The horse harness basically consists of three parts:
the part to pull with: collar with hames or breast collar and traces
the part to hold the vehicle on its shafts: pad with tugs and girths
the “breaking system”: breaching
Every harness-part has been developed and adjusted to the most “perfect” system that enables us to put a horse to a cart and drive safely enough.
The main types used are collar and breast collar.
Modern horse driving sport helps to develop anatomically even better fitting and more effective gear.
One result of this is the brollar, a very good invention.
This is a successful combination of collar and breast-collar that fits a horse exceptionally well.
How does physics of collar and breast collar actually work?
The following illustrations will help to explain it.
Let's use a collar first and see, what happens:
G is the centre of gravity of the horse and
D1 and D2 are the ideal points of draft of a collar.
Their position is depending on the different centres of resistance, W1 and W2.
Looking at this graphic it is easy to understand that a horse can pull best from a point of draft just in front of its shoulder joints.
This point is found to be in line with its gravity centre and the drawn vehicle's or object's centre of resistance.
In case of an object on the ground, e.g. a timberlog, the ideal pulling point is located at D1 in front of the shoulderjoint.
It is in line with the gravity G and the timberlog's centre of resistance, W1.
If the horse pulls a vehicle with a higher-up located centre of resistance W2, the ideal point of draft D2 moves a bit down, towards the point of the shoulder.
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Now have a look on a horse with a breastcollar:
C is the point of draft of a breast collar.
If that horse pulls a vehicle with a relatively high centre of resistance W2, the point of draft C for a breast collar is nearly in the same area as it would be with a collar.
It is quite near the ideal point of draft.
But if it would have to pull the timberlog with the centre of resistance W1 from the ground, the point of draft C theoretically would slip up towards the windpipe and the horse would choke. (see green line).
Therefor a breast collar is not good to use for pulling heavy objects low down or lying on the ground.