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Topic Date User

Cams Ripping At Arapiles - common? 15-Oct-2009 At 9:00:26 PM Richard

Message

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