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Topic Date User

Rapping with a Gri Gri - Danger? 17-Mar-2011 At 9:11:27 PM Jim Titt

Message

Bear Grylls knows nothing compared to me!

In the abseiling mode an ATC will have 3 bends totalling about (depends on the weight and rope) 337°.

A Grigri has 2 bends totalling 360° so more bending.

But 3 bends gives more resistance than 2 for the same total angle.

And the bends on a Grigri are larger radius and give lower resistance.

A wide-open Grigri has 74% of the braking force of an ATC in Lo mode and 63% in Hi mode using a 10mm rope.

The exact amount of heat energy which goes into a Grigri is very difficult to measure but it looks like it is only about 6% more than into an ATC/karabiner combination but the Grigri is less able to cope with the energy anyway as;-
The specific heat capacity of stainless steel is considerably lower than that of aluminium (0.5 to 0.9) which means the stainless steel achieves a higher temperature for the same energy input.
The thermal conductivity of the stainless is about 1/15th of aluminium and the heat dissipates away from the contact point far more slowly leading to increased temperature.
The emissivity of stainless steel is effectively 1/2 that of aluminium and so the stainless steel cools less effectively.
The contact point of the Grigri is shrouded and the cooling by conduction to the air is reduced.
However, the lower thermal conductivity of the stainless steel means that when you stop the energy tranfers from the metal to the rope more slowly so slowing the melting.

You cannot ever achieve such high temperatures with an ATC as it reaches temperature stability at a much lower level than a Grigri as the cooling is considerably more efficient, a bit of rough extrapolation from the temperature rise curves says this will about 4 to 8 times higher for the Grigri but this really is very approximate, both temperatures are much higher than you ever want to see!

The solution to all the Grigri abseiling difficulties is blindingly simple, add something above it. I usually use a karabiner brake extended on a draw with either 2 or 3 krabs depending on the rope diameter (9mm or 10mm). If you use an ATC style device usually you want to be in Lo mode as there is not enough residual force to lock the Grigri in Hi mode but you want to try this with your load and rope.

Jim