Legal gvws for straight trucks operating under the federal bridge formula face severe limitations, as we’ve seen. Calculations follow the same procedure used with tractor-trailers, except that test 1 is omitted and only the outer bridge axles 1 to 4 and the inner bridge axles 2 to 4 are figured.
In this example, we’ll assume we have a long wheelbase dump truck with three rear axles. The outer bridge is 23 feet, and the inner bridge is 9 feet.
Checking the 23-foot outer bridge on the table, we come up with 57,500 pounds allowable gvw. The 9-foot inner bridge – in effect, the tri-axle or “tridem” is limited by the table to 42,500 pounds, or approximately 14,000 pounds per axle.
That means we have to get 15,000 pounds (57,500 minus 42,500) on the front axle – if we can get that much weight transfer – to achieve the maximum allowable gvw. Practically speaking, we can’t. It’s more like 12,000 pounds with this wheelbase. So, our dump truck actually grosses 54,500 pounds.
Eastern states allow many tons more, but not so in the West, where authorities like the bridge concept. That’s why you’ll see three-axle dumps pulling two-axle pup or transfer trailers – or variations on that theme – in most states west of the Mississippi.
Space doesn’t allow getting into individual state requirements. Some follow the bridge formula using either the inner bridge or outer bridge but not both. And federal authorities have been pressuring state officials to adopt formula B for their own use, or at least enforce it on the Interstates.
If a truck operates at least part of the time – even a few miles of a regular haul – on an Interstate highway, it might be limited by Formula B. The key point to determine before spec’ing is whether you’re working under state or federal regulations – or both.
With wheelbase, especially, you don’t want to order a short, squat truck and find it should have been many feet longer. There are enough tears in the trucking world, so don’t let the Bridge Formula stain your face with more.