Here are the reasons why a tight harness is a safe harness
In the event of a collision, your body tends, by inertia, to follow its forward movement, and therefore the purpose of the seat belt or harness (hereinafter we will use one term or the other interchangeably) is to prevent this movement in order to avoid injury.
The operation of a seat belt is complex. In this link you can see a chronology of the first milliseconds of a collision. It will help understand how a correctly fastened and adjusted seat belt works. In summary, it can be said that a properly fastened belt opposes the movement of the body, which tends to be thrown forward in the event of a collision, to a certain extent. Once the pressure of the body on the belt exceeds a certain threshold, the belt tension eases slightly to prevent injuries that would otherwise be caused by the harness itself.
The device that achieves this is called a load limiter, and its working mechanism is simple: if the belt is allowed to continue to tighten on the body, without yielding, it will cause injury to the human body. If a certain (slight) relaxation of tension is allowed, the belt will follow the body it is restraining by a few millimetres, allowing the body to further decelerate, but fully protecting the occupant. Now that this is understood, we can detail the reasons why this system works only if the belt is properly tightened against the body before the collision.
Before explaining this, we should mention safety belt pretensioners. There are many types of pretensioners, but their mission is to anticipate a collision by tensioning the belt tightly against the body in a matter of milliseconds. Cars fitted with this system help minimise potential injuries. That being said, if the pre-tensioner is prevented from working properly, the system will not function as it should.
In a situation where the belt has any slack before a collision, for example, if there is a hand placed between the belt and the body, or if a clamp has been used to slightly adjust one of the straps, or if the belt is in a poor state of repair and doesn’t tighten properly, in the event of a collision the pretensioner may malfunction and activate too late, i.e., apply the tension when the body has already moved more than is advisable. In this case, the result is counterproductive.
This is also true if your car does not have built-in pretensioners, as the slack (even if only a few millimetres) will allow the occupant’s body to move before the belt is able to hold it in place. The result is that the harness itself will malfunction and cause injury.
The reasons for tightening the belt properly before setting off are clear: if the seat belt is properly fastened, the pretensioner (if available) will do its job properly; if the pretensioner is working properly, the seat belt will do its job properly; if the seat belt is doing its job, the airbag will deploy at the right time to minimise the impact. In the case of child car seats, keeping the car belt tight (in seats without ISOFIX) will help ensure the CRS itself is properly secured, and in turn, correctly adjusting the harness to the child’s body will help protect him or her better.
Adjusting the belt properly and tightening it to the body only takes a few seconds, less than the time it takes for the engine oil to start lubricating every corner of the engine. It is simply about protecting what matters most to us.