SRT Base Tied TIP Forces

Bart_

Well-Known Member
Location
GTA
Well, I finally decided to get off my duff, confined in the house in the evenings and set up a jig to measure the climber side rope tension and the base tie rope tension. This is a revisit of a hot topic back in 2013. I remembered P(r)etzl (Chicanery!!) publishing a study where they said the tip force was less than double the climber's weight. Well, I dug and found Kevin's post of the conclusions page by Pretzl where they stated the force in two stages. First they said that in SRT or DRT (they meant cinch tied SRT) the tip force was 50% more than the climber's weight. This is a reflection of bouncing while you ascend. Then they said with an SRT base tie, you increase the tip force by 50% over DRT/cinch SRT. Why not 2x? Because of friction at the tip.

Enter the test jig. I put a load cell at the base tie, a load cell at the "climber" and ran the rope 180 degree Uturn over a rounded 4x4 so the rope is sliding across the grain like a tree branch. I hooked a string and pot to the rope on the climber's side about 6" from the tip to measure rope movement/sliding. I used my medium 11.5mm lanyard. This gave the following graph:

image0 cropped.jpg

There's 15 seconds of data across the bottom, lbs force on the left scale and inches rope position on the right scale.
Follow, between 0 and 2 seconds I ramp up the climber load to about 250 lbs (the yellow line in the middle) and the lower white line, the base tie tension, ramps up to about 120 lbs. Holy Pretzl values Batman! So what gives? Enter the bollard equation. T2/T1 = exp(mu x angle in radians) i.e. e to that power. So 2 = exp(mu x 3.14 radians) i.e. 2pi radians is 360 degrees. Messing around, mu = ln(2)/3.14 = 0.22 which is about the right value for friction. Now look at the highest line and see the rope skidded about 2" during the tension rise, which I also visually watched. So takeaway? 250 lbs + 120 lbs not equal 500 lbs. It's less. So what happens next?
From about 2 seconds to 5 seconds the rope stays still but the climber side tension drops to about 100 lbs, goes back up to 200 lbs and back down to a little bump around 100 lbs. The base tie tension stays pretty much 100 lbs the whole time because it's strung tight with unchanged length between the bollard (tip) and the base tie. Close to 5 seconds the climber tension actually equals and the drops below the base tie tension - and the rope skids back again towards its pre-loading position. I slacked the system close to zero from the climber side and you'll notice the base tie side held a little residual tension - because the rope didn't want to skid around the bollard (tip).

From 7 seconds to the end is a re-try. Wash, rinse repeat. The only oddity is the ripple in the base tie tension which I chalked up to some flex in my "tree" which was a 2x10. The width of the 4x4 was perhaps enough to torque the board a bit. I did the same thing with a 2 1/2" diameter locust branch as the tip because it has rough bark. Mu started at 0.5 and quickly dropped through .44, .4, .35 etc down to between .25 and .3 as the rope smoothed the bark down. I moved the rope aside and verified the smoothing effect. It took about 6 heavy load cycles to grind the bark surface down. There was less base side ripple which suggested my torquing/bending idea might be right.

One neat thing from either the HSE report or one of its references was that bollard friction is dynamic because the rope changes its level of stretch between the bollard entry point and the exit point - because one is high-tension and the other is low tension. So the rope undergoes a stretch within the bollard wraps. Cool.

One conundrum I didn't find was that I thought I could get the climber side tension dropped to equal to base tie, then continue to reduce it and have the bollard friction hold the base tie constant at least for a while to achieve the "strung" base side that's higher than the climber side. I would achieve a small reversal and then the rope would slide. This happens at the 5 second mark. Maybe I did achieve it and it's just different than I thought it would be. This was consistent across data sets.

Here's the only piece of the Pretzl info I could dig up. I thought there was a pdf but I couldn't find anything in my archives.
Screen Shot 2020-12-15 Petzl.jpg
 

Reach

Well-Known Member
Location
Atglen, PA
Thank you for putting forth all that effort to satisfy some curiosities! I appreciate the description and the math, and experiments things like this quite fascinating.
 

colb

Well-Known Member
Location
Florida
The start of some science - I like it! Do you think the coefficient of friction changes a lot and is it the roughness of the bark or the length of the bollard circumference in contact with the rope that drives it?

Does the coefficient of friction matter to a climber ascending a rope? If a climber falls a short distance into a base tie TIP, does the coefficient of friction matter at all?
 

Bart_

Well-Known Member
Location
GTA
Colb, I'd say that a new observation is the quantity of rope rub energy dissipation I didn't realize before, due to not being able to get the base side higher than 1/2 the climber side. What that means is that the base side has more ability to absorb the hit because it's starting at a lower tension. When it gets hit, the rope has to skid over the branch for the rope to stretch up to a higher tension. Skidding equals damping. It's only a few inches of stretch. Depending how high the tip is. If the climber transients to 400 lb tension, the base side would only come up to 200 lbs. And then make waveforms like the graph as the climber bounced, reducing the tension from 400 lbs back to around 200. Bouncing is what I was trying to get at with the climber side tension being unsteady in the graph.

If I get motivated I'll swap in my tachyon.
 

Bart_

Well-Known Member
Location
GTA
Well, I swapped in my tachyon on the 2.5" diameter locust branch and got a ratio of climber side tension to base tie side tension of 2.5 or 2.6 for a mu of 0.3. Eg 250 lbs climber side had 100 lbs base tie side. Same waveforms. I immediately noticed that the tachyon was way squishier than the other unknown rope. The unknown rope was 11.5 and the guy at Shepards (the guys operating out of a barn north of Toronto) just called it zebra stripe. It was black and orange, if anyone knows the proper name for that rope - I think I spliced it with tachyon instructions, years ago. The difference was 3.8" vs 2.5" stretch for similar 250 lb "climber" in the test jig.

This leads me to want to characterize the rope a little better. Ie is it linear spring, non-linear, takes a set, cables up, has damping when stretching etc. That's going to take some ciphering as Jethro would say. I also want to change the jig to measure pulleys, BMS Belay, carabiner, roller carabiner etc. Lets just say that we just got state of emergency lock down.
 

Stumpsprouts

Active Member
Location
Asheville
Would you mind rephrasing your conclusions? Maybe it’s too early in the morning but I’m having trouble figuring out what the conclusion is on what percentage weight of load is applied to branch on a basal anchor. And it varies rope by rope?

Thank you for doing this! I also will continue to play video games in my spare time.
 

Bart_

Well-Known Member
Location
GTA
For tachyon on 2 1/2" dia locust, 250 lbs climber side and 100 lbs base tie side adds up to 350 lbs pulling down on the branch/tip, for a single tip, 180 degrees rope wrap over the top of the branch.

I swapped in my 5/8 stable braid (double braid polyester) and found a huge amount of initial settling in of the rope from loose flaked condition to cabled up like a cylindrical profile steel cable. Years ago I set up a mini zip line with this rope and it seemed to slow motion creep or sag no matter how we readjusted the line. So I was expecting to see this, but I didn't. Instead after it finished cabling up it just held solid (now this is at only 300 lbs for 8 or 16 feet of rope depending how you interpret it vs side pull loaded on the 50 foot zip line). It also had 250 lbs climber side vs 100 lbs base tie side for a mu of 0.3.

The interpreting part is because half the line is at 100 lbs and the other half is at 250 lbs so you can't really say a single tension value for the entire length of rope. Another run I took it up to 300 lbs. There were multiple runs.
 

Bart_

Well-Known Member
Location
GTA
Corollary: why does DdRT ascent suck so bad?

Well, it's the same situation. 100 lb guy, ignoring the acceleration force bump for the moment, pulls 71 lbs on the arms side while the saddle side has 29 lbs. Wait, where'd the proverbial 2:1 advantage go? But there's more. Doing some start/stops on the test rig with my 118 lb son, he had 10 or 20 lbs acceleration force, be it starting or stopping (no hard slam stops - those can be 100 lb spike if you lock suddenly and hard enough). So call it 15 lbs for our 100 lb guy. Divided 2.5:1 in the line halves, 4 lbs goes into the bridge side and 11 more lbs goes into the arms side of the rope. 71 lbs + 11 lbs = 82 lbs arm effort for a 100 lb guy on a supposed 2:1 system!

Bit smoother bark may get you 2:1 tension ratio instead of 2.5 to 1 rough locust bark. Still heavily not in your favour.
 
Bart_ thanks for doing this.
Also to keep recent things together maybe, Brian Kane, PhD, University of Massachusetts-Amherst did a TreeFund webinar on TIP loading research in February 2021.
It's available at:
Loading a Tie-in Point While Climbing
There was also a TreeBuzz discussion of the subject around then,
 
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Bart_

Well-Known Member
Location
GTA
Here's some more test data for friction. I'm keeping dataset numbers in case I want to reference my notes in the future.

W34 1/2 wrap BMS belay spool tension ratio 1.9 mu 0.20
W35 1 1/2 wraps BMS belay spool tension ratio 6.6 mu 0.20
W36 2 1/2 wraps BMS belay spool tension ratio 18 mu 0.18

W37 1/2" dia smooth steel bollard 1/2 wrap tension ratio 1.8 mu 0.19
W39 3/4" dia smooth steel bollard 1/2 wrap tension ratio 1.65 mu 0.16

W40 0.471 dia big steel biner 1/2 wrap tension ratio 1.86 mu 0.197
W41 0.375 dia half cylinder small blue Metolius biner 1/2 wrap tension ratio 2.07 mu 0.23

W42 big heavy yoke N-261 0.49" wide with edge rounds matching 1/4" diameters, flattish face, 1/2 wrap tension ratio 2.4 mu 0.28

W43 cheap green and silver w plastic pulley, 0.825" groove dia x 0.5" wide groove x 0.18" deep groove, looks like plastic on steel bushing, tension ratio 1.34 equivalent mu 0.09

W44 Petzl Partner w ball bearing, 1.06" groove dia x 0.46" wide groove x 0.13" deep groove, tension ratio 1.18 equivalent mu 0.05

W45 DMM Revolver, 0.43" groove dia x 0.41" wide groove x 0.027" deep groove, tension ratio 1.77 equivalent mu 0.18 DOH!

W46 DMM Pinto w ball bearing, 1.13" groove dia x 0.75" wide groove x 0.19" deep groove, tension ratio 1.2 equivalent mu 0.06

W47 big blue rollgliss kit pulley, bronze bushing, 1.97" groove dia x 0.58" wide groove x 0.19" deep groove, tension ratio 1.20 equivalent mu 0.06

W49 1/2" dia smooth aluminum bollard 1/2 wrap tension ratio 1.81 mu 0.19 Someone said in a post many moons ago that steel was slipperier than aluminum, this says they're identical see W37. Note lathe circumferentially smooth surface.

W50 3/4" dia not smooth aluminum bollard 1/2 wrap tension ratio 1.88 mu 0.20 Note repeated trials rubbed the small longitudinal "scratch" marks down and gave ratios 1.83 and 1.80, right back to smooth result but not equal to W39 steel 3/4" 1.65 ratio. This one says steel is slipperier by a bit.


These results are all 180 degree U turn except the BMS and lay groundwork for gizmos etc. I feel suckered buying the Revolver after seeing it vs a regular biner, even though it was years ago.

The development of the OneWay involved pinch friction and was quite enlightening but had more factors instead of being simple bollard action. Maybe another day.

take care
 

moss

Well-Known Member
Your climber-side loading MRS findings ring true. The few times I climb MRS I think, "God, SRS climbing has made me weak!", it feels like a lot more labor on ascent. As far as the MRS 2:1 goes, yes you have to move 2' of rope to move up 1' but the loading inefficiencies defeat any advantages and the slow ascent progress defeats morale ;-)
-AJ
 
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LordFarkwad

Well-Known Member
Location
Chatham Co.
I did some MRS yesterday using a rope walking setup (knee/foot) and holy cow does it feel like being on a treadmill, running in sand, etc etc.! Demoralizing.
 
Reminds me, some time ago now, Dave Stice from Wesspur made a comment in a video of his that some bounce in SRT lines is probably good, in that while ascending, it isn't so hard on your knees and joints (like walking wearing rubber heeled shoes). Don't notice this when you're younger, but it maybe helps those joints stay in the game longer? I thought about this and I think I agree - repetitive shock and strain takes it's toll. If you have a thicker rope with higher MBS, you will get less bounce. If you have more rope in your system (basal tie, over your TIP and down to you) you'll get more bounce. If you try climb a dynamic rope (alpine) you'll get even more bounce (to the point of silly sometimes). So rig smooth, climb smooth?
 

moss

Well-Known Member
To your point Ghostice I've been wondering if we're going to see more tendon injuries and other wear and tear from some of the loading the body experiences on 1:1 climbing systems. Makes sense that a less then full static line could help. I think climbing more smoothly is going to help reduce injury on even the most static lines. I climb both (bouncy) and more static, at 65 years-old I'm still liking the more static lines the best. But I haven't been climbing daily since I was 18 either ;-)
-AJ
 
. . . . at 65 years-old I'm still liking the more static lines the best. But I haven't been climbing daily since I was 18 either ;-) . . .

Which brings me then to my Wing Commander's point (have 'bout two years on 'ya), "When are you guys going to hang it up and stop trying to race a car with used parts . . . " she sez . . . .
Never, I sez . . . . :)

Never submit . . . . just Praise the Lord and pass the ibuprofen . . . .

Cheers
 

moss

Well-Known Member
. . . . at 65 years-old I'm still liking the more static lines the best. But I haven't been climbing daily since I was 18 either ;-) . . .

Which brings me then to my Wing Commander's point (have 'bout two years on 'ya), "When are you guys going to hang it up and stop trying to race a car with used parts . . . " she sez . . . .
Never, I sez . . . . :)

Never submit . . . . just Praise the Lord and pass the ibuprofen . . . .

Cheers
I put a 90 year-old on rope for a rec climb, just before I clipped him in I asked: “How’s your heart doing?” He said: “When it stops ticking I’m done” I said: “Awesome let’s climb”.
 

Bart_

Well-Known Member
Location
GTA
I think what's demoralizing about DRT is that your body intuitively measures the 80% body weight effort you put in you're going up the tree half speed. Hey! Throw in some sit back! And some wrestling while you tend the hitch!

In SRT your legs go "yeah, I'm climbing the stairs" at about 120% body weight and your brain goes "woohoo! 100% forward gain every step with no sit back at all!" while your arms keep you balanced upright at very low load.

Call it a Homer-brainer. :)
 

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