With respect to the two drawings presented by abgd, the upper drawing showing the counter intuitive "negative" caster angle, is the most common recommended configuration. I did a lot of research into this when I began to have shimmy problems. All of the articles I could find described this negative angle as being preferred. My experience verifies the favourable characteristics this way. I had sagging tail springs, which resulted in what had been a suitable negative angle, (and no shimmy problems) becoming a positive angle, and then shimmy. I re-arched and re heat treated the springs, and the shimmy ceased. That said, correct adjustment of the shimmy friction plates is important (often, the Scott tailwheels have the shimmy plate drive pins sheared off, and the plates are doing nothing). And, tire pressure. I keep my tail tire at the maximum 70PSI (for 475 pound load on the wheel) and that seems to help.