I learned about climbing from that.

[Winchman]

The other evening, I got a line to a nice TIP about seventy feet up in a very spindly pine. It's at least ninety feet tall, but the diameter three feet off the ground is only eleven inches. It's got a little lean and some slight curves along with a bunch of dead branches. There were no decent live limbs below the TIP. There's an oak tree nearby that has some limbs have been rubbing on the pine, so I wanted to trim them, too.

Yesterday morning was perfect for climbing (cloudy, light breeze, high 50s), and I spent about an hour trimming the oak limbs before taking a lunch break.

It had gotten a little misty after lunch, but I got back to work. I was about fifty feet up when suddenly the sky cleared, and the wind really started to blow. After a wild swing, I was able to grab the anchor rope coming down from the TIP, and wrap my legs around the trunk. To my surprise, the anchor rope was really, really tight. I can usually move it around a bit if needed, but it wasn't going anywhere this time. I worked my way down to the ground, and decided to stop for the day.

I noticed several some things while taking the climbing stuff apart. The figure eight on a bight for the thimble where the anchor line attaches to the basal anchor was much tighter than normal. I was able to undo it by hand, but it took a lot of effort. The figure eight on a bight that attaches the anchor rope to the upper pulley of my 3:1 was as easy to untie as it usually is. Apparently the friction at the TIP was holding all the excess tension in the anchor rope caused by flexing of the tree. The wind was bending the tree away from the side where the anchor rope was located. Since I was able to undo the knot by hand, I doubt the rope was overloaded, but it's pretty easy to imagine a similar situation with a flexing tree on a windy day might put some really heavy loads on an anchor rope.

Yeah, I know, I know. SRS with the climbing line anchored only to the TIP avoids all this. Me, I'll just check the weather.

 

[DSMc]

Movement within an SRS climbing system should be carefully analyzed, as it is more than capable of causing failures. In your case, as the tree swayed the rope slackened, the frictional force that was multiplied by your weight maintained that shortened length. Each time the tree moved enough, the down leg tightened.

In a static, base-tied setup, friction at the suspension point is working to prevent a true doubling of potential force. Movement gives enough energy potential to go beyond the 2 x load equation.

When assessing a climbing system, whether base or canopy tied, pay careful attention to any movement or the potential of movement. Unlike the elasticity built into your rope, it is not your friend.

 

[Bart_]

Maybe I can help your understanding. Your tip is a bollard with the appropriate equation for 1/2 wrap. Both legs of rope are elastics. The direction of friction force at the bollard/tip depends on the history of rope motion and whether the elastically derived forces are large enough slide the rope on the bollard. Actually its the difference of force between the two legs plus the average or so of the two legs tensions. Eg with about 200 lbs in each leg there's a substantial normal force on the bollard (tip) and it would take some number of lbs to get the rope to slip on the bollard. If you climb gentle and smooth you don't make the rope slip and your up and down tensions stay equal. If you bounce your rope leg tension up to guessing 250 lbs (25% increase) the extra 50 lbs might get the rope to slip on the bollard, but not completely (its complicated because the dynamic system has inertia and damping, overshoot or not etc) so lets say it slips enough to stretch the anchor rope leg to 230 lbs, other 20 lbs still friction and 250 still mid bounce on the down leg. The bounce recedes and with the 215 lbs average normal force the 230 lbs is locked in on the anchor leg and your climber leg gravities back to 200 lbs. This is the fabled more than double tip loading. If you climb gentle into the system initially the anchor leg never stretches into equal position and you get less than 200 lbs in the anchor leg for the much less discussed less than double the force at the tip, less by the bollard friction at the tip.

There is a limit to the increase because when the differential ( 30 lbs in our example) gets too large the rope slips backwards over the bollard to reduce it. Lets say you bounce it to 350 climber side, and achieve 320 lbs into the stretched down leg with now 30 lbs still as bollard friction, when the bounce recedes the normal force might not be able to support the 320 vs 200 = 120 lbs differential of rope tensions so the rope could back slip until the average normal force roughly agrees with the settled rope tensions difference. Rough guess 240 lbs on stretched anchor leg and of course 200 climber side. Someone with 2 or more enforcer cells could just measure all this in 5 minutes to get real values dependent on rope type, bark type, tip branch diameter, heavy/light climber. length of rope legs etc.

So rope tension depends on climber weight and bounciness on the climber's leg and on the anchor leg it depends on length, elasticity and control of that from the factors of tree geometry and the rope changing length via changing position of its contact around the bollard. The anchor leg length could change by bending the trunk or sagging the tip branch (if you're not anchored at the trunk union). Small tension changes that don't slip the bollard just elastically spring the anchor leg and settle back to where they started. Big enough changes to slip the bollard settle the system to new tension levels. The bottom line principle is that changing the length between bollard (tip) and anchor directly changes rope tension by the rope's elasticity and you have to cross a threshold level to make the bollard slip the rope, setting a new length of rope under some resultant tension. You can force this change by forcing movement of the rope or applying more or less tension until the rope position changes. 6 of one half dozen of the other.

A time when tree movement in a practical way matters is when you're climbing on a system set like a rigging system. Anchor up to high central tip, out to skinny branch tip perhaps 45 degree branch but rope coming downward to redirect spot then rope vertical down. As you weight the rope it wants to skid across the redirect bollard point but instead the branch bends downward before the rope skids (depending on the beefiness of the branch). The good part is that the rope tension up between the redirect tip and the high central tip, perhaps starts to bollard a bit at the high central tip and then the middle leg of rope triangle supports the skinny branch. That would otherwise probably snap under your weight. Your weight is distributed between the redirect tip bollard friction which is trying to bend the branch (the branch is basically mostly under axial compression), central high tip bollard friction which is trying to bend the top of the tree and downwards vector components that are axially compressing the stem of the tree downwards. So you rough out the beefiness of your vertical, how long of it you're trying to bend, the beefiness of the redirect branch, knowing you're going to spring it down a bit but keep it mostly in axial compression and rough out the side load bend you're shoving into the tree trunk from the redirect branch base. When you start to get a feel for this you can use the geometry to get out to places hitherto unexplored by man " to boldly go where no ...etc " if you're a trekkie. The only caveat is to keep an eye on verticality of the triangle formed by the redirect tip branch, rope segment and trunk segment because if you side load bend the small branch you'll know as it bends giving you a warning. It'll snap off in side load the same as pulling it down. Keep it all vertical and axial loading and you get more load capability. I encountered this on some ridiculous oak branches this fall and the redirect branch was curved - I could see the curvature changing, but I just had to get out to the tip for a trim. Acorns and glass skylights and unhappy nervous guy issue.

Also remember the advantages of SRT in terms of safety redundancy. Try to adopt the rule "always more than one tip". (I know, a concept tough to implement in a conifer, especially a bean pole) A) it divides your load b) if one tip breaks out you're not hitting the ground. Look at pg 25 of Jepson's 2nd ed. TCC book and he's explaining a method to isolate the climb line back to a single DRT tip - from what is a textbook perfect SRT double tip line placement. Which is also coincidentally a perfect rigging line placement if you work out all the vector loading stuff. Maybe not just coincidence. If it's sketchy or you're really up in the spindly's take full advantage and have your rope routed for 3, 4 or whatever tips. As Kevin said, just pull your rope out at the end of the climb.

Dark at supper = long post.

take care