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  1. #1
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    Default Recoveries and Towbars - Some Maths and Principles

    Recoveries and Towbars

    I have included some basic calculations and sketches to help illustrate the forces that can be generated during static and kinetic recoveries.
    I have also demonstrated how forces can be multiplied to dangerous levels depending on the recovery setup.

    Please donít get to fixated on the exact maths and numbers, there are many variables at play here, so rather focus on the principles involved and see how easily the wrong recovery setup can quickly multiply forces by 4-5 times and more.

    Some of the values are ideal maximums, while others can easily be further increased by gearing, shock loading, etc.

    By understanding these basic principles, it becomes clear why towball recoveries are an absolute NO. Just donít do it.

    Feel free to ask any questions if things donít make sense.

    (Forces are in N (Newton), N= kg x 9.81, it is sometimes easier to image the loading in kg or tons.

    Wishing you all safe recoveries.

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    Edit: I have reviewed and edited the first two pages, now 1A, 1B, with values and speeds that more realistically represent recovery scenarios that may be encountered out there.
    The original pages (1,2), still attached, were based on idealistic theoretical maximums and extreme scenarios, which were not to well received in the forum peer reviews.
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    Last edited by Searcher; 2022/08/02 at 11:54 AM.

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  3. #2
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    Default Re: Recoveries and Towbars - Some Maths and Principles

    I know you said don't get hung up on the numbers, but I can't help myself.

    In order to reach 40km/h in 1st gear low range (using the gear ratios and tire radius in your calculation) you would need to rev that 1HZ (or is it the 4.5EFI?) to 11 594 rpm!

    On a more serious note though, I really hope nobody ever attempts snatch recoveries at speeds anywhere near 40km/h. Even 15km/h is probably pushing it a bit.

    Now with regard to the amount of tractive effort the vehicle can produce, I think the calculations starts in the wrong place. The amount of torque the drivetrain can produce in 1st low cannot be brought to bear because one simply can't generate enough traction.
    A good road tire, optimized for grip, struggles to achieve 1G. The off-road-biased tires most of us run are probably much worse. I think most of us would struggle to achieve half your 43Kn on tar. And I suspect almost all snatch recoveries would be done in situations where traction is even further reduced.

    However, I fully agree that doing a snatch recovery off a tow ball is probably a very bad idea, and doing one off a drop plate an even worse idea.
    Last edited by Peter Connan; 2022/08/01 at 01:30 PM.
    Beat-up rat rod of a '96 Nissan Patrol that bears the evidence of many wonderful adventures (and a few stupid indiscretions).

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    Default Re: Recoveries and Towbars - Some Maths and Principles

    For those that don't know the channel, please have a look at Matt's Off Road recoveries. It shows very practically what a decent kinetic rope and proper application does in real life. Some points being made here will be proven questionable as well.
    There is never a right time to do the wrong thing and never a wrong time to do the right thing!

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    Default Re: Recoveries and Towbars - Some Maths and Principles

    All this is part of it BUT if he opened his bonnet for the recovery only the bonnet would have been damaged.

  7. #5
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    Default Re: Recoveries and Towbars - Some Maths and Principles

    Quote Originally Posted by johnnybritz View Post
    All this is part of it BUT if he opened his bonnet for the recovery only the bonnet would have been damaged.
    Well, maybe.
    Glyn Demmer used to run a recovery course where they put an old bonnet up against a tree, with the tree-protector passing through it. On the vehicle end of the snatch rope they put a bow-shackle and then a loop of relatively thin strapping designed to fail at a much lower force than the snatch strap (I forgot the exact numbers). In any case, I saw with my own eyes the D-shackle penetrate right through the bonnet and carry on at significant speed.

    And that drop-plate would have been a hell of a lot heavier than a bow shackle.
    Beat-up rat rod of a '96 Nissan Patrol that bears the evidence of many wonderful adventures (and a few stupid indiscretions).

  8. #6
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    Default Re: Recoveries and Towbars - Some Maths and Principles

    Quote Originally Posted by Francois Theron View Post
    For those that don't know the channel, please have a look at Matt's Off Road recoveries. It shows very practically what a decent kinetic rope and proper application does in real life. Some points being made here will be proven questionable as well.
    Indeed. I have been watching his channel for years.
    Another channel one can learn a lot from is Casey LaBelle.
    Both of them use snatch ropes a lot, but Casey uses winches (and multiple winches) quite often. He seems to have some formal rigging training in his background and the ideas he comes up with are truly inspirational.

    But one thing to keep in mind is that most of the recoveries these guys do are on lighter less-capable vehicles that aren't really all that badly stuck.
    There are exceptions though, and every once in a while they have to recover something that is insanely heavy.
    They have both become very good at judging how much force will be required to affect a recovery and adjusting their rigging techniques and equipment to suit the situation.
    Somebody like me who 4x4's maybe 6-10 times a year just can't gather enough experience to make those judgment calls, and for this reason I would rather play it a bit safer and employ additional equipment such as safety bridles and so on.

    On the other hand, I am not nearly as paranoid as some others I have seen.
    Beat-up rat rod of a '96 Nissan Patrol that bears the evidence of many wonderful adventures (and a few stupid indiscretions).

  9. #7
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    Default Re: Recoveries and Towbars - Some Maths and Principles

    Quote Originally Posted by Peter Connan View Post
    I know you said don't get hung up on the numbers, but I can't help myself.

    In order to reach 40km/h in 1st gear low range (using the gear ratios and tire radius in your calculation) you would need to rev that 1HZ (or is it the 4.5EFI?) to 11 594 rpm.
    Easy, with a 1HD-FTE at 177kW, 561 NM torque and autobox. Low range 2nd start or 1st high.


    On a more serious note though, I really hope nobody ever attempts snatch recoveries at speeds anywhere near 40km/h. Even 15km/h is probably pushing it a bit.
    I have personally done a kinetic recovery at around 40 km/hr, with a 30m long, 32mm diameter kinetic rope, with 25% stretch. It's actually quite a pleasant experience if all goes well. The rope is like a huge elastic, that soaks up the kinetic energy very smoothly and slowly draws out the stuck vehicle, no jerking or snatching. Not saying it's should or should not be done, but is most certainly achievable.

    Now with regard to the amount of tractive effort the vehicle can produce, I think the calculations starts in the wrong place. The amount of torque the drivetrain can produce in 1st low cannot be brought to bear because one simply can't generate enough traction.
    A good road tire, optimized for grip, struggles to achieve 1G. The off-road-biased tires most of us run are probably much worse. I think most of us would struggle to achieve half your 43Kn on tar. And I suspect almost all snatch recoveries would be done in situations where traction is even further reduced.
    A recovery or towing vehicle can achieve a pulling force of about 80-90% of the weight carried by it's drive axles. Loaded at 3500 kg, lockers on, my Cruiser can probably pull in the region of 2800-3150 kg. It has the torque, but traction would be the limiting factor. Now imagine being stuck and a friendly overlander with loaded 6-7 ton Unimog or equivalent drives by and offers to pull you out. He will easily pull in the region of 4000 kg or so.


    However, I fully agree that doing a snatch recovery off a tow ball is probably a very bad idea, and doing one off a drop plate an even worse idea.
    I like open discussions, it forces one to think and re-evaluate.
    I am purposely working with worst case scenarios, not average or your everday easy recovery.
    As can be seen by my replies in blue, these forces, speeds and other numbers are achievable, not in your everyday recovery, but certainly achievable.

    So lets not get bogged down in the math as requested.

  10. #8
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    Default Re: Recoveries and Towbars - Some Maths and Principles

    Quote Originally Posted by Searcher View Post
    I like open discussions, it forces one to think and re-evaluate.
    I am purposely working with worst case scenarios, not average or your everday easy recovery.
    As can be seen by my replies in blue, these forces, speeds and other numbers are achievable, not in your everyday recovery, but certainly achievable.

    So lets not get bogged down in the math as requested.
    Sure they are achievable. There is a video on Youtube of a Cat D11 pulling a 6x6 articulated dump-truck, two rubber-tracked farm tractors and one of those huge Case articulated tractors all at once. All with their brakes locked. And yes, at one point they even manage to snap a strap.

    But my point is this: having a thread on here that teaches people the basics of how to do reasonably simple calculations of the forces involved in any given recovery is an incredibly valuable effort. If nothing else, sitting down and doing a few basic sums gives everybody involved a bit of time to think about what they are doing. I applaud you for taking the initiative and think it should be made a sticky and expanded upon for other types of recovery and also to include the effect of angles etc.

    But if you want to teach people how to do the calculations, you need to start with reasonable assumptions and use the correct maths for the situation.

    For example, you can't add 40km/h of momentum and the maximum torque the drivetrain can produce in 1st gear low-range together because it is physically impossible for those two forces to occur at the same time.

    There is another point to consider here as well: no matter how heavy the recovery vehicle, and how fast it is going, you can't apply more force to the rope than what the stuck vehicle is applying to the other side of the rope.

    So please don't nullify your effort by being as stubborn as me.
    Last edited by Peter Connan; 2022/08/01 at 03:58 PM.
    Beat-up rat rod of a '96 Nissan Patrol that bears the evidence of many wonderful adventures (and a few stupid indiscretions).

  11. #9
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    Default Re: Recoveries and Towbars - Some Maths and Principles

    My condolences to the family.

    https://youtu.be/2rHvQykNt2M

    Nazdrovia
    Whether you are a lion or gazelle, in order to survive, you better run like hell.

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