Depends really. Length of track, expected terrain, etc. Add some measurements, pull out the lego's, and go to town. One of my first robots was track based. I learned a lot as far as how it'll all work. If you do that design for each wheel (did that too), then the top sprocket would be the drive sprocket, and a triangular frame system would be needed to keep the two surface sprockets in place. You can build the whole thing from lego's, and give yourself some pretty good ground clearance. With a standard tank like set-up, you aren't looking at much of a ground clearance, which is good for level surfaces. You can raise up the body just by building track supports an making them placed lower on the body. Just be 7 years old again and play with the lego's and see what you can come up with. I'm getting old here in my 20's, and I will preach the use of lego's in prototyping for a long time. You can construct just about everything from them just before making a permanent chassis and mechanical system for your motors and electronics to sit on.