Nothing is wrong with a metal rack. (It isn't a reprapy solution but that may not be your goal.)
By my math you have the same mechanical advantage regardless of how big or small the spur gear is. There is no way to direct drive it.
Also, I don't know if I am seeing what you are talking about but accuracy and repeatability will increase as the racks are moved apart. As a thought experiment, what happens if they get really really close? I think you will find that the stability goes down.
I have 22 bearings in the current Simpson design.
By my math you have the same mechanical advantage regardless of how big or small the spur gear is. There is no way to direct drive it.
Also, I don't know if I am seeing what you are talking about but accuracy and repeatability will increase as the racks are moved apart. As a thought experiment, what happens if they get really really close? I think you will find that the stability goes down.
I have 22 bearings in the current Simpson design.