Overweight on the Drives at the Lake Charles Scale
You are leaving a terminal near Lake Charles with a five-axle combination: three-axle tractor, tandem-axle van trailer, sliding trailer tandems with lock pins on both sides. The load is 41,000 lb of bagged sand on pallets, loaded nose to tail with the heaviest pallets against the front wall. The certified scale prints: steer axle 11,700 lb, drive tandems 34,850 lb, trailer tandems 31,300 lb, gross 77,850 lb. The posted limits are 12,000 lb per steer axle, 34,000 lb per tandem group, and 80,000 lb gross. The steer tires are rated 6,150 lb each; all drive and trailer tires are rated 4,800 lb each. There is room on the pad to slide the tandems, and the scale operator will re-weigh for a flat fee. Your appointment is 6 miles away and the receiver closes in 40 minutes.
1. Which axle group is illegal, by how much, and what is the margin left on each of the others?
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Drive tandems: 34,850 lb against a 34,000 lb limit - 850 lb over.
Everything else has room:
- Steer: 11,700 lb vs. 12,000 lb → 300 lb of margin. Tight, and worth protecting; you cannot afford to add weight forward.
- Trailer tandems: 31,300 lb vs. 34,000 lb → 2,700 lb of room. This is the reservoir you will use.
- Gross: 77,850 lb vs. 80,000 lb → 2,150 lb of room, so the gross is legal.
The gross being legal is exactly why this is a distribution problem and not an unloading problem. Nothing needs to come off the trailer; weight needs to move from one axle group to another.
2. You decide to slide the trailer tandems. Which direction, and what happens to each axle group when you do? Explain the mechanism, not just the direction.
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Slide the tandems rearward.
The trailer rests on two supports: the kingpin at the fifth wheel and its own tandem axles. Trailer weight is split between those two points in proportion to how far each sits from the trailer's center of gravity. Moving the tandems rearward lengthens the lever arm from the kingpin, so the trailer axles carry a larger share and the kingpin carries less. Less weight at the kingpin means less weight transferred down through the tractor frame to the drives - and, to a much smaller degree, to the steer axle.
So: drives go down, trailer tandems go up, steer goes down slightly. The steer margin actually improves a little, which is useful given it only had 300 lb.
You need 850 lb off the drives, so the kingpin load must drop by roughly 900–1,000 lb to account for the small steer share; the trailer tandems land near 32,200–32,300 lb, still well under 34,000 lb.
3. After you slide the tandems, what must you confirm before you pull off the scale, and what could still make the load illegal even if the three axle readings look right?
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Confirm four things:
- Re-weigh all three axles. A shift estimated on paper is a guess; the scale is the answer. Target: drives under 34,000, trailer tandems under 34,000, steer under 12,000, gross under 80,000.
- Lock pins fully seated on both sides at the same position. A half-latched pin will walk out under load.
- Tire load limits. Trailer tires are rated 4,800 lb each; eight of them carry 38,400 lb, so the trailer axles are nowhere near a tire limit - but the steer axle at 11,700 lb against 12,300 lb of tire capacity is close enough that you should not let it climb.
- Trailer height and landing gear clearance after the slide, and rear swing on turns - sliding rearward lengthens the tail swing arc.
What can still bite you: bridge formula / axle-spacing limits. Two tandems can each be legal and the pair still illegal if the spread between them is short. Check the spacing chart for the new kingpin-to-tandem distance before you accept the re-weigh.