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SMC 3-inch Doubles VS Rock Exotica Mini Machined Doubles - The showdown!


Today we are looking at comparing the actual mechanical advantage of the SMC 3" Double PMP with the Rock Exotica Mini Machined Double Pulley. We are interested in how difficult tensioning will be with both of these pulleys so we would like to find what is the maximum tension that is obtainable with them in a 5:1 pulley system. using a 3:1 multiplier.



We will be setting up a 5:1 pulley system using Mantra MKII with Alpine WebLock 2.1's as the anchor points. We will use the CMC MPD as our brake for the pulley system. The rope we will use is the PMI 11mm ACCESS PRO, which is connected to the becket of the pulleys with a 5/16" 316-Stainless Dee Shackle. We will also be using a pair of SMC Large Rigging Plate's for the anchor and a Dillon Extreme Dynamometer to measure the force. Our multiplier will be the Elite Multiplier Kit with optional Petzl ASCENSION Handled Ascender, exclusively available in the Shop.

We will setup a ~100 ft. slackline using the above equipment by first using the Rock Exotica Mini Machined Double Pulley's. We will then pull this line as tight as we possibly can and record the peak force that we reach. After a sufficient rest, we will switch out the main pulleys with the SMC 3" Double PMP's and repeat the process, recording the max tension again.



The peak force that we achieved using the Rock Exotica Mini Machined Double Pulley's was 2,770 lbf (12.32 kN).

The peak force that we achieved using the SMC 3" Double PMP's was 3,570 lbf (15.88 kN).



I was quite impressed with the efficiency of the Rock Exotica Mini Machined Double Pulley's. A force of 12.32 kN is quite good for a pulley that weighs just 141 grams. There was a bit of friction at the location where we attached the rope to the becket of the pulleys. This could have decreased our mechanical advantage slightly.

It should be said that the force we achieved with the Rock Exotica Mini Machined Double Pulley's significantly exceeding the working load limit of the pulley (8 kN - 1,798 lbf). This is not safe rigging practice and should never be done on an actual slackline. Never exceed the working load limit of your gear!

What was even more impressive was the force that we achieved using the SMC 3" Double PMP's. It's incredible that a force of 15.88 kN can be achieved by a single person with just 15:1 theoretical mechanical advantage! This is a This would mean that I would have to have exerted a minimum of 238 lbf! We will see below that this number is quite far from the actual force exerted by me.

Now for a little math: In accordance with efficiency ratings of similar pulleys, we can assume that the SMC 3" Double PMP's have an efficiency rating of 95%. We will also assume that the MPD has a 95% efficiency rating. Knowing this, our actual mechanical advantage for our 15:1 pulley system was as follows:

(1 + 0.95 + (0.95)^2) * ( 1 + 0.95 + (0.95)^2 + (0.95)^3 + (0.95)^4 ) = 12.91:1

This means that I had to have exerted a force of 3,570/12.91 = 276.5 lbs to achieve a tension of 3,570 lbf. Assuming that I put the same force on the Rock Exotica pulley system, we can say the that the actual mechanical advantage of the Rock Exotica System was:

2,770 / 276.5 = 10.01:1

Which means that the Rock Exotica Pulleys have an approximate efficiency of

( 1 + 0.95 + (0.95)^2 ) * ( 1 + x + (x)^2 + (x)^3 + (x)^4 ) = 10.01

x + x^2 + x^3 + x^4 = 2.51

By approximation, we get:

x ≈ 0.821 or 82.1%

Based on these assumptions, we can say that the SMC 3" Double PMP are 29.2% more efficient than the Rock Exotica Mini Machined Double Pulley's. Again, this information is based are large number of assumptions.

Now, what does this mean? When should you use the SMC 3" Double PMP's instead of the Rock Exotica Mini Machined Double Pulley's? Well, since the Rock Exotica's only have a working load limit of 8 kN, that should be a good delimiter. Any slacklines that have a force higher than 8 kN should be using the SMC 3's. Also, if you plan on setting up high tension slacklines (7-12 kN) and want to dynamic tricks (jumps, surfs, bounces), then the SMC 3's are a better choice. Finally, if you want to rig longer lines where the tensions will be higher for a longer period of time during tensioning, the SMC's are a better choice as tensioning is significantly easier in the 6-10 kN range compared to the Rock Exotica's.


Future Research

This test is far from representative of the efficiency ratings of these pulleys. Several other tests involving different scenarios need to be taken into consideration. In the future, I would like to conduct lab tests of the efficiency rating for both of these pulleys in order to better compare them. In order to do this, I will have to have access to a laboratory where the restraints of the tests can be better controlled.

Thanks for reading the article. Please feel free to comment below.

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17 thoughts on “SMC 3-inch Doubles VS Rock Exotica Mini Machined Doubles - The showdown!”

  • Great article. Without a Dynamometer how do you know the work load? One of these days I need to make a road trip up to Davis!

    • Hey Joey,

      We did use a Dynamometer, it was attached to the anchor. The working load limit of the pulleys is published in the product pages.

      • Sorry I meant if I don't have a dynamometer how do I know what my tension is? Or should I buy one to know exactly what my load is at?

        • You can calculate the force on your line with a slackline calculator. All you need to know is the length of your line, your weight, and how much it sags in the middle when you stand on it. A good one can be found here: http://www.slack.e30tuner.com/articles_linetension.php

          • Thanks Jerry. Really appreciate all you do. I tell people everyday to check out your website. Is there a guide line on how much tension on certain lengths?

            • Hey Joey,

              Thanks for the comment, the kind words are much appreciated!

              If you know your line's length, your weight, and how much sag there is in the middle when you are standing on your line, then you can calculate how much force there is on the line via this calculator: http://www.slack.e30tuner.com/articles_linetension.php

  • Very interesting stuff Jerry! You could probably write a book soon.

  • Great test, thanks!

    I would love to see this repeated with a different multiplier, say the rock exotica single. I understand why you used the same smc multiplier so that you could accurately compare the efficiency of just the double pulleys, but a comparison of an all SMC rig vs. an all Rock Exotica rig would be useful too.

    Also, how much do you think other brakes affect efficiency? Do you have any numbers for say the grigri or the ID vs. the MPD?

    • Hey Jeff,

      Thanks for the comments, much appreciated! I plan on doing a multiplier comparison soon, however I would keep everything in the rig the same and only swap the multiplier pulley in order to get an accurate comparison. It is my belief that we will see a MUCH bigger difference on this comparison versus the main pulley system comparison.

      Caming brakes (GRIGRI, I'D, RIG, etc...) produce a TON of friction. You should read my post about the MPD and watch the video on the comparison to the I'D to see just how much friction is introduced to the system via the brake: http://www.balancecommunity.com/Slack-Science/the-cmc-mpd-pure-brake-awesomeness

  • Jerry:

    Considering I picked up my gear from Balance Community, you already know that I regularly rig with the SMCs and Rock Exoticas that you tested. Both types of pulleys work incredibly well, including on their own and as an embedded 9:1 system. In your article you stated: "It should be said that the force we achieved with the Rock Exotica Mini Machined Double Pulley's significantly exceeding the working load limit of the pulley (8 kN - 1,798 lbf). This is not safe rigging practice and should never be done on an actual slackline. Never exceed the working load limit of your gear!" Well, that got me slightly concerned, as I use my Rock Exoticas for rigging lines up to roughly 300 ft. When I am just over 300 ft. I am obviously putting in some effort to get some good tension on the line. In these cases I am setting my line with about 7 ft of sag. I have looked at the calculators to get an idea of my tension, but just want to double check that I am not exceeding the working load limit of my pulleys! Were you able to achieve your working load limits by using the MPD? On the 300 ft. rig, I generally use the GriGri (v1) or RIG -- any concerns?

    Great article, and thanks in advance!


    • Hey Brian,

      Thanks for the comment, glad to hear you are enjoying the gear.

      If you are achieving 7 feet of sag in the middle of your 300 foot line, you should be perfectly fine. It's quite hard to go beyond 8 kN of force with these pulleys using a standard, cam-style brake. You should be fine rigging this length line with that much sag.

      Hope that helps!

  • Does anybody know if any of the Petzl pulleys can compete with the SMC 3-inch Doubles?

    • That depends on which pulleys you are referring to. Do you mean the Twin pulleys? If so, they are fairly good, but no where near as good as the SMC's. For the small price difference, you get a fairly large performance gain with the SMC pulleys. Definitely worth it.

  • The SMC 3" has a working load of 12 kN & the Petzl Twin has a working load of 12 kN (http://www.petzl.com/us/pro/verticality/pulleys/prusik-pulleys-0/twin)

    Has anybody used the Petzl Twin for serious highlines?

  • Jerry,
    Note that the figure-of-eight knot has different widths in the two principal directions perpendicular to the rope axis. This means that using a twisted shackle instead of a D-shackle for attaching the rope to the becket can reduce the friction inside the system. In fact, after testing today, I can tell you that a 10mm rope attached this way does not introduced any friction because the knot does not make contact with the 4 strands.
    Another thing I would like to try (to reduce weight) is to tigh the rope DIRECTLY to the becket using a stainless steel thimble (made for wire ropes). I do not know whether a thimble for a 10mm wire rope would fit in the becket of the P21 double pulleys. If you know the answer let me know, too :D

    • The becket on the P21 Doubles is extremely small. I doubt you will be able to fit a thimble through that hole. However, your idea of rotating the knot 90 degrees is fantastic. I will have to give that a try to see how it effects the efficiency of the overall system. I bet in combination with the Right-Angle Reeving Method, you would could have almost no friction between the strands of rope!

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