Vehicle recovery is the science of estimating and making the right assumption. The more conservative your assumptions, the safer your operations.
WHAT WE KNOW?
- Trailer is empty
- GVWR of the tractor-trailer combination is 80,000 lbs
- Tank has a capacity of approximately 9000 USG of gasoline. Gasoline weight 6lbs/USG, so the maximum payload is in the magnitude of 54,000 lbs
GVWR – Payload = Tare weight
Thus 80,000 lbs – 54,000 lbs = 26,000 lbs. Does that make sense? 17,000 lbs for the tractor, 9,000 lbs for the trailer… yeah,it does. But why not add a 10% guesstimation allowance?
26,000 lbs x 1.10 = 28,600 lbs
Vehicle is on snow. Rolling resistance on snow is in the magnitude of 1000N-3000N/ton or, in other words, 0.1-0.3 times the weight of the recovered vehicle. Leaning toward the conservative side again, let’s take 0.3.
0.3 x 28,600 lbs = 8 580 lbs, which is the force required to move the unladen tractor-trailer on a flat surface covered with snow.
The moving block tackle used here showed a veer angle of less than 30º and provided a mechanical advantage of 2:1. Computing for veer angle, friction loss and mechanical advantage, we can find the tension in the winch line:
Veer angle less than 30º : each line is 52% of the hook load. Friction loss 10% in the block.
Tension in the winch line is then approximately (0.52 * 8 580 lbs) /0.90 = 4957 lbs.
Chris Niemans used a 7/16 Super Swaged wire rope with a Breaking Strength of 27,200lbs and WLL of 7,600lbs @ 3.55 :1, which was above the required, estimated force of 4957 lbs. As the cable strength is often the limiting factor in the equation, giving yourself room for flexibility is not a bad idea.
Assumptions and estimates are not easy to do and, yes, they can vary greatly from one individual evaluation to another. The idea is to give safety the priority and be conservative with your estimates.
Now, from what you know, can you estimate the tension in the rope when Chris pulls the rear of the trailer sideways? Hint: wheels are not turning. See the table below for more details, or check out this website to help you with you estimation.