Anti-rebound systems, a safety element

First of all, there is forward movement of the occupant and the restraint system until it comes to a complete standstill. This phase is called the loading phase. In this phase there are 2 characteristic movements, one is forward, as we have already mentioned, and the other is a rotation, once the car’s restraint systems have done their job.

Once the restraint and occupant have come to a stop within the loading phase, a reverse movement begins, mainly characterized by a backward rotation. This is the return or rebound phase.

The concept of phases is applied regardless of the car restraint system used (seat belt or ISOFIX) or the child restraint system, with greater or lesser emphasis on rotation or rebound. Child seats must therefore have mechanisms that can control all the movements derived from each phase.

Now that we know the phases of a crash, we are going to focus on the solutions applied that try to eliminate (or at least reduce) these effects. With the creation of ISOFIX anchors, it was realized that the system needed a solution that would prevent it from rotating forward. There were various proposals, but the Top Tether (TT), a strap connected to the vehicle’s backrest that limits its rotation, was finally defined as an anti-rotational system.

This system is very effective in controlling rotation in forward-facing seats, but much less so in rear-facing seats. This is why many brands started to use a support leg instead of the TT, first in infant carriers and then in other seats.

We now have several systems to control the various movements in the loading phase: the seat belt and ISOFIX for the forward movement; and the TT and support leg for the rotational movement.

As we mentioned at the beginning of this article, an impact involves a rebound phase. At the beginning of the 2000s, with the arrival of the ISOFIX anchoring system in the standards, it was noticed that child seats had an exaggerated bounce, mainly due to the fact that the child seat was fixed to the vehicle by only two points, the ISOFIX anchors.

To control this, an anti-rebound system was included in the child seats. This phase causes the seat to rotate against the car seat, which can cause injury if not controlled, especially in the case of rear-facing seats. The first rebound phase controller was the handle of the infant carrier. Even today, many baby carriers still position the handle against the car seat to control the rebound phase.

Reverse-facing ISOFIX car seats generally use 2 systems to prevent bouncing. Firstly, a anti-rebound bar, which is placed in contact with the car seat and effectively controls the rearward movement of the child’s seat.

Another system used is the so-called “lower tether“. This system comprises two straps at the front of the child seat, which are connected to a fixed point on the front seat of the car. These straps prevent the seat from rebounding even more effectively than the anti-rebound bar. The only problem is that they are more complicated to install.

The non-rebound bar must be pressed against the seat with no slack, while the lower tether must be correctly tensioned and fixed to a rigid point of the car frame or front seat. Bear in mind the fact that in the case of the lower tether any variation in the tilt will require re-tensioning.

In any case, all these systems effectively prevent the rebound of the seat in the event of a collision and are, therefore, safety elements that we must pay close attention to, to make sure they work correctly.