The clamps do not fully close, by design.
I've held extensive discussions about the windshield bars with James Barth, owner of RockHard.
His position is that the weaker stock bar should be retained, as a "crumple zone", or a bar that absorbs impact by deforming. I believe that this theory has merit in some crash circumstances, but not in all circumstances in which we my find our rig upside-down. For example, in a slow offroad tumble, a stronger windshield bar would be less likely to deform, and could maintain the integrity of the rest of the cage/windshield frame while still saving your noggin.
If you decide you want stronger-than-stock windshield bars, you can get a set from Poison Spider for ~$150 that will work with the RockHard cage.