DANGER! energy absorbed on snatching!

[Engel]

How much energy does a snatch strap have to absorb on a snatching pecedure?

We have had a discussion on this and please read through to give you a better indication of the kind of energy involved when smatching.

To start off here is an extract from bartho's site, www.4x4tec.co.za

"The science behind it all:
Kinetic energy = Velocity^2(squared) x1/2 Mass
This means if a 2 ton(2000kg) land cruiser drives at 10km/h(2.78m/s) it will produce:
(2.78^2)x(1/2 x 2000)=7728.4 Joules of energy
Taking a 1000kg/g Suzuki at the same speed, gives us:
(2.78^2)x(1/2 x 1000) = 3864.2 Joules of energy
If we double the Suzuki’s speed to 20km/h, we get the following:
(5.56^2)x(1/2 x 1000) = 15 456.8 Joules of energy
This means eventhough the Suzuki is half the weight of the Land Cruiser, at double the speed it produces double the kinetic energy of the Land Cruiser and 4 times more energy it had at half the speed.
You need to be very careful when increasing the speed at which you do the recovery and don’t go too fast as the energy produced is 4 times more at double the speed."

Doug on safety equipment you have a 7 times safety rating ie a 2t bridal breaks at 14t. Now the industry standard says a 8t breaking is good for a 2000kg vehicle. Most of the American manufacturers uses the 2 x safety factor on kinetic straps. The green pofadder is a 11t rope but i am bringing it down to 10t just for incase so 5t working is then a safe figure. Now the problem on snatching starts on the above formula. So when I snatch at a fast walking speed of lets say 8 to 10 km/h I have x amount of energy. When you double the speed it goes up exponentially and now double the speed gives us 4 times the energy!

So when used correctly a snatch is an amazing tool. But now a vehicle is severely stuck so what do we do now? we get closer to the stuck vehicle and we accelerate more and more. So now we take a big laden vehicle like a kroozer that weighs in at 3t. That kroozer at 20km/h now produces 46370 joules of energy. Compare that with the suzuki @ 10 km/h that now becomes 12 times more energy so you now need a rope 12 times stronger than the suzuki at 10 km/h vs the loaded kroozer at 20km/h. So a little faster and heavier produces tonnes and tonnes of more force on the rope.

We will start working out the deceleration of the vehicles of 3000kg coming to a standstill within 2m from 10km/h and 20km/h to 0km/h. Ill look for the formulas tonight [bit rusted]

Please only add usefull info. This thread is no attack on any-one's recovery abilities.

Hope that makes it clear as mud to you

[Barto]

Hopefully by the end of the weekend there will be an imput form on my website where you can imput your vehicle's weight and the speed of the recovery to see you much force you will produce during the recovery.

[RedLineR]

I'd like to add that those energy figures are pure movement of weight.

Snatch straps/ropes have a hard life. And if you don't help a bit by doing a bit of shovel work before you start yanking...

Herman is absolutely right.
It's the same as Nieko's thread about 4th and 5th at same rpm.

Because it's velocity^2, it has a major influence on the snatch(energy). It's not linear but exponential.
You are building kinetic energy in the pulling vehicle, and transmitting that energy to the stuck vehicle through the snatch rope.
But the energy transmitted at 15km/h is 9 times more than at 5km/h.

Out of interest:
Herman, what is the official policy on failed snatches? ie, your attempt failed and the vehicle is still stuck. Do you allow the full period of rest for the rope? Do you immediately use another rope? Ican only imagine that "continue yanking till you get it out" is a no-no

[Engel]

Im not worried about repeated snatches. We get overeager and go look at all the videos on straps/ropes breaking is when a vehicle is severely stuck and the snatching process becomes more violent. That does not exclude me. And that is why you need blankets and bridals. Remember a recovery point that weighs a few grams becomes a missile. A shackle that comes loose at snatching produces more KE than a 30-06 bullet and we know what a bullet can do. The whole reason of this thread is for you to put your duckies in a row BEFORE you start jerking and snatching.

[Engel]

Hier's n tabel wat Riaan gepost het wat ook baie handig te pas sal kom

[Nobbi]

Herman,
the little flyer we hand out during our recovery training and I found them very useful. What a lot of people never consider when talking about recovery, especially when it comes to snatching (or smatching like you wrote in your first post here ) is the factor of "stuck" and "surface" resistence plus (maybe) gravity.
I also found that a lot of 4x4 drivers totally under estimate the weight of their vehicle (dry mass and especially when loaded).
You are absolutly right in saying that a snatch rope is an amzing tool when used correctly!
I only can recommend to make a printout of that little flyer which Herman posted here and keep a copy in your car. You might need it one day and then it will come in handy.
And the next thing which comes into play is those recovery points people might have or might not have on their cars. Use that little flyer and make some calculations on your own and then find out if your recovery points will do the job in case they have to.
And remember: A snatch should be only done after you failed digging yourself/somebody else out of the situation! Stay safe!

/
Nobbi

[Mike_P]

Using information gleaned from the www and the informative link http://www.whyalla4wd.org.au/Shackles.html I created the graph below for a 9m (8000kg breaking strain) snatch strap that relates the peak tension to the recovery vehicle’s mass and speed.

If my calculations are correct then using these data one would be able to limit the speed of the recovering vehicle to ensure that the working limits of the strap, shackles and recovery points are not exceeded.

[Apocalypse]

there is no danger from energy absorbed by a snatch strap during recovery.

It's the releasing of that energy that will hurt you!!!

[OFFROAD ADDICT]

What Hermon is saying in a nutshell, although we are taught to use the right methods, , use the right equipment , you still take precautions. Nothing is fool proof,

[Apocalypse]

What Hermon is saying in a nutshell, although we are taught to use the right methods, , use the right equipment , you still take precautions. Nothing is fool proof,

It's very important to contain any kind of release.

one should definitely always use a condom.

[SAROD]

Ives never looked at my speedo during a recovery, but its interesting info on that graph.

[Apocalypse]

Ives never looked at my speedo during a recovery, but its interesting info on that graph.

if you are looking at your speedo during a recovery, you are doing it wrong.

anyway, isn't this why they made rev limiters?

[Apocalypse]

Ives never looked at my speedo during a recovery, but its interesting info on that graph.

I would also ignore the graph.

E=1/2mv^2 or F = ma (mass * acceleration , acceleration being a squared measurement) means you could expect a rising or curved (upwards) graph , not a linear graph...

[Engel]

Like you said, the more you get to the end of the elasticity the bigger the chances of failure of the rope. Everything has a breaking point. Just push it.

[OFFROAD ADDICT]

A crash helmet is now part of my recovery kit

[Mike_P]

I would also ignore the graph.

E=1/2mv^2 or F = ma (mass * acceleration , acceleration being a squared measurement) means you could expect a rising or curved (upwards) graph , not a linear graph..

Yes the kinetic energy increases with the square of the velocity of the recovering vehicle however it is spread over the stretch of the strap that will increase with this energy increase in such a way that the peak stress in the strap increases in a linear fashion.
Have a look at the maths on the web site I mention or http://hyperphysics.phy-astr.gsu.edu/hbase/pespr.html

I was also surprised by the result but it makes sense when you think about it.

[Mike_P]

One interesting aspect coming out of my analysis is that for a 9m long 8000kg (breaking strength) kinetic strap a 2000kg recovery vehicle would, in theory have to be doing more than 40km/h to break the strap.

I guess one would need some fancy vehicle to achieve 0 to 40km/h over a few meters!
Perhaps something like a Fortuner!

However I suspect the strap would no longer obey "Hooks Law" at such a load and therefore the analysis would not be accurate.
(SecureTec specify their maximum stretch as 20% or 1,8m for the 9m strap)

[FLOOI]

I agree with the math so far but the figures quoted are only for the weight of the vehicles.
Should the power of the engine/drivetrain not also be added as very few people will lift off the accelerator or depress the clutch at the moment of tension on the strap.

[ThysleRoux]

One interesting aspect coming out of my analysis is that for a 9m long 8000kg (breaking strength) kinetic strap a 2000kg recovery vehicle would, in theory have to be doing more than 40km/h to break the strap.

I guess one would need some fancy vehicle to achieve 0 to 40km/h over a few meters!
Perhaps something like a Gobabis Fortuner!

However I suspect the strap would no longer obey "Hooks Law" at such a load and therefore the analysis would not be accurate.
(SecureTec specify their maximum stretch as 20% or 1,8m for the 9m strap)

Phixed

[Engel]

One interesting aspect coming out of my analysis is that for a 9m long 8000kg (breaking strength) kinetic strap a 2000kg recovery vehicle would, in theory have to be doing more than 40km/h to break the strap.

I guess one would need some fancy vehicle to achieve 0 to 40km/h over a few meters!
Perhaps something like a Fortuner!

However I suspect the strap would no longer obey "Hooks Law" at such a load and therefore the analysis would not be accurate.
(SecureTec specify their maximum stretch as 20% or 1,8m for the 9m strap)

"in theory" and "my experience" does not come close. Depending not really on your speed but rather the energy iow the speed and weight combined. A jimny at 20km/h does not produce the same energy as a loaded patrol at the same speed. Then also how stuck the vehicle is and how soon it "releases" after force is applied.


The problem is speed or too much. Lets say my Patrol weighs in at 3000kg loaded. I recover at 10km/h [2.7m/s] the first time and then at 25km/h [6.9 m/s] then get gatvol and get to 40km/h [11m/s].

Ek=1/2*m[kg]*vSQ[m/s]

At 10km/h

Ek=.5*3000*2.7sq
=10 935 J

25km/h

= 71415 J

and at 40 km/h

=181500 J

So by going 4 times the speed equates at 16.59 times more energy that you have to stop now. No standard kinetic strap will withstand the deceleration of a 181500 J in 2 to 3m. Something will break.