Wendell,
About a fourth of my layout is Unitrack. Next layout will be 100% Unitrack. The car looks best, of course, on 19" radius but it operates fine right down to 11" but, of course, I can't have another train passing on the curve at the same time! I think the "widest" radius I use with Unitrack is R7..... I aslo have two smaller radii in use but, short of lifting them off the layout, I can't tell you what they are. They did come with standard track "sets".
Dick