Why are you not supposed to drive with the side windows open?

When the windows are opened on a Grand hardtop, only the glass itself opens, not the frame. This means that if they are left open whilst driving, there is a risk of the glass breaking as the only support to the hardtop is a small clamp. The front sliding bulkhead window can be open whilst driving. If you need maximum ventilation whilst driving, we would always recommend the GLS hardtop or the ARB Ascent hardtop.