Variable Friction

How to use the greatest feature of a rappel rack.

Scott McCrea NSS#40839RL

​Out-of-control rappels, feeding rope, frustration, and exhaustion are possible results of improper rappel rack use. The friction between a rack and a rope varies throughout a rappel and needs to be adjusted in order to control a rappeller’s speed. Rope length, diameter, construction, materials, condition, and age are a few of the ways the rope can affect the friction. The angle of the descent and pack weight also requires adjustments. Variable friction makes it possible to use a rack on drops of nearly any length.

A standard 14-inch, 6-bar, "J"-shaped rack. Note: this rack is oriented parallel to the viewer.

A standard 14-inch, 6-bar, “J”-shaped rack. Note: this rack is oriented parallel to the viewer.

​Investigations have shown that improper friction variation and control have contributed to many rappelling accidents. Feeding rope, or pushing rope into the rack, is one improper technique. This will reduce the friction and allow the rack to move, but it can be difficult and result in exhaustion and frustration which leads to bad decisions. A bar accidentally getting pushed off or dropped is also a dangerous possibility when feeding rope.

For clarity, this article will feature the standard, 14-inch, six-bar, “J”-shaped rappel rack (Figure 1) oriented perpendicularly to the body.

​Figure 8s, ATCs, bobbins, micro-racks, “U”-shaped racks, and other rappel devices offer either non-variable (static) friction or limited variable friction. These devices work great in certain situations but do not have the versatility and adjustment options of a standard rack.

​Rappel racks create friction in two ways: First, with surface contact between the rope and the bars. Second, with the bending of the rope around the bars.

​There are three ways to vary the friction with a rack:

  1. ​​Increase or decrease bar spacing.
  2. ​​Adjust the rope entry angle.
  3. ​​Drop or add bars.
Fig 2. Squeezing the bars together creates more friction, spreading them creates less friction.

Fig 2. Squeezing the bars together creates more friction, spreading them creates less friction.

Bar spacing refers to the amount of space between the bars. Squeezing the bars together creates less space and more friction, slowing a rappeller’s speed. Spreading the bars apart creates more space and less friction, increasing speed. Adjusting the space between the bottom two or three bars is the most effective technique for varying friction (Figure 2). Only the last two bars need to be spread or squeezed. Trying to spread the bars above is difficult and they quickly slide back up to where they were. Always adjust bar spacing before and after making other changes such as the rope entry angle or dropping/adding bars.

​Rope entry angle adjustments vary how much of the rope touches the last bar. The angle can be adjusted so the rope only touches a tiny part of the bar or touches/wraps around the full bar. This variation is usually done by moving the rope to the left or right, which will change the entry angle and add or remove a half-bar of friction on the last bar (Figure 3). Placing the rope between the rappeller’s legs will also adjust the rope entry angle.

Adjusting the rope entry angle will add or subtract a half-bar's worth of friction. This is much easier to do on a rack that is oriented  perpendicularly to the rappeller, as shown here.

Adjusting the rope entry angle will add or subtract a half-bar’s worth of friction. This is much easier to do on a rack that is oriented perpendicularly to the rappeller, as shown here.

Dropping bars is done by unclipping the last bar and sliding it all the way down the long leg of the rack. Dropping bars should be done so that only a half-bar worth of friction is removed. For example, bars should only be dropped if the rope is only touching the top half of the bar instead of wrapping all the way around the full bar. Dropping full bars can reduce the available friction by not just one bar, but by 1½ bars. For example, if a rack is rigged with six full bars and the sixth bar is removed, the available friction instantly decreases to 4½ bars (Figure 4).

To reduce friction, follow these steps in order, one at a time, until the desired amount of friction is reached.

  1. ​​Spread bars.
  2. ​​Adjust rope-entry angle by moving rope to other side of the rack.
  3. ​​Spread bars.
  4. ​​Remove last bar.
  5. ​​Start over at 1.
Fig 4. Removing the bottom bar of a 6-bar rack reduces the friction to an equivalent of 4 1/2 bars.

Fig 4. Removing the bottom bar of a 6-bar rack reduces the friction to an equivalent of 4 1/2 bars.

When more friction is needed, adjust the entry angle for more contact with the last bar, or add a bar.

​Ideally, friction should be adjusted so that the last bar needs to be spread and held down to maintain the optimum speed. In addition to a smooth ride, this technique can provide a safety function. If the bar being held down is released, it should automatically get pulled up, squeezing the bars together and slowing the speed.

​As a rule of thumb, never rappel with less than four bars. If four bars is too much friction, this is the cave gods telling you something is wrong. Stop and figure out what it is or go home.

​Gripping the rope with the off-rack hand, pulling the rope around the hip, and a bottom belay are a few of the methods that can affect the friction from outside the rack. Using them does not take advantage of the variable friction feature of a rack. These and similar methods come at the cost of efficiency. And, depending on the situation, they may not always be an available option.

Fig 5. The two types of "J"- shaped rack frames.

Fig 5. The two types of “J”- shaped rack frames.

The most efficient way to take advantage of a rack’s options is to orient it perpendicularly to the rappeller’s body-looking at the ends of the bars with the short leg away. This makes it possible to easily move the rope to adjust rope entry angles and drop or add bars (see Figure 3). “J”- shaped rack frames are available with either a straight or twisted (90º) eye to accommodate different style harnesses (Figure 5). Most caving harnesses are best coupled with a twisted-eye rack.

​“U” shaped racks only offer limited variable friction. The bars can be spread and squeezed but dropping bars can be dangerous. Because the rope is trapped in the “U”, dropping a bar can result in the loss of 1½ bars of friction. This is a huge jump in the amount of friction available and can easily lead to problems.

​Rappel racks are the most versatile descending device ever created. They have been successfully used on drops from 4 to 4000 feet. They are infinitely adjustable when used efficiently and properly.

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