Truck Weight Limits and the Bridge Formula

Roads and bridges are built to carry a certain weight, spread over a certain distance. A truck can be under its total limit and still be illegal because too much weight sits on one axle or the axles are too close together. Knowing the limits lets you catch a problem at the scale near the shipper instead of at a weigh station 200 miles later.

Overweight trucks are harder to stop, wear out roads and bridges faster, and bring fines for the driver or carrier. On top of that, an officer can require you to shift or remove weight before you can move on.

What You Will Be Able to Do

• State the federal gross, single axle and tandem axle limits.
• Explain what the bridge formula does and use it for a simple example.
• Read a scale ticket and find which axle group is over.
• Move weight between axles by sliding the tandems or fifth wheel.
• Know where state limits differ and where to check them.

Terms to Know

Federal Limits

These limits apply on the Interstate System under 23 CFR 658.17.

Size limits work a bit differently:

A vehicle with an auxiliary power unit or idle-reduction system may carry up to 400 extra pounds over the limits, with proof of the unit's weight.

The limits do not stop at federal law. The tires also have load ratings, and the axles and vehicle have maker's ratings. You must stay under the lowest number that applies. The steer axle is the common example. The federal axle limit is 20,000 pounds, but its tires and axle rating usually hold it to about 12,000 pounds in normal tractor-trailer use.

How Much Cargo Can You Carry?

Your cargo limit is what is left after the empty weight of the truck. A typical empty five-axle tractor and 53-foot van weighs roughly 33,000 to 36,000 pounds. That leaves about 44,000 to 45,000 pounds of cargo under the 80,000-pound limit. Your actual number depends on your truck, fuel load and trailer.

The Bridge Formula

Bridges are damaged by heavy weight concentrated in a short span. The bridge formula sets how much a group of axles may carry based on how many axles there are and how far apart they are. More axles spread over more distance may carry more.

The federal formula is:

W = 500 x ((L x N / (N - 1)) + 12N + 36)

• W is the maximum weight in pounds on the axle group.
• L is the distance in feet between the first and last axles of the group.
• N is the number of axles in the group.

Worked Example

You have a five-axle tractor-trailer with 45 feet from the steer axle to the last trailer axle.

1. L x N / (N - 1) = 45 x 5 / 4 = 56.25
2. 12N = 12 x 5 = 60
3. Add 36: 56.25 + 60 + 36 = 152.25
4. Multiply by 500: W = 76,125 pounds

This truck cannot legally gross 80,000 pounds. With 51 feet between the axles, the same math gives 79,875 pounds, which the federal chart rounds to 80,000 pounds. This is why bridge length matters on a tractor-trailer. It also applies to groups inside the truck, such as from the first drive axle to the last trailer axle.

There is one exception. Two tandem axle sets may each carry 34,000 pounds if the first and last axles of the two sets are at least 36 feet apart, even if the formula gives less.

In practice, drivers and dispatchers use a chart or calculator, not hand math. You need to understand what the formula controls: the weight on a group of axles, based on the distance between them.

Balancing a Load

Legal weight also depends on where the load sits. The target for a five-axle rig at full weight is about 12,000 pounds on the steer axle, 34,000 on the drives and 34,000 on the trailer tandems.

• Low and centered: Keep heavy freight low and centered side to side, and slightly forward of the trailer axles. A high center of gravity raises rollover risk.
• Spread it out: Heavy freight bunched in one spot can overload an axle even if the gross is legal.
• Too much weight at the rear: This takes weight off the steer axle, makes steering light, can cause trailer sway, and can overload the trailer tandems.
• Too much weight at the front: This can overload the steer axle and make steering hard.
• Too little weight on the drives: You lose traction, especially on snow or a grade.

A common loading rule of thumb is to put about 60 percent of the cargo weight in the front half of the trailer and 40 percent in the rear. That keeps weight on the drive axles for traction without overloading the steer axle. It is a rule of thumb only and it is not a federal rule. The scale decides.

Moving Weight

Sliding the tandems forward also shortens the bridge length, and it can put you over a state limit on the distance from the kingpin to the rear axle. California, for one, is known for strict rules. Check before you slide.

Warning: Stay clear of the tandems and rails while sliding. After every move, check that the locking pins are fully seated. A slider that is not locked can run to the end of its rails in a hard stop.

Weighing at a Truck Scale

Weigh after loading and before you get on the highway, at a certified scale near the shipper if you can.

1. Pull onto the scale with your steer axle, drives and trailer tandems on the separate platforms.
2. Get the ticket and read each axle group.
3. Compare each group and the gross to the limits.
4. If any group is over, slide or have the load shifted.
5. Weigh again to confirm. Keep the ticket with your trip records.

Worked Example: Reading the Ticket

The gross is legal, but the trailer tandems are over. There is room on the drives. Slide the trailer tandems toward the rear to move weight onto the drives. A second ticket showing about 33,800 on the drives and 34,000 on the tandems would be legal. If the load is too far back for sliding to fix, the freight has to move.

Fuel weighs about 7 pounds a gallon. A full tank at the scale may be lighter by the time you reach the next station, and that can change your drive and steer axle weights.

Weigh Stations and State Rules

Weigh stations and portable scales check your weights. Some trucks use bypass systems, but a green light does not change the law. You still must be legal.

State rules vary:

• Heavier limits: Some states allow more weight on some roads under older laws or permits, and Michigan is a well-known example. Those limits end at the state line.
• Posted bridges: A bridge sign can set a gross or axle weight limit lower than the federal limits. The sign applies to you, whatever your permits say, unless the permit names it.
• Seasonal limits: Northern states and counties often post spring thaw limits on weakened roads. The size of the cut depends on the state, the road and the pavement. Michigan, for example, has cut legal axle weights on seasonal routes by 25 percent on rigid pavement and 35 percent on flexible pavement.
• Oversize or overweight loads: These need permits from each state, and the permit sets the route and conditions.

Check the state transportation department's website before a trip through unfamiliar territory.

Common Mistakes

Check Your Understanding

Your scale ticket shows a gross of 78,000 pounds and 36,000 pounds on the drives. Are you legal?

No. The tandem limit is 34,000 pounds, so the drives are over by 2,000 pounds even though the gross is fine. Slide the tandems forward or shift the load, then reweigh.

A six-axle truck has 30 feet from first to last axle. Does the 80,000-pound limit apply to it?

The gross limit is 80,000 pounds at most, but the bridge formula may set a lower limit. Worked out, 500 x (30 x 6 / 5 + 72 + 36) = 72,000 pounds. That truck is limited to 72,000 pounds.

You are heavy on the steer axle and light on the drives. What do you move?

Slide the fifth wheel toward the rear of the tractor if you have a sliding fifth wheel. That shifts weight from the steer axle onto the drives.

A bridge ahead is posted at 30 tons. You weigh 39 tons. What do you do?

Do not cross. Stop safely, find a legal route on your map or truck GPS, and call dispatch if you need help.

Key Takeaways

• The federal limits are 80,000 pounds gross, 20,000 on a single axle and 34,000 on a tandem.
• The bridge formula limits weight on a group of axles, based on the distance between them.
• Legal gross does not mean legal axles, so read every line of the scale ticket.
• Slide tandems or the fifth wheel to move weight, and recheck the locking pins.
• Height, some weights and seasonal limits vary by state. Check before the trip.