The main concern with shoulder belt angles is to ensure that they don't
cause spinal compression in a severe impact - what you really want is
for the shoulder belt strap to be perpendicular to the spine (i.e. to
the seatback when it's in a normal driving position). The lap belt must
be positioned to give good resistance to submarining (which means that
having it nearly vertical is bad - you can slide forwards quite a ways
before the belts begin to offer any restraining action), as well as
keeping you from lifting off the seat in case of a rollover (which means
that having the belts horizontal is bad). Therefore, 45 degrees is
probably the best compromise, and this figure will vary depending on
where you slide the seat on the tracks.
I read somewhere that the seatbelts can stretch a large amount (like 8")
in a crash, and so even if your belts are snug, in the event of a crash
you may find yourself coming into contact with interior parts you'd
never think were in reach. It's definitely important to make sure the
belt geometry is properly set to make sure that you don't first slide a
bunch and THEN start stretching the belts - that can also lead to
further whiplash injuries.
Theo
|