Rooftop cargo box bounce

[optimusprime]

2008 yukon Denali xl with the factory roof rails that came with it. Picked up a Thule Force XT Cargo box. It’s the big one. When we drive down the road, it bounces up and down quite a bit to the point where it contacts the roof. This is with and without cargo in the box. Seems like it’s the roof rails flexing. Is this normal? Does anyone else have this issue and is there a fix for it?

 

[Geotrash]

2008 yukon Denali xl with the factory roof rails that came with it. Picked up a Thule Force XT Cargo box. It’s the big one. When we drive down the road, it bounces up and down quite a bit to the point where it contacts the roof. This is with and without cargo in the box. Seems like it’s the roof rails flexing. Is this normal? Does anyone else have this issue and is there a fix for it?

I've run a large Thule box on mine and yes, the factory bars will flex, but it should still be plenty stable to keep your box in check. My hypothesis is that the nose of your box is right at the point where the airflow will keep splitting above or below it as the bars flex, increasing the amplitude of the flex as it goes. My advice would be to play around with moving the box forward and back - either by sliding the box on its mounts, or by sliding the bars themselves backward and forward, whichever is easier, to find a point where it doesn't do that any more.

 

[optimusprime]

I've run a large Thule box on mine and yes, the factory bars will flex, but it should still be plenty stable to keep your box in check. My hypothesis is that the nose of your box is right at the point where the airflow will keep splitting above or below it as the bars flex, increasing the amplitude of the flex as it goes. My advice would be to play around with moving the box forward and back - either by sliding the box on its mounts, or by sliding the bars themselves backward and forward, whichever is easier, to find a point where it doesn't do that any more.

Did you find by moving it more rearward that the bounce was reduced?

 

[Geotrash]

Did you find by moving it more rearward that the bounce was reduced?

Mine was fairly long, so moving it back any further would've caused interference with the liftgate. I had to move mine forward so that the nose overhung the sunroof by a little bit:

 

[swathdiver]

@Mickey_7106

 

[optimusprime]

Im wondering if it’s the factory bars that are the issue. Met a guy at the store who had the same box and his was mounted on a Subaru and the box was solid as a rock. His rails were thicker than mine.

 

[swathdiver]

Is it within weight limits?

 

[Geotrash]

Im wondering if it’s the factory bars that are the issue. Met a guy at the store who had the same box and his was mounted on a Subaru and the box was solid as a rock. His rails were thicker than mine.

It will make it more solid, but still won't resolve the aerodynamic effect that's causing it. It's known as aeroelastic flutter:

Aeroelastic Flutter - Flow Physics and Computation Lab

Aeroelastic Flutter Aeroelastic flutter, defined as “an unstable, self-excited structural oscillation at a definite frequency where energy is extracted from the airstream by the motion of the structure”, is ubiquitous in a wide range of engineering fields. In applications that rely … Aeroelastic...

engineering.jhu.edu

Moving the box forward or rearward will change the way the airflow over the front of the vehicle and windshield hits the shape of the box and should be enough to eliminate the problem. Moving more weight to the front of the box will also help.

 

[Kee Fuller]

Curious, how much bags can y’all get in ya cargo boxes.

 

[optimusprime]

It’s definitely not over limit. When I push on the box while the vehicle is stationary, the box wobbles like a spring board. It is firmly attached to the rails but the rails appear to have a TON of flex in them. The guy at the store had rails that didn’t budge at all.