legal scale or certified weigh station before departing.

Tridem Axle Groups: Adding a Third Axle

When the load weight falls between what a single tandem axle group can legally carry and what a two-tandem-group trailer can handle, carriers sometimes use a tridem axle group — three axles spaced closely together. Federal law does not define a specific weight limit for tridem groups separately from the Bridge Formula, so the allowable tridem weight is calculated using the same formula as any other multi-axle group. At standard 4-foot spacing between axles, three axles in a tridem group spanning 8 feet between outermost axles can typically support approximately 42,000 lbs under the Bridge Formula — a useful increment over a standard tandem's 34,000 lbs.

Tridem axles are common on the rear of heavy dump trailers and on some multi-axle lowboys. On a multi-axle hydraulic platform trailer, multiple tridem groups can be configured as needed to achieve the axle count and spacing required for the load.

Working With a Carrier on Axle Configuration

The most important thing a shipper can do to help a carrier select the correct trailer and axle configuration is to provide accurate weight information — specifically the transport weight of the equipment in its road-travel configuration (not operating weight, which typically includes full fuel, hydraulic fluid, and attachments that are often removed or reduced for shipping), and if possible, the weight distribution front-to-rear across the machine.

For standard construction equipment, carriers have this weight data for common machines. For unusual equipment, custom fabrications, or equipment where the transport configuration differs significantly from the normal operating configuration, a certified weight ticket or manufacturer specification sheet is the most useful document to share at the time of quoting.

R&RM LLC handles axle weight analysis, trailer selection, and permit applications for every load we move. If you have questions about whether your equipment will require an overweight permit, what trailer configuration is appropriate, or what the permit process looks like for your specific route, call us at (404) 987-6225 or submit a quote request. We will work through the axle weight analysis as part of the quoting process so you know exactly what to expect before the equipment is ready to ship.