Pull forward. With the trailer brakes set and the slider pins pulled, driving the tractor forward leaves the tandems farther back under the trailer, and that puts more weight on your drive axles. Backing up does the opposite and puts more weight on the trailer tandems.
The part that trips people up is that the tandems move backward relative to the trailer when you pull forward. That feels wrong until you see what is actually moving.
Which way do I pull if my tandems are over weight?
If the trailer tandems (the rear axle group on the trailer) are carrying too much, you want weight off them and onto the drives. So you pull forward.
If the drives are over and the trailer tandems have room, you go the other way. Back up, and the tandems end up farther forward under the trailer, carrying more of the load.
In both cases the trailer brakes stay set the whole time. That is what holds the tandems still while the rest of the rig moves.
Why does pulling forward move the tandems backward?
When you pull the pins, the tandem assembly is free to roll along a rail under the trailer. With the trailer brakes set, the tandem wheels are locked and can't turn. They stay put on the ground.
So when the tractor pulls forward, it drags the trailer body forward over those parked axles. The tandems haven't moved on the road, but they are now farther back relative to the trailer. Backing up does the reverse, and the tandems end up farther forward relative to the trailer.
Think of it as sliding the trailer over the axles, not sliding the axles under the trailer.
Why does that shift weight onto the drives?
A trailer works like a lever. One end rests on the kingpin (the pin under the front of the trailer that locks into your fifth wheel), and the other end rests on the tandems. Your load sits somewhere in between.
Move the tandems farther back and the load is closer to the kingpin, so more of the trailer's weight rides on the kingpin. The kingpin sits on your fifth wheel, so that weight lands on your drive axles. Move the tandems forward and the load sits closer to them, so they carry more.
How much weight moves for each hole you slide depends on the load. A heavy load that is stacked toward the nose behaves differently from one that is evenly spread. There is no one number that works for every trailer, so the scale has the final word.
What am I actually trying to get legal?
The goal is every axle group under its legal limit, not every axle group carrying the same weight. On the Interstate system, the federal limit for a tandem axle group is 34,000 pounds. That applies to your tractor's drives and to the trailer tandems alike.
The federal gross limit for the whole rig is also a factor, and sliding doesn't change the total. It only moves weight between the drives and the trailer tandems. If the rig is too heavy overall, no slider position will fix it.
Canada and individual states and provinces have their own weight and axle rules, and some are stricter than the federal numbers or measured differently. Check the rules where you are running.
How do I slide the tandems safely?
The exact details vary by trailer and by carrier, so follow your company's procedure and the manufacturer's instructions. The basic sequence looks like this:
- Park on level ground with plenty of clear room, away from people, vehicles and anything you could hit.
- Set the trailer brakes so the tandems can't roll.
- Pull the slider pins using the release handle. On most trailers that is a lever on the side near the tandems, and some trailers use an air control.
- Ease the tractor forward or back, slowly, in the direction that gives the weight shift you want.
- Stop so the holes line up and let the pins drop in. The pins are spring-loaded and should snap into the rail holes.
- With the trailer brakes still set, gently tug against them to confirm the tandems are locked.
- Get out and look at every pin to make sure each one is fully seated.
Then run the rig across a scale to see where the weight actually landed. If it isn't right, slide again.
Why check every pin if the tug test went fine?
A pin that is only partly in a hole can look fine from the cab. It may even hold through a gentle tug. But a half-seated pin can let the tandems slide during hard braking or acceleration, until they slam into the stops at the end of the rail.
That can mean a sudden change in how the trailer handles at exactly the wrong moment. Sticky or dirty pins and rails are a common reason a pin doesn't drop all the way, so keep them clean and working.
The visual check takes seconds. Look at each pin on each side and confirm it is fully through the rail.
Does sliding the tandems change how the trailer turns?
Yes. Where the tandems sit changes how the trailer follows the tractor through a corner.
- Tandems farther back: the distance from the kingpin to the rear axles is longer, so the trailer cuts inside the turn more. That is called off-tracking.
- Tandems farther forward: the trailer tracks tighter through turns, but more of the trailer hangs out behind the axles. That makes the back end swing out wider, which is called tail swing.
Neither is better in every situation. Tight docks and city corners are where you feel these effects most, so watch both the inside and the outside of the turn.
Why can't I just slide all the way back?
Many states limit the distance from the kingpin to the rear axle of the trailer. Sliding the tandems all the way back can put you past that limit even if the weight is fine. The limit depends on the state and on trailer length, so don't assume the rail's end is legal.
Canadian provinces have their own trailer dimension rules too. If you run across several jurisdictions, set the slider for the strictest one on your route, or check each state's rules before you go.
Slide for legal weight on every group, check every pin, and confirm it on the scale.