Very heavy duty quick-releases are available from theatrical rental outfits.
They're frequently used in special effects. There was even a close-up and
description of one on "Mythbusters", where they did exactly what you plan to
do, complete with Buster the myth-busting crash test dummy.

DON'T cobble up something yourself, unless you have the liability insurance
to cover you. It isn't worth it for the price of a rental.

LLoyd

"TJH" wrote in message
ups.com...
OK, perhaps this might sound strange, but I'd like to drop a car from a
crane.

I'm thinking about doing a project for a high school senior class to
show the violence of an auto crash (educational for prom week) at
roughly 30-45mph. In order to do this, I'd hoist an auto by a crane up
say 40 - 50 feet, and then drop it to the ground. But, to do this, I
need some type of quick release mechanism, because the crane owner
isn't going to risk damaging his equipment by free-falling the cable.