Everyone knows an undercammed piece is bad. Does any data exist on how bad it gets?
In other words, how does the strength of a cam placement change with the retraction angle?
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Under-camming and camming angle are two very different things.
The camming angle is constant for the whole range of the cam and is defined by the shape of the cam-lobe and can range from 12.5 degrees to up to 21 degrees (Black Diamond's C4s for example have a camming angle of 14.5 degrees.)
An undercammed cam has the same holding strength as a cam in its sweet-spot. The problem with undercamming is stability of the placement. An undercammed cam is likely to walk out of the crack with little tugs from the rope as the leader moves past the placement, and it will pull out when the direction of pull changes slightly. A good cam placement will not walk and can adjust to angle of pull.
If the only possible placement is an undercammed camming device, extending the placement with a sling can prevent the piece from walking out, but proceed with caution!
To answer your question: An undercammed cam has the full holding strength the cam is rated to.
Edit: I just looked at DavidR's link in his comment and I agree that it describes the workings of a cam very well!