Prusiks & Materials
Prusik length
Okay, on to prusiks, since there was a question last night. The guide to sizing your texas prusiks for glacier travel is in Freedom of the Hills somewhere, and I found that someone has scanned that page and put it online here: http://www.cascadeadventures.net/images/climb_school/texas_prusik.pdf It's better to aim a little on the long side, since you can always shorten them later by cutting off a little extra, or tying extra figure-8 knots in the middle to eat up length. For making hero-loops, starting with about 4'6" of cord will make a good size. Go longer with fatter diameter cords, so you can make more wraps.
Prusik material
I'm due to replace mine & make new ones, so personally I'll be using "BlueWater Titan cord" for everything (tied with a triple-fisherman's knot,) except the loop I use as my rappel autoblock. That I prefer to make out of regular 6mm nylon cord.
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Everything below this gets really geeky, if you're interested
I wouldn't overload Basic students with this much detail, but since you are Intermediate students, I encourage you to do some research, and you should choose for yourself what material you think makes the most sense.
Here's a fascinating paper to read: https://user.xmission.com/~tmoyer/testing/High_Strength_Cord.pdf
All accessory cords that I've seen so far are some combination of the following four materials:
Strengths
How strong does something need to be? As long as a dynamic climbing rope is part of the system that catches a fall, the rope will limit the peak force to the "Impact Force" given in it's specs. For example, the Mammut Infinity 9.5mm rope is a common Mountaineers club rope, and it's rated impact force is 8.7 kN, so a violent fall on to that rope won't put any more that 8.7 kN of force on the other gear involved. Be aware that ropes become slightly less-dynamic as they age. It's safe to assume that falls on a dynamic rope never exceed 10 kN. Falls that DON'T involve dynamic rope (like falling on a PA clipped directly to an anchor,) are another story. The numbers below are single-strand tensile strength. A loop of material that's free to rotate will somewhat spread the load between both strands of the loop, and will provide about 1.6 times the strength of a single strand.
Melting points
The highest temperature I could imagine ever worrying about with climbing gear is the heat from rappel friction. In a Black Diamond experiment, their most aggressive real rappel got up to 275°F, so I think it's safe to call that the maximum. As another reference point, a parked car in summer sun warms up to around 160°F inside.
There's more material attributes that I didn't go into here, like water-absorbition, resistance to sun damage, etc. You're an Intermediate student. Feel free to do more research. Choose the material that has the attributes you think are "safe enough" for your climbing.
Okay, on to prusiks, since there was a question last night. The guide to sizing your texas prusiks for glacier travel is in Freedom of the Hills somewhere, and I found that someone has scanned that page and put it online here: http://www.cascadeadventures.net/images/climb_school/texas_prusik.pdf It's better to aim a little on the long side, since you can always shorten them later by cutting off a little extra, or tying extra figure-8 knots in the middle to eat up length. For making hero-loops, starting with about 4'6" of cord will make a good size. Go longer with fatter diameter cords, so you can make more wraps.
Prusik material
I'm due to replace mine & make new ones, so personally I'll be using "BlueWater Titan cord" for everything (tied with a triple-fisherman's knot,) except the loop I use as my rappel autoblock. That I prefer to make out of regular 6mm nylon cord.
============
Everything below this gets really geeky, if you're interested
I wouldn't overload Basic students with this much detail, but since you are Intermediate students, I encourage you to do some research, and you should choose for yourself what material you think makes the most sense.
Here's a fascinating paper to read: https://user.xmission.com/~tmoyer/testing/High_Strength_Cord.pdf
All accessory cords that I've seen so far are some combination of the following four materials:
- Nylon
- Polyester (which is very similar to nylon in every attribute, so I'll treat it as if it's the same material)
- Dyneema (a.k.a. Spectra, HMPE, Dynex, and other names)
- An aramid fiber (for example, Kevlar, Technora)
- BlueWater Titan Cord is a type of tech cord. It's 5.5mm in diameter, has a sheath made out of nylon, and a core made out of dyneema. That gives it the advantage of a higher strength for it's diameter & weight, but it gives it disadvantages of heat-sensitivity and slipperiness (the slippery dyneema core can pull out of a double-fisherman's knot under load, but a triple-fisherman's fixes the problem.)
- New England Maxim Tech Cord is a type of tech cord. It's 5mm and has a sheath made out of polyester, and a core made out of Technora (which is like Kevlar.) Compared to simple nylon cord, it has the advantage of higher strength for it's diameter & weight when it's brand new and it goes overkill on heat resistance, but with the disadvantage that each time you bend it it looses strength, so it needs to be replaced before it gets 1 year old. (Yeah, sorry, the words "tech cord" are used as both a general category, and as a brand name, and it means something different depending on the context.)
Strengths
How strong does something need to be? As long as a dynamic climbing rope is part of the system that catches a fall, the rope will limit the peak force to the "Impact Force" given in it's specs. For example, the Mammut Infinity 9.5mm rope is a common Mountaineers club rope, and it's rated impact force is 8.7 kN, so a violent fall on to that rope won't put any more that 8.7 kN of force on the other gear involved. Be aware that ropes become slightly less-dynamic as they age. It's safe to assume that falls on a dynamic rope never exceed 10 kN. Falls that DON'T involve dynamic rope (like falling on a PA clipped directly to an anchor,) are another story. The numbers below are single-strand tensile strength. A loop of material that's free to rotate will somewhat spread the load between both strands of the loop, and will provide about 1.6 times the strength of a single strand.
- 5mm nylon cord - 5.5 kN
- 6mm nylon cord - 7.5 kN
- 7mm nylon cord - 10.7 kN
- 8mm nylon cord - 14.0 kN
- 5.5mm BlueWater Titan cord - 13.7 kN
- 5mm Maxim Tech Cord - 22.8 kN when new, less than 10 kN after bending 500 times
Melting points
The highest temperature I could imagine ever worrying about with climbing gear is the heat from rappel friction. In a Black Diamond experiment, their most aggressive real rappel got up to 275°F, so I think it's safe to call that the maximum. As another reference point, a parked car in summer sun warms up to around 160°F inside.
- Nylon - 428°F
- Dyneema - 297°F, though permanent strength reductions can happen starting at 176°F
- Kevlar - 932°F (this is high enough that you can't melt the ends with lighter, so you can often recognize it by it's messy frayed ends)
There's more material attributes that I didn't go into here, like water-absorbition, resistance to sun damage, etc. You're an Intermediate student. Feel free to do more research. Choose the material that has the attributes you think are "safe enough" for your climbing.
Top picture: Sharing a busy anchor on Cat in the Hat in Red Rocks, 10/8/2015