V-thread : what ice screw length ??

Data for managing ice anchor risk: 

http://hmga.gr/storehouse/word-acrobat/Ice%20Climbing%20Anchor%20Strength%20-%20MRA%202009%20-%20Marc%20Beverly.pdf

Does anyone know of another data set examining V thread strength? I am familiar with that data and it doesn't make me want to avoid horizontals for a rap anchor if they happen to be easier in any given situation.

1) it's small numbers and they tested twice as many horizontal as vertical. Who knows if the range of vertical failures would have spread out a little with more testing.

2) the 7mm cord horizontals that were >14cm were pretty bomber and seemed to drop off from there, so hit that thread deep

3) greater surface area may help given the webbing results? I suspect it's going to take a bit more to pull my ~8mm dynamic rope (threaded directly) to failure than 7mm cord.

4) even in the worst case scenario ~5kn I feel pretty OK with that on a rap anchor with a dynamic rope in the system 

I guess all things being equal (they often aren't) I'd do a vertical thread because these old data suggest vertical may be stronger but I don't really have any qualms rapping off a horizontal. I've also done that "test" in the youtube video posted earlier myself and it's not scientific but it has probably biased me.

*I am not an engineer and could be convinced otherwise. 

Matt Zwrote:

Here’s a good test: make a v-thread with a 22cm on flat ground. Put in a backup screw above with a touch of slack so you don’t go flying when the thread pops. Then thread the v-thread, weight it with a person or two, and start chopping ice away from the v-thread. See how much you need to chop away before the v-thread fails. 

Then use that experience to decide for yourself whether a 19cm screw makes a strong enough v-thread for your own risk tolerance.

Good advice - I did try that out in good ice. 8 of us pulling on the thread. Took quite a bit of ice-chopping before it finally riped - there was a loooooooooot less ice on the thread left than what I would have been comfortable rappelling with. Doesn't mean I would rap on it but...

What Mark said above is really spot on too imo.

A couple of short oddballs: an aluminum Fixe 2 & 5/16th bore X 4 & 1/4" length and a Russian titanium : 7/8" bore and 3 & 3/8" length

I have placed both just once, the Fixe took some effort, but both felt solid.

My longest fall was on a old Chouinard, about a sixty footer. A well placed screw can do miracles

I remember a demonstration by the Chinese  climber Sun Bin where he had like 6 Chinese pulling on a V thread. Bomber

IFMGA guide Mark Smiley says he avoids using the 22cm screws because any misalignement of the two screws will be bigger with the longer screws.

Tjaard Breeuwerwrote:

IFMGA guide Mark Smiley says he avoids using the 22cm screws because any misalignement of the two screws will be bigger with the longer screws.

This doesn't need to be true. It's only true if you aim the 2nd hole to the end of the 1st one. If you consider that 22cm is much more than you need, and aim closer, that actually gives more leeway.

I like the extra length to compensate for the clearance the hanger requires. At a 60° entry angle the hanger gets in the way near the end.

rocknice2wrote:

I like the extra length to compensate for the clearance the hanger requires. At a 60° entry angle the hanger gets in the way near the end.

That's why my 22 is a Grivel 360. Tiny hanger minimizes interference.

I have tended to use the 22cm in order to have more leeway for hanger clearance, for poor surface ice, and to allow me more flexibility in angle: with a longer screw, you can go a bit wider or narrower and still have a good sized “pillar” of ice left.

With the lightweight alloy screws, I don‘t see as much need to go shorter, the weight savings is so minimal(6g for the Blue Ice, 10g for Petzl), and it’s only 1 screw, max 2 per party. 

Gunkiemikewrote:

That's why my 22 is a Grivel 360. Tiny hanger minimizes interference.

I dont understand the point here ?

 I use an aliminum 22 petzle to make threads simply because longer is easier. I have used  16's many times. 13cm once because thats all the ice that was availabe.. and I have on good  faith from a friend who is still alive that 10's work.   FWIW I have  five  10cm screws on my standard ice rack..