  Non-invasive thermal noise calibration of both torsional and flexural
eigenmodes is performed on numerous cantilevers of 10 different types. We show
that for all tipless and short-tipped cantilevers, the ratio of torsional to
flexural mode stiffness is given by a constant, times the ratio of their
resonant frequencies. By determining this constant we enable a calibration of
the torsional eigenmode, starting from a calibration of the flexural eigenmode.
Our results are well motivated from beam theory and we verify them with finite
element simulation.
