This is the first post in a new series about the theory behind underwires. This series will cover several topics including wire types, wire spring/splay and choosing an underwire for your shape. This post will cover why underwires are used in bras and give a quick overview of their purpose. Other topics will be cover in depth in future posts.
In this conversation, I will be referring to classic U-shaped underwires.
The purpose of an underwire is to contour your natural breast shape by encompassing the tissue in its entirety. This helps them resist gravity and provides lift. Underwire bras are designed to bring the weight of the breast closer to the chest. Underwires also help create breast separation depending on the style used.
It achieves this via a cantilever effect which is a result of the combination of the wire, the band, the cups and to a lesser extent the straps. The underwire acts as the fulcrum or pivot point of the cantilever which is lifting the load which is the breasts. The cups, frame and the band of the bra form the lever. The straps help keep the breasts in the correct position to allow the lever to function. Their purpose is not to support the breasts. The straps should only provide 10-20% of the breast support.
Because the underwire is only part of the support system the breast will only be supported to the extent that the cups allow. A low cut cup will not provide the same support as a full coverage cup even if the same underwire is used.
The wire must be sprung open and this is achieved by the tension placed on the wire by the band and cradle. This allows the underwire to conform to the shape of the breast root and to hold the cup and frame into position. It also spreads the weight of the load over the entire wire.
If the tension exerted by the band is not enough to spring the wire open then all the weight will fall on one spot on the bottom of the wire. when this happens lift will not be achieved, the underwires will tilt forward and too much weight will be carried by the straps. A band that is too tight can cause distortion of the wire leading it to twist and snap.
* A cantilever is a horizontal beam or structure that is unsupported at its end.