Strength of uneven/offset c4 placements


Original Post
Logan Schiff · · Brooklyn, NY · Joined Jun 2012 · Points: 60

Curious if RGold or any of the other more mathematically inclined types have done any analysis on the strength of uneven/offset placements in C4s or other non-totem/non-offset cams.

I know these placements are far from optimal and there is an umbrella risk, but I really have no sense as to how much weaker they are where the only option. E.g. how strong is a .75 c4 in good rock with one set of lobes 30% open and another 70% open. And how much more solid would a totem be here? 

I'm sure there are a lot of factors that make it impossible to give a precise answer, but any thoughts greatly appreciated!

-Logan

Joe Trabucco · · Wheat Ridge, CO · Joined Feb 2017 · Points: 0

#offset cams 

John Wilder · · Las Vegas, NV · Joined Feb 2004 · Points: 1,530

It's probably stronger than you think, even if its less than ideal. Sometimes its either that or nothing, so I'll take the less than ideal placement and remember to bring offset cams or a totem the next time I climb the route. 

Once upon a time, I had to set up a hanging belay on a first ascent with a two lobe #2 and a black alien. It was pretty terrifying, and I have no idea if it would have held a high factor fall (i'm guessing not), but it held long enough to get through the pitch.

Logan Schiff · · Brooklyn, NY · Joined Jun 2012 · Points: 60

Thanks John! Obviously these placements are to be and often can be avoided, but I guess I am curious if anyone has more than a general sense of their strength.

Purely anecdotal, but I took a 10 foot low factor fall recently on said green placement equalized with another  questionable cam and it held...

eli poss · · Durango, Co · Joined May 2014 · Points: 456

In solid rock it's less than ideal but if you're climbing choss it's even worse because, depending on the cams expansion range,  you're likely to end up of two lobes. Also, it is even more important to sling it will because walking could easily put you on two lobes 

Ken Noyce · · Layton, UT · Joined Aug 2010 · Points: 2,067
Logan Schiff wrote:

Curious if RGold or any of the other more mathematically inclined types have done any analysis on the strength of uneven/offset placements in C4s or other non-totem/non-offset cams.

I know these placements are far from optimal and there is an umbrella risk, but I really have no sense as to how much weaker they are where the only option. E.g. how strong is a .75 c4 in good rock with one set of lobes 30% open and another 70% open. And how much more solid would a totem be here? 

I'm sure there are a lot of factors that make it impossible to give a precise answer, but any thoughts greatly appreciated!

-Logan

As far as the math is concerned, it is absolutely no different than a placement where both sets of lobes are the same.  In the real world it is less secure, but that is due to other factors.

Gunkiemike · · Unknown Hometown · Joined Jul 2009 · Points: 2,740
Ken Noyce wrote:

As far as the math is concerned, it is absolutely no different than a placement where both sets of lobes are the same.  In the real world it is less secure, but that is due to other factors.

Correct. Holding power is independent of cam retraction.

Alex CV · · Greater NYC area · Joined Jun 2011 · Points: 200

As others have posted, as far as holding force, the entire concept of SLCD's is to operate with equal force for all lobes regardless of crack size (within operating range). The constant angle provided by the cam shape is responsible for this. For those who aren't familiar, the principle is essentially the same as stemming a smooth chimney with your legs.

Forces internal to the cam, however, may not be the same. But any reputable company will have accounted for this in the design.

Alex

ViperScale · · Unknown Hometown · Joined Dec 2013 · Points: 235

As long as both the lobes are in ideal range it doesn't matter that they are different. Now if the rock is not flat it may make a difference or if one of the lobes is over or under cammed it will make a difference.

I guess 50pct lobe may be stronger than a 40pct lobe but I don't know if anyone has numbers on that but either one would be fine.

As far as offsets they are more for placements that would mean over / under camming of one of the lobes so you need 2 different sizes.

Greg D · · Here · Joined Apr 2006 · Points: 871
Alex CV wrote:

For those who aren't familiar, the principle is essentially the same as stemming a smooth chimney with your legs.

Alex

For this analogy to work you would need telescoping legs so that the angle at your ball sack remains constant as the width of the chimney varies. Your legs would have to get longer and shorter with no change in the angle by the taint.

Greg D · · Here · Joined Apr 2006 · Points: 871
ViperScale wrote:

As long as both the lobes are in ideal range it doesn't matter that they are different. Now if the rock is not flat it may make a difference or if one of the lobes is over or under cammed it will make a difference.

I guess 50pct lobe may be stronger than a 40pct lobe but I don't know if anyone has numbers on that but either one would be fine.

As far as offsets they are more for placements that would mean over / under camming of one of the lobes so you need 2 different sizes.

Clearly you did not attend CONSTANT CAM ANGLE CLASS.  So, you may want to sit this one out  

eli poss · · Durango, Co · Joined May 2014 · Points: 456
Greg D wrote:

Clearly you did not attend CONSTANT CAM ANGLE CLASS.  So, you may want to sit this one out  

Remember Rule #1? Education may be a better solution than insult to such ignorance.

That being said, not all cams have a constant cam angle (totems) 

Jack Servedio · · Raleigh,NC · Joined Feb 2016 · Points: 30
Ken Noyce wrote:

As far as the math is concerned, it is absolutely no different than a placement where both sets of lobes are the same.  In the real world it is less secure, but that is due to other factors.

This is absolutely true of the forces pushing out against the crack as long as the cam is within it's range, but the overall strength of the placement is lower due to the smaller surface area between the cam lobes and rock (not that an offset cam will fix this anyway). Since the placement is flaring but the cam lobes are flat, the full width of the cam lobes will not be touching the walls of the crack.

Obviously, in the real world this isn't just a problem in flaring placements and maximizing the area of the cam lobes in contact the rock is crucial in any placement.

20 kN · · Unknown Hometown · Joined Feb 2009 · Points: 1,352

Of greater concern is the surface area contact on the rock. If the placement is extremely flaring, what can happen is the offset cam may fit in the placement, but the rock has very little surface area contact on one side of the set of lobes due to the flare. This can result in a placement that appears solid when you pull on it with your hand, but will easily rip on a fall. I've encountered this in pin scars on aid lines in Yosemite on several occasions. Soft sandstone will compress a bit which will improve this condition, but harder rock will not and is less forgiving to this type of problem.

Here is a rough drawing. The preservative is if you were in the crack looking down at the cam. Notice how the left lobes have extremely little surface area contact with the rock even though in real life it may appear as if the placement is acceptable as the lobes may be within proper caming range.

Greg D · · Here · Joined Apr 2006 · Points: 871
eli poss wrote:

Remember Rule #1? Education may be a better solution than insult to such ignorance.

That being said, not all cams have a constant cam angle (totems) 

Yes, you are right.  We should never poke fun at one another, never make jokes, no sarcasm either, damn it!  This is a serious website, dealing with serious shit.  Climbing is life saving work.  

Having been around this site for more than a decade, I still can't find RULE #1.  I can't find #2 or #3 either.  Could you point me to it?

20 kN · · Unknown Hometown · Joined Feb 2009 · Points: 1,352
Greg D wrote:

Yes, you are right.  We should never poke fun at one another, never make jokes, no sarcasm either, damn it!  This is a serious website, dealing with serious shit.  Climbing is life saving work.  

Having been around this site for more than a decade, I still can't find RULE #1.  I can't find #2 or #3 either.  Could you point me to it?

Well, "guideline" #1, but it's been listed as a rule before and it's still enforced loosely as one. It's moved around over the years, but right now it's listed in the reply boxes for both the forum and route database.

"Guideline #1: don't be a jerk
Guideline #2: post relevant information about this Route
Guideline #3: limit offtopic posts, rants, or lengthy discussions to the forums"

https://www.mountainproject.com/scripts/EditObject.php?id=106044131&action=add&type=COMMENT

Greg D · · Here · Joined Apr 2006 · Points: 871

Umm.  I was kidding.

Most serious people ever.

Carry on/  

Healyje · · PDX · Joined Jan 2006 · Points: 290

Unless you've got a 3D laser scanner, caliper and calculator on a leash the math doesn't really matter squat. You either develop a reasonable feel for your pro or you don't in which case the math is again not going to help you in any way.

Benjamin Mitchell · · Unknown Hometown · Joined Mar 2016 · Points: 0
20 kN wrote:

Of greater concern is the surface area contact on the rock. If the placement is extremely flaring, what can happen is the offset cam may fit in the placement, but the rock has very little surface area contact on one side of the set of lobes due to the flare. This can result in a placement that appears solid when you pull on it with your hand, but will easily rip on a fall. I've encountered this in pin scars on aid lines in Yosemite on several occasions. Soft sandstone will compress a bit which will improve this condition, but harder rock will not and is less forgiving to this type of problem.

Here is a rough drawing. The preservative is if you were in the crack looking down at the cam. Notice how the left lobes have extremely little surface area contact with the rock even though in real life it may appear as if the placement is acceptable as the lobes may be within proper caming range.

I see what you're saying, but I feel like the fact that Black Diamond designed their offset cams without tapering the lobes to slant from one side to the other means that they didn't think this was a big concern. Obviously I'm speculating a lot here. http://blackdiamondequipment.com/en_US/climbing-cams-stoppers-nuts-hexes/camalot-x4-offset-BD26221_cfg.html#start=4

Logan Schiff · · Brooklyn, NY · Joined Jun 2012 · Points: 60
Healyje wrote:

Unless you've got a 3D laser scanner, caliper and calculator on a leash the math doesn't really matter squat. You either develop a reasonable feel for your pro or you don't in which case the math is again not going to help you in any way.

Fair enough. I have occasionally seen these uneven placements as the only option at the time look and "feel" decent, seem to have passable rock contact, etc. and have even caught falls. I just have had no clear sense beyond "feel" for how compromised they are. 

Thankfully not an issue that comes up often or is too concerning since there is usually a better placement to be had.

Anyway, some helpful insights here especially the focus on rock contact. Thanks all!

James Warner · · Tustin · Joined Dec 2014 · Points: 0

Some of the pin scars on the opening pitches of The Nose caused me to beeline for offsets as soon as I was down. However, it did seem that even offset cams would still not sit right as most of the pin scars leave no place for the cam's stem to "lie" (be oriented towards the line of fall) like a natural crack would. So, it would almost be better to have something resembling a piton as the shape would have a natural leverage causing it to lever and be bomber. Would "removable pitons" sell?

Guideline #1: Don't be a jerk.

Post a Reply

Log In to Reply