The ISOFIX system holds approved child restraint systems in place with these anchors on the lower rear part. If they rely only on these two anchors, the child seats are secured but in the event of an accident, pitch rotation or tilting forward may occur. This is precisely what the third anchor point of the ISOFIX system seeks to avoid.
The ISOFIX system could be said to comprise two anchors that secure the CRS and that are positioned on the lower rear part of the child seat The hooks are set into the back seats, normally the two side ones, at the point where the seat joins the backrest. Additionally, there is a third anchor, an upper anchor point for securing the upper part of the child seat's backrest (Top Tether) or a Support Leg (in the case of rear-facing child seats). Both the Top Tether and the Support Leg are anti-rotation systems.
What is an anti-rotation system?
In the event of an accident there are two distinct phases. One is that speed and deceleration cause the body and all objects to be thrown forward at same speed at which the car is traveling. When the car comes to a halt the child seat then recoils. This is the second phase. It is at that moment the third anchor point acts and prevents the top part of child seat from tilting or tipping forward. It avoids its rotation.
Although all of the force restraining the child seat is borne by the lower anchors, the anti-rotation system prevents the child seat and the child from being thrown forward.
A seat belt does not provide an anti-rotation system. Bear in mind that using an 'Isofix' system in combination with a seat belt cannot be referred to as a CRS with Isofix anchoring in the full sense of the phrase.
The Isofix system specifically does not use the seat belt in order to avoid errors in its installation.
Types of anti-rotation systems
- Top Tether: This is a third anchor point that forms part of the Isofix system. It is a strap that secures the upper rear part of the CRS backrest to the back of the car seat or even the trunk.
This upper anchor point is on the floor of the trunk or on the rear part of the vehicle's bench seat (see our infograph).
To find its exact location (if there is one) you should either check the vehicle manual or find the following logo:
This is what a Top Tether looks like:
- Support Leg: Another type of third anchor point. It is the alternative to the Top Tether. It is used with Group 0+ and I child seats, both in rear-facing and forward-facing versions.
- It is a leg that needs to be extended until it rests on the floor of the vehicle.
- Lower Tether: This is another system intended to prevent rotation. It is a strap that comes out of the backrest of the child seat and attaches to the floor of the vehicle. It usually attaches to the front seat guide rail.
Some manufacturers such as Römer have also developed other systems to avoid this recoil. For example, Pivot Link. In this case the connectors are not fitted to the bottom part of the child seat but are set at a certain height. This way, in a frontal collision, the rotation of the connectors pushes the child seat downwards thus absorbing the energy. It can also be used with a Top Tether or Support Leg. It has only been developed for built-in forward-facing child seats.