UKC

Metolius are no new comers to cams, and in a market which is about as saturated as you can get from cheap and cheerful cams from Eastern Europe to technical micro cams from BD and CHH with intricacies that boggle the mind, you have to pull something really special out of the bag to keep afloat.

So with their Power Cam have Metolius managed to achieve this?

Jon Griffith reveals all...

Read more at http://www.ukclimbing.com/gear/review.php?id=1776

In reply to UKC Gear:

I was a little confused by the author's description of a cam's holding power versus range. The hand-waving description given, with liberal use of the word "obviously", bears no relation to the real reason that this fact is true. If I understand the argument correctly, the statement "obviously the more compact you make the shape ..." derives from the misapprehension that it is the action of the springs in the device which provide holding power, and that these are more compressed, and provide more force, in a more compact configuration. This is not the case. Only a small amount of force is required from the springs, and this is not a relevant design limitation. The holding power arises from a camming action caused by pulling on the stem itself, and making the device seek to open in the crack. Thus, the device holds more strongly the more it is pulled. All the springs are required to do is maintain the cams in contact with the rock so that this process can be initiated.

The very simplest, rigid-body model of the operation of a spring-loaded camming device (SLCD) suggests that, by using logarithmic spirals to ensure a constant camming angle, there is a very simple design requirement, that the camming angle (defined as the angle between a line drawn between the two points of contact, and a line between any one of these and the common axle) be sufficiently small. This property would be unrelated to camming range, which can be independently increased by the use of dual axels or otherwise, and so long as a good choice of camming angle were made, the SLCD would be as strong as it needs to be. While this model falls short, and is in fact wrong in its conclusions, the description given by the review's author extends no further than this, and his instances of using the word "obviously" are unrelated to the reason that there is a relationship between camming range and holding strength.

The full description of the behaviour of cams is contained in the following excellent article, which depends on allowing the cam bodies to deform elastically: http://web.mit.edu/custer/www/rocking/cams/cams.body.html . It does explain why a larger range will be traded off against holding power, though this is unrelated to the description given in the review, and I would suggest that there is little that is obvious about it.

Che Gannararelli, research fellow,
Department of Physics and Astronomy, University College London

In reply to cmsg:

That's an interesting article but other than to prove your point that it's not 'simple' (as nothing ever is once you've scratched the surface) I'm not sure exactly what you're trying to say.

It's hardly worth getting into degree level mech eng in a gear review, surely. Only half the people reading it will be able to follow it anyway.

Al that said, I did find the handwavy explanation in the MC review tricky to follow, I guess it's not an easy idea to explain without recourse the maths.

jk
Geek.

In reply to cmsg: Che, I think (and hope) that you have misunderstood the reviewers meaning. I'm pretty sure he correctly understands the relationship between camming angle, range and 'holding power', at least for the simplistic model.

The description wasn't immediately clear to me either but I think his interlocking hand example was meant to help visualise how the effect of different camming angles. I think you need to picture the hands as a variety of different cams rather than a single cam moving for the example to make sense.

Thanks in any case for the excellent link. My understanding of cams was previously limited to the rigid-body model.

Iain, CragX

In reply to jkarran: No indeed, I'm not suggesting that the full explanation need be gone into, but rather that it should not be replaced with this purported explanation. It serves no purpose, because it is not even peripherally related to the real explanation. It's not a simplification, it's just wrong and confusing.

When I first saw the review, I was puzzled, because, if I understood what the author was trying to say, I knew that cams didn't work that way. I spent a while trying to figure out the argument, because I generally try not to assume that other people are wrong, but that I have failed to understand them. I think I am convinced, however, than I understood the review, and that it is simply misconceived. I very nearly responded by saying (on the basis of my understanding of the rigid-body model) that there was simply no such relationship. But then I stumbled upon the other web page, and was glad that I said no such thing!

In reply to IainWhitehouse:

I may well have done so.


Which is fine, but the equiangular spiral exists entirely to ensure a constant camming angle throughout the range, so it remains incorrect to attempt to use this as an explanation.

Unless we're still both misunderstanding this.

In reply to UKC Gear:
Maybe I'm oversimplifying this but is this not a more expensive DMM 4cu without the handy doubled sling? Presumably the lack of sling accounts for the weight difference or is it more complex than that?

In reply to cmsg:

In seeking to further explain something which admittedly wasn't terribly clear, I think you may be in danger of further confusing people.


The fact is that camming angle is INTEGRALLY tied to camming range, and it is only by radically changing the product design (such as by using 2 axles) that you can counteract this fundamental quality of cams. Note that in Camalots too if you changed the camming angle you would also change the range is exactly the same way and by exactly the same proportion.


The concept of being as strong as needed is just a reasonable working estimate of climbers' needs on typical rock and crack types. Slippier rock like limestone will need a device with a better cam angle to hold in a similarly flared crack. Conversely, for the same rock type a better cam angle will be effective in a more-flared cracks.

Simply put, using a better cam angle (like in the power cam) will make a similar device hold in more marginal placements, but will reduce the range of sizes that any one size will fit.

In reply to john arran:

I see. You're referring to the limitation on range due to the ends of the cam wheels impacting on the opposite side of the crack. Of course, you're quite right, that would be totally determined by camming angle. I remain a little puzzled as to how this relates to the reviewer's description, though.

As you say, the use of the dual-axle design to obviate the range limitation is a major change. As you seem to know your stuff, can you tell me whether the Camelot employs a better camming angle than most cams, given that it has less limitations on range?

In reply to cmsg:

I believe that early camalots may have used a slightly LESS advantageous camming angle (in terms of holding power) than friends, which would have accentuated the range advantage. This was a long time ago though and many changes have been made since, so it may not be the case now.

The counter-side of the camalot's increased range advantage is that it comes with an increased weight disadvantage, since each cam itself is proportinally larger.

In reply to Reach>Talent:

Yes, its a twin stem 4CU. Looks nice though, mmmmm shiney!

In reply to john arran:

I think you are right but it is not the case now. They now use the same angle as most other manufacturers. As far as I know only Aliens and Metolius now differ from the norm. (Aliens being the 15degree angle cam with 6067(?) alloy used as an example in the paper Che quoted)

Iain

In reply to cmsg:


I think we're misunderstanding each other. Or perhaps I also misunderstood his example.

In reply to Reach>Talent:

Well yes you are a bit. The major difference is the camming angle and the review explains why that is important. Che's link explains it even better.

The stem design is not the defining characteristic of a cam, any more than the spoiler is the defining element of a sportscar. Although both are doubtless important to the overall design they are just one part of it.

In reply to UKC Gear:

FWIW I have a full set of Metolius Ultralight Powercams (the full product name - see company website) which I bought to augment an existing collection of Camalots and Aliens (and a few Friends).

I have used them to a meaningful extent on granite in Canada, less so in some other places (limestone choss in the UAE, sandstone in Wadi Rum). Light weight is definitely their top attribute; however I find it hard to trust the placements as much as equivalent size Camalots. This is particularly an issue in imperfectly-parallel vertical cracks where they seem to be prone to walking where Camalots are stable. I think for people in search of weight-saving - whether mad fast 'n light alpinists or multipitch tradsters forced to carry a number of duplicate cams for a specific crack pitch - the trade-off is worthwhile. But for a primary set of cams I would suggest people look at Camalots or Friends first.