I am really interested in the Thule 840 Playpen, so I can carry stuff and also have the option of putting a bike or skies on the rack as well. But if I don't know if I need to get the extra support bars. If I need to get a set of support bars, then I can go with the ones that GM sells and save a few...

I am currently researching Roof Racks (Thule 840 Playpen Basket) for my Pontiac Vibe which I picked up last month. I am wondering if I will need to by another set of Cross Bars (an adapter), or can I just mount the basket right to the factory rails without a set to support the weight. I know the wei...