AKA Friends, these are active protection complex devices that use springs, cables and cam lobes, and which enable protection of parallel sided and even slightly flared cracks.
Friends were one of the most significant inventions in climbing since the karabiner and the name is ubiquitously applied to all camming devices. However it should only really be used for those made by Wild Country - the rest all being derived from these. Recent innovations such as reverse cam strength (that means the device will still hold a fall even when the cams are fully open - provided that is that they actually catch on the rock in some way!) are useful, and dyneema slings, forged cams and other advances have made "Friends" a lot lighter than the earlier models, whilst reductions in scale, such as Wild Country's fantastic Zeros, have to be seen to be believed.
Such camming devices enable protection of parallel sided and even slightly flared cracks rendering safe routes that were virtually devoid of protection prior to their invention.
Camming devices work due the mathematical theory of logarithmic spiral ensuring that the line between the axle and point of contact is always at a constant angle to the rock surface, irrespective of the unit’s orientation. The greater the cam angle the greater the camming range but the lesser the holding power of the cam. Manufacturers such as Wild Country and DMM have determined that 13.75° is the angle that offers the best combination of holding power and expansion range, and their cams are designed to operate at a constant 13.75° however compressed or expanded they are and whatever the angle of the rock they are in contact with. Black Diamond obtain their greater expansion range partly by using twin axles but also by having a camming angle of 14.5°.
There is a good explanation of camming angles by John Middendorf on bigwalls.net.
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