4 point harness to rollcage?

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Is it possible to mount the 2 shoulder straps from a 4 point harness directly to the factory rollbar cross bar? It is exactly the same height as the shoulder bolts that are on the side of the hard top. (out of a 73 FJ40). Or should I weld in a new cross bar a little lower for the shoulder mounting points?
 

John Smith

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You want the mount point as close to your shoulder height as possible. You want the straps horizontal as they come off your shoulers and connect to your cage.
 
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John Smith said:
You want the mount point as close to your shoulder height as possible. You want the straps horizontal as they come off your shoulers and connect to your cage.

Why then do regular shoulder straps rest off the shoulder at like a 45 degree + angle on all other 3 point set ups? I thought that if the strap went down, it would crack your collar bones in a wreck, but the more the straps are angled up, the better.
 

John Smith

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You are correct that you do not want the mount point lower than your shoulder because when in a wreck the force will want to push down and crush you. A picture can be worth a thousand words. This is a picture of a factory built cage used by the BMW factory race team in Germany. It actually does look like the mount point is somewhat above horizontal. But not by much.
cagepic.jpg
 
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John Smith said:
You are correct that you do not want the mount point lower than your shoulder because when in a wreck the force will want to push down and crush you. A picture can be worth a thousand words. This is a picture of a factory built cage used by the BMW factory race team in Germany. Notice how the harness comes from the seat about totally horizontal as it ties into the rear portion of the cage. That is what you want.

Cool. Looks like I need to take out hte welder and run another bar. Thanks.
 

treerootCO

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The ideal position is
anywhere between 5° below
and 30° above the driver’s
shoulder, as seen in part C
of Figure 2.
If the upper attachment
point falls significantly
below the driver's shoulder,
then a spinal compression
injury is likely to occur. In
an accident situation, the
shoulder belts pull down
and back on the torso as
they resist the forward
motion of the driver. The
resultant restraint force
compresses the spinal column
and will add to the stresses in the spine already caused by the force of the crash
impact.
On the other hand, if the trailing ends of the harness are too far above the shoulder
(greater than 30°), then two problems can occur. First, tension in the shoulder harness
is increased and undue stress is applied to the harness and its structural attachments.
Second, excessive angle will cause excessive motion. If the harness belts are too
far above the shoulder, they will provide little resistance to forward motion of the driver’s
upper torso. The result is impact with the steering wheel and the possibility of
neck injury. The shoulder straps should also be 3-6” apart behind the driver's neck to
prevent slippage off the shoulders.
Untitled-1.jpg
 
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slickrock said:
cool. I will use that diagram when I weld in the cross tube for the 4 point. thanks
I'm not sure what you had in mind, but I would consider adding additional tubes at angles to the top and side tubes of the roll bar or maybe an X arrangement, rather than a single tube from one side to the other. Triangles=strong.
 

treerootCO

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Triangles are great but keep them away from your head. What good is a roll cage if you smack your head into it.
 
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