as a flight engineer on a chinook i know a bit about PROPER securing, we have to secure EVERYTHING in case a crash sequence takes place. straps and chains work as follows, there is a rated value that the military uses that is based off of manufacuter recomdations for their straps. the best angle for straps is 30 degrees from the vertical, at that angle you get 75 percent of the straps rated value in a forward direction and 25 percent in the up direction, seeing that our cars are not flying in their trailer, only the forward number really matters to us, also the average weight is lets say 3,000 lbs, most people use 4 straps rated at 10,000 and hopefully the tie down points are rated at the same, so you have 20k of rated value on both ends of the car, but the adjusted value is only 15,000 lbs due to the strap angle, sound like alot, but in a crash it is easy to puch 7 g's in an impact, 7 times your 3,000 lb car is 21,000 lbs, the straps have just broke and your car is not flying towards the front of the trailer, its is a good idea to double the rear straps ad reinforce the tiedown points,i have. and all these values are based on NEW stuff not your oily and beat up ones and rusted trailer bottoms. because if all we straped our cars down for was easy driving we could just put them in park, the straps are there for the bad days, but this is just the ideas that i have, everyone has there own.
Grant