How do tricams work? What's the physics behind tricams in active mode?

I understand the general concept, and I've used them and placed them for a while, but I have a few specific questions for someone more knowledgeable in physics/math:

-I've deduced that the normal force on the lobes of tricam increases as the tricam approaches the end of its range.  Does this mean that the most secure position (so it won't rattle out) is at the outer range of the tricam, and thus that we should use the smallest tricam possible for a given placement?

-Since the lobes of the tricam do not roll like slcd's, but they actually scrape along the rock, does this mean that larger tricams with larger lobes (and hence more "angular resistance"-so to speak) will be more resistant to lock in place?  This is my suspicion as it is the smaller tricams that tend to get stuck.

-obviously, the angle of the tricams opening (call it theta) affects the amount of force on the stinger, and the direction of that force.  It seems to me that at a high theta (tricam mostly closed), the force applied to the tricam goes to nearly all shear force at the stinger.  This is far different from the normal force applied by slcd's, and I have heard that in this way the stinger can break the rock and the tricam fails.  What does the math say about the limit of this shear force on the stinger's resting spot.  

-I noticed that the channel in which the tricam's strap rests is also curved, so does this channel effectively change the fulcrum point of the tricam when it is in the smaller ranges?  How does this affect performance at smaller ranges?  Perhaps it has a mitigating effect on the shear force at the stinger?

Eric Moss wrote:

Hard to analyze, clearly.  Some starting comments below  (how do I overstrike ???)

I'm trying to understand the limitations of tricam active placement.

I understand the general concept, and I've used them and placed them for a while, but I have a few specific questions for someone more knowledgeable in physics/math:

-I've deduced that the normal force on the (lobes )Fulcrum of tricam increases as the tricam approaches the end of its range.  Does this mean that the most secure position (so it won't rattle out) is at the outer range of the tricam, and thus that we should use the smallest tricam possible for a given placement?  No, the most secure position is somwhere in between, minimizing failure from fulcrum slippage and failure from rail slippage or cutting,

-Since the (lobes) Fulcrum of the tricam does (not) roll like slcd's, but they actually scrape along the rock, does this mean that larger tricams with larger lobes (and hence more "angular resistance"-so to speak) will be more resistant to lock in place? NO  This is my suspicion as it is the smaller tricams that tend to get stuck.  The rails scrape, but the fulcrum rotates if the system moves. If  the fulcrum moves, the piece fails, or fails until it hangs up in a better angled spot.

-obviously, the angle of the tricams opening (call it theta) affects the amount of force on the stinger, and the direction of that force.  It seems to me that at a high theta (tricam mostly closed), the force applied to the tricam goes to nearly all shear force at the stinger.  This is far different from the normal force applied by slcd's, and I have heard that in this way the stinger can break the rock and the tricam fails.  What does the math say about the limit of this shear force on the stinger's resting spot.   The stingers are the points at the top of the rails.  The fulcrum is the single point.   The analysis at the fulcrum under load is very similar to that of a cam.  Both are normally immobile except for elastic strain in the system.  

 An almost open tricam will have very similar contact forces between the rails and the fulcrum, or it would move left-right.

-I noticed that the channel in which the tricam's strap rests is also curved, so does this channel effectively change the fulcrum point of the tricam when it is in the smaller ranges?  How does this affect performance at smaller ranges?  Perhaps it has a mitigating effect on the shear force at the stinger?  Yes, in a compicated way.  The force the strap exerts on the pin is directed not down, but in the strap direction, which is angled to the right in your drawing.  And the tension on the strap produces a force (perpendicular contact and tangential friction) all along the contact area.

From the CAMP Manual.

Thanks for clearing up some of the terminology for me.  I'll use the terms you presented as a standard from now on.

You say the most secure placement is in the middle range, but why?

When you say "rail cutting", what do you mean by this?  

I guess an obvious fact that I missed is that the rails and the fulcrum will have equal normal forces.

I appreciate that you mention the elastic forces.  I can't even begin to conceive of that right now.

Is there any validity to the idea that small tricams stick better?  If so, why?

Thanks for sharing!

Edit: by the way, the very crude diagram I drew assumes a constant camming angle on the lobes, that's how I replaced sin (phi) with a constant.  It's been a long time since I've done this sort of analysis, so forgive my attempt.  

You'd think Greg Lowe would be thoughtful enough to publish a whitepaper or something, haha.

Anyway, does the tricam have a constant camming angle?  Is the strap curve the purpose of this?

I'm going to try to diagram the static tricam as a big bro, and see what I get from that.  

I think the answer I seek is where the normal force is maximized, if anywhere.  This is where bite of rock into metal is greatest.  Though that still wouldn't tell me whether a small tricam bites more than a large one - that question would require a true elastic model but I'll just do physical testing instead.  Before physical testing between tricams, I must learn more about the tricam compared to itself at different levels of openness.

Eric,

Have you seen the article from "Off Belay" Feb 1978 by Bob Dill on "Abalakov Cams"?  There's a formula and design coordinates.

As I understand it, especially where the pink tricam is concerned, physics do not explain it. It is simply magic.

LL2 wrote:

As I understand it, especially where the pink tricam is concerned, physics do not explain it. It is simply magic.

Not simply magic, it is PFM .. pure f#cking magic. 

Eric Moss wrote:

Thanks for clearing up some of the terminology for me.  I'll use the terms you presented as a standard from now on.

Sorry to be persnickety, but it's too much work to get CAMP to change.

You say the most secure placement is in the middle range, but why?

First, let me say that I believe that, out in the wild,  no Tricam is placed in a smooth vertical crack.  I think that the fulcrum is always on some slight nubbin or in some slight depression.  Since the normal forces increase in wider cracks, that means that the fulcrum becomes more secure in wider placements. If rock crumbles under the fulcrum, the fulcrim will be driven into a deeper hole and become more secure.  At some point, if that happens, then there will be no contact on the other side, so failure, or the normal forces could force rock failure under the rails, ,leading to failure in a widely placed tricam.

When you say "rail cutting", what do you mean by this?    I mean that the area of the rails is smaller than the fulcrum area (for solid tricams), and the downward pull is located close to the rails, and that when rock begins to fracture under them, I worry that they could "cut" tracks into the rock, leading to failure.

I guess an obvious fact that I missed is that the rails and the fulcrum will have equal normal forces.

I appreciate that you mention the elastic forces.  I can't even begin to conceive of that right now.

Is there any validity to the idea that small tricams stick better?  If so, why?

There is certainly validity to the idea that small tricams get stuck more easily.  My worries with large tricams are with  me disturbing them.  A small tricam is kind of square.  The rail spacing is nearly the length, and the fulcrum is a big chunk of its width.  On the big guys, they are more rectangular, and the fulcrum is 2 pieces of sheet metal, and an off angle pull seems like it has a greater likelihood of disturbing things for the worse.

Thanks for sharing!

Anyway, does the tricam have a constant camming angle?  Is the strap curve the purpose of this?

The fulcrum side does not have a constant angle.  It is possible that the rail side could have a constant angle, (see the Abalakov comment above) but my big golden #7tricam has a constant radius curve when matched against a dinner plate.  It is possible that the strap wrap does help, and since the largest ones have minimal strap wrap, they may be worse in some regard.

I can flesh some of this out with some drawings, but it will take a while.

I'm going to try to diagram the static tricam as a big bro, and see what I get from that.  

I think the answer I seek is where the normal force is maximized, if anywhere.  This is where bite of rock into metal is greatest.  Though that still wouldn't tell me whether a small tricam bites more than a large one - that question would require a true elastic model but I'll just do physical testing instead.  Before physical testing between tricams, I must learn more about the tricam compared to itself at different levels of openness.

For the non-magic part, I'd guess that tricams work on the same principle as SLCDs. Sum the forces and torques, set to zero (presume the cam has 0 acceleration/rotation) & I get the same result pretty much. If the cam rails are circular? Dang. I'd have to think about that. Are the cam rails parallel to the sling channel? If the cam rails are parallel to the sling channel, and the shape is log spiral, it should not matter what the extension (crack width) is. My experience tells me that when the crack is on the wide side of the range, then the tricam wiggles too much.

For the magic part, experience tells me there are a lot more fixed pink tricams out there than all the other colors combined. I conclude that if all climbing gear was pink, it would all hold better...

I sent this thread to my kiddo, a baby ME who is in a static analysis class.  

He says that if you really want to know the answer, he'd recommend pull testing a bunch of them out of crack. :D

Eric Moss wrote:

I'm trying to understand the limitations of tricam active placement.

I understand the general concept, and I've used them and placed them for a while, but I have a few specific questions for someone more knowledgeable in physics/math:

-I've deduced that the normal force on the lobes of tricam increases as the tricam approaches the end of its range.  Does this mean that the most secure position (so it won't rattle out) is at the outer range of the tricam, and thus that we should use the smallest tricam possible for a given placement?

-Since the lobes of the tricam do not roll like slcd's, but they actually scrape along the rock, does this mean that larger tricams with larger lobes (and hence more "angular resistance"-so to speak) will be more resistant to lock in place?  This is my suspicion as it is the smaller tricams that tend to get stuck.

-obviously, the angle of the tricams opening (call it theta) affects the amount of force on the stinger, and the direction of that force.  It seems to me that at a high theta (tricam mostly closed), the force applied to the tricam goes to nearly all shear force at the stinger.  This is far different from the normal force applied by slcd's, and I have heard that in this way the stinger can break the rock and the tricam fails.  What does the math say about the limit of this shear force on the stinger's resting spot.  

-I noticed that the channel in which the tricam's strap rests is also curved, so does this channel effectively change the fulcrum point of the tricam when it is in the smaller ranges?  How does this affect performance at smaller ranges?  Perhaps it has a mitigating effect on the shear force at the stinger?

Are you asking because you're simply interested in the physics or you want to learn to place them better?

I can tell you from experience that the blue and up gets pretty "wobbly" in active mode and can pop out with rope movement (extend them), I mostly use them for anchors, but black and pink are pretty good for lead as they set well and tight. However, opening them up to max as you suggest is bad because they become "wobbly" again in that configuration.

wivanoff wrote:

Eric,

Have you seen the article from "Off Belay" Feb 1978 by Bob Dill on "Abalakov Cams"?  There's a formula and design coordinates.

I just saw the article, thanks!

It reminded me of the basic principle that if you can get a hex (or tricam) stuck into a position it will stay there as long as no deformation in the rock or metal occurs.

I have heard of a tricam shearing off a rock protuberance with the fulcrum point, and this is something that's warned against in the tricam manual.

The article mentions that 60% of the force ends up on the fulcrum. So it makes sense that overcamming is to be avoided because that would translate to highest possible shear force, unless the fulcrum is on a bomber lip.

Perhaps it's better to think of it as an expanding unit rather than a camming unit. The article states that the abalakov cams being tested had a cam angle of 70 degrees, which I presume translates to a "camming angle" (in terms of slcd's) of 20 degrees. This is a very high camming angle, and if camp tricams use the same geometry, then that would explain why cams are not super reliable unless the fulcrum is resting on a protuberance. On the other hand, you could design a tricam with a lower camming angle, however it would get stuck way worse than the existing geometery! You don't want that either.

Latro wrote:

the most secure position is somwhere in between, minimizing failure from fulcrum slippage and failure from rail slippage or cutting,

I understand this now.  I guess the answer as to where it sticks best is where you feel it stick best.

Tradiban wrote:

Are you asking because you're simply interested in the physics or you want to learn to place them better?

I can tell you from experience that the blue and up gets pretty "wobbly" in active mode and can pop out with rope movement (extend them), I mostly use them for anchors, but black and pink are pretty good for lead as they set well and tight. However, opening them up to max as you suggest is bad because they become "wobbly" again in that configuration.

Both.

I agree, except I think anything above pink is wobbly haha.  I carry one pink and two black lately.

Eric Moss wrote:

...So it makes sense that overcamming is to be avoided because that would translate to highest possible shear force, unless the fulcrum is on a bomber lip.

If it's a log spiral, the shear force does not change with crack width.

dave custer wrote:

If it's a log spiral, the shear force does not change with crack width.

In the case where the strap width is zero, I think you would be correct.  However, I believe the ratio of strap width to crack width ends up influencing the amount of shear force vs normal force at the fulcrum.  This is suggested by the article wivanoff mentioned, where the only abalakov cam that didn't stick was where the strap groove was too deep.

Interesting. Does this depend on the sling channel shape? Does it make a difference if the sling channel is parallel to the cam rails or is its own mini-log spiral? Is is possible to effectively decrease the camming angle by changing the sling channel shape--like I think is done with totem cams? Makes me think I have a summer Fusion 360 project...

I think my former colleague said it best:

So, what you're telling me, is that all those things we wrote off as negligible in our various courses, that's the stuff that keeps you from hitting the ground?

And you're on our health insurance policy?

But our pilots are not?

dave custer wrote:

Interesting. Does this depend on the sling channel shape? Does it make a difference if the sling channel is parallel to the cam rails or is its own mini-log spiral? Is is possible to effectively decrease the camming angle by changing the sling channel shape--like I think is done with totem cams? Makes me think I have a summer Fusion 360 project...

"To keep the force at the nose small, the pull must come from as close to the curved surface as possible" - from the abalakov cam article mentioned earlier.  So the sling channel is parallel to the rails, and as shallow as it can be.  

Draw a line between the tip of fulcrum and the rails where they touch the rock.  That line is basically a big bro at 20 degrees.  I believe the forces in the tricam can be simplified this way (except for all the stresses within the components of course). 

It seems the amount of torque generated by smaller tricams would be marginally smaller, so how on earth do the small ones stick so well?  I believe I have an answer.

I believe most deformation/metal bite is in the fulcrum, and the smaller tricams have a much sharper edge at the fulcrum.  This means the metal can compress better and bite into placements.  My black and pink evo have much sharper fulcrum edges than the brown evo.  I'm guessing that the manufacturer matched up the resiliency of the fulcrum (its bluntness) with the max weight of the strap.

At any rate I think it would be cool to have larger tricams that stuck like the small ones or better.  I have half a mind to sharpen the edges of my fulcrums (erring on the side of decreasing the cam angle).

Tricams are fun for "climbing theorists",

but if you want to actually climb safely, use regular active cams (like Camalots) instead.

Clint Cummins wrote:

Tricams are fun for "climbing theorists",

but if you want to actually climb safely, use regular active cams (like Camalots) instead.

Clint, that’s pretty much a nonsense statement.

Climbing “safely” involves setting good pro and traveling light.   Most commonly a few Tricams are carried and used to supplement a std rack to allow for a more optimal (safer) placement that an SLCD cam would provide or just couldn’t provide.  

For example, in this pic, the ONLY pro that would work is a tricam (And it was bomber) Not any cam, no nuts, not a pin, ball nut, big bro, tape knot... whatever.   Not theory, fact.  

If you truly want to be “safer”, add a couple Tricams to your rack.  

Clint Cummins wrote:

Tricams are fun for "climbing theorists",

but if you want to actually climb safely, use regular active cams (like Camalots) instead.

I had a camalot pull inches above a tricam which held and saved me from severe injury.  I am so glad I doubled up on the gear.

climber pat wrote:

I had a camalot pull inches above a tricam which held and saved me from severe injury.  I am so glad I doubled up on the gear.

What color was it?  I nearly pulled a grey camalot and replaced with a pink tricam that held solid.  It was a vertical crack.