Weight Distribution in a Trailer: Axle Loads Made Simple

Weight distribution in a trailer is about where the cargo's weight sits along the length and across the width. On a semitrailer the load is shared between the kingpin (carried by the tractor) and the trailer axles, so moving heavy goods forward loads the tractor, and moving them back loads the trailer axles. A good plan spreads heavy items evenly, keeps them low and on the centre line, and checks every axle group against its limit, not just the total weight.

A truck can be under its gross weight limit and still be overloaded on one axle. That is a common reason for fines at roadside checks, and it also hurts braking, steering and tyre wear.

Why weight distribution matters

• Legal limits per axle. In most EU countries, Directive 96/53/EC sets 11.5 t for a single drive axle and limits for tandem and tri-axle groups that depend on axle spacing. A tri-axle trailer group with axles spaced more than 1.3 m and up to 1.4 m apart is commonly limited to 24 t. National rules differ and some countries enforce lower or higher limits domestically, so check the rules where you drive.
• Steering and traction. Too little weight on the tractor's drive axle means poor traction in wet or icy conditions; too little on the steer axle makes steering light.
• Braking and stability. A load concentrated at one end or one side changes how the vehicle brakes and corners and raises the risk of a rollover.
• Wear and damage. Overloaded axles wear tyres, suspension and the trailer floor faster.

How a semitrailer shares the load

Think of the trailer as a beam resting on two supports: the kingpin at the front and the centre of the trailer axle group at the back. Cargo weight is shared between them in proportion to its position, like a see-saw:

• Share on the trailer axles = cargo weight × (distance from kingpin to the cargo's centre of gravity) ÷ (distance from kingpin to axle group centre).
• Share on the kingpin = cargo weight − share on the trailer axles.

The kingpin share then goes to the tractor, where it is split between the steer axle and the drive axle depending on where the fifth wheel sits. Moving the fifth wheel forward puts more of the kingpin load on the steer axle. Cargo behind the axle group lifts load off the kingpin.

Worked example: one load, three placements

Say a curtainsider has its kingpin 1.0 m behind the front wall of the load space and the centre of its tri-axle group 9.6 m behind the front wall. The distance between the supports is 8.6 m. These positions are hypothetical; measure your own trailer or take them from the manufacturer's drawing. The cargo is 22,000 kg.

These are cargo-only figures. The trailer's own weight also rests on its axles and kingpin, and the tractor's weight sits on its own axles.

• Even spread: about 14.3 t of cargo on the trailer axles plus part of the trailer's tare. Likely within a 24 t tri-axle limit, and the tractor gets a healthy 7.7 t through the kingpin for traction.
• Front-heavy: 14.3 t on the kingpin. Much of that goes to the drive axle, which also carries part of the tractor's own weight. The 11.5 t drive axle limit is easily exceeded.
• Rear-heavy: 20.5 t of cargo on the trailer axles plus the trailer's tare over them. That can exceed 24 t, while the tractor's drive axle is so light that traction suffers.

Same cargo, same gross weight, three very different outcomes. That is why planners spread heavy pallets over the length rather than loading them as a block.

Partial loads: the most common trap

Full loads of similar pallets tend to distribute themselves. Partial loads do not. Say you have 10 pallets of 900 kg (9,000 kg) in a 13.6 m trailer. Loaded all at the front wall, the cargo's centre of gravity is about 2 m from the front, close to the kingpin, so nearly all of it goes to the tractor. Spread across the length in a pattern of one or two pallets per row, it is shared between tractor and trailer.

Spreading out a partial load has a cost: gaps between pallets need more load securing. The usual compromise is to place blocks of pallets so the overall centre of gravity sits near the middle of the trailer, with blocking or lashing between blocks. See load securing basics.

Side-to-side balance

Weight distribution also applies across the width. A trailer with heavy pallets down the left side and light ones on the right leans to one side, loads one set of tyres more and is more likely to roll in a roundabout. Keep heavy items on or near the centre line and alternate heavy and light pallets in a row rather than loading one side heavy. The centre of gravity guide shows how to calculate the lateral offset.

Practical rules for dispatchers

1. 1Know your vehicle: payload, axle group limits and, for estimates, the kingpin and axle positions.
2. 2Load heavy items first and spread them along the length, low and centred across the width.
3. 3Put light, bulky items on top and in the gaps, never heavy on light.
4. 4With multiple stops, check the distribution after each drop as well. Unloading pallets at the doors at stop 1 moves the centre of the remaining load forward; if those pallets sat behind the trailer axles, removing them even increases the load on the kingpin.
5. 5Estimate axle loads in the plan, and weigh the loaded vehicle when the estimate is within a few hundred kilograms of a limit.