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Something like that maybe ?

sage: d = abs(f_v.subs(phi=-phi))-abs(f_v.subs(phi=0))
sage: sympy.sympify(d)
-sqrt((-h + s*sin(theta))**2 + (-r + s*cos(theta))**2) + sqrt((-h*sin(phi) + r*cos(phi) - s*cos(theta))**2 + (h*cos(phi) + r*sin(phi) - s*sin(theta))**2)