Ultimaker style XY gantry

I've been on and off experimenting with HBot designs, there are some videos of a couple of my prototypes on YouTube here and here if your interested. The issue that arises is that the belt geometry introduces a racking force that if the design is not extremely rigid causes crosstalk in the motion as the carriage moves across the bar in the H.

The Ultimaker design has many of the same advantages, notably static motors resulting in low moving mass,  the above video was a simple X/Y stage I put together so I could better understand the pros and cons.

The obvious con with the Ultimaker design is that the outside travel rods also rotate, so you can'y use LM8UU style bearings there. I'm using 1 inch long bronze bushings on 5/16 drill rod. Again I can't use 8mm rods because an 8mm precision rod won't fit in the 8mm hole of the 608 bearings I'm using to support the rod. The rods for the cross piece are 8mm and on them I'm using LM8UUL bearings since they do not rotate.

There is surprisingly little play in the design, the coupled belts remove almost all of slop.
The design is however extremely sensitive to alignment, towards the end of the video above you can hear one of the bushings vibrating, that's caused by the cross piece being very slightly askew resulting in increased static friction on the bushing. I fixed it just after shooting the video by loosening two of the pulleys and adjusting the alignment.

I have a couple of other issues in this prototype, the printed corner pieces aren't really rigid enough and it needs something to stop the 7/16th's rod moving in the 608 bearings, but it's pretty promising.

I have some parts here for 4th HBot prototype, my intent is to use the CoreXY mechanics this time, it looks like it resolves the issue with unwanted racking forces. So it should make an interesting comparison. I just need to make some time and finish the design.