Are you having trouble getting your carriages to ride smoothly on their axes? The root cause of the problem is most likely that the axis is over constrained. This seems to have gotten worse with the popularity of [linear bearings] and the propensity to put them on all four corners of a carriage. It's probably because cars have four wheels that we are accustomed to seeing it that way. I will attempt to explain why that is wrong and what can be done to fix it.

First of all, you should understand that it's virtually impossible to align two shafts perfectly parallel to each other. Even if you are able to get them close, they won't stay that way. Change in temperature, motion, and wear will cause them to move out of alignment.

When you put two bearings or bushings on a single shaft that is fixed on both ends (a vector), you have constrained 4 of the 6 possible degrees of freedom for that axis. There are different names for these degrees of freedom, but I like to call them Longitude (X), Latitude (Y), Elevation (Z), Pitch (rotation around X), Roll (rotation around Y), and Yaw (rotation around Z). So if we are talking about the X axis, you have constrained everything except Pitch and Longitude. You want the carriage to move freely in the X direction, so the only remaining constraint to add is for Pitch. To do this, we use a second shaft that is (mostly) parallel to the first shaft. If we only contact this second shaft on the top and bottom edges, we can stop rotation about the X axis. As long as you don't touch any other edges of that shaft, the carriage will move freely. If the second shaft is out of alignment in the Y direction, it will not have any affect on the motion. If it is out of alignment in the Z direction, the carriage will pitch as it moves left and right. To reduce this pitching, you only need to align the two shafts in the Z direction, which is a much simpler task.

With the normal vector (master shaft) and the single point (idle shaft contact), we have created a plane (the carriage surface). If we try add one more contact point on the idle shaft it will have to conform to that plane. Chances are it will not and there will be binding or something will have to bend. So resist the temptation to add a second idler.

Ideally the idler contact is centered between the two bearings to balance the load, but if the load is light, it is not that important. In the typical Mendel design a centered idler will get in the way of the hot end. So just put it off to the side where the weight of the extruder motor is. For the Y axis it can usually be centered without any problem.