DANGER! energy absorbed on snatching! - Page 2





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  1. #21
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    I assume these formulas are for a vehicle that stays stuck?

    If the snatched vehicle starts moving the force on the rope diminishes?

  2. #22
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    you decelerate the recovery vehicle. If the stuck vehicle comes out it diminishes the force on the rope
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  3. #23
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    I saw these ropes/straps are specified in kg's?, Joules and kg is not the same thing, as you indicated.
    Johan Kriel

    LCs, Hilux and Echo Chobe

  4. #24
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    It seems to me that the principles of conservation of momentum are more applicable in this case than conservation of energy. Consider that during a snatch, the snatch strap acts as a spring and nothing will happen until the strap is fully stretched out. A short duration impulsive force proportional to the rate of change of momentum will then be applied to the entire system. At that point, one of three things will happen:
    1. Something breaks
    2. The momentum in the system is transferred to the snatched vehicle which then becomes unstuck
    3. The snatching vehicle goes backwards at the same rate as it initially went forward (not a pleasant experience).

    If this is so, the amount of kinetic energy in the strap is limited to the stretched out condition and will not continue indefinitely as implied in the above calculations.

  5. #25
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    The force on a recovery point during a snatch depends mainly on the stretchability of the rope and the stuck resistance.

    The less the stretchability, the higher the force for the same energy. To explain this, replace the kinetic rope with a non-stretchable chain and see how you increase the force even at a much lower speed. This is why the manufacturer specify the vehicle weight – it must be able to stretch the rope. Correction - do not replace the kinetic rope with a non-stretchable chain to see what will happen, just imagine what will happen.

    The typical force of an average snatch recovery is between 1 and 2 ton. (of course it depends on many factors.)

    Below a paragraph from the “4x4 Offroad Reference Manual” – http://4x4offroadacademy.com/4x4trainingmanual.php

    For those interested, a Kinetic Recovery book will be released this year with many kinetic calculations and supporting load cell experimental results/case studies.
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    Last edited by Peet Hendriks; 2014/03/28 at 12:39 PM.

  6. #26
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    That diagram kinda takes the fun out of it.

  7. #27
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    I need to get a Pofadder after reading the posts

  8. #28
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    Quote Originally Posted by JLK View Post
    I saw these ropes/straps are specified in kg's?, Joules and kg is not the same thing, as you indicated.
    Energy[J] = force[N] x distance[m]
    load [kg] = force [N] / 9.81

    So again, stretchability is important - the less the stretch the higher the force for the same amount of energy.
    Last edited by Peet Hendriks; 2014/04/11 at 06:27 PM.

  9. #29
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    I have never used a snatch rope, I'm dom with this, now, is it possible to use the same rope over and over, let's say 5 times in a row? Won't it 'wear' out? I know it return to its origenal shape after some time?
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  10. #30
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    Yes you can, but with severe snatches it's better to let it settle a while. For soft snatches you can use it all day long as many times as you like. Don't be too cautious or you'll never have fun. Just stand out of harms way in case I'm wrong.

  11. #31
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    Quote Originally Posted by XJ Junkie View Post
    Yes you can, but with severe snatches it's better to let it settle a while. For soft snatches you can use it all day long as many times as you like. Don't be too cautious or you'll never have fun. Just stand out of harms way in case I'm wrong.
    Thanx for the info, I rather ask now and be little wiser as to make a mistake and hurt someone
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  12. #32
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    Just don't believe everything you read but always be safe just in case. Try to look at doing a proper training course before you attempt any serious recovery situations. The knowledge is cheaper than replacing a back window.

  13. #33
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    Quote Originally Posted by Peet Hendriks View Post
    Energy[J] = force[N] x distance[m]
    load [kg] = force [N] / 9.81

    So again, stretchability is important - the less the stretch the higher the force for the same amount of energy.
    That why question in the first place
    Johan Kriel

    LCs, Hilux and Echo Chobe

  14. #34

    Default Re: DANGER! energy absorbed on snatching!

    ... resurrecting a sticky and an old thread...

    Recovery force calculations can be made quite accurately, but we will always wonder what would it be in "real-life"
    So a few of us set out to Atlantis Dunes, equipped with a 10 ton load cell and some recovery gear.
    The expected force at 20 km/h: about 3 ton.

    1st "real-life" test was as follows:

    Load Cell: Calibrated 10 ton with peak hold function.
    Vehicle: 4800 Patrol wagon (petrol 6 cyl), Weighing 2700kg (3 people, including extras.....)
    Tyres: 285/70R17 Cooper STT Pro
    Tyre pressure: 0.6 Bar, front and rear
    Surface: Damp compacted sand. (back of Atlantis, where the big Bloekoms are)
    Anchor point for Load Cell: One very large Bloekom, 10 Ton Tree-protector, plus 2x safety lanyards.
    Patrol recovery points: 2x bolted on chassis (calculated at minimum 6 ton each) with 8t Equalizer, plus 2x safety Lanyards on towbar PIN (not ball)
    Spliced Kinetic rope from Rope World: new 9 meter long, rated at 11 ton breaking. Expected stretch at recovery, +30%
    Slack in rope: 2 meter
    Speed: to be determined but by accelerating as fast as possible in 2nd Low

    And the result was:

    1st surprise was the max speed obtained: 35km/h
    Load at peak: 4.27 ton
    Rope stretch: 45% (Total length = 13 meter)
    Rope, recovery length/time: forgot to check at intervals, but a few days later it was at 9 meter

    2nd Test:
    (After we decided that maximum acceleration in the Patrol was to much)

    Spliced End Kinetic Rope no. 2: Used but with no visible defects (bought from a big name 4x4 outlet)
    Rope Rating: 11 Ton, 9 meter long.
    Vehicle: Patrol as above
    Speed: 20 km/h; achieved by no slack in the rope and slowly building up to 20 km/h

    Result:

    Load at "peak": 3.04 ton, and then the rope broke at the load cell end of the splice point!!

    Thereafter we moved to the soft sand, as the STT Pro's found too much traction on the "black" damp sand....
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  15. #35
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  16. #36
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    Default Re: DANGER! energy absorbed on snatching!

    Quote Originally Posted by Kiri View Post
    ... resurrecting a sticky and an old thread...

    Recovery force calculations can be made quite accurately, but we will always wonder what would it be in "real-life"
    So a few of us set out to Atlantis Dunes, equipped with a 10 ton load cell and some recovery gear.
    The expected force at 20 km/h: about 3 ton.

    1st "real-life" test was as follows:

    Load Cell: Calibrated 10 ton with peak hold function.
    Vehicle: 4800 Patrol wagon (petrol 6 cyl), Weighing 2700kg (3 people, including extras.....)
    Tyres: 285/70R17 Cooper STT Pro
    Tyre pressure: 0.6 Bar, front and rear
    Surface: Damp compacted sand. (back of Atlantis, where the big Bloekoms are)
    Anchor point for Load Cell: One very large Bloekom, 10 Ton Tree-protector, plus 2x safety lanyards.
    Patrol recovery points: 2x bolted on chassis (calculated at minimum 6 ton each) with 8t Equalizer, plus 2x safety Lanyards on towbar PIN (not ball)
    Spliced Kinetic rope from Rope World: new 9 meter long, rated at 11 ton breaking. Expected stretch at recovery, +30%
    Slack in rope: 2 meter
    Speed: to be determined but by accelerating as fast as possible in 2nd Low

    And the result was:

    1st surprise was the max speed obtained: 35km/h
    Load at peak: 4.27 ton
    Rope stretch: 45% (Total length = 13 meter)
    Rope, recovery length/time: forgot to check at intervals, but a few days later it was at 9 meter

    2nd Test:
    (After we decided that maximum acceleration in the Patrol was to much)

    Spliced End Kinetic Rope no. 2: Used but with no visible defects (bought from a big name 4x4 outlet)
    Rope Rating: 11 Ton, 9 meter long.
    Vehicle: Patrol as above
    Speed: 20 km/h; achieved by no slack in the rope and slowly building up to 20 km/h

    Result:

    Load at "peak": 3.04 ton, and then the rope broke at the load cell end of the splice point!!

    Thereafter we moved to the soft sand, as the STT Pro's found too much traction on the "black" damp sand....

    So glad someone has finally done some of proper tests!

    How did you measure elongation / stretch - got to say, 45% is more than I'd expect - but does explain the relatively low forces!

    must be a hell of a thing to pull to a dead stop from 35km/h in 4m or so

  17. #37

    Default Re: DANGER! energy absorbed on snatching!

    I was actually more surprised of how high the load was. I was thinking more along the lines of 3 ton, but the "too high" speed made the difference.

    Distance was measured; firstly by taking up all slack and making a mark on the ground at the front of the front tyres. The Patrol was then reversed back to allow a slack of 2 meter in the rope. At going forward a clear impression was left in the damp sand where the Patrol came to a very brief stop and did a little digging before the kinetic energy made me go the other way.... That distance was an increase of 4 meter (snatch rope is 9 meter at rest). The snatch rope was the only stretchable rope, nylon, the rest were Polyester.

    It was a quick stop but not a sudden one, but the kinetic pull-back was quite sudden when it happened...
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  18. #38
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    Default Re: DANGER! energy absorbed on snatching!

    Quote Originally Posted by Kiri View Post
    Spliced Kinetic rope from Rope World: new 9 meter long, rated at 11 ton breaking.
    This is very interesting for me because I have that exact same rope in my recovery kit.

    Used it about a dozen times now, always surprised at how "cushioned" a recovery feels with it.
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    Last edited by Johan Slabbert; 2016/06/05 at 02:05 PM.

  19. #39
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    Default Re: DANGER! energy absorbed on snatching!

    Quote Originally Posted by Kiri View Post
    I was actually more surprised of how high the load was. I was thinking more along the lines of 3 ton, but the "too high" speed made the difference.

    Distance was measured; firstly by taking up all slack and making a mark on the ground at the front of the front tyres. The Patrol was then reversed back to allow a slack of 2 meter in the rope. At going forward a clear impression was left in the damp sand where the Patrol came to a very brief stop and did a little digging before the kinetic energy made me go the other way.... That distance was an increase of 4 meter (snatch rope is 9 meter at rest). The snatch rope was the only stretchable rope, nylon, the rest were Polyester.

    It was a quick stop but not a sudden one, but the kinetic pull-back was quite sudden when it happened...

    IT's in line with what I've always suspected.... the stretch amount surprised me though....

    effectively 10m/s = 36km/h... decelerate to zero (assuming constant deceleration) is a 5m/s average speed.

    so, stop in 4m, t = s/v = 4/5 = 0,8 seconds

    s = (at^2) / 2
    4*2 = a*0,64
    8=0,64a
    a= 12,5 m/s/s

    mass of 2700kg

    F=ma

    F = 2700*12,5 = 33 750N = 3443kg (assuming g=9,8m/s)

    so... you got 4.27tons in a real life test - as the deceleration is NOT linear on a snatch strap that would explain the higher value (i.e. it's resists more the longer you make it)

    I'd have expected the rope to stretch less to be honest , which would increase the load significantly.
    Last edited by Apocalypse; 2016/06/05 at 05:06 PM.

  20. #40
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    Default Re: DANGER! energy absorbed on snatching!

    Very informative

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