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Cams Ripping At Arapiles - common?


Richard

14-Oct-2009
1:45:58 PM

>> 13.25/Metolius
>> 13.75/Wild Country
>> 14/Trango
>> 15/Black Diamond
>> 16/CCH Aliens

So what's that mean ? Should i buy Metolius or Aliens ??

I think these differences in angles make bugger all difference:

Angle: ---- Min coeffiecent that cam will hold:
13.25 ---- 0.24
13.75 ---- 0.24
14 ---- 0.25
15 ---- 0.27
16 ---- 0.29

Given the coefficient of friction can vary between 0 to over 1, a 0.05 difference between one brand and another is probably insignifficant.

>Don't make it more complicated than it is. All the above quote is saying
>is: if the rock is so slippery that you don't trust that your feet will
>stick, don't trust your cams to stick either!

I think that's a quite untrue test - the friction of rubber boots is likely to be a lot higher than metal - after all if that wasn't the case boots would have metal soles. The fact that cams are pulling also shows this there's an ability to climb on rock that won't hold a cam. I wish this was a simple test you could use, but I doubt it is.

Cheers

one day hero

14-Oct-2009
2:21:50 PM

Richard, glad that someone has turned cam angle into coeff of friction, it's rather important and not put out there by manufacturers. However, I think you'll find that some rock type/aluminium coeffs lie inside the range you've laid out. Also, I think (from aiding experiments) that the soft lobes on aliens provide a greater coeff than hard alloy but can't actually back this up with any data.

Obviously rubber sticks better than metal. What I'm saying is that the relative friction of two different rock types can be judged by what shoes do and extrapolated to guess what cams will do. Limestone and polished quartzite are slippery for the feet and notorious for spitting cams from parallel placements. Rough sandstone, dolerite, and granite offer good friction for feet and also tend to provide solid cam placements in parallel cracks. I think you missunderstood my previous post to have meant something else.

patto

14-Oct-2009
4:47:35 PM

On 14/10/2009 Richard wrote:
>>> 13.25/Metolius
>>> 13.75/Wild Country
>>> 14/Trango
>>> 15/Black Diamond
>>> 16/CCH Aliens
>
>So what's that mean ? Should i buy Metolius or Aliens ??
>
>I think these differences in angles make bugger all difference:

Bugger all difference hey? Well if that was true then why the hell wouldn't all manufacturers use higher contact angles. Shit if you up the contact angle to 25degrees then you would have an expansion range in a regular cam equivalent to a LINK cam!!

There is and always will be a trade off between contact angles and expansion. I have read that approaching 20degrees cams will struggle to grip rock. Alien cams push the upper envelope of what is safe. They compensate by using softer lobes however this introduces its own problems of lobe deformation and is one of the reasons why alien cams often fail below rated levels.

The fact is that in most cases where a cam fails by slipping out the placement was at margin of gripping. Thus the 5%, 10% extra grip that a smaller contact angle would give should be sufficient to prevent the slip.

On 14/10/2009 Richard wrote:
>I wish this
>was a simple test you could use, but I doubt it is.

There is a simple test. Tug on the cam before moving on.

Theory suggests that if it holds for a slight tug it will hold for any tug until stuff starts breaking/significantly deforming. In practice this holds very true 99% of the time Though at the threshhold the behaviour is less predictable. The threshold level is very small and thus is quite rare. I pull on all the cams I place and have had one or two pop on testing. I have never had a cam pop under weight.


Richard

14-Oct-2009
9:00:46 PM

>Theory suggests that if it holds for a slight tug it will hold for any
>tug until stuff starts breaking/significantly deforming.

What theory where? To back up this claim you need to demonstrate that the force you can apply with one hand is similar to the force of your entire body weight with a realistic fall factor.

> I pull on all the cams I place and have had one or two pop on testing.
> I have never had a cam pop under weight.

This proves nothing if you've never placed a marginal cam that you have then fallen on. What portion of cam placements have you fallen on? One in 20? One in 50? So if you place a marginal cam 5% of the time, the odds of falling on it are 1/20 or 1/50 x 0.05 .. not very big.

I am not saying you shouldn't test, but I am saying it is still not a gaurantee...

patto

14-Oct-2009
11:09:04 PM

On 14/10/2009 Richard wrote:
>>Theory suggests that if it holds for a slight tug it will hold for any
>>tug until stuff starts breaking/significantly deforming.
>
>What theory where? To back up this claim you need to demonstrate that
>the force you can apply with one hand is similar to the force of your entire
>body weight with a realistic fall factor.

Not true. And I certainly haven't claimed that my hand applies a force anywhere near that of a lead fall!

The (maximum) friction force between a cam and the rock increases EXACTLY linearly with the tension applied to the cam. (and almost exactly linearly in practice) Thus if I tug 100N on the cam then the typical holding (friction) force might be 150N. If I fall on the same cam with a force of 5000N then the friction force would be 7500N.

Due to this fact tugging on a cam is a VERY good indicator on whether it would hold a fall. Of couse if you place it in fragile rock all bets are off.

In the case of a very smooth rock a 100N tug might only give a 90N holding force and so the cam will pop. The maginal case of 100N tug giving a 102N holding force is the cases where minor non linear aspects can come into play and cause a cam to fail during a fall.

>This proves nothing if you've never placed a marginal cam that you have then fallen on. >What portion of cam placements have you fallen on? One in 20? One in 50? So if you >place a marginal cam 5% of the time, the odds of falling on it are 1/20 or 1/50 x 0.05 .. >not very big.

True. I was not trying to suggest that my small statistical sample of my personally experience encompassess all possibilities. I was just saying.

One Day Hero

15-Oct-2009
1:01:46 PM

On 14/10/2009 Richard wrote:
>What theory where? To back up this claim you need to demonstrate that
>the force you can apply with one hand is similar to the force of your entire
>body weight with a realistic fall factor.

????

Right, I think we now have evidence that Patto gets the cam thing more than Richard who gets it more than Afj.........by the way, how did you come up with your coefficient of friction numbers Richard? I'm a bit suss about your stuff now


cruze

15-Oct-2009
1:59:22 PM

On 15/10/2009 One Day Hero wrote:
>On 14/10/2009 Richard wrote:
>>What theory where? To back up this claim you need to demonstrate that
>>the force you can apply with one hand is similar to the force of your
>entire
>>body weight with a realistic fall factor.
>
>????
>
>Right, I think we now have evidence that Patto gets the cam thing more
>than Richard who gets it more than Afj.........by the way, how did you
>come up with your coefficient of friction numbers Richard? I'm a bit suss
>about your stuff now

A bit of simplified force analysis will provide that to prevent slipping:
[coefficient of friction] >= tan(contact angle)

Regarding the tug test, while it is true that for a given contact angle the force applied perpendicular to the rock will increase in direct proportion to the force applied along the axis, I would say that the greatest benefit in tugging gear is to provide peace of mind enough to get you through to the next piece without faling at all! Other than that there probably would be some merit in tugging to break off loose surface crystals that could reduce the coefficient of friction of the rock surface during a fall. After all rock failure is probably a lot more likely than gear failure in most cases, esp soft sandstone.

patto

15-Oct-2009
2:09:41 PM


Richard

15-Oct-2009
9:00:26 PM

On 14/10/2009 patto wrote:
>On 14/10/2009 Richard wrote:
>>>Theory suggests that if it holds for a slight tug it will hold for any
>>>tug until stuff starts breaking/significantly deforming.
>>
>>What theory where? To back up this claim you need to demonstrate that
>>the force you can apply with one hand is similar to the force of your
>entire body weight with a realistic fall factor.
>
>Not true. And I certainly haven't claimed that my hand applies a force
>anywhere near that of a lead fall!
>
>The (maximum) friction force between a cam and the rock increases EXACTLY
>linearly with the tension applied to the cam. (and almost exactly linearly
>in practice) Thus if I tug 100N on the cam then the typical holding (friction)
>force might be 150N. If I fall on the same cam with a force of 5000N then
>the friction force would be 7500N.

OK, I now understand the basis for your claim, and it seems a reasonable piece of logic, though I'd like to think it through more. I have to admit I can't see the flaw in my opposing argument - the fact that cams do fail seems to sugest there is not a reliable test, but I would like there to be one. I personly feel that a hard tug is more for peace of mind than being 95% reliable, but prehaps I underrate its ability. Cams still fail as placements, I guess thats the bottom line, so while you know some placements are bomber, I don't always belive the dodgy ones are bomber even if they pass the tug test. I certianly agree that fragile rock is another situation.

>>Richards numbers were correct.

I was using the formula at http://web.mit.edu/custer/www/rocking/cams/cams.body.html
I am not taking ownership for the derivation of these formula.. they appear to make sense but I haven't validated them from first principles. I just tried to apply them to the discussion in this topic - especialy so the variation in the cam angles can be put into practical pespective.

Info on the actual friction coefficient of different types of rock would also be interesting (someone made some brief comments along these lines).

>>Obviously rubber sticks better than metal. What I'm saying is that the relative friction of two different rock types can be judged by what shoes do and extrapolated to guess what cams will do. ....I think you missunderstood my previous post to have meant something else.

Yep, I did. If your extrapolating, what you said makes sense. Maybe i took your words too literally.

Cheers


skink

15-Oct-2009
9:32:07 PM

On 15/10/2009 Richard wrote:
>I have to admit
>I can't see the flaw in my opposing argument - the fact that cams do fail
>seems to sugest there is not a reliable test, but I would like there to
>be one. I personly feel that a hard tug is more for peace of mind than
>being 95% reliable, but prehaps I underrate its ability. Cams still fail
>as placements, I guess thats the bottom line, so while you know some placements
>are bomber, I don't always belive the dodgy ones are bomber even if they
>pass the tug test.

Test all you like - tug, bounce test even - the test means nothing if the cam moves after you climb above it.

I suspect many cam failures are due to the cam, when it actually gets loaded, no longer being in that ideal position it was first placed in.

I often find cams rotated into a horizontal position when following and cleaning, and I'm pretty sure most times the leader didn't place it this way.

There are 30 messages in this topic.