3. The rails or shafting are not spaced far enough apart relative to the load.
An irregular or chattering motion may occur because the rails are too close together for a load-offset in either the y or z axis (perpendicular to the axis of travel, which is typically designated x axis).
For every inch between the rails, you need to make sure that your COG is within five inches. So if you have five inches between rails, your COG needs to be less than 25 inches or chattering may occur. To make this easier, use the 5:1 rule for the center-of-gravity-to-rail spacing. If you have room, make the spacing even wider and remember, if you were making a triangle you'd want to make it equilateral, not isosceles.
sy (or) sz must be < 5 x b. The picture on the left is incorrect because the COG is greater than fives times the distance between the rails, while the picture on the right is correct because the COG is less than fives time the distance between the rails.