The Irish Tension fatcore bobbin was designed to make it easier to spin finer yarns, by introducing the same higher ratios as the high speed bobbin, but also adding a thicker center core to simulate a half full bobbin. This helps allow the wheel to not pull in as quickly, assisting in spinning a finer yarn. Its ratios are: 6.5:1, 9.5:1 and 15:1

The fatcore bobbin will fit on all current and discontinued Louet Irish tension wheels EXCEPT: S40, S45, Victoria, S87, S90.