Chocolate fireguard
Active member
Trawling through some results obtained over many years of work by Bob & myself with the now dismantled rope drop test rig at the Bradford Pothole Club, I present the following as an honest attempt (by me) to fit numbers to the problem.
The following data came from drop tests done with semi static 9mm rope using a steel test mass.
Sorry I have no figures for dynamic rope.
I have used 6kN because the European Standards for various safety harnesses use a peak force of 6kN with a steel mass as one which most people should survive without injury. More detailed information can be found in https://www.hse.gov.uk/research/hsl_pdf/2003/hsl03-09.pdf .
Pulled to 6 kN, a Fig8 knot will release about 13 cm of rope and will absorb about 250 J of energy (that includes the energy absorbed by the released rope).
Pulled to 6 kN a metre of rope will stretch about 10% and absorb about 270 J of energy.
A long cows tail might consist of an 11 cm krab, a pair of 12 cm Fig8 knots with 40 cm of rope between the knots.
At 6 kN that 75 cm cows tail will stretch to 11 + (2*12) + (2*13) + 40 + 4 = 105 cm. So call it 1 metre.
And the energy in the cows tail will be (2*250) + (0.4*270) = 608J. So call it 600 J.
This means that a (steel!) 60kg caver will be stopped by this cows tail after a FF1 drop from an anchor, with a maximum acceleration force of 6kN. With 600N added to that, of course, because the rope will also be supporting the weight.
I have it in my mind, but can’t find the reference, that work done with volunteers (probably at forces lower than 6kN) suggests that about a third of the energy is absorbed by the body.
If that extends to 6kN then it is reasonable to believe that a 90kg person would experience the same 6kN acceleration force as the steel caver on that drop. I have no evidence, but I think that at forces well below 6kN the movable, squashy and bendy bits of the body will have done all they can.
I think most cavers use barrel knots to hold the krab. There is some data to show that while they produce a lower peak force than both a Fig8 and an overhand knot, they do release more rope in tightening up.
The following data came from drop tests done with semi static 9mm rope using a steel test mass.
Sorry I have no figures for dynamic rope.
I have used 6kN because the European Standards for various safety harnesses use a peak force of 6kN with a steel mass as one which most people should survive without injury. More detailed information can be found in https://www.hse.gov.uk/research/hsl_pdf/2003/hsl03-09.pdf .
Pulled to 6 kN, a Fig8 knot will release about 13 cm of rope and will absorb about 250 J of energy (that includes the energy absorbed by the released rope).
Pulled to 6 kN a metre of rope will stretch about 10% and absorb about 270 J of energy.
A long cows tail might consist of an 11 cm krab, a pair of 12 cm Fig8 knots with 40 cm of rope between the knots.
At 6 kN that 75 cm cows tail will stretch to 11 + (2*12) + (2*13) + 40 + 4 = 105 cm. So call it 1 metre.
And the energy in the cows tail will be (2*250) + (0.4*270) = 608J. So call it 600 J.
This means that a (steel!) 60kg caver will be stopped by this cows tail after a FF1 drop from an anchor, with a maximum acceleration force of 6kN. With 600N added to that, of course, because the rope will also be supporting the weight.
I have it in my mind, but can’t find the reference, that work done with volunteers (probably at forces lower than 6kN) suggests that about a third of the energy is absorbed by the body.
If that extends to 6kN then it is reasonable to believe that a 90kg person would experience the same 6kN acceleration force as the steel caver on that drop. I have no evidence, but I think that at forces well below 6kN the movable, squashy and bendy bits of the body will have done all they can.
I think most cavers use barrel knots to hold the krab. There is some data to show that while they produce a lower peak force than both a Fig8 and an overhand knot, they do release more rope in tightening up.