Recovery using snatch straps (snatching) is extremely dangerous if not done properly!. I will explain some fundamental physics involved, in an attempt to highlight the dangerous forces experienced.
During snatching, the momentum of the recovery vehicle is converted into strain energy, stored in the snatch strap. It's like we are using the recovery vehicle's engine to 'wind-up' the strap. What this means is that all the energy that the fire-breathing V8 recovery vehicle has produced in those few seconds of acceleration is now stored in the rope ready to be released at an instant.
The technique of snatch recovery is, in principle, very simple.
When to use it:
This technique should only be used as a last resort, as it is probably the most dangerous technique, and there have been some nasty, near fatal incidents. The pulling force that can be generated far exceeds that of any winch or conventional pulling - even if the recovery vehicle is on slippery ground.
Necessary Safety Precautions:
So why is snatching so dangerous?
As mentioned in the beginning, a huge amount of energy is stored in the snatch strap. The energy stored is half the vehicle mass multiplied by the square of the vehicle speed. Therefore, the forces exerted at say 20km/hr, are 4 times those exerted at 10km/hr! Our desire is to use all this stored energy to pull the bogged vehicle out of its mud-hole. But what if it doesn't?
The main dangers in order of magnitude are:
Some horror stories:
Some time back in SWA, now Namibia, an army tank attempted to snatch out a ldv. The result was that the chassis and axles stayed stuck, and the body was ripped off the stuck vehicle.
Then there was the story of a light vehicle (details are sketchy but I believe it involved a Suzuki), which tried to dislodge a Land Rover. He set off at pace, but was unfortunately catapulted back into the still stuck Land Rover, resulting in two broken vehicles, one which remained stuck!
I read a story of a Range Rover winching out a badly bogged vehicle. "The driver correctly insisted that his passenger leave the vehicle and stand well back. The winch cable was simply hooked over the tow ball of the stuck vehicle. At maximum stress the tow ball snapped and the cable with the round ball attached tore through the Range Rover, cutting through the roof and splitting the front passenger seat in two. The Range Rover was declared a write-off. Because of other precautions taken nobody was hurt. Tow balls are mild steel - not the correct material for high-stress pulling." This was winching, not snatching, which has higher forces!
Another story from the web: A Land Rover was stuck and no amount of winching would make the bogged landy budge. "Digging was impossible as the mud was too fluid and Hi-Lifting was impossible. So they went for snatch pulling. Even the most violent acceleration brought no results. They then decided to use two ropes (to double the length) with the result that the towing Landy reached speeds in excess of 60kph before the rope slack was taken up! Just as the Landy started to lose the battle against the increasing tension of the rope an appalling impact and what sounded like a rifle shot was heard. The tow rope seemed to have vanished. What had happened was that the towing point of the stricken Landy was pulled right out of the chassis and catapulted at awesome speed towards the towing landy. It went right through the rear door, the bulkhead and through the front windscreen, scattering bits of glass and aluminum all over the place. The towing point had actually passed within a few inches of the driver's head!! He was wearing a helmet but it is doubtful what protection that can afford against a 6 lb supersonic towook!"
A Land Rover was "de-bogging a Sammy. What happened this time was simply that maximum brute force was used right away. The Landrover accelerated about 20 feet to approx 20Mph before the rope started tensioning. All of a sudden the Sammy catapulted out of the ground, flew a distance of approximately 25 feet and then came crashing into the roof of the Landy just above the level of the tailgate. The only thing that prevented the driver of the Landy from getting killed was the substantial roll cage. What went wrong here was very simply that maximum brute force was applied first time. There was probably four times as much energy in the rope as was needed to de-bog the 'Zuki."
The bottom line is that snatching is a great way of recovering otherwise unrecoverable vehicles. If done with care, it is safe but if not, can be lethal. It is best to try winching, hi-lifting and digging first.
Now let's have a look at the theory, now that we know what can happen:
Let's base the calculations on my Land Rover 110 tdi. It's weight is 2300kg, and can accelerate to 17km/hr within 9m in 2nd gear, low range.
8 ton snatch straps break more or less at 8 ton force (no safety factor! More of this later). The stretch factor of 20% is quoted at 50% force (4 tons for our example). Some snatch straps break at a lower, some at a higher force. The straps are 9m long. 20% relates to 1.8m, and assuming a linear spring constant in the elasticity of the strap, elongation, or stretch, will be 3.6m at a force of 8 tons.
Now F= k x s, where F = force (Newtons), s = distance (stretch) and k = spring constant.
Therefore k = F/s = 4000kg x 9.81 /
==> k = 21800N/m
To calculate the stored energy and resultant forces in the snatch strap, we need to determine what the kinetic energy of the recovery vehicle is at the point where the snatch trap has enough force to stop it, ie at the point where the injected kinetic energy equals the stored potential energy in the strap.
The kinetic energy of the vehicle = 0.5 x
m x v^2 (^2 =
and the strain energy in the strap = 0.5 x F x s
Therefore, at v=17km/hr (4.72m/s), m=2300kg, and
the vehicle kinetic energy = 25644 Nm
Now, at this point, the strap strain energy = induced kinetic energy by the recovery vehicle
==> 0.5 x F x s = 0.5 x m x v^2
==> s = sqrt(m x v^2 / k) = 1.53m stretch
Therefore, the force exerted is
F = k s = 21800 x 1.53 = 3.41 tons.
This does not sound too impressive though, as we were expecting something closer to 8 tons. This force assumes that the snatch strap has a 20% elongation at 50% load. Tests have shown that some snatch straps only have 15% elongation, which would mean that for these the exerted force would be 4.5 tons instead of the calculated 3.41 tons. If we add a V8 at the same vehicle mass with a higher speed to the equation, the forces will be much higher.
Now, 3.41 tons is "not a lot", only about 50% more than the vehicle mass. But what happens if an attachment point breaks off at this "low" force?
Let's take a pin and ball attachment, fitted with inferior low tensile bolts. The attachment weighs about 1.2 kg. If it gets ripped off the vehicle, how lethal will it be?
As determined above, the total stored strain energy =
Translating this into velocity, we get v = sqrt(2 x m) = sqrt( 2 x 25644) = 226.5 m/s = 815 km/hr!
Obviously this is a theoretical figure, derived from the assumption that the elasticity of the snatch strap is perfect at 100%. Let's be pessimistic, and assume that the strap is only a third efficient. This translates to a speed of 272km/hr. Not all that fast, I hear you say.
Let's consider a cricket ball, weighing all of 155g, being bowled at 150km/hr by that Ozi wonder-boy, Brett Lee. The kinetic energy of this fast ball is 134Nm. Our 1.2kg loose projectile's energy is 25644/3=8548Nm! That's packing 64 times more punch than one of the fastest cricket balls the world has seen! We all know that this cricket ball is capable of bending and penetrating the protective screen on the helmet, and still fracture cheek bones behind it. Bearing this in mind It's not hard to imagine why that the broken vehicle attachment, now a projectile, can pierce firewalls, windscreens and seats. It will definitely be fatal to a passenger in its path!
Snatch Strap Forces (metric tons)
Vehicle Mass [kg]
These figures should be used as guidelines only, as ideal situations are used in their calculations, such as assuming an even spring constant for the snatch straps in Hook's Law formula. Many straps also have less stretch than the "industry standard" of 20%. If the stretch is less, the resultant forces will be higher as a result of the increased de-acceleration of the recovery vehicle at the end of the pull.
This also applies to the the same strap being utilised more than once within a 6 to 8 hour period. Snatch straps need 6 to 8 hours, per 10% stretch, to fully contract again. Thus, for a full 8 ton pull, the stretch will be 40%, requiring up to 24hrs to retract fully! Also, with each pull, the strap loses some elasticity. Therefore, it will never contract back to its original length again. Check the manufacturers instructions, when to discard it as a snatch strap. The ball park figure is 10%, ie if the snatch strap does not recover its length to within 10% of the original length, discard it as a snatch strap. In most cases, these straps don't lose their strength, and can be used as pull straps. Just mark them accordingly and remember that they will still have some stretch in them!
For this reason, no snatch strap should be used more than once within 6 hrs, as the elasticity is different on successive pulls. This is not always possible though - only use the snatch strap more than once if the initial stretch was minimal, or if you are aware that the successive snatches will exert more force, since some elasticity was lost in the previous snatch(es)
When connecting a snatch strap, try and spread the load by using either a 10 ton lifting strap, or strong tree protector, attaching it to two jade rings on each side of the chassis. The snatch strap loop is then slipped over this strap. This way you are distributing the high forces to both sides of the vehicle, preventing possible recovery point damage and chassis deformation in severe conditions.
The reality of the situation is that many people use snatching as their primary method of extraction, and think nothing of it. It can be safe if done properly at reasonable speeds. When you're using it, don't be in a hurry to use more power; start with slower tugs and build up speed gradually, with a new strap at each new attempt. Also make sure that you are only using rated recovery equipment on the vehicle, eg bow shackles. You should use at least a 6.5 ton rated bow shackle if the strap is to be connected to one recovery point, and 3.25t, preferably 4.75ton, bow shackles if two points are used.
Here is a summary of tests done on different snatch straps by Beaver Sales, at their Height Safety & Confined Space Testing Centre, Australia (see www.lizardlegs.com.au/blackrat/news.asp#1):
|Snatch Strap Make||Dry Test Breaking Force [kg]||Wet Test Breaking Force [kg]||Stretch
|Black Rat 8000||8277||7401||23%||$70|
|Kaymar 8000 (made by Spanset)||9288||8681||20%||$65|
|Mean Green 8000||9427||7927||15%||$69|
|Mean Green 10000||9759||9423||20%||$99|
|Repco Motogard 7500||6508||5902||18%||$59|
|Super Cheap Auto 8000 (made by Spanset)||8520||7790||19%||$70|
|Super Cheap Auto 9000||3798||3953||-----||$30|
|Don Kyatt Terrain Tamer 8000||7087||7048||18%||$55|
|Don Kyatt Terrain Tamer 11000||12022||10569||15%||$88|
Whilst some consider a strap to be good if the breaking point is more than their quoted specification, I consider the stretch factor to be more important, then followed by the breaking force. eg some rate the Bushranger 8000 highly because of its strength, ignoring the fact that it has a poor stretch factor, inducing higher forces when snatching.
The article in the Australian 4wd Monthly used the above
results to rate the straps as follows:
(I don't necessarily agree with their classification)
|Highly recommended||Mean Green 8000|
|Terrain Tamer 11000|
|Super Cheap Auto 8000|
|Black Rat 8000|
|Mean Green 10000|
|Not Recommended||Terrain tamer 8000|
|Repco Mortorguard 7500|
|Super Cheap Auto 9000|
What will be covered in the next section are safety factors. Here is a brief rundown:
Where lifting gear is used, a minimum safety factor of 5 is usually used. A 4.74 ton rated bow shackle was tested by the above mentioned Ozi crowd, and it broke at 33 tons! Legally, its minimal breaking force should be at least 5 times its rated force. This safety factor was born to protect the public, workers and equipment. Why so high? This is because material specifications cannot be guaranteed 100%, due to various uncontrollable manufacturing factors, which include material chemical compositions and heat treatment procedures.
When designing cars, weight is a major factor. To reduce excessive mass, the safety factor is reduced. To compensate, higher quality materials need to be used. In aircraft design, this is even worse, as the plane would not be able to fly at high safety factors of 5, or even 3. Here safety factors of 1.4 and 1.5 are used, but extremely high quality materials are used (which is the main reason for the high cost of aircraft)
Having digressed a bit, it can be seen that the lack of use of safety factors in 4x4 recovery gear is most worrying, and a reason why so many accidents happen. Let's hope that not too many lives will be lost because of this. The results of snatch straps investigation reveal that there are no safety factors employed with them. Most are rated at 8 tons, and most break at this force, some slightly less, and some more. Because the strap itself is not the most dangerous item when it breaks, I am actually glad that it will break at 8 tons, and not at 32 tons, the force needed if a safety factor of 5 had to be employed. This belt thus limits forces to about 8 tons, and if something breaks, we hope that it is the strap itself.
Not using safety factors on the attachment and recovery points can be fatal. This is where you don't want anything to break, especially if it can become a flying missile.