Trailer tandems are the pair of axles located near the rear of a semi-trailer. On many trailers, the tandem assembly can slide forward or backward along rails underneath the trailer, allowing the driver to adjust how the load’s weight is distributed across the tractor and trailer axles.
Proper tandem position matters because commercial trucks must comply with axle and gross vehicle weight limits. Moving the tandems can shift weight between the trailer tandems and the tractor’s drive axles, helping drivers balance a load before heading onto the road.
For shippers, understanding trailer tandems is useful when planning heavier loads, loading a trailer, and avoiding weight-distribution problems that can delay a shipment.
What Are Trailer Tandems?
Trailer tandems are a group of two axles positioned close together near the rear of a semi-trailer. Together, the axles support and distribute a portion of the trailer and cargo weight.
On a typical tractor-trailer, weight is distributed across three primary axle groups:
- Steer axle: Located at the front of the tractor.
- Drive axles: Located at the rear of the tractor.
- Trailer tandems: Located underneath the rear portion of the trailer.
The position of the trailer tandems affects how much weight is carried by the drive axles versus the trailer axles. That’s why tandem position becomes especially important when hauling heavier freight.
How Do Sliding Trailer Tandems Work?
Sliding tandems allow the trailer axle assembly to move forward or backward along rails underneath the trailer. A locking-pin system secures the tandems in position once the desired placement is reached.
Moving the tandem position changes how the trailer’s weight is distributed between the tractor’s drive axles and the trailer tandems.
In general:
- Sliding the tandems forward shifts weight from the drive axles toward the trailer tandems.
- Sliding the tandems backward shifts weight from the trailer tandems toward the tractor’s drive axles.
This adjustment can help correct axle-weight issues after a loaded truck is weighed.
The exact amount of weight shifted per hole can vary based on the trailer’s tandem-hole spacing and configuration, so drivers should not assume every trailer shifts the same amount of weight with each hole.
Why Trailer Tandem Position Matters for Shippers
Trailer tandem position may seem like a driver or carrier concern, but improper weight distribution can create problems before freight ever leaves the facility.
For shippers, proper loading and tandem positioning can help:
- Avoid overweight axle issues: A shipment can be within the gross weight limit while still placing too much weight on an individual axle group.
- Reduce rework: Improper weight distribution may require the driver to adjust the tandems, return to the dock, or have freight repositioned.
- Prevent shipment delays: Weight problems discovered at a scale can interrupt transit and affect delivery timing.
- Improve load planning: Knowing the freight’s weight and how it is distributed inside the trailer helps carriers prepare the equipment correctly.
Tandem adjustment can correct some weight-distribution problems, but it cannot compensate for every poorly loaded trailer. In some cases, the freight itself must be repositioned.
What Happens if Trailer Tandems Are Positioned Incorrectly?
Incorrect tandem position can cause too much weight to rest on either the tractor’s drive axles or the trailer tandem axles.
Potential problems include:
- An overweight axle group even when total gross weight is within the legal limit.
- The need to reweigh and readjust the trailer before continuing.
- Shipment delays if freight must be repositioned at the loading facility.
- Handling and stability concerns caused by poor weight distribution.
- Weight violations or enforcement issues if the vehicle operates above applicable limits.
Drivers commonly verify axle weights after loading, particularly when hauling heavier freight or loads where weight distribution may be uncertain. If an axle group is overweight, tandem adjustment may help redistribute the load.
How Loading Position Affects Trailer Tandem Weight
Where freight is positioned inside the trailer directly affects axle weight distribution. Freight loaded too far forward can place excessive weight on the tractor’s drive axles, while freight concentrated toward the rear can place more weight on the trailer tandems.
That’s why load planning matters before the trailer doors close. Shippers should have accurate freight weights and load freight with axle distribution in mind, especially when the shipment approaches legal weight limits.
If sliding the tandems cannot bring the axle weights into compliance, the freight may need to be repositioned inside the trailer before the truck can continue.
How Trailer Tandem Issues Can Delay Freight
Weight-distribution problems can create avoidable delays during transportation. If a driver discovers an overweight axle group after leaving the shipper, the truck may need to stop for adjustments, reweigh the load, or return to the facility if tandem movement alone cannot correct the problem.
Providing accurate shipment weights and loading freight with axle distribution in mind can help reduce these issues before the truck leaves the facility.
Shippers should also understand the difference between gross vehicle weight and axle weight. A truck may be under the total allowable gross weight while still having too much weight concentrated on one axle group.
For a deeper look at truck and axle weight requirements, see our guide to truck weight limits.
Trailer Tandems FAQs
What are tandems on a trailer?
Trailer tandems are the group of two axles located near the rear of a semi-trailer. They support part of the trailer’s weight and cargo and, on sliding-tandem trailers, can be repositioned to adjust axle weight distribution.
What are tandems on a truck?
In trucking, “tandems” generally refers to two axles positioned close together. A typical tractor-trailer has tandem drive axles on the tractor and tandem axles underneath the rear portion of the trailer.
Why do truck drivers slide their tandems?
Drivers slide trailer tandems to adjust how weight is distributed between the tractor’s drive axles and the trailer axles. This can help correct an overweight axle group and balance the load.
Does sliding tandems change the total weight of the truck?
No. Sliding the tandems redistributes weight between axle groups; it does not change the truck’s total gross weight.
How much weight can trailer tandems carry?
Federal law generally limits tandem axle groups on the Interstate System to 34,000 pounds, subject to Federal Bridge Formula requirements and applicable exceptions. State, route, permit, and equipment requirements can also affect allowable weight.
How ShipEX Logistics Can Help You
Proper weight distribution starts before a truck leaves the shipping facility. Accurate freight weights, appropriate equipment, and thoughtful load planning can help reduce overweight axle issues and avoid unnecessary delays.
ShipEX Logistics works with shippers to coordinate equipment and transportation based on the requirements of each load. Whether you’re moving standard truckload freight or a shipment with more specific weight and equipment considerations, planning those details upfront can help create a smoother move from pickup through delivery.
Learn more about choosing the right trailer for your shipment.
