There are some serious forces involved in a 4WD recovery, and they can be extremely dangerous. If you’ve ever wanted to know more about the physics of a 4WD recovery, and how to size your recovery gear correctly, this post will cover everything you need to know, and more.
This is not the normal post that we do here, and there’s a good reason for that. It’s because it was not written by me, but rather submitted very kindly by a very clever bloke by the name of Ian Norrie. So, grab a cuppa, sit back and turn your brain onto maximum!
Your 4WD is well and truly bogged. You want to know whether your recovery gear is up to the task?
This article will tell you.
Note: All the forces and loads in the article are given in kg, where 1000kg = 1 metric ton = 1 tonne = 1t (which is usually written, incorrectly, as 1T).
Gear sourced from the USA is rated in pounds (lb) where 1lb = 0.454kg or 1kg = 2.2lb
Hence gear with a 10,000lb rating = 4540kg.
We will gloss over the fact that kg and lb are units of mass, not units of force, kgf, lbf and kN (where 1kN = 102kgf = 102”kg”). Blame Isaac Newton self-isolating during a plague for discovering gravity.
Your 4WD is stuck. What is the force required (and hence the load on the various recovery items involved) to recover it? This force required to de-bog the vehicle, is a function of:
The weight of the vehicle
The type of terrain the vehicle is bogged in
The slope of this terrain
The following charts are for the three most common types of terrain you are going to get stuck in; loose rock, soft sand and mud, for slopes ranging from 0⁰ (level ground) to 30⁰ (very steep) and for vehicle weights up to 7500kg.
This weight limit allows to include bogged trailers. The 30⁰ slope limit is the angle of repose of dry sand, the equivalent of driving up the wrong face of a sand dune. In other words, bloody steep!
These debogging forces are generally in line with the “mire factors” used by the US Army in their Recovery Manual for a truck bogged to its axles.
If you get bogged to the top of your wheels, double the following, or start digging.
If you get bogged to the top of your bonnet, triple them!
The information contained in these three charts can be summarized in the following Table:
When you get bogged in your 35” muddies, you are generally going to do a really good job of it.
From these charts you can establish whether you are stuffed, or really stuffed . . .
Safe loads, breaking loads, test loads and safety factors
We now get to the heart of this article. One of the greatest difficulties for you the end user, is establishing whether the load capacity given for any particular item of recovery gear is a safe working load or a minimum breaking load.
The manufacturer test loads a sample number of their gear to failure, usually resulting in a cluster of failure load values. Based on the spread of these values (the sample standard deviation), the manufacturer can then obtain a minimum failure load that can be used with confidence.
This minimum failure load is usually defined as equal to the average test load result – 3 times the sample standard deviation.
This ensures that 99.7% of the gear will have a higher failure load than the minimum failure load quoted.
As a guide, the minimum failure load used is probably around 85% of the average test load result.
An example of an MBS derived from test loading:
8,000kg snatch straps from seven different manufacturers were test loaded to destruction by a 4×4 magazine, with the following failure loads:
9,619kg, 9,378kg, 10,486kg, 9,241kg, 9,991kg, 8,921kg and 9,839kg.
This gives an average failure load = 9,640kg and a sample standard deviation = 520kg
Hence the resulting MBS = 9,640kg – 3(520kg) = 8,080kg, which is greater than the 8,000kg then adopted and the resulting MBS is also 84% of the average failure load.
This minimum failure load for recovery gear is variously given as:
The Minimum Breaking Strength (MBS), the Minimum Breaking Strain (MBS), the Minimum Breaking Load (MBL), the Minimum Destructive Test Load, the Minimum Breaking Force (MBF), the Nominal Strength, the Ultimate Load, the Factored Load and my all time favourite, the Guaranteed Breaking Strain (GBS).
AS 1418.1, the relevant Australian Standard, defines “Rated Capacity” as the load you can apply safely.
This safe load is variously denoted as:
Its Working Load Limit (WLL), its Safe Working Load (SWL), its Manufacture’s Rated Capacity (MRC),
its Recovery Load Limit (RLL), its Allowable Load, its Permissible Load, its capacity, a Load Rating of, and its Maximum Load (see also comments under Tow Hooks)
The ratio of the failure load over its safe load is equal to the item’s safety factor (SF).
Thus SF = MBS/WLL, or MBS/SF = WLL, or WLL x SF = MBS
Safety factors vary depending on the consequences of failure.
For lifting applications, a safety factor of 5 is mandatory. This is used for cranes, lifts and hoists for example.
In particular a 4.75T rated shackle has an MBS = 4,750kg WLL x 5 safety factor = 23,562kg. You will never break this shackle. This is the reason why they should not be used to join snatch straps; anything they are attached to will break first, potentially leaving the shackle to fly around as a missile.
There is an interesting video clip by Ronny Dahl on YouTube called “mass damage snapping winch cables”, showing a 4WD being totally smashed by a flying shackle. Well worth watching.
In the truck industry for securing loads and in the commercial logging industry for winching bogged vehicles, a lesser safety factor of 3 is used.
This because the consequence of failure is less severe than that say for a lift in a building full of people failing or a crane dropping a load. This middle of the range safety factor is appropriate mainly because the force required to recover a bogged vehicle is not well defined. Nevertheless, the recovery gear industry struggles to achieve this safety factor, see below.
For static loads where the load is not moving, an even lower safety factor of 2 is used. This is because the loads are generally well defined and there is no risk of shock loading as there is with vehicle recovery.
This safety factor would be used for example for the sizing of a wire rope used in a cable-stayed bridge.
Hence the same diameter wire rope would have vastly different safe working load capacities depending on whether it was being used in a crane, in a winch or in a cable-stayed structure.
Regardless of the above, in reality the recovery gear industry appears to have adopted lower safety factors, generally around 2, for their equipment.
For example, the recovery industry’s advice is to use equipment with an MBS = 3x the vehicle’s weight.
Adopting this would give, for bogged in mud on a 30⁰ slope the absolute worst-case scenario, a safety factor of 3 135% = 2.2, which, whilst < than the 3 preferred, is still > 2 min say, hence OK.
Reducing this guidance to an MBS of say 2x the vehicle’s weight, reduces this safety factor in the worst-case scenario down to 2 135% = 1.5, which is < 2 min say, but nevertheless is still > 1 (failure).
Thus, in the worst-case scenario it won’t break, but you are not operating in what would be considered a “safe” zone.
This concept of relating the gear’s MBS to 3x the vehicle’s weight is adopted by the recovery industry for snatch straps, soft shackles and kinetic recovery rope.
On the other hand, the winch industry’s guidance to use a winch capacity of 2 to 3x the vehicles weight is more conservative (ie results in a higher safety factor SF) than the above, because winches’ rated capacities are allowable working capacities, not MBS breaking capacities.
The underlying problem in the recovery gear industry is that there is no consistent standard for conveying the strength capacity of their various pieces of equipment.
Sometimes minimum breaking loads (MBS) are given, sometimes working loads (WLL). Often the loads are designated only as “Rated” loads and you have to trust that the manufacturer is using this term correctly, viz a safe working load or WLL.
This situation is not helped by the synthetic rope industry generally quoting a “Rated” Breaking Strength (MBS) for their soft shackles. This is a grossly misleading term, essentially implying that you can safely load the soft shackle to its rated capacity, but if you do, it will break!
I suspect this sort of nonsense is written by their marketing departments to make their shackles appear “stronger”. No engineer would do this.
Recovery gear load capacity data
The following Tables give the load capacity data supplied by the manufacturers for their recovery gear.
WARNING: Attach each end of the equalizer strap to a recovery point.
Do not attach a continuous equalizer strap to your vehicle recovery points, including tow hooks. The force in the equaliser strap will remain unchanged, at 5,000kg say, but you have now added an additional 5,000kg lateral force onto the recovery points, something which they have not even remotely been designed to accommodate.
Even the ARB recovery points, which are the best of the bunch by far and have been designed for realistic lateral loading combined with the straight pull, would probably fail if loaded in this manner. This warning applies particularly to tow hooks being used as recovery points.
Note: Equaliser/bridle straps, tree trunk protector straps and winch extension straps are essentially interchangeable.
Hitch is designed to be fitted to a Class 4 Towbar with a towing capacity of 3,500kg, which is less than the 5,000kg hitch capacity.
*The hitch is attached to the towbar with a 5/8” (16mm diam) pin in double shear, with the resulting minimum allowable shear capacity of 3,265kg < 3,500kg towbar capacity < 5,000kg hitch capacity.
Hitch Pin capacity
What is known:
There is a 16mm diameter steel pin spanning a 52mm (approx) opening.
What is not known:
The yield stress fy of the steel used in this pin. This yield stress dictates the pin’s capacity in both shear and bending moment.
When loaded (correctly) using a hitch receiver, there is notionally only shear on the pin.
Assume the lowest grade of steel was being used, equivalent to a 4.6 bolt which has a yield stress fy = 240 MPa. Also assume the pin’s threads are excluded from the shear plane.
This gives the pin’s WLL capacity in double shear = 2 x Bvx = 2 x 16kN = 32kN x 102kg = 3265kg, which is the min capacity given in the text above. This could be regarded as a lower bound capacity.
I referred to notionally shear only above. There is in fact a small bending moment on the pin based on the wall thickness of the SHS section used in the hitch receiver. This bending moment further reduces the pin’s capacity marginally in combined bending moment and shear. I have ignored this.
When loaded (incorrectly) using a strap directly onto the pin, there is both the same shear plus there is now a real bending moment (not a notional one). It is this real bending moment that a hitch receiver is deliberately designed to eliminate, leaving shear only on the pin.
Pursuing this subject of the consequences of loading the pin directly via a strap, I am firstly curious as to how this can even be done using anything stronger than a 6,000kg strap which is 50mm wide. The more common 8,000kg strap is 60mm wide, so how does it even fit in?
Nevertheless, it would be helpful to derive an upper bound capacity for a pin involving bending moment and to do this we could pro-rata the capacity of a Grade S shackle, which also has a pin spanning a gap and hence a combined bending moment and shear on that pin.
Whilst I as yet have been unable to find the actual yield stress of Grade S steel, there is a strong indication from the shackle’s MBS data that the yield stress must be around an fy = 640 MPa, which, probably not coincidently, is also the yield stress of an 8.8 bolt.
As it would be most unlikely that the pin in a hitch receiver is an even higher grade of steel again, the Grade S shackle capacities would give a good indication of an upper bound capacity.
A 2T Grade S shackle to AS 2741, has a 16mm diam pin spanning an opening of 21mm and has an
MBS = 98.1kN x 102kg = 10,000kg
If shear governed and assuming a SF = 2, then the pin’s WLL = 10,000kg ÷ 2 = 5,000kg, ie the rated load of a hitch receiver if its pin had a matching yield stress to that of Grade S.
If bending moment governed, then the hitch pin’s MBS capacity would be:
= 98.1kN x 21mm/52mm = 39.6kN x 102kg = 4,040kg
Assuming a SF = 2, then the pin’s WLL = 4,040 ÷ 2 = 2,020kg, say 2,000kg.
What is puzzling, is anecdotal evidence that these pins are routinely being bent when loaded via straps. For the pin to be permanently bent means that it has reached its yield stress and has deformed (as shown on the plateau portion of a steel stress strain diagram). The pin then remains in this deformed state even when unloaded.
If the pin were to be the equivalent of Grade S, then the force required to permanently bend it would then be the MBS = 4,040kg = 4T say.
Whilst not impossible, this force does seem a tad high to apparently be occurring regularly.
If nothing else, it is a scarily high force to be applying on a strap with an MBS = 6,000kg say!
We will gloss over the fact that this force also exceeds the tow bar’s safe recovery capacity.
What this is implying, is that the yield stress of these pins may be < Grade S, but if they are then the MBS in shear only when used correctly with the hitch receiver, would then also be < 5,000kg.
We do not yet know the yield stress of the material used in the 16mm diam hitch pin, but we can bracket its capacity as follows:
When used with a hitch receiver, then 3,265kg ≤ WLL ≤ 5,000kg
When used without a hitch receiver, then WLL ≤ 2,000kg
And the above assumes that the hitch pin is attached to something with a safe capacity > 3,265kg, which exceeds a tow bar’s safe capacity under recovery of 2,800kg say.
As you may have gathered, I am not a great fan of hitch receivers, and even less so when loaded directly onto the pin via a strap.
Friction losses in Snatch blocks and Snatch Rings
I have been remiss in not stating that the capacities given above were based on the snatch block/rings being frictionless.
Snatch blocks/rings are not frictionless as has been assumed in the article. They do indeed exhibit friction and the question is then, is this friction of consequence?
The answers in a nutshell:
For one snatch, the increase in winch loading is not a lot, which is why for simplicity I had ignored it, and the debogging capacities stay unchanged
For two snatches, the increase in winch loading is the same, ie not a lot, but the debogging capacities start to decrease a little.
But for three snatches, ie towing out backwards, the increase in winch loading is again the same, ie not a lot, but the debogging capacities start to drop, a lot.
The friction losses from snatch blocks range from 8% to 10%. I will use 10% (to be conservative).
The consequence of this 10% friction loss in the snatch block is that the winch has to work slightly harder, 5% harder, to achieve the same results as for a frictionless snatch block.
Imagine that a debogging force of 2,000kg is required. For a frictionless snatch block there will be a load of 1,000kg on the winch and hence on the winch cable before it enters the snatch block, and a load of 1,000kg as it leaves the snatch block heading back to the bogged vehicle. These two forces combine to give a load of 2,000kg on the snatch block.
Using a snatch block (with a friction loss assumed to be 10%) there will now be a load of 1,050kg on the winch and in the winch cable, a loss of 100kg in friction at the snatch block leaving a load of 950kg in the cable as it heads back to the vehicle. Note that the sum of the loads in the cable = 1,050kg + 950kg = 2,000kg, ie unchanged to the frictionless scenario. Hence all the shackles, tree protectors and winch extension straps will remain unchanged.
In other words, friction does not result in a loss of force (load), but merely a redistribution of it.
The friction losses from snatch rings is slightly more, ranging from 11.5% to 16%.
I will use 15% for simplicity.
The consequence of this 15% friction loss in the snatch block is that the winch has to work a little bit harder again, 7.5% harder, to achieve the same results as for a frictionless snatch block.
There is an excellent video by Robert Pepper demonstrating how these friction losses were derived. Other blog sites come up with a similar range of values.
The consequences of these frictions for various combinations of snatches are as follows:
Furthermore, the winch loads in the above table have all increased because of the friction in the snatch blocks. To get this increase back to zero, requires the debogging forces to be further reduced slightly again. This has been done for the debogging forces revised as a consequence of friction in the relevant tables following.
Safe Recovery Options
Following are ten recovery configurations for debogging forces of up to 14,000kg (14T), with friction included where relevant.
Option 1: Snatch Strap
Option 2: Winch + stationary vehicle + winch extension (optional)
Note the following are equally applicable with the winch on the towing vehicle.
Option 3: Winch + tree + winch extension (optional)
Option 4: Winch + snatch block + tree + winch extension (optional)
Assumptions: Snatch block 10% friction loss, Snatch Ring 15% friction loss
Assumptions: Snatch ring 15% friction loss
Assumptions: Snatch blocks 10% friction loss, snatch ring 15% friction loss
Option 5: Winching out backwards
Note: This operation should only be undertaken on 4WDs with chassis rails, ie not monocoque bodies.
Rated Recovery Points:
Type A – 3,500kg Towbar (+ 5,000kg hitch receiver, optional) or 4,500kg Towhook
Type B – 4,750kg Rated Recovery Points x 2 0ff + 4.75T Shackles x 2 off
Recovery Gear Summary
The following is a summary of the recovery gear required for vehicles up to 7,400kg GVM, for either snatch strap recovery or winch recovery with and without a snatch block, as shown in Fig 1 to Fig 7.
This covers the majority of real world applications.
Note: For Recovery Points refer applicable Fig 1 to Fig 7
NA: not applicable
KRR: Kinetic Recovery Rope,
BR: Bubba Rope Mega Tow Rope
Note that the debogging force that can be achieved using a snatch strap is governed by the traction that can be generated by the towing vehicle, which ultimately is related to the towing vehicle’s weight. In the crudest possible terms, the more weight the better.
If in doubt, use a winch, a tree with a protector strap, plus a snatch block and an equaliser strap, together with soft shackles wherever possible.
Better still, dig and use recovery boards.
A note from Aaron:
Now, if you made it to the end, and understood even a portion of this you’ve done well. I’m still looking at the tables and picking up different bits of information! A massive thanks to Ian, who has gone above and beyond developing this. Safe recoveries!