NHRA-legal roll bar

Forward or rearward "lozenging" or collapse is prevented by the two diagonal (as viewed from the side of the car) bars (rear braces) which connect to the top of the B bar and run down through the package tray and into the trunk area in the rear, or through the cab rear window/rear cab sheetmetal onto the floor of the bed if we're talking pickup truck, forming a triangular support to the B bar when the car/truck is viewed from the side. Lateral (sideways) collapse is prevented by the cross bar which runs from the left (or driver's side) of the B bar to the right (or passenger's side) of the B bar. This cross bar is installed at the driver's shoulder height with the driver seated in his/her driving position and also serves as the shoulder belt anchor point. It should be noted that many builders also install a diagonal "D" bar (as viewed from the front or rear of the car) from the "B" bar corner at the driver's helmet down to the other side of the car at the bar end/floor to further prevent structural failure.