Reverse Winch - It Works! 

bleepinjeep sure! I don't have any super good pics of the mounts themselves, So I'll take some tommorow of the mounts and throw them on your FB page.

you can almost see it stretching when he hops in the passenger seat.

Nope. As he explained, the winch is taking up the "slack" behind the jeep. The force of the which is pulling the jeep backwards, not forward. There wouldn't be any forward pull until the "slack" behind the jeep is all taken up and the line between the two trees is straight.

Actually half the rate but twice the force overcoming the force at the front

your are just wrong if it was about force it would just be trying to rip the jeep in half

+matt byrne Actually, it is trying to rip the Jeep in half... The Jeep moves backwards and to the side to take up the slack, but if you kept at it, then eventually you would run out of slack and start disassembling your vehicle. It would be interesting if someone put a load sensor on each of the snatch blocks to show the forces involved. It would be easy enough to create a model of what is happening by just using a block of wood to simulate the vehicle and then a bobbin on the top of the block to simulate the winch. Then run a string from the bobbin to some fixed points simulating the fixed anchor points and back to the block.

Watch "Reverse winch" on RU-vid ru-vid.com/video/%D0%B2%D0%B8%D0%B4%D0%B5%D0%BE-_4tI3HtgdCY.html

@@CurmudgeonExtraordinaire You're right, technically, but as you said, it would only try to rip the Jeep apart until after all the slack is gone. At that point in time the Jeep would already have been recovered. So at that point, yes it would be out of slack at ripping it apart 🤷‍♂️

+WonderingAimlessly You asked for a physics major, and now you have one (BSE in Engineering Physics from Case Western Reserve University). This would be much easier to describe with figures, but I will do my best with words. The reason the Jeep moves at all is the same reason anything moves (or changes how it moves), it has a net external force applied to it (Newton's First Law). First let us consider all the forces acting on the Jeep. Obviously you have the tension force from the three winch lines, but you also have gravity (the Jeep pushing on the earth), the normal force (the earth pushing back, Newton's Third Law), and friction. Next, let us assume the Jeep is on even, level ground. This makes gravity and the normal force cancel out so we don't have to worry about them. There is no way to get rid of friction but the only thing it affects is the tension required to get the Jeep moving, not the explanation of why the Jeep moves. Another assumption is that the pulleys in the snatch blocks are massless and frictionless and that the cable is massless, giving the cable equal tension along its entire length. The error in the tension from this assumption is small compared to the tension itself and makes the math much easier since it allows us to use the same tension for each line. Also, assume that the winch, snatch blocks, and hook are all at the same height. That simplifies this to a two dimensional problem. With all that out of the way, we can now just consider the forces from the tension in the lines. We will only consider the forces that act along the axis of the Jeep. Based on the geometry, most of the tension forces are along the axis and the components perpendicular to the Jeep are not likely to overcome the static friction between the tires and the ground. We cannot simply assume, however, that all of the tension is along the axis of the Jeep, as we will see that there is a limit to how far back you can go with the setup used in this video. Now for some math. First off, we need a sign convention. Let's take the positive direction to be the direction the Jeep would travel if driven forward. We also need to define reference angle. Let's use straight ahead from the Jeep's reference as zero and angles increasing in the counterclockwise direction. Now for notation, let F=total force along the axis of the Jeep, f=friction force along the axis of the Jeep, t=tension in the cable, a_i=angle of the line, where i=1 for the line from the winch to the first tree, i=2 for the line from the second tree to the Jeep, and i=3 for the line from the Jeep to the third tree. The total force on the Jeep is given by multiplying the tension in the line but the cosine angle of each line and add them together along with the friction. F = t*cos(a_1) + t*cos(a_2) + t*cos(a_3) + f (1) Factor out the t from the three lines F = t*[cos(a_1) + cos(a_2) + cos(a_3)] + f (2) Based on the geometry and sign convention, cos(a_1) is near 1 since the line, while not going out straight ahead still has most of its force along the axis of the Jeep (a_1~0 degrees). Similarly, cos(a_2) and cos (a_3) are near -1 since they are mostly going straight out behind the Jeep (a_2, a_3~180 degrees). Approximating these as 1 and -1, respectively, and plugging them into equation 2 above, we get F = t*[1 + -1 + -1] + f (3) F = f - t (4) Once the magnitude of the tension is greater than the magnitude of static friction, the total force term is negative and the Jeep moves in the reverse direction. The reason we bother with the angles of the lines at all is because as the Jeep backs down, a_2 approaches 270 degrees and a_3 approaches 90 degrees (if the Jeep went back far enough, it would be between the trees). As those angles approach their respective limits, the cosines of those angles approach 0. If the snatch block on the back of the Jeep is between the snatch block on the second tree and the hook on the third tree, there is no longer any pull backward. Also, as you back away from the first tree, a_1 approaches 0 and the cosine approaches 1. If you back infinitely far away form the tree, it is directly ahead. At some point, the Jeep reaches a position where the forward forces from line 1 and dynamic friction equal the backward forces from lines 2 and 3 and the Jeep stops. If you connect lines 2 and 3 to the same tree, a_2 and a_3 so not 270 degrees and 90 degrees, respectively (the Jeep just backs toward the tree), and you can back all the way to the tree. That turned out to be a lot longer than I expected but I wanted to be as thorough as possible.

Great Work! I am an EE, but definitely needed the Statics refresher. Most of the trig and calculus will go over a lot of people's heads, though, but it's great for cancelling out sins and cosines and solving for all solutions. What helped me understand the physics is taking your equation (3) and simplifying it in a different way for demonstration purposes: F = 1t - 2t + f When you place a snatch block behind the rig, you change the direction of the pull relative to the vehicle (sign convention) and using 3 snatch blocks, you basically cancel out the additive effect most people are used to with snatch blocks. I was also working on a way to use simple math for practical purposes. I think one can assume a tension force of 9000,10000,12000 lb (I think those of lbf's) or whatever your winch rating might be. The f to overcome friction would be the gross vehicle weight. (again.. i think)

Protective HT You are right, in the reason it works.

bleepinjeep Oh hell...all those snatch blocks and tow ropes are like 200 bucks! LOL!!

sweet, so is mine, I have a 97 xj 2 wheel drive. got one hurdle over with. the 32r trans craped out on me so after doing research I swapped it out for a 4l60. really don't know whats next im going to do it in instalments, I think its convencing the wife into letting me go it. lmao.

Chuck a Barra in it. Australian Ford 6. 4 litre.

Unless it's a straight pull it's gonna put an immense side load on the hitch and probable cause failure. The cost of a second winch, running wiring, etc, would be immense for the occasional useage

Options are always good

well wont he be in revese helping the winch in that case?

+bosshoss69lee you mean it was going slow? that's because the geometry of the set up gives the winch a mechanical advantage ( less effort but also less pull speed)

Yeah grrrrinding Long dynema and spanish windlass is best

I have the same question as well. I understand that D-rings create more friction, but enough so as to not allow the winch to even move the vehicle a bit backward? I bet he's getting tired of revisiting this topic, but a re-revisit seems in order. Say with one less snatch block at the rear, then two less, to see if there is a critical point where snatch blocks are needed (I have t imagine the front one is critical, but maybe it's the one at the hitch of the Jeep since it is handling the true slack?), or if it's about friction vs. winch strength.

Watch "Reverse winch" on RU-vid ru-vid.com/video/%D0%B2%D0%B8%D0%B4%D0%B5%D0%BE-_4tI3HtgdCY.html

@@Spearfisher1970I just did a reverse winch today as a training exercise for something to do. I used winch rings and they must cause too much friction I think because I wasn’t able to get it to go backwards with an uphill in the reverse. But it wasn’t so steep that I couldn’t drive it backwards. We did manage to get it to work on basically flat ground. As far as I can tell the technique is practically useless. But maybe with snatch blocks there would be less friction, but even on flat ground it wasn’t very convincing.

fusilikevin they both wanna pull on something

they both have snatch blocks