This might be just me, but I still see a tiny bit of backlash, maybe <0.2mm. best way to check is to print a filled calibration test model.
Like a filled circle. But a few layer hig, not 1, and see if you have tiny holes at 5 and 11 oclock.

Ultimaker 2, which is a pretty expensive printer and unlike Solidoodle actually cares about good engineering and quality, uses MXL and its advertised to print up to 300 mm/s. And btw, it also has closed loop belt for the motor and drive rod.

Main issue with fishing line is I haven't figured out how to use it on the motor pulleys (closed loop, http://www.soliforum.com/topic/8199/odd … s/page/23/ ), I've tested fishing lines instead of side belts, isn't hard to do.
Using fishing lines is like using belts with no teeth. Well, not really but easy to think of it that way. So yeah, I see why it would work at considerably faster speeds.
Side note, will probably want to add a fan at these speeds even for ABS.

I've printed calibration tests at 30 mm/s, the backlash is the same...

Again, if you can find an (ebay) seller that will ship the Gt2 pulleys (both 5mm and 6mm holed) and belts together I'll be glad to do a comparison test for everyone. Preferably around $50 excl shipping.

I found the real problem tonight. Here's my results at 100 mm/s. This is with only the bearings in the tensioner as a mod, so these results could easily be achieved by a totally stock unit.

I don't know how we missed this.

Main issue with fishing line is I haven't figured out how to use it on the motor pulleys (closed loop, http://www.soliforum.com/topic/8199/odd … s/page/23/ ), I've tested fishing lines instead of side belts, isn't hard to do.
Using fishing lines is like using belts with no teeth. Well, not really but easy to think of it that way. So yeah, I see why it would work at considerably faster speeds.
Side note, will probably want to add a fan at these speeds even for ABS.

I've printed calibration tests at 30 mm/s, the backlash is the same...

Again, if you can find an (ebay) seller that will ship the Gt2 pulleys (both 5mm and 6mm holed) and belts together I'll be glad to do a comparison test for everyone. Preferably around $50 excl shipping.

no, i have not done it to my printer. but the library at my university has a fusion 3 which uses a similar kind of line, and I've seen it fill a 12x12 build area at 300mm/s+ travel with extreme accuracy. other parts of that printer make it far more accurate than a stock solidoodle can ever be, but they could not use belts with those speeds, all else being equal.

As for the GT2 belts... again, Ultimaker and few other printers use MXL too. I read those articles and they make sense, but maybe the difference seems negligible in practice.

However, if you find a place or eBay seller that sells an indentical GT2 looped belt for the motor, around 2m GT2 belt for replacing the side belts, 8 GT2 pulleys (20 tooth?) to replacd stock MXL ones (x3 6mm holed for the back rod and x5 5mm for the motors and screw ones) all shipped together do let me know. I've wanted to switch to GT2 and do a comparison for a while.

Here's the pics of the parts in the setup.

If you can move it with a screw, don't you think the Y motor and all the weight it's slinging around can as well?

The question is how much pressure I need to apply to the tensioner screws to be able to move the pulley screw and how much it is compared to the pressure from the belts.
If it is less, then you're right, if it is more, then probably not.
Do you know what it is?

Printing 2 Solidoodle 3 style brackets and testing if using them solves anything won't be hard. Try it out and see. I'd do it too but my bed heater has completely messed up to print even small stuff.

Any movement of the bolt pulley changes tension. This is just by definition. To have minimal lash, the belts need to retain their tension as much as possible in all directions.

Can you adjust the belt tension without loosening the nut on the screw? If you can, then it is too loose.

How do we know it is too loose?

I plan to replace my idlers with something different for fishing line. I have the nut tight enough that the screw cannot rotate, but I cannot get it tight enough that the screw has absolutely NO forward/backward movement without rubbing the pulley and causing the axis to bind up.

If you mean the pulley is loose on that screw and that might cause problems, then there's a solution for that as well, though others who have switched to direct drive or just fixed their backlash issue somehow haven't had an issue with that.
Print this, http://www.thingiverse.com/thing:391490 , get bearings, put them in place and tighten the pulley to the screw with a set screw this time.

EDIT: on my phone so I can't link, but if you chop the belts and put them under the keepers it's no different than the method pirvan came up with in hacks/mods.

I still don't understand how your tensioner works.

One thing, what kind of pulley did you use on the motor to make room for adding two on it? There's not enough space on the shaft of the motor for two stock pulleys.

Could you reword this please?
I don't understand what you mean by "bottom front", front tensioner doesn't have room for two pulleys either and whole setup is still confusing.
And a photo is worth thousand words if you have one or can make one.

And I think the topic title is a bit inaccurate at this point.