Cam engagement

Ted Pinson wrote:shearing out of soft desert sandstone seems like the more likely cause for failure.

Shearing out would generally not cause the damage on the cam as evident. This is evidence of 'umbrellaring' which implies that the cam at some stage was loaded undercammed.

(Though certain scenarios of shearing out could lead to a secondary 'umbrella' loading on a cam, so you can't rule it out.)

I would assume umbrellaing would only happen if the cam slipped backwards and opened up than was fallen on or it could have been placed perfectly but pulled and caught a lip after it opened up.

I don't know where this happened but you always have to take into consideration that it just pulled and as it was falling down the wall it caught an edge somewhere.

Does anyone know the reason why they don't just overlap the cams on the 0.3 and smaller so they can't do this? I know they only have 1 bolt going through the smaller ones but it still seems like they could put some metal sticking out on the edge that would prevent this or at least try to prevent it (I know there isn't alot of metal on the small cams to work with)

I know the picture isn't of X4 but still why don't they overlap the cams like this one to prevent it.

You mean like a C3 (What the top one looks like)?

Yea I don't understand why they don't do that on all the x4. It would prevent or at least make it alot harder to happen. I couldn't imagine it weakening the attachment to the bolt going through them enough for the benefit it would add. Once you hit 0.4 x4 and larger they move to 2 bolt and the cams themselves overlap like that.

grog m wrote:Here is a pic of my top cam that blew.

Looks like it either umbrellaed, or you just broke a couple trigger wires as it pulled. It's possible that the springs will push lobes a little further than the trigger wire allows it to, so without the wires it looks a little umbrellaed.

If the springs, lobes, and axle all are still in good shape your cam isn't exactly 'broken'... Just 'worn out' (which can happen in one bad fall, especially in micro sizes). If anything but the trigger wires is actually damaged it's probably a sign that it did umbrella.

I have a related question to tack onto this post.

Why does this fellow in the following link feel that it is ok and safe to under cam rather severely in very large size cams that would be number 5 Friend and larger?

Crack School Episode 5 - Gear and Gear Placement - by Tom Randall and Pete Whittaker
vimeo.com/40654007

Relevant section at 5:25.

My understanding is like a lot of others here. 50 to 90% retraction for medium to large size cams. Almost completely over cammed for small to micro size cams. I have though climbed with people that feel it's best to always place a cam at the 90% retraction no matter the size. There is nothing really unsafe about that, but I feel for the medium and larger size cams you lose out on a lot of usable range and probably limiting your cam size options.

anotherclimber wrote:I have a related question to tack onto this post. Why does this fellow in the following link feel that it is ok and safe to under cam rather severely in very large size cams that would be number 5 Friend and larger? Crack School Episode 5 - Gear and Gear Placement - by Tom Randall and Pete Whittaker. Relevant section at 5:25. My understanding is like a lot of others here. 50 to 90% retraction for medium to large size cams. Almost completely over cammed for small to micro size cams. I have though climbed with people that feel it's best to always place a cam at the 90% retraction no matter the size. There is nothing really unsafe about that, but I feel for the medium and larger size cams you lose out on a lot of usable range and probably limiting your cam size options.

I've heard others say the same as in the videos as well. I think the logic has to do with another property of the logarithmic spiral shape. The best way to explain it is to look at a cam chart . Notice that the the larger a cam is, the larger it's range is in inches. This means that if i retract a #6 camalot to 80% of it's range (undercammed), then it can expand roughly 0.5 inches before it is completely tipped out. If we take a #0.3 camalot and retract it to 80% then it can only expand less than 1/10th of an inch. Therefore, a tipped out #6 has a larger margin of safety than the tipped out #0.3.

However, that's not really the whole story either. The larger axles and lobes on the #6 probably have more flex in them than a smaller cam would, which will diminish its margin of safety a bit (no idea how much).

anotherclimber wrote:50 to 90% retraction for medium to large size cams.

You are missing out on a hell of a large range by limiting yourself to 50% minimum retraction.

Another thing to consider...
The main reason undercamming is bad (in my mind) is that cams walk. Obviously, an undercammed cam provides a very small margin of safety against walking. You could apply the same logic as in my last post to conclude that a tipped out large cam is safer than a tipped out medium or small cam when it comes to walking. But I don't think that is a good conclusion.

The reason is that a large crack also tends to have larger features/flares than a small crack. Therefore, a large cam must be able to cope with those larger variations of crack size.

It just goes to show you that YOU MUST carefully evaluate every cam placement you make. If you are climbing perfectly parallel cracks at Indian Creek, then you may be able to get away with tipping out your large cams a bit more than you normally would (but not your small cams). But you better be sure that it can't walk into a wider position.

I want to add on to what I said earlier and what has been said here. I agree that my failed cam was undercammed and umbrella-d. The placement is unique and at the surface a 0.2 fits nicely, but deeper in the crack you need to have a 0.3 size. This is because of wear and tear from cams wearing away the sandstone. For small cams, 0.5 and less, undercamming is never a good idea, and in sandstone is just dangerous. You can get away with it on bigger pieces but smaller pieces should be almost overcammed for safety.

kennoyce wrote: correct with one nitty picky change to say that a more "crammed" placement will exert more outward spring force, and thus prevent it from walking as much. I say this to point out that the outward force exerted in a fall will be the same either way, but the outward force of the cams on the rock due to the spring tension will be higher.

This is a fantastic correction and point kennoyce. I'm sure that's what Greg G probably meant in his post. Your correction though makes it a lot clearer.

I'd also like to add that different cam manufacturers spring their cams differently. I have Black Diamond C4's and Metolius Ultra light Master Cams and I actually prefer to climb with the Master Cams as they are sprung much stiffer than the C4's and seem more resistant to walking because of that. On the other hand I've also heard C4 users not like Master Cams for that very same reason. They feel the trigger bar requires too much effort and force to retract the cam and it doesn't feel like a C4. Each to their own.

I think Tom Randall is pointing out that it can be done (undercamming larger cams) and is regularly done.. but I don't believe that is safe by any stretch.

I mainly place 5s and up when I climb (for some strange reason..). And the fact is, most of the time you place what you've got. The crack gets huge, your cam gets tipped and you say a prayer and go.

Ray Lovestead wrote:I think Tom Randall is pointing out that it can be done (undercamming larger cams) and is regularly done.. but I don't believe that is safe by any stretch. I mainly place 5s and up when I climb (for some strange reason..). And the fact is, most of the time you place what you've got. The crack gets huge, your cam gets tipped and you say a prayer and go.

So you think that this is done because they don't carry anything larger than a #6 cam on them and that is the largest piece of protection they've got to protect large cracks with? So he protects it as well as he can to the point of under camming and move on?

I've carried Valley Giant 9s that I tipped out and made do. A lot of times that is the end of the story. You can keep climbing or give up.

Here's a slide I made as part of a "Physics of Climbing" presentation I gave last semester to my students. It provides equations for the forces on a cam lobe as a function of the camming angle beta. You choose the shape of the cam lobe such that beta is the same no matter how much the cam lobe is retracted. Thus, you will have the SAME amount of force on the cam lobe regardless of retraction. Side note: the shape that accomplishes this is the logarithmic spiral.

Of course, this is the ideal world where everything is infinitely rigid. In reality, the rock and the cam lobes will compress a little bit when force is applied. Thus, the cam will move a little bit and the lobes will rotate about the axle when it is loaded. Therefore, you better make sure that this rotation doesn't compromise the cam's function. You retract a cam by >50% to ensure that it still functions under these deformations.

Forces on a cam lobe

By the way, just to put in some numbers, Black Diamond uses a camming angle of 14.5degrees. Plugging that into the equation for N (the normal force acting on the cam lobe), gives N to be approximately 2F_applied. So for a given force, you do get a multiplicative effect. You can redo the whole analysis for a flared (not an ideal parallel sided crack) crack and it turns out that the multiplicative factor increases with increasing amounts of flare.

Bob Johnson wrote:You retract a cam by >50% to ensure that it still functions under these deformations.

Except the deformations are nowhere near the degree where you need >50% retraction.

It's been a while. Why isn't there a vertical component to the reaction force at the cam axle? Why half the applied force?

Also. The buckling equations are relevant to your VGs.

EJD wrote:It's been a while. Why isn't there a vertical component to the reaction force at the cam axle? Why half the applied force?

Because it's a diagram of forces ON the cam lobe. The lobe doesn't feel the reaction force, but the axle does. And it's half the applied force because, in the ideal case, half the force is applied to each lobe (the left and right).

patto wrote: Except the deformations are nowhere near the degree where you need >50% retraction.

Right. I just assumed 50-80% was a "rule" that was established by empirical testing of the devices. Does anyone have more information about where that range comes from?