Calculated Risk & Climbing: Cam-Style Belay Devices
The modern belay device comes in basically one of two styles: tube & camming. When used correctly, they will both allow for a belayer to catch a fall, & the climber to be caught & lowered. Both require a hand on the brake side of the rope AT ALL TIMES, and all climbers (excluding you pebble wrestlers) know how to use at least one of these types of belay devices.
Not all climbers know why belay devices work. Let’s take a look at the more complex of the 2 styles: cam-style, and break down why this style of belay device does & does not work.
One of the more common cam-style belay devices is the Petzl Grigri. First Ascent has a Grigri 2 on every top rope in the gym, & this author owns one, which is why it’s going to be used to demonstrate why cam-style belay devices work.
What is a cam?
A cam is any device that translates linear motion into rotational motion. In climbing, linear motion is the motion of ascending or a fall, & consequently is the motion of the rope. Rotational motion in climbing is hopefully limited to the motion of certain devices, like SLCDs (cams), pulleys, & the action of a Grigri braking.
Why does a cam work?
Most cams in climbing use a logarithmic spiral shape to ensure that the contact point between the cam & the other thing (rock, friction plate) remains constant. The shape of this spiral has been optimized such that the force of a fall is always translated to a greater force by the cam. The Grigri uses this action to compress the rope between the cam & friction plate to generate friction to arrest a fall.
Like math? Read about the finer details of cams (SLCDs) in Phillip Anuta’s thesis for his Applied Physics degree.
Why do I care?
Without a cam, your Grigri would just be a US$100 rope ‘redirector’ & 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 cannot act like a tube style belay device.
Why else won’t my Grigri work?
The second part of a Grigri that makes it user-friendly is the spring that holds the cam in the position it’s in when the Grigri is open (see above image). This small amount of force allows the rope to slide through the device when you want , like when you’re paying out slack for your leader.
This is also the amount of force that needs to 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. Not holding the brake side of the rope can compound these problems.
If you are not gripping the brake side of the rope, the motion of the rope is in two directions. Imagine that you are at the crag, belaying, and the brake side of the rope falls down the embankment you’re atop. The force generated could be enough to snap the cam open, as if you had pulled the brake release handle suddenly. If your climber was hanging, your climber is now falling.
Keeping your hand on the brake side of the rope means you control the motion of the rope at all times, whether that means no motion, ascent, or descent
These situations can happen even when using a grigri properly. Keep your hand on the brake side of the rope at all times.
Finally, all cam-style belay devices are rated for a certain range of rope diameters: if the rope is too thin, the cam cannot generate a force on the rope.
What can I do with this knowledge?
Climbing is about making informed decisions to manage the risks you are taking by calculating whether they increase the risk to you, your partner’s, or other’s safety.
Now that you know both how to use a cam-style belay device, & why it works, you can better assess the condition of your own belay device, your belayer, situations that could result in a loss of control of the belay & why you always have a hand on the brake side of the rope.
Climb safe, climb more, & climb happy!
