Here are some sketching I did for the elbow.
You would think the elbow would be an easier than the wrist since it is a hinge type, but the fact is that its sharp angle makes it difficult. For us, when we bring our arms together the flesh moves out of the way to allow a sharp angle. That will not be the case for a BJD.
To compensate for the lack of moving flesh because of the sharp angle there are a few options...
- Aim for a 90 degree angle instead of 120+ degrees. So less flexibility, but better aesthetics.
- Cut parts of the arm that would meet at the sharp angle, but this is not as aesthetically pleasing if it ruins the shape of the arms. The sharper the angle, the more of the arm needs to be cut away.
- Make it a double joint. What results you get depends on how you go about it.
The below picture illustrates how much needs to be cut away from the arm depending on how much of an angle you go with. The left side is a paper cut out that shows the 60 degree angle on the left and the 90 degree angle on the right. Then the drawn image on the right shows how much you will need to remove if you wanted to cut away the arm to allow for 120 degree. You don't want to weaken the joint my having very little holding it all together. The nice thing about using a single sphere is that you get a nice shape for where the elbow is.
On the other hand, if you go with a double joint there are a lot of things to consider.
In the above picture you can see how I tries to achieve a 120 degree angle. The image on the left shows what happens if the middle of the circle is also the edge of the other circle. The image on the right shows what happens when the center of the circles are brought closer. The orange highlight where the two arms overlap if the arms were the same width as the circles. The pink highlight shows the overlap when the arms are wider than the circles.
I am losing sleep just thinking about the double joint. Just so many things to consider and on paper it is really not easy to think about. I need to experiment on my carving wax arm to see how it works instead of thinking theory.
What I want to accomplish with the elbow is:
- 120 degree elbow range
- A noticeable point that is the elbow. Although this is harder to do with the double joints, the closer the centers of the circles are to each other the better it gets. Obviously if both the centers are overlapping perfectly, it is as if there is one circle. I basically don't want a flat elbow or double elbows.
- Cut away some of the inner joint area of the inner arm. This adds to the range of motion and would be appropriate in looks since out arms are naturally cut away in that area.
- Inhibit range of motion that is unnatural.
- Not to forget this is a hinge joint, so unless added flexibility gives a natural motion which is not able to be made at its origin of movement. For example, the upper arms twists while the elbow stays in the same position.
You may be wondering why I even bother figuring out joints when I can just borrow someone's idea that already exists. Chances are someone has already thought out what I will end up using or maybe it is a duh! answer. But I have certain goals. I want to know why certain joints work better than others for my situation. It is part of the learning process that I enjoy.
I am making plaster molds of wooden balls so I can start testing the wrist and elbow joints.
Zen & The Art Of Articulating Dolls By Using Balljoints
There are a few ideas listed for the elbow joint.