Three factors, which are at least theoretically under your control, determine the amount of friction that your belay will produce. The strength of your hand grip is one factor. Since the belay ampli
Fig. 7-27. Comparison of some dynamic belay methods
HAND GRIP, pounds
Fig. 7-27. Comparison of some dynamic belay methods fies this grip, your choice of belay method is a second factor. Both of these factors are shown in figure 7-27. For example, virtually anyone can grip a rope hard enough to hold a 50-pound pull. A standard slot device or the Munter hitch will amplify this grip so that your belay can hold a 350-pound steady pull before the rope begins to slide. (But as you grip harder, the Munter hitch grips harder than slot devices do, as shown by the diverging lines.) Figure-8 devices come in a wide variety of sizes and surface finishes, so their belay varies from 250 to 430 pounds, with a 50-pound grip. A standard slot device with two carabiners holds 700 pounds. (Different ropes can produce somewhat different numbers. For example, using a half rope, forces are reduced by an average of 20 percent.)
The third factor determining belay friction is the angle between the ropes entering and leaving the belay device or the Munter hitch, or the total number of degrees in all the wraps of the rope in the hip belay. Varying this factor produces a continuous change in the total grip, with no angle at which a sudden change occurs, for any belay method.
The belay method is the most important of the three factors, because it operates for every fall. Hand strength and rope angle can usually be varied only when a fall is expected by the belayer. Taken by surprise, a belayer will react as he has trained himself to do, with full braking efforts.
Although you can predetermine the level of friction to some extent, it would be an overstatement to say that you can precisely control it. Probably the most you can do is to seek to avoid the extremes. High friction is justified whenever protection is known to be bombproof. Low friction might be called for when protection and anchors are poor, or rope drag (acting just like the belay itself) is very high, or for a very long fall on uncertain protection, provided the extra fall length is not itself a hazard.
Most belays are chosen and executed by a given belayer in that belayer's customary way, to allow concentration on other aspects of climbing, as it should be. But armed with a knowledge of how belays work, informed climbers can recognize extreme conditions and may decide to vary their belay accordingly.
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