How much Force/Load doing a Snatch recovery...? - Page 2




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  1. #21
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    Default Re: How much Force/Load doing a Snatch recovery...?

    Thanks Kiri for this thread. The thing that is important here is that the forces involved is much higher than some of us(me) thought it would be. That means that we should keep this in mind when recovering a vehicle, by whatever means.
    What the forces are and how it is worked out doesn't really matter, does it?

  2. #22
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    Default Re: How much Force/Load doing a Snatch recovery...?

    Some good responses so far, and with a few physics guys too.

    Willem T calculated it to me 4.5 ton, which came close to what was measured by a Load Cell on peak hold for the below real life exercise. (below readings would have been different with a less stretchy strap)

    1st run:
    The rope of 9m stretched to 13m, 45%. (11 ton Ropeworld rope)
    Load Cell peaked at: 4.27 ton

    2nd run:
    Repeating the below exercise but at 20 km/h (11 ton 4x4 MW rope)
    Load Cell peaked at: 3.04 ton, but the rope snapped at the splice (rope was only used once before and stored as specified)

    The reason for the bold idea of hooking the snatch strap to an unmovable object like a massive bloekomboom with a 4.8 Patrol accelerating faster than should be on the other end, was to determine "worst case scenario"....

    Recovery 4x4 Weight: 2700kg (close approximate)
    Recovered/Stuck 4x4: Unmovable (dead weight)
    Recovery 4x4 speed: 35km/h before the load slowed it down. (approximate as per GPS and speedo) (yes I know this is too fast. "regulation" speed is about 20km/h)
    Recovery 4x4's traction surface: Damp compacted black sand. (= good traction)
    Recovery 4x4's tyres: 285/70R17 MT's, deflated to 0.6bar (just to get even more traction)
    Centre transfer locked for 4x4 (as it's a part time)
    Manual hubs locked.
    Low range 2nd.
    Front and Rear diffs not locked.
    Auto box, so it can keep pulling without stalling.
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  3. #23
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    Default Re: How much Force/Load doing a Snatch recovery...?

    When you're talking about stopping a moving vehicle, and you want to calculate the force required to stop it, you need more information than what is given. For starters you need the distance over which that force is applied to affect the stop. This being a kinetic strap/rope, that is impossible to determine, as the force increases with the rope stretching. Because that happens, the vehicle slows down, and/or your engine applies additional force, but more than likely, a combination, which makes an accurate calculation even more difficult.

    You've got a better chance of calculating the force required for a non-kinetic strap. I'm not volunteering to do a snatch recovery using a tow strap.

    Calculations for a kinetic strap will always come out higher than the real world measurement, purely because it's a progressively increasing force, and not a constant. The rate of force increase would be directly related to the elasticity of the rope. Most calculations are based on a constant force.

    With enough tests you would be able to establish a factor with which you can divide the calculated force to get a closer guess at the real figure. That factor would of course be unique for every rope/strap type, and will change with use and condition.
    Sakkie Coetzee

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  4. #24
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    Default Re: How much Force/Load doing a Snatch recovery...?

    So if my understanding of the above is correct I would need a minimum of 10 ton rope for my 2.5 ton Fortuner say 3 ton when loaded. I think I'll buy a 12 ton to be safe.

  5. #25
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    Default Re: How much Force/Load doing a Snatch recovery...?

    Get yourself a 12 ton poffadder. More important, make sure you know when and how to use it safely.
    Sakkie Coetzee

    Some people say I have a "short temper"....I see it as swift and assertive reaction to Bull!

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  7. #26
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    Default Re: How much Force/Load doing a Snatch recovery...?

    11.8 tonne snatch strap tested. It broke at 12.7 tonnes

    https://www.youtube.com/watch?v=epbLwnUOP68

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  9. #28
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    Default Re: How much Force/Load doing a Snatch recovery...?

    Interesting to note that the specified force is break strength, not SWL as per lifting equipment standards. (which require as huge safety factor)
    Sakkie Coetzee

    Some people say I have a "short temper"....I see it as swift and assertive reaction to Bull!

  10. #29
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    Default Re: How much Force/Load doing a Snatch recovery...?

    Quote Originally Posted by Sakkie View Post
    Interesting to note that the specified force is break strength, not SWL as per lifting equipment standards. (which require as huge safety factor)
    Will be very difficult to describe a SWL on a "stretchy" system as max energy is stored in the system only at the point where 100% of available stretch has been used. The fibres from which the strap / rope is made will have a certain elastic modulus which in turn describes how much the system can or will stretch under a given load. Once that has been exceeded the system will fail fairly quickly.... hence the use of mean breaking strength.

    If you are looking to work out the energy stored in a kinetic strap / rope you will need to know the elastic mudulus (K) of the material the strap / rope is made of and then use Hooke's law to calculate the energy stored in the strap at any given elongation factor. This modulus will vary from manufacturer to manufacturer.

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    Default Re: How much Force/Load doing a Snatch recovery...?

    Why I mentioned it, is that a lot of people are under the misconception that these straps/ropes have a 300% safety margin.
    Sakkie Coetzee

    Some people say I have a "short temper"....I see it as swift and assertive reaction to Bull!

  12. #31
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    Default Re: How much Force/Load doing a Snatch recovery...?

    With you on that one.....

  13. #32
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    Default Re: How much Force/Load doing a Snatch recovery...?

    The 3rd test was done under more realistic conditions.

    We buried a Hilux weighing about 2.4 ton in soft sand by digging all 4 wheels into the sand till all 4 were spinning freely and the Lux was on its chassis.
    Instead of a rope we used a Snatch Strap.
    Again the Patrol was used, but this time the traction surface was the same soft sand.
    There was a slight incline of about 5 deg.
    The aim was to recover at a speed of 20 km/h, which was to difficult to guarantee with only only a 1 meter "S" slack. So we took a long run-up but maintaining 20km/h, or as close to as possible.
    The Hilux did not assist in the recovery and was left in neutral.
    The Load Cell peaked at 2.7 ton

    The Hilux was un-bogged, but the Patrol partially dug itself in as it came to a standstill. Even though not tested, the assumption was made that if a more stretchy rope or lower rating strap was used, it would have prevented the Patrol of a potential bog.
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  14. #33
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    Default Re: How much Force/Load doing a Snatch recovery...?

    Using a kinetic rope, you'll have to be rather silly if you get the recovering vehicle bogged down to the point where the tension in the rope won't pull you out backwards.
    Sakkie Coetzee

    Some people say I have a "short temper"....I see it as swift and assertive reaction to Bull!

  15. #34
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    Default Re: How much Force/Load doing a Snatch recovery...?

    One of the points of this thread is, why use 10 and 12 ton kinetic equipment when based on our exercises, you don't see half even while doing an "unsafe" recovery. Safety factor in kinetic equipment does not serve the same purpose as other recovery equipment. The higher the breaking strength of the snatch rope or strap the lesser the stretch, which equals less of its intended purpose.

    So for us that did those exercises, the conclusion was; use a lower (within limits) breaking strength rope, not strap for a better outcome. This was inline with the below findings, accept in SA we have evolved to a more elastic Rope than the Aussie Strap.

    Similar tests were done in sand in Australia, and the loads they recorded were between 1.3 and 2.1 using the "regulation" run-up distance.

    https://www.outbacktravelaustralia.c...oading-on-test

    "Conclusion
    We discovered that elasticity is more significant than ultimate breaking strain in assessing snatch straps and that highly elastic straps impose lower loads on the vehicles involved than do stiffer ones. The next question we need to answer is the life of a snatch strap: how many recoveries can a strap make before it’s ‘dead’? We’re working on it."
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  16. #35
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    Default Re: How much Force/Load doing a Snatch recovery...?

    Quote Originally Posted by Kiri View Post
    On request from ChrisStoffel from CT here it goes:
    Many 4WD'er, seasoned or new to the "game" are either too scared to do a Kinetic recovery or to relaxed when doing it...

    Snatch recoveries were discussed for a few years now and there are still many opposing views, so best to start fresh.

    Doing a Snatch Recovery with a snatch strap/rope (kinetic strap/rope, usually nylon) what is the expected load in kg or ton for the below scenario?

    Recovery 4x4 Weight: 2700kg (close approximate)
    Recovered/Stuck 4x4: Unmovable (dead weight)
    Recovery 4x4 speed: 35km/h before the load slowed it down. (approximate as per GPS and speedo) (yes I know this is too fast. "regulation" speed is about 20km/h)
    Recovery 4x4's traction surface: Damp compacted black sand. (= good traction)
    Recovery 4x4's tyres: 285/70R17 MT's, deflated to 0.6bar (just to get even more traction)
    Centre transfer locked for 4x4 (as it's a part time)
    Manual hubs locked.
    Low range 2nd.
    Front and Rear diffs not locked.
    Auto box, so it can keep pulling without stalling.

    In the above scenario, where a vehicle is so deeply stuck, a snatch recovery is not an option. But this is a scenario to determine what is the expected maximum load during such an "unsafe recovery"

    Recovery force calculations can be made, but we will always wonder what would it be in "real-life"...??
    There are many other forces missing on either side of the = here. Coefficient of friction on its own is a moving target here with many considerations. You have various materials at play with various coefficients of friction - you need to distribute and weigh proportionally. Without all the data hard to calculate.
    Last edited by ryanbe; 2018/04/19 at 11:27 PM.

  17. #36
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    Default Re: How much Force/Load doing a Snatch recovery...?

    .... because of all these variables and uncertainty around snatch recoveries, we did a few "real life" tests....
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  19. #37
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    Default Re: How much Force/Load doing a Snatch recovery...?

    Just a quick scan over the calculations and you can see for yourself why recover with the towhitch is such a bad idea!

  20. #38
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    Default Re: How much Force/Load doing a Snatch recovery...?

    Kiri, I wonder why aussies consider it a discovery that highly elastic ropes straps work better, I thought this was school stuff (but then you have to attend school ).
    As mv=Ft then the stiffness shortens your elongation time which makes the force shoot up to infinity (read - damage) like if tow strap was used.

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