There are several ways to rig a double rope bobbin on a single rope for extra friction during a rappel. Each of these methods involves making extra turns around the bollards where the second rope would normally run. When rigged in this manner double rope bobbins provide substantially more friction than the normal "S" rigging. Some of these methods have the highly undesirable side effect of forcing the main rope to rub against itself, and cannot be recommended. Another way to increase friction is to add a second carabiner or maillon from the seat maillon to the tail end of the rope as shown in the figure. On the other hand, a "C" rigging can be used on either type of bobbin to reduce friction if desired.

The diameter (D) of the bollards is chosen as a compromise between the desire for compactness and the need to provide a large enough bending radius to prevent damaging the main rope. Most bobbin bollards are about 30-35 mm. in diameter, or three times the diameter of a standard 11 mm. caving rope. Since the rope takes a 240° bend over each bollard, these may appear to be rather sharp bends; however, tests performed by Tomaz Planina (Obraba vrvi pri spuscanju z vrvnimi zavorami, Nase jame, 17, 1976, pp. 15-22; Climbing ropes wearing out with rope brakes, Nase jame, 19, 1977, pp. 15-22), indicate that bobbins may damage the rope less than several other common devices, including rappel racks and figure eights. Although Mr. Planina’s tests results are very informative, he rigged a few devices in nonstandard fashion during his tests, and so any conclusions should consider the anomalous arrangements.

I find that most bobbins work best on flexible 9 mm. ropes, although they tend to be faster on these thinner, more flexible lines. The larger 11 mm. ropes can be used, but larger ropes do not fit well in several of the bobbins. Furthermore, stiff ropes such as PMI Pit Rope are more difficult to rig, although they are still quite usable.

It is convenient to divide bobbin bollards into two categories. The subdivision is somewhat arbitrary; essentially determined by the inside diameter (d) of the rope groove. If this diameter is larger than the rope diameter we have a U-groove; if it is smaller the sides of the groove are formed into a V, yielding a V-groove. The essential difference is that other factors being equal, V-groove bollards have a higher effective coefficient of friction due to ropes wedging between the walls of the groove. As a result, a V-groove bollard increases the braking friction of the bobbin. Now both bollards on a bobbin have essentially the same friction angle. Since the friction (and hence heating) is higher on the bottom bollard (with the higher rope tension), most manufacturers attempt to even out the load by using a U-groove for the lower bollard and a V-groove on the upper.

The bobbin has one unusual characteristic which may present a hazard to the unwary. A number of cavers utilize the questionable practice of providing "bottom belays," where a caver positioned in the rockfall zone at the base of a drop stands ready to apply tension to the rappel line if the rappeller seems to accelerate out of control. This works for most devices where the braking friction of a device is given by the well-known formula

where F_brake is the braking force created by the device, W_caver and W_rope are the weights of the caver and the rope below the device respectively, T_belay is the tension supplied by the bottom belayer until he gets killed by a falling rock, µ is the dynamic coefficient of friction and θ is the friction angle of the device. In most devices the bottom belay merely increases Tbelay, thus slowing the caver, but with bobbins the increased tension can result in the bobbin rotating, which reduces θ and hence reduces the braking force. This could lead to the rappeler accelerating even faster. Note that the additional tension might also arise from the rappeler’s own braking effort (in this case the formula is modified slightly). The over-rotation is limited if the rappeler uses a second maillon to clip the rope below the descender to the main seat maillon, as illustrated in the "increased friction" figure above.

I prefer to attach bobbins to my seat harness so that the rope comes out to one side, rather than away from or towards me. Since my seat harness is designed for use with standard rappel racks, the seat maillon sits in a "horizontal" position. This necessitates the use of an additional maillon between the bobbin and the main seat maillon in order to turn the bobbin 90°. The extra carabiner also provides clearance for the mandatory safety carabiner connecting the trailing end of the main line to the seat maillon (The Raumer Handy is a specialty carabiner specially designed for this purpose). The major drawback to this arrangement is that it lengthens the hardware chain. Other descenders, such as the Figure 8, do not require all this extra hardware. When comparing the bulk of the bobbin to other devices one should really penalize the bobbin by the size and weight of a maillon and a carabiner.

Petzl developed another credible alternative to the second seat carabiner - the Freino. The Freino is essentially a double carabiner. The main (locking) carabiner attaches the bobbin to the seat maillon while the trainling end of the rope passes through the subsidiary (wire-gate) side. Although the Freino eliminates the need for one carabiner, the subsidiary side is not large enough to pass the bights needed for locking a bobbin off in the standard manner.

If a bobbin is not properly closed, it can open and let the rope escape, resulting in an air rappel. I lost a long-time caving friend when he allowed this to happen. Note that the carabiner can press against the safety gate - this makes me nervous as well. Since bobbins are popular among the rebelay crowd, there are many opportunities to overlook whether the side plate is closed, whether the safety is engaged, and whether the carabiner is resting against then gate or not.

Warning: Be careful to ensure that the bobbin is closed with the safety catch engaged before trusting your weight to the device.

Under no circumstances should the autostop feature of so-equipped bobbins be used to control one’s rate of descent! First, the amount of control provided is generally insufficient. Second, to increase friction one must release the control handle. This is contrary to natural instinct, particularly if one starts accelerating out of control. For this reason, one should never rely on the autostop feature as a safety device. There is too much chance of responding in the natural, instinctive manner rather than the proper one. Finally, the autostop feature should not be relied on to hold one stopped in mid-rappel, since it is too easy to bump the handle causing an unintended descent (the Tracson is the only device discussed here where the handle is not easily bumped, in fact, it requires lifting one’s weight off the device before the stop feature can be released). In fact, most of my autostop bobbins slip under my own weight. Frankly, I'm not sure what I can recommend the stop feature for, even though I've used a Petzl Stop descender far more than the non-stop models.

All my comments are oriented towards using these devices for their design purpose. Unless I specify otherwise, this is limited to a single person plus equipment descending ropes within the 9 to 11 mm. diameter range. My comments do not apply to descender abuse, such as use in rescue lowering systems.

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