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Slackline Load Measuring Tests - Test 1

Date: 5/10/2008
A = End By scale
B= Opposite End from Scale
C = Center

Testing Conditions:
53oF, 30% Humidity, Wind 6mph, Partly Cloudy

Test Length In Feet Sag In Feet Weight in Pounds Pre-load in Pounds Standing at A Standing at B Standing at C Predicted Load Percent Diff Surfing at A Surfing at B Surfing at C Jumping at A Jumping at B
1 31.5 ft bottomed out 220 68 362 252 342   -100%          
2 31.5 ft 3.50 220 150 446 408 492 507 3% 480 460 540 511 420
3 31.5 ft 3.00 220 200 500 496 572 587 3% 552 528 634 632 564
4 31.5 ft 2.67 220 300 570 532 680 658 -3% 664 634 734 712 660
5 31.5 ft 2.17 220 400 656 644 730 805 10% 756 672 782 776 802
6 31.5 ft 2.08 220 500 732 734 806 840 4% 814 826 862 880 874
7 31.5 ft 1.92 220 600 810 792 900 909 1% 830 800 946 912 926
8 31.5 ft 1.50 220 700 906 872 954 1160 22% 934 972 1014 1014 1022


System Used

I used an old orange mil-spec webbing that was semi-static and a 2" ratchet w/ tensioning add-on to reset bite. The line has been exposed to salt water under tension which has since made it seemingly "low stretch" and stiff from general observation and comparison and is generally considered in rough shape. My choice in line is to help reflect a "well loved" line that may commonly be used. Slackers rarely have a new line so I wanted to see how a heavily trafficked line stands up. Relatively new mil-spec webbing was used at the anchors as a safety precaution. The trees were only 10 inches in diameter so minimal elasticity should have been expected from the anchoring webbing. Under visual inspection they did stretch a small amount under use but bounced back to their prior measured state when unloaded.


The 150 lbs pre-load was a very mushy line but very suitable for people looking for a very large arc for surfing.

The 200 lbs pre-load test was about ideal for a surf line and let me get a good wide arc as well as aggressive movements.

The 300 lbs pre-load test was a good general line. Suitable for surfing and tricks but a bit mushy feeling for jumps.

The 400 lbs pre-load was a good jump line without being too harsh for general use. Subjectively, that's about where I usually set my lines.

The 500 lbs pre-load was an aggressive jump tight line and started to be harsh for barefoot landings.

The 600 lbs pre-load was a bit on the extreme side of tight and was fairly harsh and felt very twitchy for sit starts. I found myself having to take several attempts to get adjusted to the tension.

The 700 lbs pre-load was very harsh and felt awkward, much like a real tight rope. Even under minimal 2-4mph wind it chattered and vibrated on its own thus pulsing the pre-load numbers. Had I not had the load scale on there I would have been concerned about breaking hardware at this stage. For anything beyond this I believe that shoes will be a requirement for further testing.

Testing Notes

I could have continued at higher tensions but feet started hurting on high tension lines. When the system was tensioned to 700lbs it was perceived as insanely tight, far beyond what I would normally recommend for slacking. I was however surprised that even though it felt like it was tightrope solid that it had a full foot and a half of dip in the center.

Point A and Point B were 3.5 feet from the end of the line. C was measured as dead center in the line and was used to provide the line sag measurement.

To obtain the forces in the table above I intentionally landed as harshly as I could and surfed aggressively where possible. On tight lines it was difficult to get a large surfing motion close to the trees. For all standing measurements we went with a steady value. For jumping and surfing we took the highest number of any attempts. Oddly we seemed to get lower numbers when only sitting on the line than standing. Additional testing will have to be done to explain why. My theory is that sitting encouraged me to subconsciously sit closer to the tree than the standing point, which would have lowered the system load. While slacking however it was not perceivable.

Due to line stretch the preload had to be adjusted back after each measurement. In almost every case it needed 1/4" taken up (1 click on a ratchet) after each measurement to raise the pre-load to the desired test amount. When the line was tensioned the force would slowly begin dropping on it's own due to the slackline's webbing relaxing under the load. This was with a system that provided zero line slippage from the tensioning system. I do not believe that I could have maintained a consistent preload easily with most other tensioning systems as that 1/4" often changed the pre-load by 40-60lbs. If the pre-load was higher than needed we often only had to wait a minute for the load to be where we needed it.

Common thought in the past has been that 5 minutes under load is enough to stabilize the line stretch, this did not seem to be the case as even with what I would normally consider a "static" and heavily used slackline the numbers continued to drop while the load was in place even over several hours of testing. This observation hints that future tests must also have the pre-load adjusted in between each measurement to maintain consistent results. Without it, the loads would decrease after every dynamic movement on the line or even under a slow static load. A separate testing series may be needed to determine how long a line needs to fully "relax" it's stretch to provide a static system. For now I am terming this phenomenon "Pre-load Decay". I believe that we did our best to keep the test accurate despite this condition. However, under larger loads the rate of the pre-load decay became much faster and may explain the deviation between our expected results on the 700lbs pre-load test and the expected results. It may be possible that in the ten seconds it took from setting the line to getting the test results that we had lost enough pre-load to skew the results as under these higher loads the pre-load decay seemed to accelerate.

Despite the pre-load decay I do not believe that it warrants switching materials until further tests would actually confirm that other materials such as spectra or dyneema blends would outperform mil-spec nylon given the trade offs and costs involved. On the plus side for safety, the elasticity of mil-spec seems to reduce peak shock from dynamic loads such as jumping nicely. I suspect that if a more static webbing was used, we may see much higher peak loads and thus a higher chance of breaking hardware. I suspect that even the blends that are available on the market that advertise a lower elasticity will still have this effect at play, but perhaps at a somewhat slower rate. 

The first test was excluded since line drop could not be determined accurately due to bottoming out in the middle.

All load numbers produced by a professionally calibrated 10,000 lb capacity digital load scale that displays in increments of 2 lbs. Predicted force loads were provided by trigometric force calculator from 

Our Force Calculator Vs the Real World

Refer to the chart below to see our Field Tests of the Slackline Force Calculator for a nifty chart showing how our mathematical predictions stacked up against the real world. Overall, I'd say it is accurate enough to continue recommending as a method of locating a force baseline for slackline systems. Most of the values were within 3% which is exceptional. As mentioned above, Pre-Load Decay may have attributed to the deviation for the last test of 700 lbs pre-load and in theory could actually be attributed to nearly all of the variation from our predictions.

Force Calculator Vs Testing 

Tested By: Joe Kuster - Owner - Slackline Express
Results Confirmed by - Teale Niemeyer - Production Manager - Slackline Express
All measurements are displayed in US Pounds and US Feet