Roof Hum - Ladder Rack - Silverado 2500HD Ext Cab

Ok, I have a problem and I don't know how to fix it. I need a ladder rack on my Silverado 2500, but everytime I put one on the buffeting of the wind
over the front cross bar causes the roof to vibrate and hum horribley. I have tried two different racks ont he truck, and both do it. One has a square bar, and as you would think its worse. The one with a round bar is pretty bad to. At speeds over about 35 or 40 miles per hour the hum gets so loud that its pretty unpleasant. You can put your hand up aganst the ceiling of the truck and feel the roof vibrating very strongly.
Basically the roof of that truck is very flexible and any air tubulence is going to cause it to become a large sound diaphram. My Colorado has ridges molded into the roof to make it more rigid, but the Silverado is basically a plain flat surface. I have considered adding a wind deflector like a lot of people pulling fifth wheels use, but man that a big ugly device. I have also considered making the roof more rigid by glueing some heavy rubber strips to the roof. I was thinking the rubber used on the keel of boats to protect the fiberglass on boats that are routinely beached might do the trick. Probably about 6 or 7 such strips run from front to back. (that would be a pretty pricey solution, and I don't even know for sure it would work,)
Any other suggestions?
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Bob La Londe wrote:

Instead of trying to "fix" the roof I would concentrate on modifying the air flow between the ladder rack and the roof. It sounds like there is some kind of resonance happening. Try putting some kind of air dam under the rack toward the roof. Try a "bug shield". You just want to disrupt the air flow some. You might also try to stuff some batting up under the plastic part of the head liner.
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On Mon, 10 Jan 2005 07:46:53 -0700, "Bob La Londe"

Have you tried it with ladder(s) on the rack? Any difference?
I'd suggest experimenting by altering the airflow over the front of the rack. Either make it smoother or more turbulent, see which way makes an improvement.
To make it smoother, fair the rear surface of the front crossbar(s). The idea here is to minimize the turbulence off the trailing edge.
To make it more turbulent, add some "tabs", protruding into the airflow. The idea here is to disrupt the airflow to possibly eliminate or change the frequency of the vibration(s).
You could also fashion some sort of "air dam" between the rack and the roof. Depending on the design of the rack, you may have to add an additional, strategically located crossbar. It may be as simple as a piece of suitably sized pipe insulation (foam) over the crossbar.
DJ
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On Mon, 10 Jan 2005 07:46:53 -0700, "Bob La Londe"

If all the external suggestions don't work, consider pulling the headliner from the inside and putting some padding on the inside?
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I did this to my 97 Tahoe, I injected some "Great Stuff" expanding foam between the double wall roof panel, that made the roof more solid. You will need to take down your headliner, Then verify that you have a double wall roof berfore can start injecting. I drilled a small hole in spots where I knew the foam would not reach then put a piece of duct tape over the hole to prevent oozing. If you use the regular expanding foam be careful because it has alot of expanding power and you could buckle your roof.
Regards William
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If I tried that, I'ld use the "Door and Window." It will expand to fill the space, but doesn't have he force of the regular stuff.
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Companies that make roof racks for sporting goods (bikes, skis, etc.) like Yakima and Thule have as an accessory an air deflector that clamps on to their front cross bars. Go to a large sporting goods or other store that carries these racks and check it out, or use go to www.yakima.com and select Products, then Rack Systems, then Accessories. Check out their Fairings.
Cheers - Jonathan
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