From manual:
Entities are allowed to rotate around axis 1 and axis 2. Use this constraint for setting up wheel-like behaviour instead of using two separate PH_HINGEs. For a car, axis 1 is the steering axis (left/right), while axis 2 is required for forward/backward movement. Only axis 1 can have angle limits and can have its orientation queried (phcon_getposition).

I think this means that you can only query the angle offset of the ph_wheel and not the "shock absorber" slider.