After trawling the internet looking for info, I decided to develop my own spreadsheet for designing the leaf springs for my new loco. It turned out to be surprisingly simple. The main thing I found is that a leaf spring having the same number of leaves with scale thickness as in the full-size loco will be way to stiff for practical purposes unless used to also support the operator.

My planned loco will be a small 5" gauge steamer with a total anticipated weight in the order of 50kg. By way of comparison, my 5" gauge petrol loco which has similar dimensions weighs 36kg. To maximise weight on the drivers, the leading and trailing pony trucks will only carry about 5kg each, leaving 40kg on the two driving axles.

Using steel strapping from packing cases with a thickness of 0.7mm and a width of 10mm, with a spring length of 66mm, the calculations indicate I would only need 3 leaves to provide the combination of load-carrying and deflection needed. The full-size loco has 10 leaves.

I'd be happy to share this Excel spreadsheet with anyone who wants it, but make no guarantees on the results obtained. I have not carried out any proof testing to verify the results, but will do once loco construction is further advanced.