01
Only the groups that have names get checked
The drive tandem, the trailer tandem and the gross weight are the three things everybody checks, because they are the three things that have names in a yard. The formula applies to every set of two or more consecutive axles, which on a five-axle combination is ten sets. Seven of them have no name, and it is usually one of those seven that binds.
Worked example
Worked example for Only the groups that have names get checked.| Subsets on a five-axle combination | 10 |
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| Subsets a typical check covers | 3 |
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| Interior four-axle group at 38 ft | 67,500 lb limit |
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| Result | a load 200 lb over on a group nobody looked at |
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This is not an exotic failure. It is the ordinary one. A load can clear the steer axle, both tandems and the gross with hundreds of pounds to spare and still be over on the four-axle group that runs from the front drive axle to the rear trailer axle.
What to do about it
Enumerate. Take every run of consecutive axles of length two or more, from the tightest pair to the full outer spread, and evaluate all of them. It is a loop, not a judgement call, and the cost of running the seven extra checks is nothing.
02
The tandem exception is applied in the wrong direction
Two axles spaced 8 ft or less are capped at 34,000 lb. It is a cap. It can only ever lower a limit, never raise one. Implementations get this wrong in both directions, and the two errors have opposite consequences.
Worked example
Worked example for The tandem exception is applied in the wrong direction.| Two axles at 8 ft, formula result | 38,000 lb |
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| Two axles at 8 ft, limit after the cap | 34,000 lb |
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| Two axles at 10 ft, cap does not apply | 40,000 lb |
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| Two axles at 3.5 ft, formula below the cap | 33,500 lb |
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Return the formula result for a pair at 8 ft and you have granted 4,000 lb that does not exist. Stamp 34,000 lb on a pair at 10 ft and you have denied 6,000 lb that does. And on a tightly coupled pair the formula itself returns 33,500 lb, which is below the cap, so the cap is not the answer there either.
What to do about it
Compute the formula first, then apply the cap as a minimum operation, and only where the spacing is 8 ft or less. Note also that a pair spaced under 40 in is not a tandem at all for weight purposes and is treated as a single axle group, which is a different rule again.
03
The rounding is done any way but to the nearest 500
The formula result is rounded to the nearest 500 lb. Implementations truncate instead, or round up, or carry the raw figure. It looks like a detail. It is worth up to 250 lb of legal capacity in either direction, on a check that is routinely decided by less than that.
Worked example
Worked example for The rounding is done any way but to the nearest 500.| Four axles at 50 ft, raw formula | 75,333 lb |
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| Rounded to the nearest 500 | 75,500 lb |
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| Truncated downwards instead | 75,000 lb |
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| Four axles at 38 ft, truncated | 67,000 lb, against a real limit of 67,500 lb |
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Truncating the 38 ft group gives 67,000 lb. A load carrying 67,200 lb on that group is legal by 300 lb and a truncating implementation calls it over by 200 lb. The load gets broken down and rebuilt for no reason, which costs a crew, a dock slot and a delivery window.
What to do about it
Round to the nearest 500, with the usual tie handling, and test it explicitly. Two test cases catch every version of this error: one group whose raw result rounds up and one whose raw result rounds down.
04
The 80,000 lb cap is left off
For long groups the formula returns a number larger than the federal gross limit. The cap is not optional and it is not implied by anything else in the calculation. Leave it out and the outer bridge check silently stops being a check.
Worked example
Worked example for The 80,000 lb cap is left off.| Five axles at 54 ft, formula result | 82,000 lb |
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| Applied limit after the cap | 80,000 lb |
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| Capacity granted by an uncapped implementation | 2,000 lb that does not exist |
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An uncapped check passes a load at 82,000 lb gross, which is 2,000 lb over the federal limit. The outer spread is exactly the group where the cap matters, and it is exactly the group people assume is safe because it is the longest.
What to do about it
Apply the cap to the result of every group, not only to the outer one. It costs one line and it converts a class of silent failure into no failure at all.