schwede 0 #1 February 19, 2004 Can somebody tell me how much load a chest strap can take in hybrid dives??? As this type of dives get more and more common I believe this could be a point of safety issue...Schwede "Das Leben ist schön, nicht immer aber immer öfter" Quote Share this post Link to post Share on other sites
SkymonkeyONE 4 #2 February 19, 2004 you mean until it breaks? that would depend on the type of webbing you have. Quote Share this post Link to post Share on other sites
schwede 0 #3 February 19, 2004 The "normal" single webbing on a T-drop, if I would be talking about my own gear. But perhaps I'm overcausiousSchwede "Das Leben ist schön, nicht immer aber immer öfter" Quote Share this post Link to post Share on other sites
Brains 2 #4 February 19, 2004 Most of the hybrids WE have been doing invlove hanging from laterals, mlw, or leg straps now. Hanging from a two way would be the only time i hang from a chest strap now. I am not an expert at hybrids by any means, just what i have observed. Never look down on someone, unless they are going down on you. Quote Share this post Link to post Share on other sites
riggermick 7 #5 February 19, 2004 QuoteCan somebody tell me how much load a chest strap can take in hybrid dives??? As this type of dives get more and more common I believe this could be a point of safety issue... Properly consructed chest straps,either type 7, type 8 or type 17 will hold up just fine. Type 17 (2500 lbs) is the "weakest" material comonly used in chest construction, it's still way stronger you will ever need. The true "weak spot" on the chest strap is the hardware. the MS 70101-1 quick fit adapter (for 1 3/4" webbing) is only rated at 500 lbs, which is still more than you need. The reason for this is that the horizontal load component of the harness is farily low when compared to the vertical one. Webbing will generally be cut and shredded by the chest strap hardware before the sliding bar will yeld and pop out. But generaly only this happens during opening shock on very very hard openings with very very tight (read difficulty breathing)chest straps. It's a geometry thing and it's extremly rare. So unless someone gets a limb caught in a harness of another jumper (a whole other story)I think you'll be ok. Mick. Quote Share this post Link to post Share on other sites
cvfd1399 0 #6 February 19, 2004 you will never break "in service" webbing, the only thing I would worry about is loosening the strap, or somehow unthread it. Quote Share this post Link to post Share on other sites
FrogNog 1 #7 February 19, 2004 Quote... only rated at 500 lbs, which is still more than you need. Someone hanging from a chest strap during an accidental high-speed deployment could probably greatly exceed this. One issue is the "telephone wire" effect - that tension on a line is related to the pull force perpendicular to the axis between the ends by a sinusoidal function that is much higher than unity for small angles (as formed between the bent real line and the ideal, straight axis between the ends). This means someone pulling 200 pounds on a chest strap (e.g. a 200 pound person holding onto the strap while an unexpected deployment results in 1G of acceleration) directly away from the sternum could conceivably cause a tension of a couple thousand pounds on the line, if the ends were held taught enough to keep the line from bowing much. I'm not saying something bad _will_ happen, but for some time now I've felt chest straps were designed with the assumption they would never be taking signficant loads. Attaching a human - even tenuously - to the chest strap and having a premature deployment may violate this assumption. (Particularly at hybrid speed, with a slammer on an already quick-opening reserve) As they say, "Skydiving equipment can fail and kill you in ways you never even thought of." -=-=-=-=- Pull. Quote Share this post Link to post Share on other sites
Samurai136 0 #8 February 19, 2004 I recall Billy Webber lecturing a skydiver who had routed his seatbelt thru his chest strap in the otter at summerfest, "Do Not route the seatbelt that way! The chest strap is not rated to withstand the forces put on it by the seatbelt in an accident." My recollection is that either the hardware or stitching attatching the chest strap to the lateral would fail. It's probably ok for hybrids. I suspect that a skydivers' grip/ hand would fail before the chest strap in a reserve deployment scenario. But that only a speculation...Ken"Buttons aren't toys." - Trillian Ken Quote Share this post Link to post Share on other sites
bch7773 0 #9 February 20, 2004 Quote I suspect that a skydivers' grip/ hand would fail before the chest strap in a reserve deployment scenario. But that only a speculation... I think you're right. unless someone has a good enough grip that they could do a pullup with a couple 150 lb people holding onto them (500 lbs total), then i think the hangers grip would break before the strap. MB 3528, RB 1182 Quote Share this post Link to post Share on other sites
FrogNog 1 #10 February 20, 2004 QuoteQuote I suspect that a skydivers' grip/ hand would fail before the chest strap in a reserve deployment scenario. But that only a speculation... I think you're right. unless someone has a good enough grip that they could do a pullup with a couple 150 lb people holding onto them (500 lbs total), then i think the hangers grip would break before the strap. Trigonometry says a person need not put a 500 pound load on a strap to exert a 500 pound tension. Load to tension is only 1:1 when they are pulling on the strap in-line, i.e. hanging onto one end of it. I believe the tension is equal to the inverse cosine of the angle at which the force is applied to the strap, multiplied by the force. As the strap approaches horizontal (i.e. it is flat across the wearer's theoretical square chest), the angle of the force to the strap approaches 90 degrees, and cosine approaches zero, and the force multiplier approaches infinity. This is what I think is bad. As the strap approaches vertical (i.e. it stretches away from the wearer's chest and down toward the hanger), the angle of force to the strap approaches 0 degrees, and cosine approaches 1, and the force multiplier approaches 1. In between, if the chest strap hangs down 30 degrees away from its anchor points, the force multiplier is 2. Now someone need only weigh 250 pounds and undergo a single gravity of acceleration - the equivalent of merely hanging by his hand or hands - to exert 500 pounds of force. On the side of reducing the likelihood of failure, I find: * most jumpers will weigh less than 250 out the door, but maybe not a whole lot. * the hardware is rated to 500 lbs and will probably have a safety factor before it truly breaks, but maybe not a whole lot. * the angle of the chest strap _will probably_ increase as a load is applied to it. This will reduce the force multiplier. Unfortunately, I don't know how much the chest strap angle will increase. It depends on stretchiness of the strap, harness geometry, and the deformation of the body of the wearer underneath the harness. On the side of increasing the likelihood of failure, I find: * many jumpers can hang their out-the-door weight from one hand (i.e. remain hanging while under one G of acceleration). Some of them may be able to do it under a little more. * the chest strap angle may be less than 30 degrees from horizontal. It doesn't seem likely, but I don't have any proof either way on chest strap angles with hangers. And as Cosine heads from .5 to 0 as the angle (in this case the angle of applied force, the complement to the chest strap angle) goes from 60 degrees to 90, the force multiplier exhibits hyperbolic behavior. That is, the force multiplier goes up increasingly quickly as it trends toward infinity. For various chest strap angles moving toward horizontal in 5 degree increments, the force multipliers are: 30 degrees: 2.00x 25 degrees: 2.36x 20 degrees: 2.92x 15 degrees: 3.86x 10 degrees: 5.75x 5 degrees: 11.47x So a 50 pound pull on a chest strap 5 degrees from horizontal would cause 573 pounds of tension if the strap couldn't stretch or the endpoints couldn't move inward. Realistically, it may well be safe to hang from chest straps. This math has not been adjusted to apply to the bendy nylon webbing, and there are safety factors, and etc.. But I don't like what the theory says here. -=-=-=-=- Pull. Quote Share this post Link to post Share on other sites
Martini 0 #11 February 20, 2004 Eugene, I love ya man. Your dedication to science is really admirable. I constantly amazes me that you are more fascinated by skydiving physics than anyone I know of. Keep it up dude. Also you are welcome to hang from my type 17 chest strap whenever you have the skill, it's not that hard but it requires good sit/stand control. Even if I throw and have a slammer while you're hanging on your fingers will tear off before the strap or hardware fails. Assuming it's in good shape which it is. I took a hybrid 3-way a little low a couple of years ago while hanging. One of the belly fliers mentioned that he wasn't too concerned, he'd just remove me at 3k by throwing if I was still there. I believe him.Sometimes you eat the bear.............. Quote Share this post Link to post Share on other sites
bch7773 0 #12 February 20, 2004 Quote For various chest strap angles moving toward horizontal in 5 degree increments, the force multipliers are: 30 degrees: 2.00x 25 degrees: 2.36x 20 degrees: 2.92x 15 degrees: 3.86x 10 degrees: 5.75x 5 degrees: 11.47x So a 50 pound pull on a chest strap 5 degrees from horizontal would cause 573 pounds of tension if the strap couldn't stretch or the endpoints couldn't move inward. I think something might be wrong in your calculations... according to your table, we might guess that a 1 degree angle would have a force multiplier of around 25... this implies that if you take a piece of chest strap webbing and keep the ends perfectly level and horizontal, it would take less than 20 lbs to break it. I can't believe that... would u mind posting your formula for the force multiplier? I can't figure out what you did from your writing (of course, its probably me... it is 1 AM here) and of course, you have to remember that during a hybrid, the hanger will NOT be putting a force of 1 G on the strap... since the jumpers are both already at 1 G, the "relative G-force" would be 0. and remember, the hanger will only fall faster than the belly flier for maybe a half a second, until the pull on the chest strap brings the belly flier to the new hybrid speed. we'd have to calculate the acceleration the belly flier experencies from the hanger... I would just guess its less than .5 Gs. MB 3528, RB 1182 Quote Share this post Link to post Share on other sites
Salamander 0 #13 February 20, 2004 Take a look at this picture http://www.skydyv.com/AZ2004/008.jpg I know this may be an exaggerated example, however, I am not sure that 30 degrees is an accurate representation of a likely angle of deflection when the chest strap is under tension (ie. not slack) due to a hanger flying relatively relative (hehe). I would assume that an impact loading (high forces in small time frames) could have significant consequences. Depending on how the forces distribute through the hardware this could be a concern. As for the webbing - I am not current on impact loadings with respect to fabrics. Any ideas??? Quote Share this post Link to post Share on other sites
schwede 0 #14 February 20, 2004 Your pic caught my eye.. This is what I had in mind when I wrote my mssg.Schwede "Das Leben ist schön, nicht immer aber immer öfter" Quote Share this post Link to post Share on other sites
mr2mk1g 10 #15 February 20, 2004 I cant see that as being right. Its the hardware that is rated to 500lb. For your logic to be right you have to pull on the hardware at an angle other than that which it is rated in. The problem is you cant really do that very easily. It is not fixed rigidly to the MLW so as you pull on the chest strap the harware simply rotates round to face the direction in which the force is coming from. Therefore its quite dificult to put force on the harware at an angle. Quote Share this post Link to post Share on other sites
Salamander 0 #16 February 20, 2004 QuoteTherefore its quite dificult to put force on the harware at an angle. The angle at which you pull on the hardware will not negate the forces that it is exposed to. I don't have my rig in front of me so I can't verify this, but the angle may change any likely failure planes depending on how the hardware rotates to accomodate for certain forces. Although, I can't see the forces being negated, where would they go and how would they get there? Quote Share this post Link to post Share on other sites
FrogNog 1 #17 February 20, 2004 QuoteEugene, I love ya man. Your dedication to science is really admirable. I constantly amazes me that you are more fascinated by skydiving physics than anyone I know of. Keep it up dude. Also you are welcome to hang from my type 17 chest strap whenever you have the skill, it's not that hard but it requires good sit/stand control. Even if I throw and have a slammer while you're hanging on your fingers will tear off before the strap or hardware fails. Assuming it's in good shape which it is. Quote When I get decent at sitting, I may take you up on that offer. See, as long as we're not close friends, I don't too much mind hanging onto your chest strap. Just y'all leave my chest strap alone! I totally admit the theory I'm working with isn't accurate and isn't the whole story and therefore probably nothing will snap. But if the theory is even partially correct (which theories tend to be - partially correct I mean), then it adjusts the numbers people were talking about before: "Nobody can hold on with the 500 pounds of force required to exceed the hardware." And, if I were hanging on and someone threw and slammed, I damn well hope I would fall off! -=-=-=-=- Pull. Quote Share this post Link to post Share on other sites FrogNog 1 #18 February 20, 2004 QuoteTake a look at this picture http://www.skydyv.com/AZ2004/008.jpg I know this may be an exaggerated example, however, I am not sure that 30 degrees is an accurate representation of a likely angle of deflection when the chest strap is under tension (ie. not slack) due to a hanger flying relatively relative (hehe). The angles of those chest straps is definitely close to the "safe" end of the angle range. That is good. -=-=-=-=- Pull. Quote Share this post Link to post Share on other sites Samurai136 0 #19 February 20, 2004 I also completed a physics degree.While it's nice to see you've analysed the chest strap forces you don't comment on the forces involved in the arm joints and hand. The hanger doesn't even need to have a solid 'grip' just a placed/ cupped hand on the strap will do. In that scenario my expectation is that the jumper would not have sufficient time to get a 'better' grip and would probably let go as the pain rapidly increased or just slip off... Ken"Buttons aren't toys." - Trillian Ken Quote Share this post Link to post Share on other sites FrogNog 1 #20 February 20, 2004 QuoteQuote For various chest strap angles moving toward horizontal in 5 degree increments, the force multipliers are: 30 degrees: 2.00x 25 degrees: 2.36x 20 degrees: 2.92x 15 degrees: 3.86x 10 degrees: 5.75x 5 degrees: 11.47x So a 50 pound pull on a chest strap 5 degrees from horizontal would cause 573 pounds of tension if the strap couldn't stretch or the endpoints couldn't move inward. I think something might be wrong in your calculations... according to your table, we might guess that a 1 degree angle would have a force multiplier of around 25... this implies that if you take a piece of chest strap webbing and keep the ends perfectly level and horizontal, it would take less than 20 lbs to break it. I can't believe that... would u mind posting your formula for the force multiplier? I can't figure out what you did from your writing (of course, its probably me... it is 1 AM here) and of course, you have to remember that during a hybrid, the hanger will NOT be putting a force of 1 G on the strap... since the jumpers are both already at 1 G, the "relative G-force" would be 0. and remember, the hanger will only fall faster than the belly flier for maybe a half a second, until the pull on the chest strap brings the belly flier to the new hybrid speed. we'd have to calculate the acceleration the belly flier experencies from the hanger... I would just guess its less than .5 Gs. It does appear you are under the influence of late night. The formula for the multiplier is 1 / ( cosine of angle of applied force). This comes from drawing triangles and figuring out the answer to this question: what force resists the pull on the chest strap? When the hanger pulls down on a chest strap, even standing on the ground, something has to pull up on the chest strap equally to keep the chest strap from accelerating. And we can all agree that at some point, the chest strap is not accelerating - for example, standing on the ground and not moving, if you pull on someone's chest strap and hold onto it, you are exerting force, the chest strap has mass, but it's holding still. So there must be another force acting on the chest strap, counteracting the hanger pulling. And there is, and it's obvious to us all: it's the ends of the chest strap affixed to the MLW. The MLW effectively "pulls" on the ends of the chest strap, and this counteracts the downward pull of the chest strap. (For now we will ignore the force on the MLW, which is connected to the jumper, which is either standing on the ground or falling through the air.) This is where a significant sub-question arises: how can the MLW, which is at the ends of the chest strap, counteract the downward pull of the hanger? This should be confusing because the MLW can only pull directly on the ends of the chest strap, i.e. exert force in a direction parallel to the chest strap, while the hanger pulls down, which is perpendicular to that. And the initial answer is the MLW cannot directly counteract the pull of the hanger, because the MLW can only exert force perpendicular to the hanger force. The secondary answer is that because the MLW cannot directly counteract the pull of the hanger, there is a net downward force on the MLW and it accelerates, which over time means it has velocity, which over time means it changes position. This will happen until equilibrium is restored. But to simplify all this, we can simply say "the chest strap moves downward in the middle where the hanger is holding on." And this matches up with video, so the freefliers may still believe me. Once the chest strap is no longer horizontal, the direction the MLW can exert force along the chest strap - parallel to it - is no longer perpendicular to the force applied by the hanger. This is the chest strap angle I'm talking about. When the chest strap is at an angle, a fraction of the force the MLW applies to the ends of the chest strap can be thought of in the horizontal direction, and another fraction in the vertical direction. But the MLW really exerts its force parallel to the MLW. This is where sinusoidal functions come in: the hypotenuse is the MLW force, and sine and cosine are the vertical and horizontal force components. (I see now looking back at my math that I was using the cosine of 90-degree complement of the chest strap angle. Well, the cosine of a 90-degree complement is the same as the sine, so that was an unnecessary complication.) Anyway, the "force multiplier" comes from the force component issue: if there is a 5 pound downward force on the chest strap, in order to put the chest strap in equilibrium, a 5 pound upward force must be exerted on the chest strap. But only the vertical force component of the MLW's force on the strap ends counts. The vertical force component is sine of the chest strap angle, and we want that to be 5 pounds. The relation of the vertical force component to the overall force exerted by the MLW on the chest strap (which we will call chest strap tension) if the same ratio as sine of the angle. We want to solve for the hypotenuse, given the opposite, so: sine (angle) = opposite / adjacent = 5 / x if sine (angle) = y [a decimal], then we have: y = 5 / x or y / 1 = 5 / x and by cross-multiplying,: 5 = yx. then dividing both sides by y: 5/y = x or 5 * ( 1 / y ) = x Since x is the tension on the strap, and 5 is the force of the hanger (which could just as well have been z), this gives my formula for the "force multiplier" between the hanger pull and the strap tension being the inverse of the sine of the angle the chest strap makes with its MLW-MLW connection axis. For very small angles, i.e. very horizontal chest straps, the MLW would have to exert huge tension on the strap to counteract small hanger pulls. This means if the ends of the chest strap were held so they could not move closer, e.g. the chest strap were attached to a couple of steel beams cemented into the ground, the MLW (or in this case the steel beams) would exert so much force on the strap trying to counteract a small hanger force that the strap hardware would fail. Now, this is clearly not like real life. When someone pulls on a chest strap, the ends are at first relatively free to move toward each other (squishing our armpit flab) as the middle of the chest strap is pulled down. As this happens the angle increases and the MLW does not have to pull as hard to counteract the hanger. This continues until the MLW exerts enough force on the chest strap to hold the hanger. How much force the MLW needs to exert depends on the weight of the hanger and the angle the chest strap forms, and this has to do with how far apart the MLW connection points are, how long the chest strap is compared to that distance, and how flabby the wearer's pectoral area is. The classical experiment is to get a piece of string a couple feet long, hang a weight from its middle, and hold the ends of the string in your hands, then move the hands slowly apart to try to make the string horizontal. The weight does not change the force it exerts, but holding the weight up as the string gets more horizontal requires more and more pull on the string. I guess my advice from this would be: 1. get the strongest chest strap, hardware, and stitching you can. 2. leave your chest strap as loose as safety otherwise permits, so it can form the greatest angle from the MLW-MLW axis. 3. have flabby pects. (This and #2 work together, so you can substitute a bit here or there.) And finally, a 1 degree angle would have a force multiplier of 57.29. Perhaps I should construct a stiff steel frame to which I can attach a chest strap and show its hardware being broken by a 50 pound pull. Of course, it would be a real trick to keep the strap horizontal as the nylon stretched under load and the hardware stretched as it deformed. -=-=-=-=- Pull. Quote Share this post Link to post Share on other sites FrogNog 1 #21 February 20, 2004 Quoteand of course, you have to remember that during a hybrid, the hanger will NOT be putting a force of 1 G on the strap... since the jumpers are both already at 1 G, the "relative G-force" would be 0. and remember, the hanger will only fall faster than the belly flier for maybe a half a second, until the pull on the chest strap brings the belly flier to the new hybrid speed. we'd have to calculate the acceleration the belly flier experencies from the hanger... I would just guess its less than .5 Gs. I totally agree that during the hybrid, the force on the chest strap will be low. (It will not be zero, because although the two jumpers will be in velocity stasis once at terminal, their "free" terminal velocities will be unequal, and the difference will be made up as the hanger "pulling" on the wearer.) My concern was really during an unexpected reserve deployment, because I consider that to be the worst-case acceleration one could expect: a canopy that is supposed to open relatively quickly, and a deployment at the highest airspeed expected during the dive. During a brisk opening, it is further true that the duration of the acceleration will be brief (as the pair slow down or, more likely, all or the vast majority of the hanger detaches from the chest strap), and that may allow the hardware to yield without completely failing, or the tension rating of the strap (and stitching) to be exceeded without completely separating. So chances are good that this practice does not comprise a significant increase in risk over the other things we do in skydiving. But without empirical or circumspect theoretical evidence, we are being harness test pilots by doing this. -=-=-=-=- Pull. Quote Share this post Link to post Share on other sites Martini 0 #22 February 21, 2004 Instead of trying to analyze this thing that has lots of variables why not take a piece of webbing and anchor it firmly between two solid points to create some reasonable representation of a chest strap angle, even an extremely flat one, and try to break it. Using hard anchors will eliminate speculation of deflection and provide an extreme case scenario. Hang from it, jerk on it, use any means possible using only your body as the pulling force. Betcha a 6-pack of Red Hook that you can't break even a type 17 strap. The theory doesn't mean squat until it's tested anyway.Sometimes you eat the bear.............. Quote Share this post Link to post Share on other sites tbrown 26 #23 February 21, 2004 We laid the base for a hybrid last weekend and my friend was wearing an older rig. I realized that his chest strap is not folded over and stitched the way it would be on a more recent rig. Instead, it has a patch of velcro (this is a rig from the eighties, it was on the cutting edge in it's day). If the adapeter failed to stop the chest strap, there would be no folded & stitched end to hang up in the adapter. Probably not a good idea for him (he IS looking for new gear). Your humble servant.....Professor Gravity ! Quote Share this post Link to post Share on other sites webracer 0 #24 February 21, 2004 Most chest strap hardware is rated at 500lbs load. Your chest strap should not cause a premature deployment if your rig fits right and has the proper reserve ripcord. You shouldn't use the chest strap to buckle into the plane (500lb load max), use your harness (my pet peeve).Troy I am now free to exercise my downward mobility. Quote Share this post Link to post Share on other sites Join the conversation You can post now and register later. If you have an account, sign in now to post with your account. Note: Your post will require moderator approval before it will be visible. Reply to this topic... × Pasted as rich text. Paste as plain text instead Only 75 emoji are allowed. × Your link has been automatically embedded. Display as a link instead × Your previous content has been restored. Clear editor × You cannot paste images directly. Upload or insert images from URL. Insert image from URL × Desktop Tablet Phone Submit Reply 0 Go To Topic Listing
FrogNog 1 #18 February 20, 2004 QuoteTake a look at this picture http://www.skydyv.com/AZ2004/008.jpg I know this may be an exaggerated example, however, I am not sure that 30 degrees is an accurate representation of a likely angle of deflection when the chest strap is under tension (ie. not slack) due to a hanger flying relatively relative (hehe). The angles of those chest straps is definitely close to the "safe" end of the angle range. That is good. -=-=-=-=- Pull. Quote Share this post Link to post Share on other sites
Samurai136 0 #19 February 20, 2004 I also completed a physics degree.While it's nice to see you've analysed the chest strap forces you don't comment on the forces involved in the arm joints and hand. The hanger doesn't even need to have a solid 'grip' just a placed/ cupped hand on the strap will do. In that scenario my expectation is that the jumper would not have sufficient time to get a 'better' grip and would probably let go as the pain rapidly increased or just slip off... Ken"Buttons aren't toys." - Trillian Ken Quote Share this post Link to post Share on other sites
FrogNog 1 #20 February 20, 2004 QuoteQuote For various chest strap angles moving toward horizontal in 5 degree increments, the force multipliers are: 30 degrees: 2.00x 25 degrees: 2.36x 20 degrees: 2.92x 15 degrees: 3.86x 10 degrees: 5.75x 5 degrees: 11.47x So a 50 pound pull on a chest strap 5 degrees from horizontal would cause 573 pounds of tension if the strap couldn't stretch or the endpoints couldn't move inward. I think something might be wrong in your calculations... according to your table, we might guess that a 1 degree angle would have a force multiplier of around 25... this implies that if you take a piece of chest strap webbing and keep the ends perfectly level and horizontal, it would take less than 20 lbs to break it. I can't believe that... would u mind posting your formula for the force multiplier? I can't figure out what you did from your writing (of course, its probably me... it is 1 AM here) and of course, you have to remember that during a hybrid, the hanger will NOT be putting a force of 1 G on the strap... since the jumpers are both already at 1 G, the "relative G-force" would be 0. and remember, the hanger will only fall faster than the belly flier for maybe a half a second, until the pull on the chest strap brings the belly flier to the new hybrid speed. we'd have to calculate the acceleration the belly flier experencies from the hanger... I would just guess its less than .5 Gs. It does appear you are under the influence of late night. The formula for the multiplier is 1 / ( cosine of angle of applied force). This comes from drawing triangles and figuring out the answer to this question: what force resists the pull on the chest strap? When the hanger pulls down on a chest strap, even standing on the ground, something has to pull up on the chest strap equally to keep the chest strap from accelerating. And we can all agree that at some point, the chest strap is not accelerating - for example, standing on the ground and not moving, if you pull on someone's chest strap and hold onto it, you are exerting force, the chest strap has mass, but it's holding still. So there must be another force acting on the chest strap, counteracting the hanger pulling. And there is, and it's obvious to us all: it's the ends of the chest strap affixed to the MLW. The MLW effectively "pulls" on the ends of the chest strap, and this counteracts the downward pull of the chest strap. (For now we will ignore the force on the MLW, which is connected to the jumper, which is either standing on the ground or falling through the air.) This is where a significant sub-question arises: how can the MLW, which is at the ends of the chest strap, counteract the downward pull of the hanger? This should be confusing because the MLW can only pull directly on the ends of the chest strap, i.e. exert force in a direction parallel to the chest strap, while the hanger pulls down, which is perpendicular to that. And the initial answer is the MLW cannot directly counteract the pull of the hanger, because the MLW can only exert force perpendicular to the hanger force. The secondary answer is that because the MLW cannot directly counteract the pull of the hanger, there is a net downward force on the MLW and it accelerates, which over time means it has velocity, which over time means it changes position. This will happen until equilibrium is restored. But to simplify all this, we can simply say "the chest strap moves downward in the middle where the hanger is holding on." And this matches up with video, so the freefliers may still believe me. Once the chest strap is no longer horizontal, the direction the MLW can exert force along the chest strap - parallel to it - is no longer perpendicular to the force applied by the hanger. This is the chest strap angle I'm talking about. When the chest strap is at an angle, a fraction of the force the MLW applies to the ends of the chest strap can be thought of in the horizontal direction, and another fraction in the vertical direction. But the MLW really exerts its force parallel to the MLW. This is where sinusoidal functions come in: the hypotenuse is the MLW force, and sine and cosine are the vertical and horizontal force components. (I see now looking back at my math that I was using the cosine of 90-degree complement of the chest strap angle. Well, the cosine of a 90-degree complement is the same as the sine, so that was an unnecessary complication.) Anyway, the "force multiplier" comes from the force component issue: if there is a 5 pound downward force on the chest strap, in order to put the chest strap in equilibrium, a 5 pound upward force must be exerted on the chest strap. But only the vertical force component of the MLW's force on the strap ends counts. The vertical force component is sine of the chest strap angle, and we want that to be 5 pounds. The relation of the vertical force component to the overall force exerted by the MLW on the chest strap (which we will call chest strap tension) if the same ratio as sine of the angle. We want to solve for the hypotenuse, given the opposite, so: sine (angle) = opposite / adjacent = 5 / x if sine (angle) = y [a decimal], then we have: y = 5 / x or y / 1 = 5 / x and by cross-multiplying,: 5 = yx. then dividing both sides by y: 5/y = x or 5 * ( 1 / y ) = x Since x is the tension on the strap, and 5 is the force of the hanger (which could just as well have been z), this gives my formula for the "force multiplier" between the hanger pull and the strap tension being the inverse of the sine of the angle the chest strap makes with its MLW-MLW connection axis. For very small angles, i.e. very horizontal chest straps, the MLW would have to exert huge tension on the strap to counteract small hanger pulls. This means if the ends of the chest strap were held so they could not move closer, e.g. the chest strap were attached to a couple of steel beams cemented into the ground, the MLW (or in this case the steel beams) would exert so much force on the strap trying to counteract a small hanger force that the strap hardware would fail. Now, this is clearly not like real life. When someone pulls on a chest strap, the ends are at first relatively free to move toward each other (squishing our armpit flab) as the middle of the chest strap is pulled down. As this happens the angle increases and the MLW does not have to pull as hard to counteract the hanger. This continues until the MLW exerts enough force on the chest strap to hold the hanger. How much force the MLW needs to exert depends on the weight of the hanger and the angle the chest strap forms, and this has to do with how far apart the MLW connection points are, how long the chest strap is compared to that distance, and how flabby the wearer's pectoral area is. The classical experiment is to get a piece of string a couple feet long, hang a weight from its middle, and hold the ends of the string in your hands, then move the hands slowly apart to try to make the string horizontal. The weight does not change the force it exerts, but holding the weight up as the string gets more horizontal requires more and more pull on the string. I guess my advice from this would be: 1. get the strongest chest strap, hardware, and stitching you can. 2. leave your chest strap as loose as safety otherwise permits, so it can form the greatest angle from the MLW-MLW axis. 3. have flabby pects. (This and #2 work together, so you can substitute a bit here or there.) And finally, a 1 degree angle would have a force multiplier of 57.29. Perhaps I should construct a stiff steel frame to which I can attach a chest strap and show its hardware being broken by a 50 pound pull. Of course, it would be a real trick to keep the strap horizontal as the nylon stretched under load and the hardware stretched as it deformed. -=-=-=-=- Pull. Quote Share this post Link to post Share on other sites
FrogNog 1 #21 February 20, 2004 Quoteand of course, you have to remember that during a hybrid, the hanger will NOT be putting a force of 1 G on the strap... since the jumpers are both already at 1 G, the "relative G-force" would be 0. and remember, the hanger will only fall faster than the belly flier for maybe a half a second, until the pull on the chest strap brings the belly flier to the new hybrid speed. we'd have to calculate the acceleration the belly flier experencies from the hanger... I would just guess its less than .5 Gs. I totally agree that during the hybrid, the force on the chest strap will be low. (It will not be zero, because although the two jumpers will be in velocity stasis once at terminal, their "free" terminal velocities will be unequal, and the difference will be made up as the hanger "pulling" on the wearer.) My concern was really during an unexpected reserve deployment, because I consider that to be the worst-case acceleration one could expect: a canopy that is supposed to open relatively quickly, and a deployment at the highest airspeed expected during the dive. During a brisk opening, it is further true that the duration of the acceleration will be brief (as the pair slow down or, more likely, all or the vast majority of the hanger detaches from the chest strap), and that may allow the hardware to yield without completely failing, or the tension rating of the strap (and stitching) to be exceeded without completely separating. So chances are good that this practice does not comprise a significant increase in risk over the other things we do in skydiving. But without empirical or circumspect theoretical evidence, we are being harness test pilots by doing this. -=-=-=-=- Pull. Quote Share this post Link to post Share on other sites
Martini 0 #22 February 21, 2004 Instead of trying to analyze this thing that has lots of variables why not take a piece of webbing and anchor it firmly between two solid points to create some reasonable representation of a chest strap angle, even an extremely flat one, and try to break it. Using hard anchors will eliminate speculation of deflection and provide an extreme case scenario. Hang from it, jerk on it, use any means possible using only your body as the pulling force. Betcha a 6-pack of Red Hook that you can't break even a type 17 strap. The theory doesn't mean squat until it's tested anyway.Sometimes you eat the bear.............. Quote Share this post Link to post Share on other sites
tbrown 26 #23 February 21, 2004 We laid the base for a hybrid last weekend and my friend was wearing an older rig. I realized that his chest strap is not folded over and stitched the way it would be on a more recent rig. Instead, it has a patch of velcro (this is a rig from the eighties, it was on the cutting edge in it's day). If the adapeter failed to stop the chest strap, there would be no folded & stitched end to hang up in the adapter. Probably not a good idea for him (he IS looking for new gear). Your humble servant.....Professor Gravity ! Quote Share this post Link to post Share on other sites
webracer 0 #24 February 21, 2004 Most chest strap hardware is rated at 500lbs load. Your chest strap should not cause a premature deployment if your rig fits right and has the proper reserve ripcord. You shouldn't use the chest strap to buckle into the plane (500lb load max), use your harness (my pet peeve).Troy I am now free to exercise my downward mobility. Quote Share this post Link to post Share on other sites
