In a 5-point harness the sub belt's
function is to keep the lap belt in place while it is being pulled up by
the shoulder harness tension loads. It is not intended to restrain one by
one's crotch. It should be approximately vertical to serve this function,
because before it becomes effective it must align with the pull of the
shoulder harnesses. If it wraps under the body it is useless. A 5-point
harness is appropriate for the upright seating position of track driven
street cars. Unfortunately relatively few street seats provide the
correct provisions for a sub belt (my Recaro SRD's are an exception).
A 5-point harness will not work properly with an extremely reclined
seating position. Imagine the extreme case of a fully reclined position.
In this case the lap belt and shoulder harness serve no function but to
hold the body down. The occupant will slide out from under the lap belt.
In this case a 6-point harness acts like a parachute harness and it
restrains the occupant by the pelvis. For a 6-point harness to function it
must wrap under the legs and not straight down.
In a street car the upright seating position renders a 6-point harness no
more effective than a 4-point harness. A 4-point harness is dubious when
it comes to motorsports as the shoulder harness pulls the lap belt up and
off the hips; however this tendency is somewhat reduced if the lap belt is
installed at a 45 degree angle to horizontal (as it should be in all cases
anyway). The 45 degree angle means that a forward load on the driver
results in a downward component on the belt, helping keep the belt on the
hips. This works well for a 3-point restraint where the shoulder harness
load is put into one of the lap belt anchors directly. With a 4, 5 or
6-point system the shoulder harnesses upset things by introducing an up
load into the middle of the lap belt.
On the question of belt lengths - belt stretch reduces the loads
experienced by the occupant. Obviously carried to an extreme stretch
amounts to loss of restraint if it allows the occupant to strike other
parts of the car. Just as extremely stiff chassis increase the loads the
driver sees in a collision, so do stiff (ie short) belts. The Schroth
belts I use incorporate a short section of webbing in the shoulder harness
that is doubled over and stitched to allow controlled elongation (energy
absorption) in an impact. Kevlar webbing could be used to essentially
eliminate all elongation but it is not as the elongation provided by nylon
or polyester webbing is a positive feature. Modern street cars
incorporate load limiters that provide this same function by controlled
deformation under load.
James Mewett
89 RX-7 Turbo II
jcm@airtechcanada.com
