Fold up chasses allow you to easily and correctly align the sideframes,
with just a couple of careful bends. After a quick check you can
then solder in a few spacers to form a strong, rigid structure, ready to
take further components. The picture shows the essential chassis pieces
from a Worsley Works kit (14.2mm version) for a Great North of Scotland
Class V locomotive.
Some fold up chasses include spacers as fold up pieces within the main
chassis fret, which makes things even easier. This example is the main
etch from the Brynkits GWR 2251 chassis kit (12mm version).
One alternative to the fold up approach is the slot-and-tab chassis.
Tabs on the sides of spacers fit into matching slots in the sideframes,
which helps align things correctly. This approach requires a
bit more care, to ensure the spacers are properly bedded, and a bit more
soldering, but is still quite easy. Unlike fold up chasses, you don't
normally need different kits for different gauges; the different width is
catered for by different width spacers, as in this fret from the Society
Modelex-produced GWR 45XX chassis kit.