A scale ticket that reads 35,000 pounds on the drives doesn't mean the load is wrong. Often it just means the trailer tandems are in the wrong place. This tandem slide calculator works out how many holes to slide, which way, and what the scale will read afterwards.
How it is calculated
The trailer's weight sits on two supports: the kingpin and the trailer tandem. With the same load and the same center of gravity, the tandem's weight times its distance from the kingpin stays the same. So when you slide the tandem forward and that distance gets shorter, the tandem carries more:
- New trailer tandem weight = current tandem weight × current distance ÷ new distance
- New distance = current kingpin-to-tandem distance − holes × hole spacing (forward is positive)
- Weight moved = new trailer tandem weight − current trailer tandem weight
- New drive weight = current drives − weight moved × (1 − steer share)
- New steer weight = current steer − weight moved × steer share
The steer share depends on where the fifth wheel sits; a few percent is typical with the fifth wheel just ahead of the drives. Gross weight doesn't change, only where it sits.
A worked example
- Ticket: drives 35,000, trailer 32,500
- Drives over
- Tandem 40 ft from kingpin, 6 in holes
- ~420 lb/hole
- After 3 holes forward: drives
- 33,797 lb
- After 3 holes forward: trailer
- 33,766 lb
Two holes would leave the drives 208 pounds over. Three gets both tandems under 34,000. Sliding forward also shortens the kingpin-to-rear-axle distance, which helps in California, as the kingpin setting calculator shows.
When sliding won't fix it
If the drives and trailer tandem together weigh more than two tandem limits, no slide works. The calculator says so; the load has to be reworked or lightened. Check the whole picture, including gross and bridge formula, with the axle weight calculator.
Sliding tandems for dummies, in one list
- Slide on level ground, trailer brakes set, tractor brakes released.
- Pull the release handle and confirm the pins are out.
- Move the tractor slowly; the trailer box moves over the tandems.
- Count the holes, set the pins, and confirm each one is fully seated.
- Tug test, then scale again.
More detail, with photos of the slider, is in sliding tandems on a trailer. Doubles balance weight differently; see double trailer truck. Pulling preloaded trailers means checking this at every hook, part of power only dispatch.
Results are estimates from your ticket and trailer measurements. Always scale again after sliding.
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