Big Bros in Horizontals

So I was climbing in the great state of new jersey today and placed my #1 big bro in a wide horizontal. When I placed it I suddenly had a thought of whether I should have the screws placed up or down. I went with screws down because of the potential for sawing the cord, but I'm still wondering, if cord sawing was not an issue, which is better: screws up, screws down, or does it matter at all?

Threads up, cord down. It's either in the instructions or in one of Craig Luebbin's books, I don't remember which. It's stronger, you're pulling straight out in the piece's most stable position. With the cord on top the force comes at an angle that directs more of the force into the threads instead of into the rock.

To add to my previous post, it's likely physically strong with cord side up, but in my experience it's a harder to get a stable placement. I've got a pair of similar pics, and in a similar situation. In the second image you can see that there is also the potential to load over a sharp edge. There may be situations where it becomes a judgment call, but based on the instructions and my own experience placing Big Bros I've always placed cord side down.

In testing various positions in a less than perfect placement the Big Bro is less stable with the threaded aside down. It has more of a tendency to rotate into a less stable position with the cord side up.



This is a fairly sharp edge, but even when loaded under body weight there was no damage to the cord. There is the possibility of damage to the cord in a fall, but this is true of any deep horizontal placement with a sling running over an edge. I'd trust this cord over most slings in this situation anyway.

Suppose the BigBro is placed in a horizontal crack. If the thread is up, the direction of the load is more or less outward. If the thread is down, the direction of the load is more or less downward.

The outward component of the load is converted by the Big Bro into force against the sides of the crack. The force pushing against the sides of the crack is greater in magnitude than the force pulling outward because the angle formed by the Big Bro with the normal to the crack's sides is small.

The downward component of the force, on the other hand, is balanced by an upward component of equal magnitude applied by the lower face of the crack.

Since the outward component of the load is amplified and the downward component is not, the compression of the thread is lower when the thread is down. The chance of shear is also reduced.

In the thread-up placement, the cord's friction against the rock will reduce the load on the BigBro, but the obvious drawback is possible abrasion.

EDIT: Stability may indeed be an issue if the load has a sideways component.

EDIT 2: Mark, that looks like a #2, not a #3.

Use a camalot...

chosspector wrote:Use a camalot...

Man, that 10" crack sure is well protected now!

Jackass.

brenta wrote:Suppose the Big Bro is placed in a horizontal crack. If the thread is down, the direction of the load is more or less outward. If the thread is up, the direction of the load is more or less downward.

Actually, the opposite is true.

I agree with the rest of your analysis.

Greg D wrote: Actually, the opposite is true.

You are right. I meant the opposite. I'll fix it. Thanks.

No problem. should I delete my post?

Greg D wrote:No problem. should I delete my post?

As you prefer. On the one hand, the thread would be a little cleaner; on the other hand, keeping your correction may help those who might have been confused by my mistake.

About your post: Rotation of BigBros is relevant in that it may either tighten or loosen the collar. In that regard, the two pictures you annotated are the same--both show a tightening of the collar if I interpret your annotation correctly. What am I missing?

Actually, I'm not referring to rotation of the collar at all.

Since the cord is intentionally on one side of the tube, a force on the cord results in a rotation (or camming) of the piece. This results in the outward force on the rock similar to a cam. The "angle" designed into the tube ends are a critical part.

See updated photo above.

Got it. Thanks. I was thinking of rotation about the tube's axis, having just read in the instructions that the inner tube should be placed against the right side of the crack if possible, to reduce the chance that rope movement loosen the collar.

brenta wrote:EDIT 2: Mark, that looks like a #2, not a #3.

My bad. Corrected in photo desc.

Good info btw.

chosspector wrote:use a camalot...

Brian Scoggins wrote: Man, that 10" crack sure is well protected now! Jackass.

nah.. both are wrong,
drill a bolt.