With the aid of elementary beam theory a Lagrangian equation of motion was developed to relate the crack velocity to cleavage energy in a double‐cantilevered single‐crystal specimen when shear contributes significantly to the cleavage energy. For a constant cleavage energy the equation predicts that the square of crack length is proportional to time when the crack is long with respect to transverse specimen dimensions. For shorter cracks, crack length is proportional to time.