Ken Gano wrote:
>In Dan Poynter's The Parachute Manual he states
>that Type I nylon fabric loses 94% of its strength after three weeks
>exposure to the summer sun (see chart in section 9.3.1.8.15, first edition).
>Of course when you figure that most seat belt webbing is initially rated for
>between 3,000 and 8,000 pound break strength it still not a major problem
>and webbing is much thicker that Type I (the outer fibers shielding the
>inner fibers), but clearly, the safer approach is to replace seat belts and
>to do it fairly often. Certainly well before they become badly sun faded.
>
>
>
Actually, it is a HUGE problem. When you consider the force multipliers
involved with a collision, those faded belts suddenly become very flimsy.
If a 8000 lb belt has lost 80% of its strength due to age and UV
exposure, it is now rated at 1600 pounds.
The question now is how many "G"s are exerted upon the restrained human
body during a collision? It would only take a 9 "G" force to cause a 180
pound body to exceed the breaking point of the degraded belt.
According to research on this matter, the G forces produced in
collisions are incredibly high.
In a sideswipe of broadside collision, the following numbers are generated:
10 MPH - 2 G's
20 MPH - 4 G's
30 MPH - 7 G's
50 MPH - 11 G's
70 MPH - 15 G's
Impact forces in head-on collisions. The speed is that of each vehicle.
20 MPH - 17 G's
30 MPH - 24 G's
50 MPH - 40 G's
70 MPH - 60 G's
In other words, if you have a "mild" head on collision at 40 mph
combined, you ARE going through the windscreen.
So, let's not be screwing around with old seat belts, eh? It's not worth
your life.
Here's a web site with new belts. I used them to put harnesses in my
vintage Mercedes. http://www.andoauto.com/
For those of you more adept than I, here is the force equation used to
produce these numbers.
The impact forces, F, are calculated from the law of elementary
mechanics that relates the force generated by an impact to the rate of
change of momentum:
F = m (vi )/D t (A1)
Where m is the mass of the passenger, vi is the velocity of the vehicle
just prior to the collision, and D t is the duration of the impact (how
long it takes the passenger to come to a stop after the collision). D t
is assumed in a broadside to be a fraction of a second, and in a head-on
collision a few milliseconds.
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