by Takeo De Meter
(a.k.a. "Snatch" recovery).
Basic Physics: What is Kinetic Energy?
Kinetic energy can also be defined as mass * height * gravity . The easiest example for this is a seemingly harmless bucket full of bolts and spares stored on the top shelf in your garage, being full of static energy. As long as it stays there, it is completely harmless. BUT when this innocent looking bucket of bolts is put in motion by, for instance, the Land-Rover Laws Of Gravity, static energy in this bucket is immediately transformed into Kinetic Energy, which can be a killer. Ever had a bucket full of bolts fall on your empty, bald and worthless skull ? No ? Well, 20 kg of bolts, dropping from a height of 2 meters, accelerating at 9.6 m/s^2 does a lot of harm. Believe me. So what has a bucketful of bolts to do with a nylon rope smashing your windshield ? Lemme explain.
Another definition of Kinetic Energy is m/2 * velocity^2. Example: the golf ball that your neighbour sliced through your drawing room window and straight onto your still empty skull. Owch. Yes. Another example of the effect of a kinetic energy application is the use of a Porsche as your front crumple zone. Heheheh. See ? We are slowly getting there.
A third definition is 1/2 Force * length. Also known as a Knuckle Rapper. You are better off breaking off a bolt head with a short 1/2 " spanner than that larger 1 1/2: bolt where you had to use a 3 ft length of plumbing pipe on your wrench to get some movement into it. In the latter case it is usually your own fist that sells you an olympic uppercut.
Now I had the pleasure to attend the Bramsche Land-Rover meet in Germany in September and I must say that I thoroughly enjoyed it. Friendly people, nice trucks, good atmosphere and a nice little playground with sand and water where I witnessed some of the most suicidal attempts at Kinetic Energy Recovery that I ever saw. Gawd, I love the sound of nylon rope slapping a windshield in the morning.
So, Annette and I dug into our old school books and came up with the following:
"Nylon" is the commercial name by DuPont De Nemours for their polyamide polymer. NY = New York and LON comes from London.
Polyamide ropes are either PA6 made from caprolactam or PA6.6 made with adipic acid as a base. These are the polymers that your so-called snatch straps or ropes are made of.
Some properties: (averages)
stretch: 100 %
Using these basic figures and a length of 10 metres by a diameter of 25 mm for the sake of the argument, the energy (W) set loose by the breaking of such a rope is in the order of magnitude of 75 tons, when using a 2-ton vehicle. This means that by then your 10 metre rope has a length of 20 metres while snapping back at you, possibly carrying 2 or 3 kg of the other Land-Rover with it. So the word is : "DUCK !"
On the other hand, using a rope with much less stretch, such as a polyethylene rope, this danger is greatly diminished and the energy is much more effciently transferred to the other vehicle.
While winching, the 0.2 % stretch of wire rope at breaking point is neglectable as a potential danger. Winching wire ropes snapping back are usually caused by another factor such as deformation of the object being winched out.
K.E.R. rule # 1.
Now what do you do with these useless straps once you found out they're not to be used for recovery?
We recently bought a brand new washing machine. It works fine, but it just did too many square dances on the scullery floor. So we bought a 12 mms drill, two stable rings and a five ton ratchet strap. It works.
Our clothes did land safely ever since. Even the cats aren't scared stiff any more of the dancing washing machine.