Like math? Read about the finer details of cams (SLCDs) in Phillip Anuta’s thesis for his Applied Physics degree.
Why would a Grigri fail?
There are a few reasons why a Grigri might fail to secure a climber who is depending on the rope.
Reason #1: Without a cam, your Grigri would just be a US$100 rope ‘redirector,’ and you or your belayer would be left to catch a fall with just your grip strength. Have you seen that video of a belayer locking out his Grigri with his left hand? The results are not pretty.
The cam in the Grigri must be allowed to rotate, or it can not create a force on the rope to arrest a fall. A Grigri will not act like a tube style belay device – at least not a very good one.
Reason #2: One of the design features that make Grigris user-friendly is the spring that holds the cam in the open position (see above image). The small amount of force applied by this spring allows the rope to slide through the device when you want it to, like when you’re paying out slack for your lead-climber.
Ultimately, that force must be overcome for the cam to engage. A light climber may not always generate that force due to rope drag. A hanging climber may start a slow descent that becomes uncontrolled.
Reason #3: All cam-style belay devices are rated for a certain range of rope diameters. If the rope is too thin, the cam cannot generate appropriate force on the rope to arrest its movement through the device.
What can I do with this knowledge?
You can do two things to significantly narrow the margin for error with a Grigri:
#1: Keep your hand on the brake side of the rope at all times when belaying. Maintaining control of the brake end of the rope means you have control the motion of the rope at all times, whether that means no motion, ascending or descending.
#2: When lead belaying, always use the Petzl recommended technique for paying out slack. You should only override the Grigri’s cam when paying out slack, and you should use the recommended technique at all times to avoid inadvertently dropping your climber if a fall were to occur while you are paying out slack.
Ultimately, climbing is about making informed decisions to manage the risks you are taking by evaluating whether they increase the risk to you, your partner’s, or another’s safety.
Now that you know both more about Grigri mechanics, you can better assess the condition of your own belay device, the preparedness of your belayer, the potential for loss of belay control, and the necessity to use proper technique when belaying, such as maintaining a hand on the brake side of the rope at all times while belaying.
Climb safe, climb more, and climb happy!