Beam Driven by Two Wheels

A beam is supported by two wheels, which are counterrotating "inwards" and cause opposite frictional forces given by

FR = μ FN .

At the symmetrical position the two forces cancel each other. But if the beam is displaced by x0, the loads on the supporting wheels are different, which leads to a linear restoring force. i.e. to harmonic oscillations. Their frequency depends on the height h of the beam, the distance l of the wheels and the friction coefficient μ.

Use the applet to find the relation giving ω = ω(h, l, μ). Can you explain your result?