I am using 3 point harnesses anchored at the mounts in the boot section behind the rear seats.
The MGB does not have a crumple zone so crash forces are largely transferred to the occupants. If you restrain the occupant further with point harnesses the load is increased on the head and neck. But you don't hit the dash!. You pays your money and takes your choice!
In order to decrease these loads I have designed a progressive restraining system. Based on remote anchoring the eye rings on a plate to give a resistance as the plate bends and finally locks up over about 3 inches of movement.
6
inch x 6 inch x 8mm H30 machine alloy plate
This plate is hard to bend even using a professional press, a V block and a solid steel bar were used to form the curve. We had to anneal the plate as our first attempts did not touch it.
Original Style mount
The original mount is not strong enough to cope with the bending forces caused by off-setting the leverage point. So I cut along the top edge, folded it all the way back and ground off the original captive nuts.
6mm steel plate welded under original mount
Using the original mount as a template, I made a 6mm steel plate, with holes and captive nuts aligned with the original. This was seam welded in behind, and the original folded down and welded over the top. Might have been easier to cut the original off and replace it entirely.
The plate was cut about two thirds of the way down the middle to allow the system to work even if only one occupant is in the car.
The plate will bend through the wooden boot cover, which should cause little resistance. But I may cut a square out of the cover to give a clear run.
How thick, wide, leverage and grade of metal all effect the resistance of the plate to bend. Too stiff and it wont work, not stiff enough and it will lock out before its done its job. No destructive testing was not done to see what kind of force is required to bend the plate, its all best guess on the principle that something is better than nothing!
Hypothetically if it takes 1/2 ton (500kgs) to make the plate move and my upper torso weights about 50kgs, it would become effective at anything over 10 g's. If anyone has a better theory than that let me know?
