Level curves of total mechanical energy
In addition to stability and steer torque, the plot below includes two level curves of total mechanical energy (kinetic + potential). The values I chose for these two curves are the total mechanical energy that the upright, zero steer, benchmark bicycle parameter set has at the weave and capsize speeds. At the origin these two curves seem to have the same slope as the boundary of the stable region, similar to the zero steer torque curve. Away from the origin, these curves do not bound the stable region but instead pass through it.
Click on the image to see a hi-res version.