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Alex, your calibration cube is very impressive. I just recently got an Ender 3 and will do a direct drive conversion. I have some questions regarding your operating principles for the SpeedDrive design.
1 - Orientation of the motor (shaft) should be perpendicular to the X extrusion for good absorption of rotational inertia.
I am assuming you mean the motor shaft of the extruder stepper? The extruder moves relative slowly, does it really have that much of an effect? Wouldn't the gyroscopic effect of cooling fans be more of an issue?
2 - The motor's centre of mass should be as close as possible to the middle of the X extrusion for absorption of its significant linear inertia at speed.
If this is the case, have you considered moving the two rollers further apart so that the stepper could sit inbetween the rollers? This would put the stepper closest to the X extrusion and also widen the stance of the assembly. It may come at the expense of the build volume in X. We can also use the stepper itself as structure of the frame and increase overall rigidity.
Also, is this statement actually true? The linear inertia should be absorbed by the belt. Ideally, the centre of mass of the whole assembly should be at the attachment point of the belt which is the near the bottom side of the X extrusion so that the assembly only see a force in the X direction back and forth. Any rotational inertia is currently taken up by the rollers. Perhaps if the centre of mass is shifted up by the stepper being on top, maybe it makes sense to relocate the attachment points of the belt to the top of the extrusion. I'm just thinking out loud here. Am I making sense?
Edit: Another question, what are you setting your belt tension to? Are you using 3600/L formula and measuring acoustically?