So I have a forearm with a number of small bones children of the forearm proper, that control the loops of cps along the arm, these bones are constrained to the hand bone with a rotate like so the motion of the hand when turning drives the shape of the forearm - this works pretty well until a certain angle is reached at which point the bones "snap" and the forearm goes all pretzel shaped, I think it's the dreaded gimbal lock or something like that.

Any suggestions? How do people normally rig forearm motion?

render a brief animation of this happening so we can see it better. Shaded mode with Bones Visible ON

What it sounds like is that the bone is rotating enough to flip. Using a combined "translate to" and "aim at" constraint on a bone will make it more prone to flipping, sometimes giving it a roll target will be required to maintain the 'Z' orientation. You could try setting the "Roll-Method" of the bone to "Roll-History" (the default is "Z-Singularity).

If that doesn't fix the problem, you could use several bones to take up some of the rotation. You do that by making copies of the bone, make those copies parents of the bone in series and have each one rotate a percentage of the overall rotation so that none of them reach the flipping point.

I hope that explanation made sense...if it didn't, take a look a the finger end "geom" bones in Squetchy Sam (the "aim_fix" bones do what I described).

Hope that helps.

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The geometry bones in the Squetch Rig forearms are set up in a similar manner to what you describe...taking a look at Squetchy Sam's forearms may also help you.

Just guessing, but an Euler constraint can be used to limit the rotation of a bone. You could set the X and y limits to 0 and a bone would rotate on Z only then.

Using Roll like instead of Orient like would do the same thing.