Stew, Jason,
Thanks for that.
Actually I missed a bit when I said made over scale. I meant that the width of the wheel was usually wider than scale, not the diameter. I realise that He had the diameter close enough for all practical purposes, 4.19" as to 3.9", but my whole point is what is the correct scale ? If it is fairly flexible then I'll go for 16 :1.
I don't intend to redesign the Stroudly, The drawing from NRM provides most of the dimensions. But I will need to convert all these to the appropriate scale for the guage I eventually choose.
I'll keep an eye open for the Martin Evans book.
Dave.