Moments on a Beam

This principle considers the importance of where mass is located within or on a trailer.

The principle is fairly straightforward. Everybody knows that on a playground see-saw, a child sitting at one end of the see-saw can balance an adult sitting near the middle on the other side.

In physics, the distance each person on the see-saw is from its pivot is called a ‘moment’ and the effect is called a 'moment arm'. For RV purposes however, we are more interested in how items of the same mass behave differently at different points, or moments, along the see-saw (or beam) when an external force is applied.

The single axle trailer (or twin axles located close together) behaves like the see-saw's ends pitching up and down, with the axle being the fulcrum (pivot), but with the added complexity that the trailer's front and rear may also yaw (sway) sideways under tow as well as up and down.

If we carry (say) a tool box weighing 20 kg directly above a (6 metre long) trailer’s fulcrum (the axle) and the trailer starts to pitch or sway, the tool box behaves like a 20 kg tool box: i.e it has next to no effect.

If we carry the tool box half-way toward the back of the trailer and the trailer then sways, the tool box also moves but with an effective mass of around 40 kg, because it is further away from the fulcrum. 

If we carry the tool box on a 10 kg rack at the rear of the trailer, the tool box will have an effective mass of around 80 kg and the rack an effective mass of around 40 kg.

The tool box and rack in effect becomes four times heavier at the back of a 7 metre trailer compared to in the middle and exerts such additional force when the trailer pitches and yaws (sways).

The effect is similar at the front of a trailer, although the potential for sway here is to some extent reduced by the resistance of the coupling and rear tyres of the tow vehicle.

For this reason it is never a good idea to store heavy items, particularly tool boxes, spare wheels and full jerry cans at the very front or rear of a trailer.

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