Force on Seat Belts During a Crash

[John Rogers]

Since we have a lot of autocrossers, racers and those who just like to drive fast I did some calculations on the force the driver gets if there is a crash and it is pretty scarey. The formula is: F = (-1/2 * m * (v * v))/d
Where 'F' is the force of the crash, 'm' is the car's(or driver's) mass = object weight / 32, 'v' * 'v' is velocity (ft per sec) squared and 'd' is stopping distance. I calculated that if I (220#) am traveling at 90 mph and it takes the car 1 foot to stop (crumple) and my belts stretch 6 inches that I have:
F = (-1/2 * 220/32 * (132 * 132)) / 1.5 = 39,930 ft pounds of force = 19.965 tons! Some handy numbers: 30 mph = 44 ft/sec, 60 = 88, etc and the 32 is the gravitational constant to convert weight to mass. Finally tests have shown that racing belts, even if tight can stretch 6 inches in a crash. This is why we inspect the belts for tears, make sure they are not out dated and very solidly mounted!