Step 1.   

As usual gather together your reference material (photos/plans etc). With this info, you need to work out the length between spring end eyes and the basic curve of the spring and the width of the spring. Mark out this info onto the MDF/wood piece. Drill and insert some brass rod at the spring eye points, and one at the apex of the curve (see photo), for 1/25  - 1/24th scales use 0.75mm (.030") rod (approx), adjust to suit smaller scales, or an alternate to brass rod is dressmakers pins.

Step 2.   

Either find some brass strip of the required width and thickness or cut a strip from some brass sheet.

Handy Tip: Brass sheet can be cut using the scribe and snap technique the same as plastic sheet, with an old blade (or machinist scribe) scribe the required line on both sides (do a few passes) then clamp the sheet in a vise and bend back and forth till it snaps. Then just a quick swipe with a file or W/D sandpaper to clean the edge. For 1/25 - 1/24th scale models I normally use 0.4mm (.015") brass sheet/strip. Cut the strip longer than needed, this is to allow for the bending step.

Step 3.   

Anneal one end of a brass strip, and with the pliers wrap around one of the brass rods/pins in the jig to produce a closed eye. Reharden the brass when happy with the eye produced. Then gently bend the strip to create the curve. Next up is to anneal the other end and wrap around the other brass pin - this will require doing a half wrap then trimming to the required length and then finishing the wrap to close the eye. Reharden this end.

Step 4.   

Now you can lay the individual spring leafs. Using some plastic strip of the same width and thickness (0.4mm x 3.2mm (.015" x .125") looks good for this scale) epoxy the first leaf to the brass leaf. Clamp in place and allow to harden then continue adding each additional leaf (using plastic glue) till the required number/thickness is reached. Before gluing the plastic strip, gently pull over the edge of the workbench in a slight downward motion as this will give the plastic strip a gentle curve making it easier to mate to the already curved piece. If the 1:1 version has shaped ends and you want to recreate this, its best to trim before gluing the strip. Also if you want to depict slightly splayed leafs don't glue the full length. Glue sparingly as you don't want fused plastic seeping out between the leafs, if this does happen, when cured just run an Olfa cutter along the join lines to tidy.

Step 6.    Attach to the modified kit spring hangers or use some scratch built hangers. Continue with your build as usual. If the model will not sit level (ie. all wheels touching the ground) a very gentle bend at the spring centre, or a very careful "tweak" at the eye will solve that problem.

Conclusion

As with most scratch building techniques all that is required is a little patience and thought before committing knife to plastic. Although not an essential modification to a kit I personally think its just one of those little details that helps create a truly individual model with the added bonus of a strong component. Of course there is little point in doing this if the body panels cover this detail, but for a 1920's - 30's engine that has exposed springs (especially the fronts) it will certainly create a more realistic appearance.