Jim,
Am I right in thinking that to get the vehicle at the correct height, and the bearing carrier sitting at mid stroke, you need to juggle with the diameter of the spring and the weight of the wagon?
Scott,
Yes - that's about it.
You may remember that I did a piece in the S Scale Gazette about springing using Russ Elliot's data from the CLAG web site whereby you can select the gauge of guitar string depending on vehicle weight, spring length, etc. (December 2011) However, I found from experience in Scale7 that
fitting springs then adding ballast to the wagon until the springs were at the correct height was a good method, with the ballast eventually being hidden as lead strip in the wagon underframe. So it can be a bit of a mix between spring selection and ballasting. Current thinking in Scale7 at the time I was fitting springs was to make the vehicle quite heavy to get the best effect from springing and I will be applying the same idea in S scale.
In the underframe I built to illustrate the Gazette article, I used Russ's formula to calculate the gauge required and it turned out to be pretty well spot on, with the springs in mid deflection where they should have been. I had made the underframe quite heavy, with brass section for solebars and buffer beams, so was going some way down the Scale7 thinking of a heavy weight vehicle, but I think I will add additional ballast to any further coaches and up the gauge of spring to get better running.
I suspect that in practice, since most S scale wagons weigh much the same in any case, that I shall ballast all my wagons to the same weight and work out the gauge of spring to suit this weight so that the same gauge of wire is used for everything.
Jim