Scary stuff

[frp88]

the other thing too remember is how diffrent the tow bar are fitted to the vehicle

[DR Frankenstine]

the other thing too remember is how diffrent the tow bar are fitted to the vehicle

What ???

[HotFourOk]

F = M x A

= 1000KG x 7.84m/ss
= 7840KGM/ss

so a 1tonne trailer behind a car doing 100Km/h exerts 7.84tonnes of force on the tow ball.

You can't just calculate a force in Newtons then say that is the mass in kilograms acting on the towball.

As you stated, a force is made up from a mass and an acceleration.
The units are Kg.m.s-2 ... so you cannot neglect the other components by just saying Kg.

If you actually mean kilogram-force... 7840N is equivalent to 800 Kg-f acting on the towball.

[lay80n]

As if a trailer braking from 100km/h is ANYWHERE NEAR a force used in a snatch recovery...

F = M x A

Braking distances are calculated using a friction coefficient of 0.8xG. this is based on new (very good) tyres on a dry road.

So the braking force is 0.8 x 9.8m/ss = 7.84m/ss

F = M x A

= 1000KG x 7.84m/ss
= 7840KGM/ss

so a 1tonne trailer behind a car doing 100Km/h exerts 7.84tonnes of force on the tow ball. Remember, this is the actual force and can actually happen.

Even assuming a braking coefficient of 0.5xG (mud tyres prolly) the force is 4.9tonnes.

And this is on a 50mm ball rated at 1tonne. So what can a 3.5tonne ball handle? As you say a 3.5tonne trailer is required to have brakes so the same maths cannot be used. But assuming the brakes on the trailer failed, that 3.5tonne trailer would be pushing the car infront with a force of 27.4tonnes.


7.84tonnes is 57% more than a 10,000LB snatch strap is certified to handle.

Just out of curiosity, shouldnt this be 7840 Newtons of fource, Where did the tonnes of force come from.

Layto....

[sudso]

So who's right here?
Tow balls and tongues have been known to snap off during a snatch recovery, not just the 2 piece balls either.
Also the construction of some towbars wouldn't like taking the forces of snatching too often. i.e if the recovery was at the wrong angle the strap can twist the tow bar down and brake it, off it comes at a million miles an hour.

Not saying some tow balls cant take the load, if none did then we'd be hearing stories of accidents with them all the time as it's obvious a lot of people do it this way.
I myself wont be recovered this way or would I recover someone else like that simply because there are safer ways and the fact that tow balls, tow bars and tongues have broken from snatching in the past.
Loop of the strap thru the hitch sleeve and pinned or straight on rated recovery hooks for me please. No shackles either, Ever!
If there's no rated hitch points on the other vehicle I'd just winch myself out

[eye-for-detail]

crikey
after all that
2 questions
1: What do the pins holding the tongue on a class 4 towbar rate at &
2: haven't read anyones' experiences with towballs flying off yet

[HotFourOk]

1: What do the pins holding the tongue on a class 4 towbar rate at

If a peice of metal is not stamped, it is not rated.

[ISUZUROVER]

crikey
after all that
2 questions
1: What do the pins holding the tongue on a class 4 towbar rate at &
2: haven't read anyones' experiences with towballs flying off yet

As I wrote in the other thread

has anyone EVER seen a towball fail? not read it on the internet or heard it from so-an-so down the pub? i think its become more of an internet myth without any real stories to back it up

btw i've done it before, mainly on the beach where its quick and easy and saves pulling out shackles and the loads are light. but all the rigs that we go wheeling with regularly dont even have towballs, so we use the hooks/eye's

There was an article in the courier mail a while back, about a fraser resident who died when he was hit in the head by a tow-ball that broke off. He has bogged his 4x4 on the beach and was trying to recover it backwards using a snatch strap attached to the tow ball on the 4x4.

There was another case mentioned on exploroz (oh no, I only read it on the internet) - where a guy said he saw an attempted towball recovery - the tow ball broke off, went through the both f/r windscreens of the vehicle doing the recovery, lightly clipping the driver on the ear on the way through.

[HotFourOk]

I was just pondering this question....

Why did you use acceleration due to gravity to calculate F=ma?

It should be the acceleration of the vehicle.. not due to gravity.

Using only motion in the horizontal plane...

We take a new sports sedan which brakes from 100km/h to 0km/h in 6 seconds. (This will of course be a lot greater for a heavier car and a vehicle towing a 1000kg trailer).
This gives us an acceleration of -4.67m/s/s, being generous.

Anyways, using this figure, this gives a force of 4670N on the towball. (Neglecting friction forces in the trailers wheels/tyres etc)

Now i don't know how you assumed Newtons = kilograms...

But this shows you, it is much lower than you calculated.

Inertia must then be calculated for the trailer, and the force calculated which is opposing the normal force supplied by the towball....

[GUJohnno]

No doubt a NEW tow ball is very strong. Probably capable of with standing the forces.
But how would this strength be effected with a couple of scratches and marks that you would normally get on it if you use it for towing?

The less links in the chain the safer it will be. By using the tow ball and this shakle you are adding three links to the chain as opposed to using the pin through the eye of the strap.

Just use your common sense at take extreme care in any recovery situation.
BTW. I have had the GU Nissan front recovery hook brake off before. Luckily, when I went to use it it had already broken, otherwise it would've become a very dangerous missile....

[sudso]

1: What do the pins holding the tongue on a class 4 towbar rate at

If a peice of metal is not stamped, it is not rated.

I'd be more worried about shearing the weld that joins the tongue and the hitch sleeve.

1: What do the pins holding the tongue on a class 4 towbar rate at

The haymens are high tensile.
When you hook the snatch on the tow ball it is still putting the same pulling force on the pin! It's still there holding the sleeve, the tongue and the tow ball.
I agree with GUjohnno, the less links in the chain the better when snatching, just in case something does break.

[Shadow]

A Newton is defined as 1 KGm/ss http://en.wikipedia.org/wiki/Newton_(unit)

Gravity is used to calculate the firctional force applied to the tyres (thus the vehicle) during braking.

The acceleration of the vehicle is directly related to the frictional force between the tyres and the road surface.

It is accepted in the automotive industry (and used in pretty much all manufacturers calculations in vehicle braking specifications) that a vehicle will have an acceleration due to the frictional force of the tyres with the road of 0.8 x G. THis is of course on a dry road with good tyres etc.

[ISUZUROVER]

The haymens are high tensile.
When you hook the snatch on the tow ball it is still putting the same pulling force on the pin! It's still there holding the sleeve, the tongue and the tow ball.
I agree with GUjohnno, the less links in the chain the better when snatching, just in case something does break.

There are a number of differences between snatching from a tow ball and a hitch pin:
(1) The hitch pin is in DOUBLE SHEAR, the tow ball only in SINGLE SHEAR
(2) The load on the tow ball is being applied further away from where the shear stress is applied, introducing some leverage
(3) The pin is locating the strap securely
(4) As stated, one less link in the chain

Hot4 - much better go at the maths. So, for a 1tonne trailer, we have an absolute maximum of about 450kg load on the tow ball. Compare that to snatching a 4x4 - not only are you trying to pull something much heavier than 1t (even a sierra would weigh more?), it is also stuck, requiring even further force to move than would be implied by its mass alone. As well as that you are using a device which applies significant shock loading.

[Shadow]

Just out of curiosity, shouldnt this be 7840 Newtons of fource, Where did the tonnes of force come from.

Layto....

Kilograms or Pounds is a measure of mass, or the force of gravity (9.8m/s/s) exerted on a mass.

I used Newtons as Kilograms simply to relate the calculation back to the towball rating.

What does a towball rated at 3.5tonne mean? Does this mean it can carry 3.5tonnes of weight with gravity exerted on it? if so then a 3.5tonne towbal is actually rated at closer to 35,000N.

[rangemann]

As well as that you are using a device which applies significant shock loading.

Are you talking about a snatch strap? Isn't the point of a snatch strap to reduce shock loading. The same advantage as jumping of a bridge with bungee cord instead of a rope tied round your legs?

Perhaps I've been using the term shock loading in the wrong way....

And just for the discussion, I snatch according to the conditions and from anything that's accessible. If you're unsure just duck your head below the dash

[Jimbo]

Shadow wrote:
Jimbo wrote:
Under straight shear the towball is very strong but its under bending as well shich greatly reduces its strength


recovery hooks are under the same stress and i dont know anyone thats bent a recovery hook.


Didnt you just read that towballs are not made for sudden forces.. whereas recovery points/gear is designed to cope with these forces.

JUST DON'T USE IT! It is really so simple.

If you do use your towball.... dont expect any sane person to snatch you out "

Why did you have a go at me??? I'm trying to say that a tow ball is not safe!!!!!!

Shadow your calcs are a good start BUT what ISUZUROVER said is right about the different forces.


A snatch strap DEFFINATELY applies more of a shock than towing a trailer (even under braking). You ahve to consider the forces needed to pull a heavy vehicle from a standstill out of deep thick mud. That initial shock to get the vehicle moving is huge...Have you not see what happends to rated shackles that break off during a snatch gone wrong???? and they are rated at 3.2 tonne but their breaking strain is more like 28 to 30tonne!!!! They still fail before 10,000lb straps.

There is no use having an argument, putting a shackle around a tow ball is asking for some serious trouble (initial point of post)

Jimmy

[-Scott-]

As if a trailer braking from 100km/h is ANYWHERE NEAR a force used in a snatch recovery...

F = M x A

Braking distances are calculated using a friction coefficient of 0.8xG. this is based on new (very good) tyres on a dry road.

So the braking force is 0.8 x 9.8m/ss = 7.84m/ss

F = M x A

= 1000KG x 7.84m/ss
= 7840KGM/ss

so a 1tonne trailer behind a car doing 100Km/h exerts 7.84tonnes of force on the tow ball. Remember, this is the actual force and can actually happen.

Even assuming a braking coefficient of 0.5xG (mud tyres prolly) the force is 4.9tonnes.

And this is on a 50mm ball rated at 1tonne. So what can a 3.5tonne ball handle? As you say a 3.5tonne trailer is required to have brakes so the same maths cannot be used. But assuming the brakes on the trailer failed, that 3.5tonne trailer would be pushing the car infront with a force of 27.4tonnes.


7.84tonnes is 57% more than a 10,000LB snatch strap is certified to handle.

As noted, F = M x a. You won't get your 0.8G deceleration with a brakeless 3.5t trailer behind a vehicle. To decelerate from 100km/h (27.8m/s) to standstill at your 0.8G means you'd pull up in 3.5 seconds. Isn't going to happen.

Also, a 3.5 tonne bar is designed to drag a 3.5 tonne trailer - with wheels, and only a certain amount of rolling resistance. That doesn't mean it's rated to vertically lift a 3.5 tonne dead weight.

As for snatching from a tow ball, and a "modern" 3.5 tonne ball being OK? Perhaps it is. But some dufus who barely has two brain cells to rub together may see you snatching from your one piece 3.5 tonne ball, and decide that he can do the same from his 1200kg two piece ball which has been on the back of his 40 series for thirty years.

Snatching at any time is potentially deadly. We should all be doing as much as we can to minimise risks, which includes setting an example for others to follow.

Don't use a tow ball.

Scott

[Shadow]

Shock loading as I understand it is defined as an impact force, or a large force applied over a short time, like hitting a nail with a hammer.

I belive its completely the wrong term to be used when speaking of a snatch strap or any device which is elastic. A snatch strap is an ellastic force and is very much not shoock loading.

http://www.landyspares.co.za/files/snatch_straps.htm

This page has some very good calculations which tell us that a v8 landcruiser using a 9m snatch strap with 20% stretch will generate an elastic force in the snatch strap of about 4.2tonnes. This is based on the v8 landcruisers acceleration of 3m/s/s(0-100 in 11seconds).

So is a force applied during a snatch recovery greater than that applied during an emergency braking situation with a trailer coupled? Very similar according to my calculations, however there are a large number of uncertanties.

The snatch strap strech rating, if it only stretches 15% instead of the industry standard of 20%, then the force would be about 6tonne.

What rating is the towball? What does a towball rating mean (still cant find exactly what a tow-ball rating of 3500KG actually means.)

I was merely looking at the device from an engineering viewpoint. Everyone was quick to shoot his device down but no-one could provide data which said his device was bad/wrong/lethal. My calculations tell me his device is operating within the working limits of a towball. But i do agree with others that say it increases the number of uncertanties and potential problems in a recovery.


I have and always will simply use the hitch pin as my rear recovery point as it takes me about 20 seconds to hook a snatch strap up this way, and certainly looks alot safer ato me as the strap cannot slip off the end of a ball etc.

[HotFourOk]

Isn't the point of a snatch strap to reduce shock loading.

No.

The point of a snatch strap is to store kinetic energy when it is stretched.

This stored energy is then realeased when the maximum elongation is reached (usually 20%), which in turn produces more snatching force compared to a solid tow strap.

A side effect of using a snatchy is that it removes the jolt as if you were to use a solid strap and started to drive away in a recovery (which you should never do), but this is not the main benefit.

[HotFourOk]

My calculations tell me his device is operating within the working limits of a towball. But i do agree with others that say it increases the number of uncertanties and potential problems in a recovery.

Your calculations were found to be incorrect, and you still have not confirmed how you converted the Force into a kilogram number.

You said 1 Newton = 1 tonne (1000Kg)...

I dont see how this works, units do not cancel.

Kg.m/s/s = Kg ....... Is this quation balanced?

Even kindergarten Johnny can see it is not.

Please clear this up for me.

Nick.

[HotFourOk]

Does this mean it can carry 3.5tonnes of weight with gravity exerted on it? if so then a 3.5tonne towbal is actually rated at closer to 35,000N.

You have your terminology mixed up man.

Weight already has gravity acting on it. Weight is a force.

Mass is a number which describes how heavy an object is.

Weight is Mass multiplied by the gravitational effect of the place in question.

Your statement should read: "3.5tonnes of mass". Which gravity then acts upon. Weight already has gravity taken into account.

[Jimbo]

No.

The point of a snatch strap is to store kinetic energy when it is stretched.

This stored energy is then realeased when the maximum elongation is reached (usually 20%), which in turn produces more snatching force compared to a solid tow strap. "

Bugger you beat me to it...i wsa gonna say that but i would have explained it too long and gone off the point. But you are right!!!

That link is crap http://www.landyspares.co.za/files/snatch_straps.htm They did not take into consideration the force required to actually move the vehicle from the bog situation ie the suction from mud)

This is some info from another tech site. Not saying its 100% accurate but gives you an idea
(it does assume a few things though)
Stuck (mire) resistance:

A pull of 100% of loaded weight will be required if the truck is stuck to a depth of the sidewall on the tires.

A pull of 200% of loaded weight will be required if the truck is stuck to the hubs.

A pull of 300% of loaded weight will be required if the truck is stuck to the frame.. http://www.pirate4x4.com/tech/billavist ... index.html

Plus he (first link) uses 9 meters of distance to get up to his 17km/h speed which i feel is too long. Most of the speed is gained in the first couple of meters.

Anyway back to work boss is looking at me funny

Jimmy

[Shadow]

That link is crap http://www.landyspares.co.za/files/snatch_straps.htm They did not take into consideration the force required to actually move the vehicle from the bog situation ie the suction from mud)

Thier calculations assume the stuck vehicle does not move.

If the vehicle does move the energy stored in the strap will be less.

[Shadow]

My calculations tell me his device is operating within the working limits of a towball. But i do agree with others that say it increases the number of uncertanties and potential problems in a recovery.

Your calculations were found to be incorrect, and you still have not confirmed how you converted the Force into a kilogram number.

You said 1 Newton = 1 tonne (1000Kg)...

I dont see how this works, units do not cancel.

Kg.m/s/s = Kg ....... Is this quation balanced?

Even kindergarten Johnny can see it is not.

Please clear this up for me.

Nick.

ok,

What does a snatch strap rating of 8000KG mean, as you say KG is not a force.

I said 1 Newton = 1KG because I believe that is what is meant in the rating of the snaatch strap and the tow ball. Is a Snatch strap actually rated at 8000N ?

[Jimbo]

By the way 1kg = 9.8Newtons


"Thier calculations assume the stuck vehicle does not move.

If the vehicle does move the energy stored in the strap will be less" Do you know what you are talking about? Honestly (not having a go)

jimmy

[lay80n]

My calculations tell me his device is operating within the working limits of a towball. But i do agree with others that say it increases the number of uncertanties and potential problems in a recovery.

Your calculations were found to be incorrect, and you still have not confirmed how you converted the Force into a kilogram number.

You said 1 Newton = 1 tonne (1000Kg)...

I dont see how this works, units do not cancel.

Kg.m/s/s = Kg ....... Is this quation balanced?

Even kindergarten Johnny can see it is not.

Please clear this up for me.

Nick.

ok,

What does a snatch strap rating of 8000KG mean, as you say KG is not a force.

I said 1 Newton = 1KG because I believe that is what is meant in the rating of the snaatch strap and the tow ball. Is a Snatch strap actually rated at 8000N ?

There is no saftey standard applied to straps, unlike as1353.1. that applies to lifting slings in industry, so in reality they can almost right anything within reason. 4wd monthly tested straps ages ago and there were a fair few big names that broke up to 2 tonnes early. I would guess that the ratting of 8,000KG or whatever is what that strap would support , as in you could pick up 8 tonnes with it before it would snap. The equvalent force of 8 tonnes being - F=MA : F=8000x9.81 : f= 78,400N

Layto....

[lay80n]

My calculations tell me his device is operating within the working limits of a towball. But i do agree with others that say it increases the number of uncertanties and potential problems in a recovery.

Your calculations were found to be incorrect, and you still have not confirmed how you converted the Force into a kilogram number.

You said 1 Newton = 1 tonne (1000Kg)...

I dont see how this works, units do not cancel.

Kg.m/s/s = Kg ....... Is this quation balanced?

Even kindergarten Johnny can see it is not.

Please clear this up for me.

Nick.

ok,

What does a snatch strap rating of 8000KG mean, as you say KG is not a force.

I said 1 Newton = 1KG because I believe that is what is meant in the rating of the snaatch strap and the tow ball. Is a Snatch strap actually rated at 8000N ?

There is no saftey standard applied to straps, unlike as1353.1. that applies to lifting slings in industry, so in reality they can almost right anything within reason. 4wd monthly tested straps ages ago and there were a fair few big names that broke up to 2 tonnes early. I would guess that the ratting of 8,000KG or whatever is what that strap would support , as in you could pick up 8 tonnes with it before it would snap. The equvalent force of 8 tonnes being - F=MA : F=8000x9.81 : f= 78,400N

If im wrong feel free to whinge

Layto....

[Shadow]

By the way 1kg = 9.8Newtons


"Thier calculations assume the stuck vehicle does not move.

If the vehicle does move the energy stored in the strap will be less" Do you know what you are talking about? Honestly (not having a go)

jimmy

I think so.

They use busche borche whatevers law to calculate the force exerted on the centre of the snatch strap ( ----><---- ).

Thier calculations are a worst case scenario assuming that the snatch strap reaches its maximum stretch rating of 20%. In most recoveries this wont occur as the vehicle will be pulled free by the building energy in the strap. As soon as the energy stored in the strap is enough to overcome the forces holding the vehicle "stuck" the vehicle will come free which means the energy in the snatch will drop or at the very least plateau.

The force required to move thye vehicle is irrelevent in thier calculation as it assumes the stuck vehicle does not move, thus allowing the snatch strap to stretch to its absolute maximum stretch of 20% with the vehicle at some velocity at the end of the stretch. If the vehicle is still stuck and the pulling vehicle attempts to apply even more energy to the strap it will over stretch and snap.

Why does 1KG = 9.8Newtons? This is the only thing i dont get at the moment and is why i cant relate my calculations to the snatch strap and tow ball ratings, so i really am interested in this definition.

On Wikipedia it lists the unit of force "Kilogram-force or Kilopond" which is equal to 9.8 Newtons.

Are they actually saying a 8,000KG snatch strap can withstand 8,000KG's of weight? Where weight's actual scientifc unit is KGm/s/s also known as a Newton. So can a 8000KG snatch strap withstand 8000N ? or is it as you say 80,000N?

[-Scott-]

Why does 1KG = 9.8Newtons? This is the only thing i dont get at the moment and is why i cant relate my calculations to the snatch strap and tow ball ratings, so i really am interested in this definition.

On Wikipedia it lists the unit of force "Kilogram-force or Kilopond" which is equal to 9.8 Newtons.

Are they actually saying a 8,000KG snatch strap can withstand 8,000KG's of weight? Where weight's actual scientifc unit is KGm/s/s also known as a Newton. So can a 8000KG snatch strap withstand 8000N ? or is it as you say 80,000N?

Weight is one of those horrible "common" terms which gets used interchangeably with mass and force. We humans measure our "weight" in units of mass - but the devices are actually measuring the force exerted by the mass, due to gravity. Go somewhere with less gravity and your mass is the same, but your "weight" is less.

Straps and shackles etc should be rated in terms of force, but your average person has no concept of 80 000N. But 8000kg they can understand - so devices are rated according to the mass they are rated to lift, with gravitational constant of 9.8m/s/s (or thereabouts.)

This is where I think towball ratings fall down - they're rated according to the mass of the trailer they're rated to drag, presuming the trailer has wheels which roll easily. A 3.5t towbar won't necessarily pull a 3.5t mass along a surface with a friction coefficient of 1 - which is what would be required to generate 3.5t (equivalent) force in the towbar. This is also equivalent to using said towbar to drag said 3.5t mass up a vertical cliff.

Scott

[Jimbo]

8000kg = 78,480N on earth!

I'm pretty sure their calculations are way off

They give acc as 3m/s/s which may be fine as an average over the total time of the recovery but the initial shock from the strap will be higher as the initial acc will be higher. I'm talking about the first few milli seconds after the strap has been streched (to its full 20%). The shock on the strap during the 20% stretch maybe correctly calculated but after that is where things get interesting

In an easy snatch the vehicle being recovered will start moving befor the 20% elongation has been reached so there is not much shock, but for a really stuck vehicle the force needed to elongate the strap by 20% is not enough to move the stuck vehicle. So the strap stretches...stretches then bang it cant stretch any more and the shock that was once absorbed by the strap is transfered to the car.

Jimmy