Remarks about Van Zytveld, 1975: 
His approach to derive the equations of motion is very similar to what I 
have done in my hand calculation in 2004. He does not give full details 
about his derivation. His first equation of motion appears to be perfectly 
correct. His second equation appears to be correct, except for the terms 
with the robot lean angle \psi. The first should have dots, and the moment of 
inertia should be an inertia product, as in the next equation of motion, 
to make a symmetric mass matrix (with the sign of the second equation 
reversed), and the second term should not be there, to make the constant 
stiffness matrix symmetric. The third equation appears to be correct, except 
for the interpretation. I^R_xz is not the product of inertia with respect to 
the centre of mass, but with respect to a point on the hinge axis directly 
above the rear wheel. 

If the robot is removed, his equations agree with ours. Note that his sign 
conventions and the direction af the axes are different. 

JPM, Nottingham, 8 Nov. 2005.