The External Bridge, or distance between axles 1 to 5

We know the weight on all the axles totals 80,000 pounds. We apply the bridge formula or look at the table. The “L” is 51 feet; “N” (number of axles) is 5.

After doing the calculations, or looking it up in the table, we find the answer is 80,000. Conclusion: with the 51-foot spacing, we just meet the bridge requirement. Had it been 50 feet, we would have been limited to 79,000 pounds gcw. Studying the table, we see that 51 feet is the least spacing necessary between axles 1 and 5 to gross 80,000 pounds.

The table continues with increasing weights beyond 51 feet, but in most states it does no good. Eighty thousand is as high as you can legally go with total gcw.

Finally we get to test 3 – the inner bridge between axles 2 to 5. Here, each of the four axles is loaded to 17,000 pounds, so 68,000 pounds is the actual weight.

We have an inner bridge of 34 feet. The table says that for four axles, we are limited to 64,500 pounds. Since our actual weight is 68,000 pounds, we’re not legal. We must either reduce our payload so the four axles scale out at 64,500 pounds, or we must use a longer trailer so we can increase our inner bridge dimension to at least 39 feet.

If we’re fortunate enough to have a sliding tandem on our semi trailer, maybe we can move it rearward five feet, which will meet the requirement. That’s not likely, since the tandem might have been there in the first place.

Again, approaching this from the spec’ing perspective, we can see that for an 80,000-pound gcw rig we must have a minimum outer bridge (test 2) of 51 feet, and a minimum inner bridge (test 3) of 39 feet. There is an important exception to the inner bridge rule, which we’ll cover in a bit.

In spec’ing out a tractor and semi trailer, it’s mandatory to work out trailer dimensions first as they control the inner bridge. Deciding on how long a wheelbase the tractor must have, and whether you can or can’t use a setback front axle will be governed by the length you must have to meet the outer bridge.

Remember also that if you choose to spec a long wheelbase conventional with a big sleeper box, you’ll have to set the fifth wheel as much its 28 inches ahead of the tandem center line if you are to get sufficient weight onto the front axle to make it to 12,000 pounds – the only way to legally gross 80,000 pounds. But moving the fifth wheel forward reduces the inner bridge dimension. You can see how closely these all interrelate.

Now there are exceptions as noted in the footnotes of the Formula B table.

The Federal Highway Administration says 34,000 pounds can be carried on two consecutive sets of tandem axles, or 68,000 pounds total, with an inner bridge (axles 2 to 5) of 36 feet or more, not 39 feet. (The table shows a maximum of 66,000 pounds for 36 feet; 66,500 pounds for 37 feet and 67,500 pounds for 38 feet.)

This 68,000 pounds at 36 feet is known as the “ATA bump,” as it was sought by the American ‘Trucking Assns. specifically so fleets could use 40-foot trailers and gross 80,000 pounds. It was the only way tank operators could reach 80,000 without adding axles.

A recent ruling by the FHWA states that dimensional and weight data can be rounded off to the nearest foot or nearest 1,000 pounds. Thus, an inner bridge dimension of 35 foot, 6 inches actually is treated as 36 feet; and when the rounded-off wheelbase dimension is inserted into the bridge formula, the weight is rounded upwards. For example, 79,500 pounds is rounded up to 80,000 pounds.

On the basis of the ATA bump, the minimum dimension for an 80,000-pound rig must be:

This last item really is the clincher in many cases. It often isn’t practiced to stretch out a rig to meet the bridge requirements, then offset the fifth wheel enough to get maximum weight transfer to the front axle.

What we end up with is a “practical” gross weight. That is, 34,000 pound, on each pair of tandem axles and perhaps 9,000 to 10,000 pounds on the front axle.

An example is a five-axle tractor-dump trailer. The tractor has a 187 inch wheelbase and the semi trailer is a short 27 feet. The rig has a 37-foot external bridge and, with a 12-inch fifth wheel offset, a 24-foot internal bridge. A check of the bridge table shows that the legal maximum for axles 1 to 5 is 71,000 pounds, and for axles 2 to 5 it’s 58,000 pounds.

You can see that the inner bridge limits the two sets of tandem axles to 58,000 pounds. That’s 29,000 pounds per tandem, or 5,000 less than the normal maximum.

Weight transfer to the front axle is limited by the wheelbase and that 12-inch fifth wheel offset. Through separate calculations, it’s determined that the front axle can only be loaded to 9,450 pounds. Maximum allowable gcw then is 67,450 pounds.

But that’s only theoretical. Even 67,450 isn’t “practical” because it requires loading our dump trailer very precisely, with about 1,800 pounds more in the front than the rear. The “real world” legal gross weight on this rig is more like 63,000 to 65,000 pounds, depending on the skill of those who load it.

Longer dump trailers – usually 32 or so feet are used to achieve higher gcws. But they still can’t get 80,000 pounds, and 40-foot dump trailers aren’t safe because they become unstable when they’re tipped.