screamers on snow anchors

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mountainsandsound

 
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screamers on snow anchors

by mountainsandsound » Sat Feb 18, 2012 12:27 am

Does anyone out there ever use screamers on snow anchors? I know someone is going to be tempted to answer with:
"Pickets are worthless, your best protection is not falling."
Great. Got it. But I like to increase my chances of surviving a screw-up. Seems like Yates screamers might have a good application for snow. Especially if one is simul-climbing hard snow with only one picket in between you and your partner.
Any experience with this scenario?

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Re: screamers on snow anchors

by sharperblue » Sat Feb 18, 2012 1:04 am

Pickets pounded correctly into neve will hold an elephant; they are VERY useful in the right circumstances (eg, protecting a steep traverse over a fatal fall zone) and are a pretty great form of protection that works where nothing else will at times. Never heard of using a screamer on one though; if they deploy at 3kN or so I suppose they would work just fine on a picket; certainly no worse than using a standard sling, IMO

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Re: screamers on snow anchors

by mountainsandsound » Sat Feb 18, 2012 1:42 am

What got me to thinking about it was a report by the New Zealand Mountain Guides association:

http://www.sunrockice.com/docs/Snow%20Anchors.pdf

Probably old news to some, but they did some extensive testing of picket positions for snow anchors. They found that MSR coyotes pounded in vertically on 45 degree neve slopes sometimes failed (due to shearing of the aluminum) at forces around 6 kN. Which seems like alot, but apparently you could have forces like that if someone falls on a slick slope. Especially if simul-climbing with a lot of rope out like most people do.

Anyway, if you haven't seen it, the report is pretty cool, but more so if you like physics. They have an interesting recommendation for picket orientation in snow that is less than rock-solid neve hardness.

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Re: screamers on snow anchors

by ExcitableBoy » Sat Feb 18, 2012 1:45 am

I was just going to link to that report. 6 kN is not a huge force.

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mattyj

 
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Re: screamers on snow anchors

by mattyj » Sat Feb 18, 2012 1:58 am

Keep in mind, though, that screamers only dissipate a limited amount of energy - I have some figure like "reducing the equivalent fall length by 4 feet" in my head but that might be wildly off. Climbing physics is based on fall factors - rope length increases with the fall distance, and it's the ratio that matters - but I don't think screamers fit neatly into this. My understanding is that a screamer will be more effective at protecting a short factor X fall than a long one.

If you have one picket acting a running belay there will likely be a lot of rope out and a long slide distance, which means the screamer's effect will be more limited than, say, protecting the first piece above an anchor. One thing working to your advantage is that because the leader is sliding rather than falling and the second will be giving a dynamic belay, forces should be less than a traditional climbing fall.

Bear in mind I haven't thought this through too much and may be wrong.

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Re: screamers on snow anchors

by mountainsandsound » Sat Feb 18, 2012 2:00 am

ExcitableBoy wrote:I was just going to link to that report. 6 kN is not a huge force.


I was surprised to see pictures of coyotes mangled up after that. I figured if screamers activate at 2 kN like they are supposed to, and reduce peak forces on the anchor by 3-4 kN (or 6-7 kN for the zipper screamers) maybe a screamer on a picket would be a worthwhile system.

Seems like a reasonable way to stack the odds in your favor just a little more...
Last edited by mountainsandsound on Sat Feb 18, 2012 2:11 am, edited 1 time in total.

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Re: screamers on snow anchors

by mountainsandsound » Sat Feb 18, 2012 2:07 am

mattyj wrote:One thing working to your advantage is that because the leader is sliding rather than falling and the second will be giving a dynamic belay, forces should be less than a traditional climbing fall.


I would think the dynamic belay would probably help a lot, but I was surprised to see in the report that they estimate a fall on a hard snow slope of 45 degrees would generate 70% of the force on an equivalent vertical fall through air. If the slope was more like 50 or 60 degrees, seems like you could be approaching free-falling.

I read some accident reports about the "token picket" getting yanked out. But maybe it had more to do with the snow consistency than anything else.

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Re: screamers on snow anchors

by ExcitableBoy » Sat Feb 18, 2012 2:31 am

That article made me think about picket design. I've been modifying Coyote pickets, and have come up with some 'improvements' to the current designs so they are easier to place, more durable, and overcome the weakness and failure points discovered by the New Zealand testing folks. I'd love to test them to see if they are actually stronger and less likely to fail catastrophically, but I have no way of doing that.

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Re: screamers on snow anchors

by mattyj » Sat Feb 18, 2012 4:29 am

ExcitableBoy wrote:I'd love to test them to see if they are actually stronger and less likely to fail catastrophically, but I have no way of doing that.


As in, you actually have physical pickets you've modified, not just a design? I'd be happy to help you out with that, but it might not be until next year as the snow around Tahoe would make for a poor test.

My SAR team has a dyno that we occasionally use in practice rope systems to sanity-check forces and have also used to break gear to failure. You could probably find a local mountain rescue team that has one and would be interested in incorporating and your pickets into an on-snow training. Backtie everything, run the rope through a 90 degree redirect for extra safety, and then build a giant pulley system or hook it up to a truck winch. Teams like that tend to have at least a few gear/physics geeks who would be very curious whether your design actually improves things.

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Re: screamers on snow anchors

by mattyj » Sat Feb 18, 2012 4:35 am

mountainsandsound wrote:I read some accident reports about the "token picket" getting yanked out. But maybe it had more to do with the snow consistency than anything else.


If it's actually a token picket, it's probably a vertical top-clip, which are easy to place on the go. They're total garbage in anything other than rock-hard snow, and if you place one in softer snow, a screamer isn't going to help you.

BTW in case it wasn't clear, the main point of my post wasn't about reduced anchor forces due to incline/dynamic belay/etc, but rather that a screamer's effectiveness drops off as the energy to be dissipated (i.e. fall length) increases.

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Re: screamers on snow anchors

by mountainsandsound » Sat Feb 18, 2012 2:55 pm

mattyj wrote:BTW in case it wasn't clear, the main point of my post wasn't about reduced anchor forces due to incline/dynamic belay/etc, but rather that a screamer's effectiveness drops off as the energy to be dissipated (i.e. fall length) increases.


OK. My physics teachers were pretty bad and boring, so I might not be understanding this very well. Are you saying that the amount of force a screamer reduces for the anchor depends on the length of the fall and not so much the amount of force on the anchor? If a long fall on steep snow puts forces in excess of 2kN on the anchor, and the screamer rips at 2kN, the screamer is working but is not reducing peak loads nearly as much for a fall like that than if it was a shorter fall? I think I got it now if that's it.

ExcitableBoy wrote:I'd love to test them to see if they are actually stronger and less likely to fail catastrophically, but I have no way of doing that.


Hmm. Sounds like a good proposal for a Master's Thesis for an engineering student. Especially if you had a nice grant with lots of cash for equipment.

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Re: screamers on snow anchors

by ExcitableBoy » Sat Feb 18, 2012 3:49 pm

mattyj wrote:
ExcitableBoy wrote:I'd love to test them to see if they are actually stronger and less likely to fail catastrophically, but I have no way of doing that.


As in, you actually have physical pickets you've modified, not just a design? I'd be happy to help you out with that, but it might not be until next year as the snow around Tahoe would make for a poor test.


Yes and no. I have made some of the mods, haven't finished the others. What I have done so far is to cut one end at 90 degrees, rivet a steel striking plate on top to give a larger striking surface and to avoid the mashing of the aluminum when hammered in. I have cut the bottoms at a 50 degree angle, to make a sharper point for better penetration. I still need to source steel collets or grommets to insert into the top three holes to reinforce them, as these seemed to be a weak point. I will then swage a long cable through the center hole which can be used in a T-trench or Sierra style picketted orientation. If hammered straight in, the cable will be run through one of the upper, colletted/grommetted holes. The idea is the swaged cable or possibily tied 5.5 mm tech cord running through the steel reinforced holes would be less likely to fail than a carabiner through the naked aluminum.

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Re: screamers on snow anchors

by mattyj » Sat Feb 18, 2012 5:46 pm

The longer you fall, the faster you're going and the more energy you have. The quicker you stop, the more force you put on the anchor. Your acceleration is determined by (fall length / rope length), which allows us to deal with fall factors when talking about anchor forces. A longer fall at the same fall factor doesn't put more force on the anchor, but it does load that anchor for a longer period of time.

A screamer works by extending your fall length; since you're stopping over a longer distance, forces on the anchor are reduced. If you slip immediately above the anchor and take an 8' fall, a screamer ripping and extending the fall length by 4' has a huge impact on the effective fall factor. If you take a 100' fall and the screamer extends it to 104', not so much.

Reality is messier because the screamer will only affect part of the fall. It will work great until it's done screaming, and then your fall will be back to normal. It will reduce the overall load your anchor sees, but the degree to which it affects the peak load depends on when it deploys, how peaky v. flat forces are during the catch, how the rope stretches during that time, and a lot of other variables. The physics get very messy very fast and I'm not going to pretend to understand it, but the bottom line is that its effectiveness is related, though probably not linearly, to (fall length / deployed screamer length).

The following user would like to thank mattyj for this post
Fire4x4, mountainsandsound


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