Modem passenger cars offer a high level of occupant protection compared to minibuses and coaches. Whereas a typical car is now equipped with lap and diagonal seat belts and airbags for the front seat passengers, the majority of minibuses and coaches in use have either no restraint system or simple lap-only belts that offer no upper torso restraint in frontal impacts.

Following several tragic accidents involving minibuses, raised awareness has led to a demand for seat belts to be fitted to existing vehicles. This has resulted in a number of companies providing a service to retrofit seat belts, or fit new seats with integral belts.

Bolting seat belts to an existing seat is generally unsatisfactory because the seat and its mountings were not originally designed to withstand the loads imposed by the occupant under impact conditions. Thus the seat either detaches from the floor structure or collapses onto the occupant. Specially designed structural seats with integral belts provide the maximum occupant safety, but the vehicle floor generally requires signiñcant strengthening to successfully transfer seat belt loads into the structure.

The majority of companies that convert vehicles mount these replacement seats directly to the sheet metal floor. Research work at MIRA has confirmed that this method is insufficìently strong to prevent the seats from tearing out of the floor when subjected to the quasi-static seatbelt anchorage test, ECE Regulation 14 (M2 vehicle loads).

To deal with this installation problem, MIRA has developed a system which enables the ñtting of replacement seats with integral belts into minibuses. An 'under floor’ solution was adopted, to prevent the loss of headroom and weight penalty incurred with alternative 'over floor' framework designs. The design, development and subsequent system validation process is described, illustrating the methods adopted for transferring occupant belt loads through the sheer metal floor, into the body structure.