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Analysis: Slackline Brake Efficiency
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Jan 28, 2013, 3:16 AM
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Analysis: Slackline Brake Efficiency
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This study illustrates the efficiencies of eight slackline brakes with two ropes and two base pulley systems. I examined this issue by applying 100 lbf on the pull strand of the test tensioning system, and by recording the peak load subjected to the load-side of the tensioning system. I conclude the study by finding the ATC performed the worse, all other locking brake options performed similarly and the combination pulleys performed the best. In addition, the 3/8” static rope performed worse than the 9mm.


Many experienced slackliners place great emphasis on choosing efficient pulleys for tensioning systems—for good reason too. Inefficient pulleys can rob net mechanical advantage from a tensioning system. However, few place much thought into the efficiency of a brake. The efficiency of a brake is almost equally as important as the efficiency of a multiplier pulley because the brake also affects the mechanical advantage of a tensioning system’s compound multiplier system.


We will examine two 5:1 pulley systems: the CAMP 2 3/8” doubles and the SMC 3” PMP doubles; and two ropes: the 3/8” ABC Polyester Static and the 9mm Edelweiss Speleo. We will compare those two groups of equipment against eight brakes: Petzl I’D S, Petzl Rig, Trango Cinch, Petzl GriGri 1 and 2, Petzl Reverso 3, Petzl Mini-Traxion, Petzl Pro-Traxion and the base system without a brake or multiplier. The multiplier used in this test was the SMC 2” Russ Anderson pulley, and I used it for every test exclusively.


I placed a load cell on the load-end of the tensioning system, and another load cell on the pull strand. I then tensioned the pulley system until I reached 100 lbf of applied force on the pull strand, while the indicator interfaced with the load-end side was logging the force subjected to the cell. After testing every brake sample, I then retested all of the samples in order to confirm the repeatability of my methods. After conforming the test’s repeatability was within an acceptable limit (5%, assumingly limited by a change in the rope’s modulus of elasticity from repeated tensioning and detensioning,) I then swapped out the static ropes and restarted the test cycle.


Unsurprisingly the ATC performed the worst; likely because of the large amount of friction created by the rope running over itself in the auto-block mode. The locking belay devices performed similarly with
the GriGri 2 being the most efficient option. Interesting to note is that the GriGri 1 and 2 performed almost identically. Also unsurprisingly, the combination pulleys greatly outperformed the locking belay devices due to the presence of an integrated pulley.

The 3/8” ABC Polyester Static rope performed noticeably worse than the 9mm Speleo, likely as a function of three issues: more rugged sheath pattern, significantly less flexibility and greater surface area. Keep in mind that 3/8” is thinner than 11mm, which is the most common diameter amongst slackliners. However, it is worth noting that the ABC Polyester is a special new breed of static rope that features a 100% polyester make-up (previously, “polyester” static ropes had a nylon core.) Consequently, the rope is quite stiff. However, it is not any stiffer than other 11mm stiffies.

The following graph represents everything shown on the previous graph, plus the integration of a secondary vertical axis that shows the total efficiency of the tensioning system.

The last graph represents the same tests performed above, but with the CAMP 2 3/8” pulleys added on the secondary axis. Worthy to note is that the 5:1 base test without the multiplier showed that the polyester static rope performed slightly worse than the 9mm Speleo, whereas with the 3” SMC PMPs, they performed equivalently. This is possibly because the smaller sheaves force the somewhat inflexible rope over a smaller radius, which causes friction. Imagine trying to bend a 1/8” steel cable around your finger. Now imagine trying to bend a 1/2” steel cable around your finger. That is the general premise. However, it is certainly possibly, probable even, that there is far more to this then just the radius presented to the rope.


The ATC is rather inefficient and slackliners should avoid them, except in situations where value is the most appealing attribute. The majority of the other mechanically locking belay devices performed rather similarly, and therefore it matters little which device a slackliner chooses (from an efficiency perspective). The I’D performed below average with the thicker rope. However, the I’D S is pretty overkill for a slackline anyway, so there is little functional purpose in choosing one over the Rig. The combination pulleys vastly outperformed the locking belay devices. In fact, switching from a GriGri to a combination pulley appears to yield a greater increase in net mechanical advantage then switching from CAMP pulleys to top-of-the-line SMC PMP 3” pulleys.

(This post was edited by USnavy on Jan 21, 2014, 1:09 AM)

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