Over on the Drives, Room on the Tandems
You are running a five-axle tractor-van combination with a 53-foot trailer, loaded by the shipper with 42,000 lb of bagged seed on pallets running from the nose to about four feet short of the doors. The shipper's scale is closed for the night, so you weigh at a certified scale three miles down the road before getting on the highway.
Scale ticket:
| Axle group | Weight | Limit |
|---|---|---|
| Steering | 11,600 | 12,000 |
| Drive tandem | 34,600 | 34,000 |
| Trailer tandem | 32,800 | 34,000 |
| Gross | 79,000 | 80,000 |
The trailer tandem is six feet forward of the rearmost hole and the fifth wheel is in its middle position. The distance from the drive axles to the trailer tandems is 31 feet, so the bridge formula is not your problem. The port of entry is 30 miles ahead, and the shipper is closed until morning.
1. Which axle group is illegal, and which way do you slide the trailer tandems? Explain the reasoning rather than just the direction.
Read the worked answer
The drive tandem at 34,600 lb is 600 lb over the 34,000 lb tandem limit. The steering axle has only 400 lb of headroom and the trailer tandems have 1,200 lb of room.
Think of the trailer as a beam carried at two points: the kingpin at the front and the trailer tandems at the rear. The share of the load carried at the tandems rises as the tandems move closer to the load, and the share carried at the kingpin - which the tractor's steer and drive axles then share - falls. So to take weight off the drives, you slide the trailer tandems forward, toward the nose. That moves weight off the tractor and onto the trailer axles, which have room for it. The steering axle will also lighten slightly, which is fine because it is already near its limit.
What you must not do is slide the fifth wheel forward to fix the drives. Moving the fifth wheel forward does lighten the drive tandem, but it throws that weight onto the steering axle, which has only 400 lb of headroom. You would trade one overweight axle for another and probably lose steering capacity on top of it.
2. You slide the tandems forward and reweigh. The new ticket reads: steering 11,450, drives 33,500, trailer tandems 34,050, gross 79,000. Now what?
Read the worked answer
You fixed one problem and created another: the trailer tandem is now 50 lb over. Two lessons are worth taking from this ticket.
First, do not guess at the rate. Weight transfer per inch of slide depends on where the load actually sits, and 42,000 lb of bagged seed is not perfectly distributed. The reliable method is small adjustments with a reweigh after each one, not a single big move based on a rule of thumb.
Second, you overshot because you were chasing a number, not a range. The better technique is to slide in small increments and aim for the middle of the legal window on every axle group, not the edge. A 50 lb overshoot on a certified scale is still an overweight axle at the port of entry - the scale does not round in your favor.
So: slide the tandems back a few inches, reweigh, and confirm the drives are still under 34,000 before you leave the scale. If the holes available will not let you satisfy both groups at once - for example, if the next hole back puts the drives over again - the load itself has to move. Note that any change to the load means checking the securement again, because the tiedowns and their working load limits have to match what you actually did.
3. The gross is 79,000 lb, under the 80,000 lb limit. Why doesn't that settle it, and what else can be checked if sliding is not enough?
Read the worked answer
Gross weight is only one of four independent limits you must satisfy, and being under on one does not excuse being over on another. The four are:
- Gross vehicle weight - 80,000 lb here, and you are under it.
- Axle and axle-group limits - 12,000 lb on the steering axle and 34,000 lb on each tandem. These apply regardless of gross.
- Tire ratings - each tire's sidewall rating, multiplied by the number of tires in the group, is its own ceiling. A group can be under the axle limit and still be over the tire capacity, particularly on a steer axle running 11,000 lb on two tires.
- Bridge formula - the federal spacing rule, and states may impose lower limits on their own roads. Your 31-foot spacing clears the federal formula, but that does not guarantee you clear a state table on the route ahead.
If sliding the tandems through the available holes cannot get every group legal, the remaining option is redistributing the load itself - but you are the driver, not the shipper, and the shipper is closed. Do not start rearranging 42,000 lb of pallets by yourself in a parking lot unless you can do it safely, keep the load blocked and secured to the same standard, and reweigh afterward. The honest answer is: if you cannot make it legal and verify it on a scale, it does not move. Call dispatch with the numbers and let them decide whether to reschedule or send someone to help.