The choice of the model of the bars depends on:
• by load capacity: 50, 75, 100 Kg. in any case, the load capacity is conditioned by that of the roof of your vehicle which should be indicated in the characteristics of your car
• 
the need for the attachment system for the accessories (some accessories such as bike racks, ski racks require the "T" fixing system) The bars that support the "T" system are fitted in a 20 or 22 mm wide channel where the screw with square head supplied with the accessory for its fixing
• by design: Standard or Aerodynamic and colour. The design in addition to the purely aesthetic aspect such as the black or silver color affects the silence. An aerodynamic shape will offer less resistance to the wind and consequently will reduce the noise that is heard during movement of the car.
The width of the bars. If you need to mount accessories that use the U-shaped coupling system, it is important to know the width of the bracket in order to choose the 8 or 6 cm wide bars