Oh Oh, shit! [Music] All right so turns out grigri can slip in a complete No Hands situation and I finally found a way to demonstrate that and all of this came from me trying to understand the physics of how grigri works and when it won't so hopefully, this is gonna be the last grigri video I have to make and in case you haven't seen the previous ones here is a quick recap; first I showed you that if you press on a grigri's cam and the climber takes a fall at that moment the grigri won't lock even if you're a small girl belaying a big dude and then I showed you this; this is a hand of my viewer after belaying like this; where you bring a big loop of slack and press on grigri's cam grigri won't lock until all of that loop goes in and while that loop is going in this rope is potentially running through your hand and I did an experiment on that Oh shit! and yes I managed to burn my gloves and then I myself got involved into an incident where I was dropped half of the route and after my pants dried out I did a little investigation and I found that if you hold the grigri like this where the rope goes through your hand but you don't actively hold the rope grigri might slip even without the need to press on the cam and now finally I want to address this people using grigri completely hands-free. So let's start with some physics. First let's see what happens when we apply the tension to both strands of the rope for that let's divide the rope into multiple segments the first segment is pulling the second one up but at the same time the second segment is pulling the first one down and since these forces are opposite they cancel each other out and this process continues as we travel down the rope until the rope starts to curve at this point vectors are not in a straight line anymore and adding them creates a little tiny force to the center of the cam and this happens along entire curve of the cam now notice that the pivot point of the cam is off center and there is way more force on the left side of it which makes the cam to rotate and then the right side of the cam will squeeze on the rope and at this point the harder I pull on the rope the more the cam will pinch on it which is pretty smart design, however, let's see what happens if I don't hold the brake side of the rope before the cam engages. Let's say we have 80 grams of rope hanging down from the grigri and the rope also needs to come over this arc which adds extra resistance but for simplicity of this demonstration let's use one newton as a force on the brake side of the rope now the forces that I've already mentioned will be applicable here as well, however, they will be very low and that's because the total tension in the rope is very low let me explain, the maximum tension on the load side before the rope starts to slip is equal to the friction plus the tension on the brake side. And if we increase the tension on the load side the rope will start to slip. Now a quick refresher on friction let's say we have an object on a flat surface being pulled by the force the friction will only depend on how hard the object is being pressed into the surface and the friction coefficient which is basically a roughness of the surfaces and notice that friction does not depend on the speed if the object would accelerate faster and faster and faster friction force will still stay the same this will be handy a bit later in this video. So let's try to find the maximum friction in our example since we don't have a flat surface here but the Rope goes around the circular object we can use a capstan equation which states that T1 must be equal T2 times a constant and this constant depends on the friction coefficient and the angle the rope has to go around and in the case of grigri this would be roughly equal to two so in our example the maximum force we can pull before the rope starts slipping would be around 2 newtons while it's really hard to test these super low forces but here I have a setup of 200 grams and if I let go these 200 grams are being pulled up which means that there is 2 newtons of force in that direction so I'm gonna replace these grams with the grigri so now I have two newtons pulling up from the grigri and if I let go the break strand it will start slowly slipping through the grigri now you might be thinking well if I pull a really hard the grigri cam will engage what's the big deal so hold on to that thought I will come to that and it will make sense. So the tension in the rope in our example is gonna be about 2 newtons and the torque generated by this force is gonna be much lower than the tension in the cam's spring so the cam would never engage, however, there is another component in this situation every point on the rope that's in contact with the cam is moving in this direction so the frictional forces are making the cam to move in the same direction which also results into extra torque now remember I said that the friction force depends on the friction coefficient and how hard the objects are being pressed together also known as a normal force well in our case the normal force is actually the red force which means that our green frictional force depends on the red forces and this is where things get interesting the total torque is gonna be the sum of these forces and since the green force depends on the red one and the red one depends on the tension on the brake side of the rope that means that the total torque only depends on the tension on the brake side of the rope. So all of that means that cam engagement depends only on what's happening on the brake side of the rope and has nothing to do with how hard you pull on the climber's end which, eh, was kind of hard to believe so to test this theory I made an experiment where I let the brake side of the rope to free fall essentially making the tension on the brake side close to zero and I pulled on the climber's end of the rope as hard as I could and the grigri never locked I repeated the same experiment with the weights falling and the grigri engages only when there is tension on the brake side. [Music] So yeah, if there is absolutely no tension on a brake site the cam will not engage, however, in real life there is always a little bit of tension here because the rope has weight so then the question is how much tension you need for the cam to engage I tested this with different ropes and different loads and I found that depending on the rope tension needed to stop any fall was between 100 to 200 grams so yeah, just couple of hundred grams on the brake side of the rope is enough for the grigri to engage which is nice, however, the weight of the rope from the grigri till the ground is only about 80 grams which means if there is no kinks or twists in the rope at all the grigri could slip and before I demonstrate you that here are a couple more interesting things I noticed. First the angle at which the rope enters the grigri is really important notice the speed of the rope and now if I would reposition the rope it starts to go much faster. In the first case the rope has to go through two arcs and a change of Direction while in a second case it's one big smooth arc which significantly reduces the tension and then the second thing I noticed is that the direction which the rope leaves the grigri is also very important if I pull straight up there is significantly less friction and the cam is less likely to engage compared to pulling forward. So with all of this theory it was time for experiments I stood directly under the anchor and tried my best to position the rope on the side of the grigri. However, the rope was very keen on jumping forward and as soon as it jumps forward the grigri cam engages so I kept trying and trying and after multiple of attempts I finally got this so even though the rope did jump forward but somehow it kept dancing around the grigri and finding a low resistance path in and the grigri never locked so I tried again Jesus Christ! And well this is something that you should definitely not try at home. [Music] And since it was so much fun I decided to increase the weight okay would be nice to not let go when it's like swinging towards me [Laughter] Oh, shit! Yeah! obviously brilliant idea [Music] [Laughter] Now all of these tests were done with a brand new Petzl Volta rope which is very popular rope but at the same time it's very soft and skinny so the following days I wanted to try different ropes. The first one was Mammut Sender it's a much stiffer rope but also quite skinny and the results were quite similar sometimes it locked and sometimes [Music] it did not lock and I also collected a lot of these beautiful shots of grigri slipping and I'm gonna make them into reels and shorts so you can share with your friends who have no patience in watching all of this long video. Thank you! Next I tried with thicker ropes first one was a used 9.5 Mammut rope and although I got it to slip it wasn't going far that's why I decided to keep my hand on the grigri I'm not pressing on the grigri's cam but I'm not also actively holding the rope and this time this slippage was much bigger What! My hand was acting as a guide for the rope and since not all 9.5 ropes are equal next day I tried with the Petzl 9.5 and this happened. So yeah, even the same diameter ropes can behave very differently and I noticed that softer ropes are more likely to slip they can probably navigate and bend inside the grigri's cam easier and as a good reminder if you hold your hand like this but you don't actively hold on the Rope the grigri is way more likely to slip and then the big question was how does the weight of the falling object or the climber affect the grigri's engagement as we saw before it shouldn't it should only depend on what's happening on the brake side of the rope so I had to test with heavier weights. This time I had 50 kilograms of weight which even bringing it up was a challenge on its own. [Music] Then the next challenge was how to unlock the grigri so I decided to use a jumar. My jumar just went up! And this is what's holding all this weight now. So yeah, it kind of failed a little bit, however, the grigri was slipping. The impact on the last fall was so hard that I cannot untie the knot and this usually doesn't happen on the real climbs. Now while testing with even heavier weights would be nice, however, I noticed that my wooden beam at the top was flexing a lot already so I didn't want it to risk breaking something and having objects falling on my head, however, extrapolating my test data into the real world scenarios isn't very hard but first let me give you an example of friction. Okay, 34 kilos 34 kilos that means that approximately half of my weight can support my weight over one single quickdraw and now this is a fun one I created this little zigzag here to imitate more friction but I removed the weight only to 16 kilograms and that is enough to support my weight just this little zigzag here so as you can see friction can play a big role in entire system so let's do a quick math. The force which pulls on the grigri is basically the weight of the climber minus the friction that the rope has to go through and in my test setup I had two pulleys at the anchor each of 85 percent efficiency making my setup around 70 percent efficient and from my test on real climbing falls the ratio between the force to the climber compared to the belayer is usually between one and a half to three and a half times. So my 50 kilogram test is equivalent to 56 to 130 kilogram climber falling with zero slack in the system. Yeah! Could happen! And now I have a quiz for you here are two identical grigris one is brand new and the other one is used a lot it has a lot of wear on the cam already. Which one do you think is more likely to slip? I thought that the used one will slip more because the cam has already rounded edges and it will probably provide less friction, however, it turned out to be the opposite and here is why so this is an old grigri and I'm gonna load its cam with a specific weight that I already measured carefully on this weight I can push the rope in and the cam is grabbing on the rope now let's try with the new grigri I'm gonna load the cam with the same weight very carefully and if I try to put the rope in cam doesn't bite on it. So yes, the old grigri has looser spring which means that it's more likely to engage which also means that the older the grigri is the safer it gets? This is not a message that Petzl would say so they don't take it too seriously and remember I said that friction does not depend on the speed that means is that if the grigri is already slipping it can go faster and faster and it's not guaranteed to lock, at least in theory so in search for more information I wrote to petzl and their answer was that the speed at which the rope moves through the belay device is not directly related to friction and if you're not using proper belay technique by holding the brake side of the rope then there is no guarantee that grigri cam will engage. So the three final points for the video first of all making the grigri fail in this way you've seen is really hard even though I knew exact conditions required it still took me a lot of attempts to make the grigri slip which brings me to the second point that grigri is still one of the safest belayed devices on the market and my next video is gonna be a long awaited where I'm gonna show how other belay devices fail where I've seen quite a few surprises. And the final point is that to make this video I was in contact with multiple physics professors so huge thank you to them, unfortunately, this was the language we were trying to speak to me and my brain felt like this guy in his notes but I was inspired I wanted to learn more and he suggested to try brilliant.org for a quick fix and honestly that platform is brilliant we have thousands of super fun interactive lessons to increase your scientific thinking and the best part is that it doesn't matter what's your level for example I like this one you have a yellow cup of water submerged into a big pot of boiling water and the question is will the water in the cup boil as well and the answer is that it doesn't you can see that the water is boiling in a big pot but it's not boiling in the small one and the reason is probably not what you might be thinking at least my thinking was wrong so if you are a curious person I highly recommend brilliant.org and since we are sponsoring this video If you visit brilliant.org/hardiseasy you can find the link also in the description you can get a free 30 days trial nothing to lose and first 200 of you can also get 20 off their annual subscriptions. So yeah, enjoy learning fun stuff and see you in the next video
