The hydrodynamic forces on millimeter sized air bubbles undergoing zigzagging and spiraling path oscillations are deduced from precise experimental measurements of the bubble trajectory. We find that the lift force exhibits hysteretic dependence on bubble speed. We also observe an abrupt transition from rectilinear to oscillating path for increasing bubble size. These two facts suggest that a subcritical bifurcation may underlie the bubble path instability dynamics. We also measure unexpected dynamical changes in the drag experienced by the bubble which play a role in the path instability.

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