Rack
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.
Bobbin
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).
Bobbin Only
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.