Just a quick note. I’ve been having trouble with yugaeri since changing my tenouchi. Today my teacher helped me work on that, trying various things with various degrees of success. After lunch I decided I need to take a breather from that so I brought out my other bow, which I hadn’t really used since changing the nigirikawa (the leather grip). Then on the first shot… wham! Fully turn. And same deal with every other arrow.
What seems to make the difference is just that on my regular bow the nigirikawa has become so smooth that the bow may be rotating in my hand from daisan to kai. That reduces the torque on the bow that would normally lead to yugaeri. Since the other bow still has a relatively rough surface on the grip, that doesn’t happen, and the torque is enough to spin the bow fully.
So the bottom line: don’t let the grip get too smooth or it starts to affect things. Some of my friends chance theirs about once a month (not really on a calendar basis but just when it feels right to do so), and one teacher changes hers before any taikai or shinsa. I, on the other hand… have sometimes used the same grip for months and months, until holes get worn through the leather. Although certainly economical, I guess that wasn’t such a good idea…
So, lesson learned.