A Rolling Plotter

This is my attempt at making a rolling plotter. I am just getting started with the design & think I have the Y-axis movement worked out. The wheels are currently 3D printed plastic 60mm diameter with an 88 tooth timing pulley along the center of the wheels. I used 60mm 3D Printed Wheel to get the starting point for the profile for these wheels. Each wheel is 3 printed parts, the main plastic wheel & 2 TPU plastic printed tires. I will use 2- 2020 V-slot for the X-axis starting with 500mm lengths & move to 1meter lengths after the bugs have been worked out. There shouldn't be much weight on the X-axis 2020's, so it seems like they would be sufficient. The drive belt for each side will be a closed loop GT2 belt which is currently should be in the 420mm length. The only place I have found them so far is China site on ebay, so I need to make sure I have the correct lengths before ordering any. I have 6mm height tension adjustment with the motor & can add some adjustment for the bottom center idler. I initially did not have that center idler in there, but thought the belt might rub on the ground. There is probably only .5mm-1mm clearance of the belt hitting the ground which is if it is tight should be enough. If not, I can make the TPU tires a little thicker. Looks like the other rolling bots listed on this site are using around 100mm wheels which I would guess gives better resolution. I could make them 100mm or any size that fits on my printer, but will start with 60mm for now. At 100mm diameter the tooth count would probably be around 152. I am not sure what the step settings will be for the Y-axis, but should be able to calibrate it with some trial & error.
Here are some screen shots of the current design.


2/26/2020 Update
Think I have worked out all the design problems for this except for the servo connection to the pen movement. I had to lower the location of the wheels by 10mm because of the X-axis belt interference with the pulleys. 5mm might have been enough, but gave it a little extra for comfort zone. The X minimum limit switch I just placed in the design & will just make a simple mount for that for now. Looks like I will get around 35" of x-axis with 1 meter extrusions with current configuration. I switched the tires to use #327 O-rings instead of the 3d printed TPU. I am showing the model with 300mm length V-slot for better visibility of all the parts & have that as a variable length in the design in case I want to change it for viewing.

3/16/2020 Update
I attached a zip file to the files tab of the1st version of STL files & test file of BOM list for this build.