Centre of gravity

The stability of a load

Following recurring debates in certain forums on pack animals, we felt it necessary to provide some technical clarification to help novice hikers understand the issue.

First of all, let’s define things clearly: a pack saddle is not stable on its own; it is the combination of the pack saddle and the luggage that is stable or unstable.

Of course, due to their design and shape, some packsaddles are inherently more stable. This is the case with Randoline packsaddles with their downward-sloping arches. The packsaddle wraps around the animal’s body and is difficult to turn, as it is locked onto the coat. However, if you put a large 20 kg bag on top of the packsaddle, even the best packsaddle in the world will lose all stability.

When loading an animal, there is one golden rule to follow: the centre of gravity. If you don’t respect this fundamental law of physics, you’ll end up with tipping and trouble, and you’ll put your poor animal in an uncomfortable position.

To recap: the stability of a load depends on several factors:

• the pack saddle and its harness

• the saddlecloth

• the side packs

• the top packs

• the positioning of the packs on the pack saddle

• The pack saddle and its harness: this is the heart of the carrying system. Its quality is essential to ensure the animal’s comfort while working.

There are many different types of pack saddles. Traditionally, the most stable ones are those with downward-sloping arches that wrap around the animal’s body.

It must be solidly constructed to withstand the weight of the load, constant swaying and impacts from obstacles.

The harness is an important part of the pack saddle. The strength of the straps, their attachment to the pack saddle, the number of adjustment buckles and the quality of the leather are all essential.

• The carpet: the pack saddle rests on a carpet, which acts as a cushion between the animal and the pack saddle’s skids.

The mat also softens the difference in shape between the saddle on the back and the shape of the pack saddle’s pads.

It must be thick enough to soften the weight of the equipment, but not too thick so as not to unbalance the load.

In some older packsaddles, the carpet was sewn to the packsaddle, so that the two elements became a single object. Randoline is extending its range of equipment to include this concept, as it gives the luggage remarkable immobility and stability.

• Side luggage: In the equestrian world, saddlebags, rucksacks, baskets and even hard cases are often hung on either side of the pack saddle.

For obvious reasons of balance, the weight of this side luggage must be more or less identical. This calculation must be done constantly if you open a bag during the day to add a tin or a large loaf of bread. If you do not pay attention to this detail, you will see your pack slowly tilting. This phenomenon will be less pronounced if you have a pack with descending arches. With a cross-bar pack saddle, the penalty is immediate as soon as you hit the first puddle.

To improve the stability of a load, you can lower the position of the saddlebags on the pack saddle vertically, either by lengthening the suspension straps or by lowering the attachment points along the arches.

• Top luggage: Hikers often place excess luggage that does not fit in the panniers on top of the pack saddle. This third pocket is often shaped like a large tube called a duffel bag.

Caution: this is often where stability problems begin. As long as this top bag contains light items (sleeping bags, pyjamas), everything will be fine. But if you start to pile heavy items (boots, cooking equipment, tinned food, etc.) into this bag, you will destroy the stability of the pack saddle.

This point will be developed further in the section on centre of gravity.

• Positioning luggage on the pack saddle: Each item must be in its place and, if possible, should not be moved. Nothing is worse for the stability of a pack saddle than items swinging around in all directions.

This lack of secure fastening will also cause discomfort to the animal. In the worst case, rubbing could cause injury.

So, before setting off, fasten everything securely.

• Centre of gravity: The diagram opposite sums up better than any words the balance that must be achieved to prevent the pack saddle from tipping over.

The centre of gravity of each pannier must be as low as possible, either by placing the heaviest items at the bottom or by lowering the vertical position of the panniers.

The overall centre of gravity, which is the component of the centre of gravity of the saddlebags (plus, incidentally, the weight of the pack saddle), must be approximately 20 cm BELOW the spine.

If this rule is followed, the pack saddle will not move and will not be able to tip over. Whenever it tends to lean, the law of gravity will bring it back upright.

This is how the mule drivers of old worked, as did the military mule companies. The loads they placed on the mules were enormous, but perfectly balanced. For example, a machine gun mount on top was offset by two 40 kg ammunition boxes on each side.

Incorrect loading:
a very heavy bag on top of the pack saddle

In the example shown in this sketch, if the third bag contains duvets, nothing will happen.

But if this third bag is heavy, even the slightest difference in weight between the bags will cause the whole load to start tilting.

If you don’t intervene immediately, the entire load will tip over.

This phenomenon is accentuated by what is known as the lever effect. In this sketch, the bag on the left, moving away from the animal’s body, will ‘weigh’ more heavily and accentuate the tipping.

Summary table of loading options