shear reduction device

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shear reduction device

Postby Bob Thrun » Oct 23, 2016 11:13 pm

I was looking at vertical gear and came across the CMI shear reduction device . It is available from several dealers. CMI and the dealers give specifications about its strength and that it will take 1/2 inch rope. None of them explain what shear reduction is and why I would want to use this particular device to reduce shear.

Can anyone explain why, where, and how I would use the device? Who first described shear, invented the term, and made some group of rope users aware of shear? What group of rope users is most concerned about shear?
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Re: shear reduction device

Postby driggs » Oct 24, 2016 1:43 am

A shear reduction device is used to provide a larger diameter when a moving rope is routed over an acute change of direction, typically on a "ropes course". The typical application is the top anchor in a top-rope rock climbing belay. Typically in sport usage, climbers will use a carabiner or doubled carabiners. The hypothesis behind this type of device is that it prolongs rope life by increasing the diameter of that inflection point.

I believe that in a ropes course, the "climber" is moving over and across many obstacles, and this shear reduction device follows them on a cable-mounted trolley, ensuring that they are always belayed from above.

Why not just use a pulley? Presumably because you want the additional friction to assist with your belay.


Singing Rock makes a device called the Red Block, and their instructions (with diagram of ropes course top-rope rigging) are online: ... 016_04.pdf

CMI also makes a slightly less confusing version which appears to be a pulley that allows you to lock its sheave in place: ... ducts/shrb

In their defense, these products are categorized on the CMI website under the "Trolleys / Ropes Courses" category. Not designed for caving!
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