Belfry Bulletin

Search Our Site

Article Index

 

Floating Cams.

By Nick Thorne

The floating cam is a subtle innovation in prusiking methods.  It was first introduced by American, Kirk Macgregor for the purpose of speeding up a ropewalker ascents.  The modification enabled him to set several records.  The floating cam takes the form of a length of elastic from the shoulder to the knee ropewalker, as shown in figure one.  The knee strap, previously used to raise the ascender, is then discarded, and the ropewalker lifted by contraction of  the  elastic.  This  idea  seemed fine  for    prusik racing, but was perhaps thought of as being a little remote from caving.  Experience has shown this not to be the case.  With judicious attachment of the elastic to the ropewalker, several distinct advantages over the knee strap method arise, in addition to the convenience of the lifting mechanism.  These advantages, of which I will explain later, make the floating ropewalker far superior to the fixed knee variety.  Although racing rigs deal mainly with ropewalkers, for caving purposes, sprung earn ascenders are justifiably popular.  These too can be floated in suitable systems (e.g. the Mitchell method). The floating operation with sprung earn ascenders is much simpler than with ropewalkers, and again, is very effective.

A discussion of the setting up of a floating ascender obviously centres around the elastic. For materials, it is worth experimenting with most types of elastic fibre.  Ordinary rubber bands work well (1).  These may be tied in parallel and end to end, and this allows for convenient alterations to the elastic length and tension.  The major disadvantage is that the bands wear out and break quite easily. They have recommended thick surgical tubing, despite its vulnerability to cuts (2, 3), and I have even seen inner tube rubber used with some success.  The best material however, seems to be shock cord. (1, 2, 3) 5-7mm thickness seems to be the most appropriate diameter.  The elastic properties of the shook cord are perdurability makes it the best choice.  Some makes of shock cord have sheaths that expose the underlying rubber when under tension.  These types would be less suitable for caving, I think, due to abrasion of the rubber.

Once a suitable material is chosen, length and tension considerations occur next.  The two main properties of the elastic that are important are the tension at full working extension, and the slack threshold, i.e., the upper limit of the ascender movement.  The tension at full working extension can be as high as eight to ten pounds force (1).  This may seem a lot and certainly a straight vertical lift of the ascender can be performed with a much smaller force.  Problems arise however on sloping pitches, where only a component of the elastic tension pulls the ascender up the rope.  Consequently, for general purposes, the tension mentioned above is recommended. This tension is not difficult to judge, but for those who cannot estimate what feels 'right', the tension can be measured quite easily using a fishermanÂ’s spring balance.

The other important consideration concerning the float elastic is the slack threshold.  The elastic should come just slack when the foot using the floating ascender is raised well above that involved in normal prusiking.  This makes for a good, clean lift, entirely within the linear region of the elastic.

Some experimentation will be required to obtain the optimum properties of the elastic.  In addition to length variations of the elastic itself, tension and slack threshold variations can be made by altering the position of the upper and lower attachments of the elastic.  This may seem obvious, but a few qualifying statements need to be said.  It is nice to have the upper attachment of the elastic.  This may seem obvious, but a few qualifying statements need to be said. It is nice to have the upper attachment of the elastic within easy reach when the time comes to de-float.  An attachment to the front of the sit harness therefore provides a very convenient attachment point.  Unfortunately, the resulting length of elastic will often be too short to supply sufficient tension.  This in turn can be slightly offset by placing the ascender lower down. This obviously leads to shorter steps, which may be undesirable, and it may also place the ascender effectively out of arms reach, which may prove awkward at times.  Alternatively, the upper attachment of the elastic can be raised to the shoulder as mentioned earlier.  This involves a longer length of elastic and is still convenient from a handling point of view.  For some elastic materials however, the resulting length of elastic may still be insufficient.  If this is the case, then the next step is to pass the elastic up and over the shoulder and attach it to the back of the sit harness.  This attachment may be more awkward to reach, but the ascender can be temporarily de-floated by slipping the elastic off the shoulder.  This action may prove adequate for short sections of cave between pitches, instead of a complete removal of the elastic. The elastic can be prevented from slipping off the shoulder accidentally by placing the attachment more in the middle of the waistband, at the back.

 

Passing the elastic over the shoulder does however raise a subtle complication the resulting tension in the elastic becomes a function of the friction between the elastic and the clothes worn.  On one surface practice, whilst wearing one of a well known Mendip retailer's plush 'boiler' suits (low friction) I set up the precise length of elastic required for an 'over the shoulder' attachment.  Because of the low friction between the elastic and the suit, the elastic was fairly evenly tensioned along its length.  However, on the first underground outing with this particular set up, I was of course, wearing a wetsuit (high friction). Consequently an uneven tension in the elastic resulted.  The elastic from the shoulder to the back of the sit harness was almost slack, and that from the shoulder down to the ascender was very highly stressed. It didn't take long for the lower attachment to fail.  The solution to the problem seems to be to sheath the elastic with some flexible hose. This I have found does not add to the practical complexity of the set up, and it does make the float elastic performance independent of the clothing worn.

The actual type of attachment mechanism for the elastic at its upper end is not critical.  Any hook and eye arrangement should do.  I have found the hooks from standard car top carriers to be quite suitable, especially after bending over the end to make a more secure, barb type of structure.  Less likely to unfasten accidentally would be some form of snap link arrangement, but the potential increase in awkwardness of operation should be borne in mind.

The lower attachment of the elastic i.e. that to the ascender, is unlike the upper attachment inasmuch as it must be totally secure.  Failure of the float elastic at its upper end is generally fairly innocuous as far as physical injury is concern.  If the lower attachment gives however, the elastic is nicely primed to flick up into the face, bringing with it whatever hooks and the like that may be tied onto it.  Total security I have found, is only genuinely obtainable by actually tying/or lashing the elastic to the ascender.  As well as being extremely unlikely to fail, this mechanism avoids any extra metal or other parts that may be potential projectiles.  The method does however, have certain difficulties associated with its permanent nature.  If the ascender were to be used for other purposes (such gear hauling) the elastic may be a bit of a nuisance.  The permanency of the attachment also makes replacement by a spare more impracticable. Consequently, many practitioners again use some sort of hook and eye or ring and snaplink arrangement just like the upper attachment.  This seems perfectly suitable providing that it can be made secure enough.

As for the exact part of the ascender that the elastic should be tied to, then this obviously depends on the type of ascender.  For all slung cam ascenders commonly available (i.e. Jumar, Petzl, Clog.)  All have krab holes conveniently placed at the top of them.  The only point to note is that with the Petzl ascender, only one of the two top holes should be used.  Any hook placed through both would mean that the ascender would have to be de-floated in order to remove it from the rope.  This is an unnecessary procedure, and should be avoided.

With ropewalkers such as the Gibbs, the ideal attachment position is less obvious, and is subject to several considerations.  These are:- the spring loading of the cam; the attitude of the cam when the ropewalker is disassembled (assuming an a attachment method is used that avoids de-floating to dismantle the ropewalker) and a possible increase in cam wear. Consequently several attachment positions are possible, but here, I only propose to outline what I consider to be the best method.  Other methods of attachment are given elsewhere along with discussions of their various pros and cons. (2, 4).  The discussion essentially hinges on whether the elastic lifts the cam directly or indirectly via the body or the pin.  The former is by far the better method.  Lifting the cam directly is the only way to spring load the ascender (i.e. make the cam action like that of a Jumar) whilst, at the same time, not critically increasing the cam wear.  This can be achieved by the attachment show in figure two.  The cam is sprung onto the rope by the couple of the foot pressure down, and the elastic tension up.  (The weight of the body of the ropewalker is small enough to be ignored.) When foot pressure is released, the upward travel of the ascender is accompanied by some release of the cam pressure on the rope.  This effect is more marked than with other attachment methods and so it leads to lower cam wear.  (Particularly suspect for cam wear would be the attachments to the pin or body of the ropewalker.)  This attachment method also has the added advantage that the cam is held nicely poised in space when the ropewalker is dismantled.  This means that the cam accepts the other parts more readily and so a faster on/off time for the ropewalker is achieved.

Finally, a few words of warning are needed to those who wish to set up a floating cam system. Firstly, check the attachment methods, particularly the one to the ascender.  Before 'Kitting up' test the attachments under loads well above those expected in normal use.  Practise on the surface first-this should go without saying, and be able to cater for an elastic failure.  With sprung cam ascenders this is no problem.  The hand that was freed by the use of the elastic simply comes back into action.  With ropewalkers, a spare elastic or knee strap should be carried.  When it is necessary to de-float the elastic, do so from the upper end first.  The other way could be dangerous if the elastic is stressed and it slips out of the hand.

In conclusion therefore, I hope I haven't deterred any prusiking cavers from trying this innovation for fear of getting a black eye!  To put elastic failure into context, then with a carefully set up rig, it is an extremely rare event.  Once this is appreciated the full advantages of the floating ascender can be enjoyed. Ropewalking cavers can ascend faster than before.  Gone will be that flicking motion required to make the cam bite, and gone too will be those annoying holes on the inside of the knee of the wetsuit.  With sprung cam ascenders, the freeing of a hand will be found most welcome.  There will be no tired upper arm any more, as the hand pushing the upper ascender can be alternated, or both hands can be used in combination.  Additionally, on those sloping pitches, there will be no need to have your nose rubbed into the rock as the free hand can be used to 'fend off'.  Make the initial effort to set up a floating ascender system, try it, and you'll be convinced.

References:

(1)                Macgregor, K. - Personal communication, I.S.C. September 1977.

(2)                 Montgomery, N.R. - 'Single Rope Techniques - a Guide for Vertical Cavers'. pp. 86-88 and p 90.  (N.B. When Montgomery discusses floating ropewalkers, I doubt if he has tried all the methods he shows. His conclusion about cam wear is valid, but not for the reason he gives.  Some of his arguments concerning the 'spring levering' and slippage of the cam are incorrect. (p.86, 87 fig, 131C, fig. 131D).

(3)                Halliday, W.R. - American Caves and Caving. pp. 217-219.

(4)                Thorne, N. Floating Cams for Prusiking. Cambridge Underground 1978