When you are picking out an underwire to go with your bra pattern you are in essence trying to reverse engineer the pattern. Why? Because when a bra pattern is drafted you start with the underwire you intend to use in the final product. Now if you know what type of underwire the pattern designer used and you can get the exact same type then great get the wire size that corresponds with your pattern size. But that’s not always possible so we need a way of figuring things out.
You might think no problem just use the wire that matches the seamline on the cradle exactly. If you pick your wire like that you will end up wire that is too big for your pattern and it’ll dig into your underarm. Why? Because you haven’t taken wire spring into account. When you put your bra on the wings stretch and applies tension to the bra which flexes the underwire open so it matches the natural shape of your breast root better. This also explains why a bra with a band that is too big might feel like the wire is too small because there is no tension opening the wire. You should always fit the band first for this reason if you have multiple alterations to do.
The photo above is of a cradle from a self-drafted bra pattern and the underwire it was drafted around. You’ll notice that wire matches the seamline towards the center line but is about 15mm away from the seamline even though this is the exact wire the pattern was designed for. That’s because I’ve taken wire play into account. This is what you want to look for when matching a wire to your pattern. For this reason, you also shouldn’t worry if you can’t find an exact fit since there is some flexibility in the wire once it’s installed in the underwire channeling. You might not be able to bend the wire with your hands but once on your body, it’ll spring open to fit.
This diagram shows how you would create your pattern to allow for wire spring for those of you getting ready to draft your own patterns or if you want to alter a pattern for a specific wire. The 5mm gap from the top inside edge is to allow for wire play but that’s a whole ‘another concept.