Setting up a forearm roll joint - method 2

Here is a second method for setting up forearm roll joints. It's a follow on from this post which covered how to set it up using the rotateY attribute of the wrist joint. This second method is the way I implement a forearm roll system, but I wanted to post the other version simply because it's a great way of looking at how different rotation orders can have implications for your overall rig set up, stability and usability.

So, to set up the forearm roll:

The basis for this is a single aim constraint, and is pretty quick to implement.

Setting up a forearm roll joint - method 1 of 2

The problem with roll joints is always one of driving them, and that comes down to being able to separate out the twist rotation from the source joint (upper arm/wrist etc. etc.).
The upper arm is pretty tricky in this respect due to the large range of movement and so requires a more complex set up. The forearm, however, is quite a lot easier because the rotation range of the wrist joint within its local coordinate system (of the elbow) is reasonably small.

There are two ways you could set up the twist system. The first is super-quick and 99% stable (but is completely dependent on your wrist joint rotation order for it to work), the second is just as quick but completely stable and doesn't rely on you having optimal wrist rotation orders set.

Actually, if you want to set up a forearm roll joint system I'd definitely do it this way.
The main aim for this post is that it serves as a really great way of exploring rotation orders and the implications they have for your rig. It's quick-to-make practical example of how different orders can affect stability and useablility of the rig and also your own sanity when building more complex systems.