How to Slide Trailer Tandems
Sliding the trailer tandems is the one adjustment a driver can make at the scale without touching the freight, and it fixes more overweight-axle problems than anything else. This guide explains what sliding actually does, which direction moves weight where, how much a hole is worth, and walks through a worked example. The per-hole numbers here are conservative, commonly cited estimates — every trailer is a little different, so the scale always has the final word.
What sliding the tandems actually does
Picture the loaded trailer as a plank resting on two supports: the kingpin at the front (which passes weight through the fifth wheel onto the tractor's drive tandem) and the trailer tandems at the rear. The freight pushes down between those supports, and how the load splits between them depends on where each support sits relative to the freight's center of gravity.
The tandems ride on a sliding rail with a row of holes. Pull the locking pins and the trailer body can move over the axles, changing where the rear support sits under the load. The axles never move along the ground — the body slides over them — but the effect is the same as moving the rear support point forward or back under the plank.
Which way does the weight move?
- Tandems forward (toward the nose): the rear support moves closer to the freight's center of gravity, so it carries more. Weight moves off the drives, onto the trailer tandems.
- Tandems back (toward the doors): the rear support moves away from the load. Weight moves off the trailer tandems, onto the drives.
A memory hook that survives a 2 a.m. scale visit: the tandems take the weight wherever they go. Slide them toward the heavy end and they relieve it; slide them away and they hand the weight back. The steer axle barely moves in all of this — it mostly carries the tractor, so tandem slides shift weight between the drives and the trailer tandems and leave the steer nearly alone.
How much is one hole worth?
A commonly cited rule of thumb: each hole shifts roughly 250 lb between the drives and the trailer tandems on rails with 4-inch hole spacing, and roughly 400–500 lb on rails with 6-inch spacing. Treat these strictly as estimates for getting in the neighborhood — the true number depends on the trailer's geometry and where the load sits. Count your holes, do the rough math, slide once, and rescale rather than creeping one hole at a time.
A worked example
Say the scale ticket reads: steer 11,400 lb, drives 34,600 lb, trailer tandems 31,800 lb — gross 77,800 lb. Gross is fine, but the drives are 600 lb over the 34,000 lb limit while the tandems have 2,200 lb of headroom. The fix is to move at least 600 lb rearward-support-wise: slide the tandems forward so they pick up load the drives were carrying.
On a 4-inch-spacing rail at roughly 250 lb per hole, three holes forward is about 750 lb. After the slide you would expect something near: drives 33,850 lb, tandems 32,550 lb — both legal, each with a few hundred pounds of cushion. Rescale to confirm; if the estimate ran hot or cold, you are one small adjustment away rather than guessing from scratch. If the drives had been over by more than the tandems' headroom, no slide could fix it — that load needs to be physically re-worked, which is the situation a load planner exists to prevent.
The limits on sliding: kingpin-to-tandem rules
You cannot always slide as far back as physics would like. Several states limit kingpin-to-tandem (KPT) distance — the length from the kingpin to the center of the trailer tandems — with California's roughly 40-foot rule the most famous. A tandem position that scales perfectly in one state can be an instant length violation across the line, so West Coast lanes are often planned around the KPT cap first and axle weights second. Sliding also interacts with the federal bridge formula, which rewards spreading axle groups apart; see our guide to US axle weight limits for how those pieces fit together.
Practical habits
- Know your trailer's hole spacing (4″ and 6″ are common) so the per-hole estimate means something.
- Plan the load so a legal tandem position exists before the truck is loaded — sliding rescues small misses, not fundamentally nose-heavy or tail-heavy loads.
- After any slide, verify on a certified scale. Estimates get you close; the ticket is what counts at the DOT.
Pass the Scale simulates all of this before the first pallet goes on: set the tandem position in the advanced settings, watch the drive and tandem gauges move, and load the trailer so the scale visit is a formality.