Been away doing a few other things in life.

To further clarify what I say in the "long post" - every SAFE mechanical system must have a working limit (whether it is known as "SWL" or "WWL").

A mechanical system is a combination of components or assemblies that are employed to work together to do work.
For example, an assembly comprising of a 9000Kg bridle chain or strap connected to a vehicle by 3200Kg bow shackles at each end and a 8000Kg winch strap connected to the bridle chain or strap by a 6000Kg bow shackle and to a 8000Kg tree protector with a 6000Kg bow shackle is a simple mechanical system. If a 6000Kg load is applied to the winch strap (it does not matter how it happens) each of the bow shackles to the vehicle will be loaded to about 3000Kg, the bridle strap or chain will be loaded to 6000Kg, 6000Kg load to the bow shackles either end of the winch strap, 6000Kg to the winch strap, 6000Kg to the tree protector and 6000Kg to the tree - the tree has become a part of the mechanical system.
The system described in the example is what I would call a "balanced system" i.e. the load on any part of the system is greater than 6000Kg - the known maximum load that can be applied to that system by the winch you are using (for example).
If the bow shackles at the ends of the winch strap are rated to 4500Kg, the example system would be "unblanced" i.e. a 6000Kg load would exceed the load the (example) winch could apply to the system.
An unbalanced system is unsafe.

You might say that a 4500Kg bow shackle has a safety factor of 600%, so that a load of 6000Kg would merely reduce that safety factor to say 300%.
Unfortunately, physical forces do not work in that way, the safety factor might be reduced to 200%.
Now, say the vehicle slips during recovery and there is a "bounce" in the winch strap, it is possible that the "bounce" will create a instantaneous load of say 7000Kg - the safety factor would be reduced to about 45% (at best) and that would be an unsafe system.

From MudrunnerDT's figures and others, I think it can be accepted that loads on a recovery kit (read "mechanical system") may easily exceed 6000Kg and the load may sometimes be double that if the vehicle being recovery (as deadweight or being driven) slips or jumps during the recovery process.

Rating of products is usually with a constant load, which involves completely different stresses and strains to the dynamics of a vehicle recovery.
For that reason I tend not to increase any "rating" beyond the WWL (of a bow shackle, for example).
Similarly, straight line pull tests of snatch straps are a bit of nonsense in my view. The only way to truly test a snatch strap is to simulate the peak loads of an actual recovery.
In addition, it is generally acknowledged that the performance capabilities of snatch straps change after each use, including a progressive loss of their elasticity.
In short, a snatch strap is not a ratable product - its performance is always changing.

A 9000Kg bow shackle might seem at first to be excessive, but I look at it in three ways -

1. The extra weight is not significant.
2. The extra expense of the purchase is not huge.
3. I want to be alive at the end of the day and I do not want to kill or injure anyone.

The recovery kit you choose ought be determined by the total load your winch can apply to any mechanical system you can assemble using that kit as the WWL of that system.
Cost and weight are irrelevant - safety rules.

Just a word on "helping" others who do not have the appropriate kit.
The law of negligence arrives here and will hold you liable for all of your actions (however well intended) and the safety of your equipment.
It is one of the maxims of equity that the law does not assist a volunteer.

Just a parting thought. I keep simple line drawings of different double and triple pull systems along with my recovery gear - partly to stop me making a simple mistake and partly to help me not to miss the obvious.




The working limit of any mechanical system is the