I drilled new mounting holes in the middle of the left side and moved one bearing bracket to the middle-left position. I kept two bearings to the side closest to the belt to minimise any turning motion the belt might introduce with less alignment constraint on one side. The base plate looks like a piece of Swiss Cheese at this stage with various experimental reworks, but you should be able to make out the new bearing mount positions in the photo above. A smaller change I've also made is the reversal of the y-belt at the idler end (far side), to give a smoother belt surface contact with the far idler bearing(s).
With the most firm supports for the heated bed and top plate now in a triangular shape I felt the next step was to reduce the spring loaded adjustable supports to three also. I've always felt that levelling the print bed would be a lot easier with three points of adjustment rather than four, and this was an ideal opportunity to explore that design. You can see a new hole in the centre left edge of base plate to take the spring loaded levelling bolt.
Manual movement of the y-carriage was much smoother through the entire travel length, a full 200mm distance. My only final compromise was to not completely tighten the bolts on the left bearing bracket, allowing it some minor horizontal play. I think it may be best to replace that 'smooth' rod eventually as it seems to have some imperfections.
Levelling the print bed:
Levelling the bed was a dawdle! Now with only three adjustment points it took no time at all to get it level. I started on the right hand side where there are two adjustment screws, positioned the print head to the right and manually moved the y-carriage back and forth adjusting the right screws until the bed was level. I then moved the print head manually left and right, adjusting the left bed adjuster until the bed was completely level. I did this first pass levelling visually with the print head about 5mm from the bed. I lowered the print head and repeated the process to fine-tune the levelling.
Print bed levelling was made even easier with these 'trapped nut' thumb wheels, visible in photo above.
Conclusion: In the case of y-carriage bearings and print bed levelling, three points of contact is as good if not better than four. The only proviso is that you have a sufficiently ridgid print bed plate, so that it doesn't droop in the unsupported corners.
The movement of the y-axis feels, and even sounds smoother with only three bearings. It may be a better compromise solutions if you are using rolled stainless smooth rods as against precision ground rods.
My current config:
To show the new y-carriage in action, and to give a general overview of my current configuration here's a short video clip. It's printing the Turks Head from Thingiverse.
Thanks for viewing. Now I'm back to printing! :)