[asid61]

Interesting. The weight is quite high,, but the simplicity of this design is really good. Excellent, even, espcially considering that it's made in SketchUp. Look into getting some SolidWorks licenses for the team.
That being said, there are a few changes I would highly recommend:
1. Change the wheel to a single wheel in the center. Having such widely seperated wheels will contribute greatly to wheel "scrubbing" while turning.
2. Make those pulleys the correct diameter and width. Those look like 5mm wide belts right now or something, which is way too thin. Go with 9mm wide or 15mm wide belts.

What size box tubing is that? Can you source it? Do you have thrust bearings or thrust washers on there?