Seat Belts (and Physics!)

Most of today was spent installing the front seat belts. The bus does have factory mounting points since seat belts were an option from the factory, but the factory mounting points are for lap belts only. It’d be perfectly legal to keep lap belts only, but with a metal dash and sitting so close to the windshield I see shoulder belts as a necessity. To add the shoulder belts I had to add a 3rd mounting point to the B pillar on each side. The 3rd mounting point consists of a nut welded to a chunk of 1/4″ steel plate, a slot is then cut in the B pillar, the plate & nut installed, and the slot welded shut. It’s important that the mounts are strong, time for some physics:

Stopping in 2ft from 45mph (a crash) creates a deceleration, which can be calculated as follows:
(EndSpeed)² = (InitalSpeed)² + 2(acceleration)(distance)

45miles per hour = 20.1168meters/sec
2feet = 0.6096meters
0 = (20.1168m/s)² + 2(acceleration)(0.6096m)
(20.1168m/s)²= -2(acceleration)(0.6096m)
404.6856m/s²= -1.2192m*(acceleration)
(acceleration) = 404.6856m/s²/-1.2192m
(acceleration) = 404.6856m/s²/-1.2192m
acceleration = -331.9272m/s²
331.9272m/s² = 33.83G’s

When decelerating at 33.8G’s, a 200lb mass (i.e. a person) will impart a force on the seat belt that’s calculated as follows:
F=M*A
200lb = 90.7185Kg
F= 90.7185Kg*331.9272m/s² = 30111.9377N
30111.9377N = 6769.4329lbs = 3.3847TONS

Assuming the force is spread evenly across the 3 mounting points, each point would need to withstand a little over a ton of force in this scenario. Granted, you wouldn’t want the mounts to be right at their breaking point, so I estimate each point should be able to withstand at least 5 tons. With 1/4″ plate steel I believe this has been achieved (and then some).

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