N891JF Shoulder Belt Mount

created January 2014

When I bought N891JF, the shoulder belt attach points were located on the narrow shelf just behind my shoulders. The harnesses came over the shoulders and then dropped 3.5" to the shelf, at an angle of about 45 degrees. The recommended angle is something like 30 degrees ABOVE the shoulders, and 5 degrees below as a minimum angle. This is because in a frontal impact, your body is going to move forward, and as the shoulder belt tightens, it's going to stuff your torso down and compress your spine...not good! So I moved the belt attach point back 40" (two bays), so now it's a 5 degree angle (inverse tangent of 3.5" / 40" = 5.0)

The idea is to build a "hardpoint" in the junction between longeron and crossmember, and install an aluminum bushing (to spread the load to the spruce without having to use a heavy a 5/8" bolt). I've also doubled the cross piece (not shown yet here). I'm trying to include as much structure as possible. T-88 was used, of course, but this photo was a dry fitup. I never clamp T-88 tightly...it's better to lightly clamp it to allow the T-88 to penetrate the wood as deep as possible, rather than squeezing it out.

Here's the "assembly", but it will be epoxied in place one piece at a time, but all within a few minutes so no epoxy will intefere with another part. Total weight for both sides is 10.2 ounces.

Here are the pieces. You get the idea. This is 12 ply 1/4" Finnish plywood and a spruce block for the aluminum (6061-T6) bushing . the bushing is drilled out to fit a 5/16" bolt.

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