Roller Bearings vs Ruby Bearings: Why Trackballs Feel Different Near the Edges
The left video shows a roller-bearing trackball, while the right video shows a static ruby-bearing trackball.
In the left video, the Z-axis spin is mostly suppressed, so the ball tends to roll more cleanly in the intended up/down direction. You can see this because the marker stays relatively parallel while moving up and down.
In the right video, the ball rotates not only in the up/down direction, but also around the Z axis. In other words, it twists. You can see this from the way the marker itself rotates/skews during the motion.
I think this difference is not just about friction or smoothness. It also affects how the finger motion on the surface of the ball gets projected onto 2D cursor movement.
With roller bearings, even when you operate the ball away from the “sweet spot” and closer to the edge, you can expect more consistent motion. The roller bearings mechanically resist unwanted Z-axis spin, so more of the motion remains useful for X/Y pointing.
With static bearings, as you move closer to the edge of the ball, the motion can start to feel “duller” or less responsive. That is because part of the finger motion turns into Z-axis spin, and a normal X/Y sensor does not use that spin for cursor movement. In practice, some of the motion is lost.
I am not saying one design is always better than the other. A well-made static bearing trackball can feel extremely smooth and effortless. Roller-bearing designs, on the other hand, usually have some amount of friction, mechanical feel, and noise.
But if someone is used to one type and is thinking about switching to the other, I think this is a useful difference to understand.
P.S. I would expect BTU-style designs, using ball transfer units, to behave more like the static-bearing example, since they allow the ball to spin freely in multiple directions instead of mechanically suppressing Z-axis twist.
