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16 - Common 4x4 Myths Busted

Discussion in 'Podcast Episodes' started by Patriot, Jun 23, 2016.

  1. Patriot

    Patriot Super Moderator

  2. smokebelch

    smokebelch New Member

    Some good info but what happened at 41min 43sec? You were talking about Traction Control and it was just getting good when the podcast jumped to you talking about towball weights
     
  3. badams

    badams New Member

    Hi Folks,
    Found the talk on snatch blocks a bit confusing.
    I always thought part of the benefit was that a snatch block could double the pulling force. If the winch goes out around the snatch block then hooks back to the car, that effectively double the pulling power. See attached.

    In my case a winch that isn't really up to the job, so I've always relied on this to make up the short fall.

    Brett
     

    Attached Files:

  4. Tank

    Tank New Member

    To gain a Mechanical Advantage (MA) from a Snatch Block (SB) the SB has to be moving with the load, but you are gaining a MA using the SB as you described in your setup. The MA comes from the fact that, lets say you had your SB anchored to a tree 25' away, that would mean that you have run out 25' to the SB and 25' back to your 4wd, a total of 50'.
    Now if you have a look at the specifications that came with your winch when you purchased it, it should have the rated "Line Pull" for each layer of rope or cable wound onto the winch drum. So a 9000lbs. rated winch will have 9000lbs. capacity to pull with one layer on the drum, by the time you have wound 100' of rope or cable to a full winch drum, that 9000lbs. at the first layer is now down to around 3000lbs. to 4000lbs pulling power.
    By running out 50' of rope/cable you have increased the pulling power of the winch because you are closer to the centre of the winch drum. If you had just run 25' of rope/ cable out to the anchor point on the tree you wouldn't have had as much puliing power.
    But if you had run out 50' of rope/cable to a tree further away without the need of the snatch block, you would have gained the exact same winch pulling power and had the extra gain of not having 5% of the load through friction caused by the pulley on it's shaft in the SB.
    So same pulling power with 50' straight line pull with none of the friction caused by the SB and still have 50' to wind back in with SB, plus less damage to cable getting rolled around a pulley.
    The formula for working out the MA by using a SB is, "the number of parts of rope/cable SUPPORTING the MOVING block", so to get a MA of 2 with a single vehicle you will need 2 SB's one attached to an anchor point (tree up front somewhere), run winch cable or rope up to anchored SB through it and back down to a second SB attached to the stuck vehicle, through that SB and back to end at the first anchor point.
    Or if you have 2 vehicles you could attach the SB to the front of the stuck vehicle run your rope/cable down and through the SB and back to your vehicle, this is a MA of 2 and you are pulling to advantage (towards yourself) Now I have seen (too many times the first one) this setup called a 3 line pull, it is a 2 line pull as the number of parts of rope/cable supporting the moving SB is only 2, the third rope is called the Lead rope and does not support the moving SB.
    A single ANCHORED SB does not halve the load on a winch, they only allow the winch to work closer to it's centre of pull where it generates more pulling power. Using a SB to change direction of pull can be extremely dangerous if the triangle formed by anchoring a SB to a tree and pulling another vehicle at an angle, if you have to do this the triangle created will form an Apex at the anchored SB, if that angle is 90 degrees or more you may be doubling the load on the rope/cable and god knows what weight the anchor strap holding the SB to the tree is copping. Always try and keep this angle below 90 degrees, same principle applies to Bridle straps for towing, hope this helps, Regards Frank.
     
  5. Ali Ozbay

    Ali Ozbay New Member

    Hi Frank,
    What you said about a single SB not changing anything apart from decreasing the number of layers on the drum is not right. The SB does not have to move, because the in this case it is the car that is moving. SB does not increase the pulling power, to be more exact, it halves the tension on the rope, which is directly equal to the force the winch tries to overcome. You would be right in saying the SB does not make a difference only if you hooked the end of the rope to another stationary object instead of the car. But since the rope starts from the winch, goes thru the SB and then is hooked back onto the stuck vehicle, the tension on the rope is actually halved (assuming there is no friction loss due to SB). Basic rule, (as long as the end of the rope is hooked onto the car being recovered), number of lines between the car and the tree is the number by which the tension on the rope is reduced. If 3 lines run by using 2 SB, then tension decreases to 1/3 of that of a single line case. Again, omitting the friction in each pulley, which is not too great anyway.

    Ali
     
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  6. Tank

    Tank New Member

    Ali, if you read my post you should have noticed that I said that to achieve a Mechanical Advantage (MA) that the formula for calculating the MA of Pulleys/Sheave Blocks is "The number or PARTS of Rope/Cable SUPPORTING the MOVING Block".
    Now Ali, I didn't dream up or conjure this FORMULA out of nowhere.
    I am Quoting OH&S Riggers guide, Tafe riggers guide, every published document on Rigging blocks and pulleys, plus 40+ years as a Qualified Class 1 Ticketed Rigger. So Ali if I'm wrong, all these learned institutions and Newton's 3rd law of Physics, i.e. "for every action there is a equal and opposite reaction", is wrong also.
    Now that you have stated that what I posted and explained is wrong, how's about you proving that your Laws of Physics are correct and Newton is wrong.
    I can physically prove what I have quoted as correct, now it's up to you to do the same, just saying so doesn't show or prove anything, let's see your statement proved, BTW who told you that what you are saying is correct or did you just see someone do it and noticed the bogged vehicle pulled out easier than with a straight pull, it is easier because the winch is pulling from a closer point to the centre of the axle driving the winch drum. the same result (without 5% friction from the SB.
    All of which I have already said, so when you come back and prove YOUR idea is right I will admit I'm wrong along with Newton and all his mates, Regards Frank.
    Here is a Link you should check out:
    https://en.wikipedia.org/wiki/Mechanical_advantage_device
     
    Last edited: Jul 7, 2016
  7. Ali Ozbay

    Ali Ozbay New Member

    Frank, there is no need to get upset. You are simply missing an important point, and that is, the end of the rope is hooked back on to the recovered car in our case. That makes all the difference. Have a look at the sketches I attached. Sorry about the poor quality.

    You're not applying the rules of physics correctly. Please look at the wikipedia link you sent a second time. The hook is not attached to weight block in any of the examples in the picture. That is all that makes the difference. Try to think of it as such: There is a heavy load you want to lift with a rope. But the rope is not strong enough to lift it. In order to lift the load using the same rope, instead of running a single line off the load, you'll double or triple the number of lines running off the load. So effectively you'll decrease the tension on the rope. Still the same rope, but by doubling it up, you'll lift the weight. Same goes for the winch case.

    If you want to involve refer to Newton, the laws of physics and all that, try approaching the problem from energy&work point of view. To move the car a certain distance, you have to do a fixed amount of work. The work done wont change whether no snatch blocks are used or 1 SB is used or 2 SB are used. And work equals force times distance (W=F*x). Lets say the force you need to apply to the car to get it moving is equalt to 2T. And lets say you want to move the car a distance of "x".

    In the single line pull case (no SB) the work will be equal to W=(2T)*(x) = 2Tx
    In double line pull case (with one stationary SB hooked to the tree) W=(T)x(2x)= 2Tx

    The work stays the same in the second case. Because the winch will actually have to work 2 times more (as you have already explained). That is why it will keep applying the force for a distance 2 times the first case's distance.

    I don't know if there is any other way I can express the matter. But I strongly suggest you do some research as well. I'm not saying OH&S Riggers guide, Tafe riggers guide, every published document on Rigging blocks and pulleys are wrong. I'm saying that you got it all wrong.

    And as far as the degrees go, I'm a Civil Engineer, so I know a little bit when it comes to high school physics.

    Cheers,
    Ali


    image2016-07-07-051911-1.jpg image2016-07-07-051937-1.jpg
     
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  8. hiluxdriver

    hiluxdriver Well-Known Member

    cool, a "who's got the biggest d*** " argy bargy. Where's Captain Correction?
     
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  9. discomatt

    discomatt Well-Known Member

    Yep who wants some pop corn, salt AND butter if I remember correctly;)
    As far as the discussion I have NFI and would need a set of heavy duty scales to actually test the theories :p
     
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  10. Ali Ozbay

    Ali Ozbay New Member

    Hi Brett,

    If you listen to 38:10 in the podcast, you'll note that he bases his arguement on the case that you don't tether the hook back onto the car being recovered. And he is totally right. If you DONT hook the cable/rope back on to the recovered vehicle, you will NOT gain any mechanical advantage. And you'll end up puling the anchor point (i.e. tree) with a force almost 2 times (at the most, depending on the angle between the two sections of rope i.e. before and after the SB) . However, for this case to take place, the most possible scenario would be that the car being recovered will not have a winch, and another vehicle will recover the car while running the rope thru a snatch block while doing it.
    However, as I explained before, as long as the car being recovered has a winch, uses a snatch block AND ATTACHED THE HOOK ONTO ITSELF, there will be mechanical advantage.

    This is the difference.

    Frank, what do you think?

    Cheers,
    Ali
     
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  11. Tank

    Tank New Member

    To prove the statements I've made is simple, if you are bogged, instead of grabbing the old Snatch Block (SB) out and hooking it to a tree 25' away, run your cable/rope out 50' away and you will have the same MA as if you had run your rope or cable round a SB 25' away.
    I recover broken and bogged 4wd'ers for a living and if I'm on a steep hill that I can't tow up (30% or more) I will position my truck so that when I wind out enough cable to reach the broken 4wd I will only have one layer of cable left on the drum, because this is where the winch pulling power is at it's highest.
    Or if I can't get back far enough I attach my SB to the recovery point on the broke 4wd, run my cable down to the SB and back to a tree next to my truck. This way I have a TRUE MA of 2 and I'm pulling to advantage and from the strongest part of the winch's pulling ability.
    Yes you do get a MA using a SB 25' away hooked to a tree, But only because you have more rope out and lower on the drum, which I have already explained. Think about it a SB is just a bloody wheel, no magic gears inside to halve loads, because most 4wd'ers believe that a SB halves the bogged load, some commonsense is needed here./
    Running a wire cable around a SB also damages the wires, the outer part of the cable is stretched and the inner wires closest to the bottom of the pulley groove get compressed, heavy use of a SB, especially when you can get the same advantage with a straight pull, will cause quicker than usual deterioration of your cable/rope.
    Put the popcorn away I have outlined what I've learned over 40 years in the rigging game and what is available in text and technical books and Google an UTUBE what is taught at TAFE rigging courses, make up your own minds, I've said all I'm going to, Regards Frank.
     
  12. Ali Ozbay

    Ali Ozbay New Member

    Frank, as I have repeated many times, the case we're discussing here is a bogged car WITH A WINCH on it and no other rescue vehicles. I understand your case, and you're right in all you say when you recover another car using your winch on your truck. So I believe you misunderstood the scenario we have been discussing. This whole conversation started because you responded to Brett's question, which was about a car recovering itself using a snatch block, in a way that would be true if the car was being recovered by ANOTHER car, as in your daily case.

    However, still, you are wrong when you say "Yes you do get a MA using a SB 25' away hooked to a tree, But only because you have more rope out and lower on the drum". You get the mechanical advantage because of the reasons I tried to explain to my best abilities above.

    If the bogged vehicle uses a snatch block to recover itself:
    1- The total force required to move the car won't be halved (we both agree on this)
    2- But the tension in the rope will be halved (you don't agree on this, but it just will. Think about buying 6 bottles of wine and putting them in a single plastic bag. The handles will snap. But if you put them in a second bag, the number of handles will double so the load in each handle will be halved and happy days)
    3- The car will start to be pulled from TWO points, on which only half the total force is applied. Two separate points with half of the total recovery load on each. The sum makes (0.5+0.5=1) the total force. So the math still holds.
    4- Since the tension in the rope is equal to the force the winch will try to overcome, you can say the recovery capacity of the winch will be doubled by using a snatch block.

    Apparently I can't say enough times, so I'm saying again. This holds for the case where a car recovers ITSELF. This does not hold for when a vehicle recovers another vehicle by simply running the rope thru the SB, which is hooked to a tree.

    On the other hand, in the case where you run the cable off your truck, attach the SB onto the bogged car and run the cable back to a tree next to your truck is physically very similar to the case where a single car recovers itself with a SB. There is MA in both cases due to the same principle.

    Anyway, if you ever have to recover YOUR truck one day, using the winch ON IT, give this a go see what happens.
     
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  13. Ali Ozbay

    Ali Ozbay New Member

    Hi All,

    Just to clarify Franks comments which are
    "Using a SB to change direction of pull can be extremely dangerous if the triangle formed by anchoring a SB to a tree and pulling another vehicle at an angle, if you have to do this the triangle created will form an Apex at the anchored SB, if that angle is 90 degrees or more you may be doubling the load on the rope/cable and god knows what weight the anchor strap holding the SB to the tree is copping. Always try and keep this angle below 90 degrees, "

    Please have a look at the figures below. It is quite easy to misunderstand which angle is which. The term "change in direction of pull" might easily be interpreted differently by different people. So decided to explain with some sketches...

    First of all: The force required to free a car from where it's bogged does not vary with the angle of ropes you attached to it or somewhere else. It is constant in each case, as the variable determining the force are mainly, weight of the car, friction between the surface and the car, suction created when pulling and the angle of the slope etc.

    Secondly: What you can change by changing the direction of pull is the total force exerted on the SB and the tree. If you look at the diagrams below, you'll see that the maximum load on the SB is achieved when two segments of rope are parallel. As the angle between two segments of rope increase, the total force exerted on the SB decreases. So It is SAFER to make the ANGLE between two segments of rope as LARGE as possible.

    Hope this helps


    image2016-07-07-231635-1.jpg

    image2016-07-07-232552-1.jpg
     
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  14. Tank

    Tank New Member

    So if the vehicle needs 1000kgs to move it you are saying that it only takes 500kgs. to do so, how much weight is on the rope supporting the SB, Regards Frank.
     
  15. Ali Ozbay

    Ali Ozbay New Member

    I'm saying; if the vehicle needs 1000kgs to move it, in the setup below, the winch will have to overcome 500kg plus half the friction loss in the SB.
    And the weight on the rope supporting the SB will be 1000, which is the total force to move the car.

    Edit Note: I initially said the winch needs to overcome 500 plus the friction. That's incorrect. The correct answer is: 500 plus half the friction as in the revised figure

    View attachment 39185 image2016-07-08-023723-1.jpg
     
    Last edited: Jul 8, 2016
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  16. bigblockford545

    bigblockford545 New Member

  17. bigblockford545

    bigblockford545 New Member

    Hope this helps
    [​IMG]
     
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