Roof Rack & Cross Bar Strength

[5.9 Litre]

I bought an Maggiolina-small tent(www.autohome.it) for my 1998 Grand Cherokee Ltd. a while ago.

I installed the tent on my jeep’s standard roof racks' cross bars:
http://promerg.com/photos/CIMG0045.JPG
http://promerg.com/photos/CIMG0042.JPG
http://promerg.com/photos/CIMG0046.JPG
http://promerg.com/photos/CIMG0047.JPG

It weighs 115 pounds and it has a pretty good aerodynamic design which is supposed to be fixed on standard roof rack rails of Grand Cherokee, without any extra equipment -so I'm told.

I've never installed/carried anything on my car before? Are they strong enough? Can they handle that weight at speeds up to 90 miles/hour without any extra equipment to fix on racks or raingutters?

I've drived mostly short distances and haven't had any speeds yet. First thing I noticed was the front part of tent; it slightly oscillates up and down when I'm driving(I see it through sunroof)
Will it cause metal tiredness at cross bars in time and possible damage or is it normal?

[5.9 Litre]

I had the system examined by a Jeep service mechanic and he strongly advised me to find a solution for front oscillating and warned me it might caused cross bars to rip off in high speeds due to excessive wind load.
Following his recommendation, I used a lashing strap with tensioning ratchet that goes all around front part of tent and inside car you tighten it. I also put some rubber at touching surfaces of tent-front and roof racks to further absorb tention.
When I showed the final to service man he agreed it was more than OK.

[NulodPBall]

Can you strap gear to the top of the tent when it's closed? If so, I imagine that the weight will keep the tent from "floating". If you can't put gear on top, I guess the "lash" would be a cheap and easy fix but maybe one of those wind deflectors that you see for car roof bike racks to prevent wind from getting underneath your tent, or maybe you could work some magic with a bug deflector...just some thoughts.
---also, I'd like to hear how well you like your tent

Ray

[Dsilver]

From what I know as a mechanical engineering major, and looking at those pictures there is way too much room under your tent thing for the air/wind to create lift forces on it, so yes I would say your mechanic is right, your cross bars could rip off after enough time of being stressed by those lift forces from under the tent thing, so you either need to fashion some kind of wind deflector in front of it, or you will just have to take it off.