Isofix vs sikkerhedssele/seatbelt


Er isofix mere sikkert i kollsioner ud over at være nemmere at montere rigtigt? Det spøgsmål har jeg stillet Robert Bell, som har over 35 års erfaring indenfor autostole. Hans svar kommer her:


Which configuration is safer in collisions when you disregard ease of installation? I posed this question to Robert Bell who has worked in the child car restraint industry for more than 35 years. This is his reply.

Posted here with kind permission form Robert himself:


Hi Lena

The answer is both easy and a little complicated. The basic idea with ISOFIX
is to reduce the risk of misuse, and we have a few studies that make us
pretty sure that ISOFIX systems do get installed correctly more often. As a
result, they are better in crashworthiness.

But even if we disregard misuse, there are some reasons for believing that
the actual performance of the product improves. The basic goal for child
restraints is to restrain, to keep the child from moving in the vehicle,
while the vehicle structure itself crumples and absorbs energy, retarding
the movement of the passengers in a way which the automotive engineers
design to yield the best chances of reducing injury. You tie the passengers
as tightly to the car chassis as you can, and you let the chassis be the
primary means of protecting the occupants.

This is one of the ways rearward facing systems outperform forward facing
systems, entirely ignoring the issue of head and neck loads. The forward
facing system, typically anchored to seat belts, moves forward in a crash as
the seat belts extend, the plastic in the seat itself stretches, the child
stretches the internal child harness in the seat and the vehicle seat
cushion collapses under the load of the seat. The outcome is that the child
moves forward, close or beyond the 550 mm extension that is allowed in the
standard. What this means is that during perhaps a third of the crash the
child cruises, motion unchecked, forward in space, and the retardation ends
up taking place in a shorter period of time, resulting in a higher load on
the child during the 40 ms or so that remain.

This is just physics, and the rearward facing system exploits the physics of
the vehicle better.

The rearward facing seat, when placed on dash, moves very little in the
crash, perhaps as little as a centimeter or two and as a result the child is
retarded throughout the entire ride down. The loads on the child are
therefore much lower than they would be in a forward facing system. It is
also important to remember that the loads on the fragile neck and head are
reduced dramatically in relation to forward facing systems. But one of the
additional big advantages of those rearward facing systems is that they
restrain more effectively. Rearward facing seats in the rear are
disadvantaged by the rear seat back, which moves more than the dashboard,
but they are still superior to forward facing CRS in this regard.

Now you can improve both systems with ISOFIX. But the improvement is not
very dramatic for a rearward facing system, particularly one that is
installed on dash in the front seat. A forward facing system, on the other
hand, is much more tightly tied to the vehicle in the case of a European
style hard ISOFIX installation, when compared to the traditional seat belt
installed versions. It is also important to remember the contribution of the
upper tether/ floor support in reducing the motion of these ISOFIX forward
facing systems. Some of these features have also migrated to belt installed
systems, so they get a least part of the improvement.

Lastly, in studies performed by the UK Transport Research Lab, improvements
were noted in side impact performance for various types of ISOFIX seats. The
reason is most likely linked to the lateral stiffness provided by the hard
European ISOFIX anchors. These seat move less in side impact and are
therefore likely to perform better in the real world.

How is all this expressed in actual crashes? In the case of the rearward
facing systems we will be hard pressed to note improvements. Excepting fires
and the like, and accidents in which the car is crushed flat by a lorry or
cut in two by a tree, we never see fatalities in the seats now in use in
Sweden. Most every summer we have a dramatic crash in which families are
decimated in frontal accidents, except for the toddlers that are seated in
rf systems. We had one this summer in Piteå, parents killed, two siblings,
one killed and one seriously injured, rear facing toddler unharmed. So we
don’t expect major improvement in the real world of rear facing systems as
ISOFIX becomes more common.

In jurisdictions with forward facing systems, I expect that we will see a
reduction of fatalities and serious injuries among children. Most
specifically, I think we will see a reduction in compressive head and neck
injuries, as these may occur in frontal crashes when the child is flung
forward and impacts the back of the front seats with the head. ISOFIX seats
will reduce the chance of that sort of impact and the resultant injury. My
guess is that ff systems will be very much improved in the real world by the
increased use of ISOFIX equipped CRS.

Does this make sense to you? Let me know if you have questions on this

Robert Bell
Sakta AB
Fleminggatan 67
112 33 Stockholm


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