I need advice from those that have mounted the Thule, Yakima or other luggage/cargo boxes on the factory rack crossbars. I have read from some others (not specific to Expeditions) that they recommend using thin foam between the box and the crossbar to eliminate knocking or vibrating and to use rubber tubing around the steel u-bolts to keep them from sliding or marking up the bottom of the crossbars. I have mounted my new Thule box in the garage to adjust the spacing of the crossbars and see how it installs and it seems very tight once everything is locked into place but I don't want to figure out different on a road trip beginning in a week. Thanks for the advice and suggestions.
I've got about 7,000 miles with my Yakima SKYBOX21 and it fits nice and tight and doesn't vibrate on the factory cross bars.
My Thule box also fits very tight once I get all 4 steel u-bolts clamped down, so I'm hoping it stays that way on the road because I'm down to the last day before the road trip and don't have time to look into a large enough piece of rubber between the box and the crossbar of the roof racks. I did put a thin walled rubber (pond liner - wrapped in electrical tape) around the u-bolts because I could get a little sideways movement with the box when the u-bolts were in metal-to-metal contact with the bottom of the crossbar. I'm hoping this provides a little bit more grip between the u-bolt and crossbar. I'll update once we return from the trip.
The u-bolts are the main reason I purchased the Yakima Skybox, no tools to install or adjust. I've put this on 10 different vehicles and can make the adjustments is just a couple of minutes without tools. I rent my skybox out for $45 a week for a little extra money by advertising on the local craigslist.
After 2,000 + mile trip I wanted to quickly follow up. All I did before the trip was cover the steel u-bolts with a thin pond liner rubber (where the bolt is in contact with the cross bar) with electrical tape. I tried to be careful when I installed the box on the crossbars to get the u-bolts nice and tight (pushed down inside the box when clamping). Even with many hours of strong head and side wind travel the Thule box wasn't noticeable and did not move at all on the racks. Very nice to have for overflow luggage.
Thule has a lobster claw type of assembly on their newer boxes. You clamp it down from inside the box. I've never had a rattle or shake from them, nice system in my opinion. You can drop the box on any crossbar and be on the road in 10 minutes.
I have a Thule North Cape box and it used to rattle around on the factory rack, so I called Thule and they sent me rubber covers made to fit the steel brackets. Now the rack is tight and quiet (and it does not move around on bumpy roads). Thule customer service was great.