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KINETIC ENERGY and RECOVERY

by Takeo De Meter

K.E.R.- (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)

Breaking stretch: 100 %
Elasticity modulus: 2.5 kN/mm^2

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.
NEVER, under any circumstance, use a Polyamide (Nylon) rope with a diameter less than 50 mm !
Polyamide ropes have the tencency to act like a rubber band, with some Polyamide types having an elongation possibility of up to 60% before breaking. That is one hell of a slingshot that may hit the puller or the pullee or anyone standing in-between.
Of course, it is a "comfortable" pull and many like the soft feel of a Nylon rope stretching. Some people also like the soft pull on a gun trigger.

K.E.R. rule # 2.
Use a Polyethylene (PE) or Polypropylene (PP) rope of sufficient diameter (40-50 mm) in a length of about 1
5 metres.
Polyethylene has some "stretch" in it, enough as not to act as a wire rope, and enough to give you a good "feel" of what you are doing. If you use Polyamide you dont feel Jack Shit. Polyethylene ropes are usually colored bright orange.
Polypropylene is a bit stiffer, but is as strong as PE. I prefer to use it on a capstan winch because of its better fibre distribution within the rope. Polypropylene are often found in green, brown and black.
The lesser stretch will also give a much more effcient energy transfer from one vehicle to another instead of wasting this same energy in the stretching of a rope that then acts as an energy absorber / damper, rather than an energy transfer device.

K.E.R. rule # 3.
One thing NOT to do when practicing K.E.R. is breaking transmission parts. The L-R drive train is as good as any and better than most, but it is designed to move the vehicle under its own power over any terrain. It is NOT designed to absorb the shock loads incurred in K.E.R.
The philosophy of K.E.R. is to utilize the energy stored in a moving vehicle by transferring it to a stuck vehicle by means of a rope, applying a shock load to the rope and the stuck vehicle, through the ROPE, not through your transmission.
This means that you give your tow rope as much slack as possible, launch your vehicle (the puller) at the maximum speed obtainable within the length of your tow rope and applying the momentum, gained by the mass of the pulling vehicle to the one stuck in the same boghole that you have been trying to avoid all day. Adding some road traction to the moving mass of the vehicle sounds theoretically good, but most trannies and half-shafts are simply not up to it, neither are central diffs.


SO,
try this: attach your vehicle to the sad looking guy's vehicle in the bog by means of a tow rope of suitable length. Drive backwards as near as possible to to the stuck vehicle and then slowly stretch your tow rope by advancing your vehicle. When the rope is stretched, look for a recognizable mark on the ground just below your side window, or make a mark with your boot yourself. Now you know when and where the rope will be stretched. Back up your vehicle as far as possible again and launch it at the maximum speed you can get. BUT, before reaching that mark on the ground, push in your clutch pedal as fast as you can and as far as it will go, so that when the load is applied to your vehicle, it is not going to your transmission. Then you will be using the kinetic energy stored in your vehicle only and you may make some substantial savings on half-shafts, central diffs and gearboxes in general.

K.E.R. Rule # 4.
If the above does not work, try winching the sucker out. If winching don't work, go get a bulldozer or a tank to pull him out. And if this does not even work, leave it where it is, take the top off and plant some geraniums in it.

Have fun and work safely !


Annette & Takeo.

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.