Half rope for the gym?

I got a really good deal on a Beal 9mm half rope, and I'm wondering what peoples opinion is on using it for my gym lead rope. Not a double rope but a half rope I wanna cut in half and share with a friend. My gusses is it's safe but for how many falls. Opinions please.

I think you'll be wasting a really good alpine rope. You can buy a gym rope for like $50 that will be a tank.

Plus 1/2 ropes are meant to be used in conjunction with another rope. Hence the 1/2. Sure they can take a fall on their own, but it's part of a system.

Keep the 9mm for something else and get a gym rope for the gym. Plus do you really want 5-10 feet of dynamic strech in the gym?

Brian Snider wrote:I got a really good deal on a Beal 9mm half rope, and I'm wondering what peoples opinion is on using it for my gym lead rope. Not a double rope but a half rope I wanna cut in half and share with a friend. My gusses is it's safe but for how many falls. Opinions please.

Are you talking about a 60m single rope chopped to 30m, or a half/double rope (half and double mean the same thing, twin is different)?

If you are truly talking about a half/double, you almost certainly would not be allowed to use it in a gym, and I definitely wouldn't recommend it.

Sorry. Half/double not a twin rope. So if I can't use it in the gym and don't use doubles outside anyone wanna buy or trade a nwt 9mm rope.

You could chop it in half and use both halves as a double rope setup in the gym. I would probably just clip them like twins in this case since the bolts are bomber.

Do not use them as twins... It will hurt. You are doubling the forces by doing that. Bad idea, even in the gym.

Nikolai Daiss-Fechner wrote:Do not use them as twins... It will hurt. You are doubling the forces by doing that. Bad idea, even in the gym.

My thoughts exactly.

Nikolai Daiss-Fechner wrote:Do not use them as twins... It will hurt. You are doubling the forces by doing that. Bad idea, even in the gym.

Not true. Forces are not doubled by doubling the ropes.

Randy Riha wrote: Not true. Forces are not doubled by doubling the ropes.

Thank you for your strong research-backed insight.

I doubt there is any good technical reason not to use a half rope as a gym rope. Gym falls are low fall-factor; a half rope is plenty good for that and is used that way in the field. I know of folks with half ropes that have singly probably caught hundreds of short falls.

Of course, it is a thin rope, it will wear out quicker than a beefy gym rope with a sturdy sheath.

A pair of half ropes could be clipped as twins, but in theory that will increase peak impact loads by about 40% (not 100% as suggested earlier; the multiplication factor is about the square root of 2, not 2). There doesn't seem to be any reason to purposely make the system stiffer.

The one potential drawback is that, at 9mm, the rope is thinner than a typical gym rope, and will generate less friction in a belay device. Falls will be harder to hold, and the potential for failed belays will increase. Make sure the belayer isn't using an old-style ATC-type device that was made for 10.5 and higher diameters.

By the way, I'd guess that the reason the rope was on sale is because nowadays 9mm is way too fat for a half rope.

Price wrote: Thank you for your strong research-backed insight.

Just because the research was not provided doesn't mean it wasn't there ;).

Actually, I was planning on getting on here later to go into detail WHY the doubling the rope does not double the force. I wanted to refute the false statement immediately though. Looks like rgold gave a pretty good response, though he didn't go into the math.

rgold wrote:I doubt there is any good technical reason not to use a half rope as a gym rope.

I think you hit on some of the reasons in the rest of your post.

Both with belaying and lowering with a skinny cord takes way more input from the person holding the rope than a thicker cord. Ie, less prone to user error.

And, if the weight difference is significant, it magnifies the problem of controlling the rope.

I'd rather see folks (myself included) stuggle with a fat cord in the gym, than have smooth sailing with a skinny cord that has a much higher chance of being hard to control in a fall or lower. Especially given the lack of belay saavy and attention...

This is besides the point but the reason it was cheap is I think the guy bought it on accident and now doesn't climb. I talked with him for just a minute and don't think he had a clue what a half rope was. So for 70 bones I got 18 brand new sport draws 2 lockers 2 non lockers 2 rope bags a junk rope and a new with tags half rope plus a bunch of slings and webbing I'll probably toss out. After selling half the draws and a rope bag I'm only in to this stuff for 25 ducks. So trashing a half rope in a short season is the least of my worries. Btw will trade for a new style bd #3

Brian Snider wrote:This is besides the point but the reason it was cheap is I think the guy bought it on accident and now doesn't climb. I talked with him for just a minute and don't think he had a clue what a half rope was. So for 70 bones I got 18 brand new sport draws 2 lockers 2 non lockers 2 rope bags a junk rope and a new with tags half rope plus a bunch of slings and webbing I'll probably toss out. After selling half the draws and a rope bag I'm only in to this stuff for 25 ducks. So trashing a half rope in a short season is the least of my worries. Btw will trade for a new style bd #3

Sounds like he stole it. . .

shoo wrote: Sounds like he stole it. . .

Not what I got of it. More like he really really wanted to be a sport climber but didn't have a clue and quit after buying the wrong rope and sucking at it for less than a year

rgold wrote:A pair of half ropes could be clipped as twins, but in theory that will increase peak impact loads by about 40% (not 100% as suggested earlier; the multiplication factor is the square root of 2, not 2). There doesn't seem to be any reason to purposely make the system stiffer.

rgold, I just want to thank you for the great post, and also for the fun task of determining why the peak load will increase by the square root of 2 and not by 2. For those of you who would like to know the reason why, I am including the math I used to come to the same conclusion (correct me if I'm wrong rgold).

I simplified this problem by assuming that a rope is a spring that follows hooke's law (which of course is not the case, but it makes it much easier to deal with). The potential energy stored in a spring is given by:

PE = 1/2*k*x^2,

where k is the spring constant and x is change in length of the spring (the amount the rope stretches). The spring constant, k, is equal to F/x, where F is the applied force, and x can be found as:

x = F*L/(A*E),

where F is the applied force, L is the length of the spring (or rope in this case), A is the cross sectional area of the rope, and E is the youngs modulus of the rope (again we're assuming that a rope has a constant E). I will call the peak force on a single rope F1, and the peak force on the doubled up rope F2, and of course we are looking for the ratio of F2:F1.

The fall generating the force is with the same mass and from the same hight meaning that the initial gravitational potential energy is the same in both cases, so we can simply equate the spring potential energy for both cases, or:

1/2*k*x^2 = 1/2*k*x^2

Now remembering that the area associated with F2 is double the area associated with F1 and substituting in for the k and x we get:

(1/2)*(A*E/L)*(F1*L/(A*E))^2 = (1/2)*(2*A*E/L)*(F2*L/(2*A*E))^2

which simplifies to:

F1^2*L/(2*A*E) = F2^2*L/(4*A*E)

Now solving for F2/F1 we get that F2/F1 = 2^(1/2) or the square root of 2.

I hope that someone out there finds this interesting because I certainly did.

Brian Snider wrote:and sucking at it for less than a year

Wait....we have to be good at climbing?? crap!

Scott McMahon wrote: Wait....we have to be good at climbing?? crap!

Being good at climbing means having fun as often as possible.
Sucking at it means not going enough and selling all of your gear.

Brian Snider wrote: Being good at climbing means having fun as often as possible. Sucking at it means not going enough and selling all of your gear.

Win!

kennoyce wrote:The fall generating the force is with the same mass and from the same hight meaning that the initial gravitational potential energy is the same in both cases, so we can simply equate the spring potential energy for both cases, or: 1/2*k*x^2 = 1/2*k*x^2

This is slightly incorrect, because the stiffer spring will stretch less and the falling climber will lose a little less potential energy, but in first approximation it's OK to ignore such difference.

Said otherwise, the ratio of sqrt(2) applies to the case in which the two springs store the same energy, which is not quite the case in a fall.

Said in yet another way, in Wexler's equation, which gives the exact solution for the ideal spring case, the impact force is not exactly proportional to the square root of the rope modulus.

brenta wrote: This is slightly incorrect, because the stiffer spring will stretch less and the falling climber will lose a little less potential energy, but in first approximation it's OK to ignore such difference. Said otherwise, the ratio of sqrt(2) applies to the case in which the two springs store the same energy, which is not quite the case in a fall. Said in yet another way, in Wexler's equation, which gives the exact solution for the ideal spring case, the impact force is not exactly proportional to the square root of the rope modulus.

You are correct, and thanks for pointing this out. As you said though, this was just a quick and dirty first approximation. I also think (I can't be certain because I can't quantify this) that treating a rope as a spring will screw up the results a lot more than the difference in potential energy due to the shorter fall from less rope stretch. I would guess that all of the invalid assumptions I made in order to solve this would pretty much make the difference in potential energy negligible.