We present a sawtooth model that explains observations where the central safety factor, q0, stays well below one, which is irreconcilable with current models that predict a reset to q0 = 1 after the crash. We identify the structure of the field around the magnetic axis with elements of the Lie group and find a transition to an alternating-hyperbolic geometry when q0 = 2/3. This transition is driven by an ideal MHD instability and leads to a chaotic magnetic field near the axis.