Federal law provided that any two or more
consecutive axles may not exceed the weight
computed by the Formula even though
single axles, tandem axles, and gross weight
are within legal limits. In other words, the
axle group that includes the entire truck--
sometimes called the "outer bridge" group--
must comply with the Bridge Formula. But
interior combinations of axles, such as the
"tractor bridge" (axles 1, 2, and 3) and "trailer
bridge" (axles 2, 3, 4, and 5), must also be in
compliance with weights computed by the
Formula (Figure 3).
The most common vehicle checked for
compliance with weight limit requirements
is shown in Figure 3. While the Bridge
Formula applies to each combination of two
or more axles, experience shows that axle
combinations 1 through 3, 1 through 5, and 2
through 5 are critical and must be checked.
If these combinations are found to be
satisfactory, all of the others on this type of
vehicle will normally be satisfactory.
The vehicle with weights and axle dimensions
as shown in Figure 4 will be used to illustrate
a Bridge Formula check.
Before checking a vehicle for compliance with
the Bridge Formula, its single-axle, tandemaxle,
and gross weight should be checked.
Here the single axle (number 1) does not
exceed 20,000 pounds, tandems 2-3 and
4-5 do not exceed 34,000 pounds each, and
the gross weight does not exceed 80,000
pounds. These preliminary requirements
are thus satisfied. The first Bridge Formula
combination is checked as follows:
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