A rack is a very specialized descending device typically used by cavers who are utilizing SRT
or single rope
technique. It has the added bonus of giving the user the ability to adjust the friction on the rope
no matter how much the rope
weighs, which is invaluable for longer rappels.
Figure Eight / Whale Tail
"Normal" Figure Eights
This figure eight by Black Diamond is the most typical device you will come across. However, there are many modifications that manufacturers include to make them safer, easier to use, or more versatile.
The Petzl Huit Antibrulure has a special heat resistant tab on the top, so you can grab it without burning yourself.
Modified Figure Eights
The rescue eight has two "ears" at the top that allow a user to tie the rope
around the device and come to a complete stop without holding onto the rope.
Another useful feature of most rescue eights is the double clip at the base,
which allows the user to clip it into multiple points without having to take it off the carabiner.
It also has an extra large ring, designed to provide less friction so that it moves easier down a high angled slope than a typical figure does. This design isn't necessarily purposed for vertical use, however, because of the lack of friction, rescue eights are a very typical sport rappelling piece simply because they'll go faster than any other device.
The Petzl Pirana allows for three different braking positions, each with varying amounts of friction, and has a lock off feature that is easier to use than the rescue eight. The pirana's optimal uses are canyoning and rescue work.
The biggest advantage of a bobbin style descending device is that they put gradual bends on the rope
and therefore increase the life of your rope.
Unlike a figure eight, a bobbin style descender is specialized for SRT,
or single rope
technique and cannot be used with a dual rope
system. These devices are ideal for rappelling under most conditions until you get in to situations where you have to descend with a lot of extra weight, or have an especially long drop (the extra weight of the rope
will lock the device).
The Petzl Simple works by applying friction gradually over a long piece of the rope.
By doing so, it minimizes the amount of friction on any one spot on the rope
(by distributing it more) and thus increases rope life.
Bobbin and Cam
The Petzl Stop has the same advantages of the Simple (above), but also has a camming mechanism that automatically stops the device unless its disengaged by the user (by depressing the brake lever). This makes this device an ideal setup for almost all types of rappelling.