Caster is a tricky thing. More + caster will give you added steering feel, or driver input, but, it will make it harder to turn. With modern power steering, the additional effort needed to steer is not reall noticed. The overall goal is to keep the tire contact patch as large as possible. Remember that as you turn, all sorts of forces are acting on the tires, pushing and pulling them around. As the sidewalls and tread flex, the patch gets smaller, and you loose traction. Setting up in anticipation of that flex is what were trying to do. I would be willing to give up some of the contact patch/traction when going straight, and max traction isn't needed, to gain some in a corner entry where it is needed. Stock trucks came with 1* or 1 1/2* + caster. The new Hotchkis kit will push that up to 9*+. For a "street driven" truck, I think 5* or 6* is the right range. Roll center hieght is a toughr goal to hit. Lets think about what is "reasonable" to acheive. It would be awsome to set the RC at the same height as the spindle pin centerline, but thats not going to happen. As a general rule, "higher is better", for RC. Try to keep it above 1/2 of the spindle height. So, if the tire diameter is 27", spindle height will be around 13", try to keep your RC above 6 1/2". This means drop spindles in favor of springs. Try to keep the swaybars close in size. 1 1/4" front with 1 1/8" rear is good for more streetable trucks, 1 1/2" front with 1 1/4" rear for more aggressive handling. - Remember that increasing caster will raise the pivit point ot the outer Tie Rod End, and could create additional bumpsteer, which will ned to be adressed in your design calcs.
