The Dangers of snatch straps and the forces involved.

[GeeTee]

When lives are at stake I'm not going to mince words. I'd bet my CT that the fencing wire went on as a stop-gap solution and was simply left there.

The injured man could have been saved by one of these, not hard to come by. Or just weld the chain closed - a one million acre property will have a welder, and someone who knows how to use it - or how about just using a webbing strap, or a rope... anything but a snatch strap!

I'm not sure if a picture of a cheap, bright-plated, unrated D shackle is appropriate in this thread!   

[Bird]

Put together by AJ and his team yrs ago..

http://www.whyalla4wd.org.au/Shackles.html


Checking Stretch of Snatch Strap.

If all the Kinetic Energy of the Recovering 4WD is transferred to Elastic Energy in the Snatch Strap then the stretch of the strap can be determined. (Assuming no losses due to friction)

From Energy Conservation,

Kinetic Energy of Recovering 4WD = Elastic Energy in Snatch Strap
KE4WD=PEStrap
3472 J = 1/2 k x2
therefore x = 0.579m
or approximately 6% extension.

And from Hookes Law the Force Stored in the Snatch Straps is,
F = kx
= (2.075 Tonne/m) (0.579m)
= 1.2 Tonne



Determine Speed of Shackle if Snatch Strap Breaks.

If all the Elastic Energy of the Snatch Strap is transferred to Kinetic Energy in the Shackle then the peak Speed of the Shackle as the Snatch Strap returns to it's 9.0m length can be determined. (Assuming no losses due to friction)

From Energy Conservation,

Elastic Energy in Snatch Strap = Kinetic Energy of Shackle
PEStrap = KEShackle
1/2 k x2 = 1/2 m v2
where k = spring constant
x = distance stretched (meters)
m = mass of shackle (kg)
v = velocity of shackle (m/s)
1/2 (20750N/m) (0.579m)2 = 1/2 (0.68kg) v2
v = 101m/s

or Peak Speed of Shackle   v = 364 km/h

[Rumpig]

. Just sorry that the guy who had nothing to do with putting that contraption together was the one who copped the chain in the head.

where does it say he had nothing to do with putting it together?
being it's his property the accident happened on, wouldn't you think he knew exactly what was happening?
i was on that property a few months after the accident happened, and was having a drink with Rhett at his bar whislt he told me all about what happened, to look at him you wouldn't believe the person in the first photo is one and the same. Rhett was having some vision problem still at the time back then, but other then that looked pretty healthy to me. his boy (it says grandson in the article , but i pretty sure he was his son?) was driving the vehicle, Rhett saw what was happening but couldn't get him to stop in time, the rest is history.

The tyre must of hooked on something, the wire (assume it was 2.5mmHT) has a breaking strain of some 1100LB.

according to the conversation i had with Rhett, that's exactly what happened. the tyre got hooked or wedged on / between something (i can't remember exact details) and he didn't have time to tell his son (who was driving) to stop (or he didn't stop in time) before that accident happened. being the kid driving was only something like 16 years old from memory, it'll be a lesson he'll never forget after nearly seeing his old man die right in front of him.

[Jason B]

In addition to the shackle, a winch extention strap would also be alot safer as it wouldn't store the energy that a snatch strap would.

[Squalo]

where does it say he had nothing to do with putting it together?
being it's his property the accident happened on, wouldn't you think he knew exactly what was happening?

Hi Rumpig, I read the account of the accident quite a few months ago and did not remember the exact details - thanks for clarifying, and good to hear that he is recovering so well. I'm also appreciative of the fact that the accident details are online - they will get some flack about it, but it will also save others from making the same mistakes.

Here's why I am so hardnosed about snatch strap misuse - that little head sticking up in the back of my car is my daughters - 5 years old at the time. She was supposed to be ducking down low in her seat, but kids being kids...  you can see that the strap is at full extension with my car having been pulled over a metre to the left under tension. I nearly had a heart attack when I saw this pic, and I've always cleared the vehicle of extraneous occupants since this occasion. I didn't get them out because we were on a rising tide at Hook Point and literally only had minutes to save the other vehicle, but now I know better!

I got away with making a huge mistake there, and I'd hate to ever think that someone else might make a similarly disastrous mistake simply because I'd kept my mouth shut about the dangers of snatch straps.

[Rumpig]

Here's why I am so hardnosed about snatch strap misuse -....

all good with me mate, if more people were hardnosed about snatch straps, then we wouldn't see some of the "accidents" that we see in the media at times. a lot of people don't realize how dangerous they are, they just think connect it up and floor it to extract a stuck vehicle, if they had half an idea they'd know speed has nothing to do with safely using a snatch strap.

[jim-m-72]

Not good to see someone hurt like that but it is good to see him getting better and hope a lesson learned by all about saftey
Jim

[Squalo]

July edition of 4x4 Australia has a spread on Corella Station, Rhett discusses the accident in one paragraph.

[Grunta]

Not good

I always use a dampener bag over my snatch strap

[cheif carlos]

one thing to think about if you have wagon have a cargo barrier, they will reduce/stop this sort of thing from happening if you don't have gear piled to the roof. for those with a ute umm.

from a safety point of view they could place a board at the back of the car which could be mounted in the tow hitch for easily removal. make the tyre mount to this so that it cannot be used without the board.

Sorry I am stuck in safety mode all the time. I have seen too many idiots doing stupid things and not willing to listen to what other people have to say.

Jason

[BigJules]

I was watching a clip from All 4 Australia on youtube, his snatch strap broke and really smashed up his rear door. The damage done was like hitting a roo, and without even a shackle involved.

This one.

Australia's Best 4WD BOGS

[Symon]

Hardly surprising considering there was a knot in the strap!  Doesn't that derate the strap by a third or something?