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What is the bridge formula?


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The bridge formula is: W = 500 ((LN/(N-1)) + 12N + 36).

W = the overall gross weight of any two or more consecutive axles to the nearest 500 lbs.

L = the distance (in feet) between the outer axles in a group of two or more axles.

N = the number of axles in the group.

A bridge formula helps determine the maximum axle weight different types of commercial vehicles can safely carry. 

Bridge formulas help prevent commercial vehicles from overloading bridges and other roadways. 

A bridge formula permits less weight for axles that are spaced closer together. 

A shorter truck will cause more stress to bridges an roads than a longer truck of the same weight.

What can poor weight balance lead to?

Improper weight balance can make operating a commercial vehicle extremely dangerous. 

Cargo that is piled too high will raise your vehicle’s center of gravity. A higher center of gravity will make your vehicle more likely to tip over. 

Cargo that is poorly balanced can cause problems as well. Too much weight on the steering axle will make the vehicle hard to control. 

Too little weight on the drive axle may lead to poor traction. 

Too much weight on one side can damage the suspension and cause handling issues as well. 

Cargo should be loaded as low as possible with the heaviest items on the bottom and evenly distributed between axles. 

You can find more information about this topic in the Transporting Cargo Safely (3) section of your official driver’s handbook. 

A commercial vehicle that is improperly loaded or balanced can:

  • Be hard to control.
  • Brake poorly.
  • Steer poorly.
  • Slow to gain speed. 
  • Take longer to stop. 

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