Horrible reverberations between roof and RTT

[dylan.h.brown]

Hey dudes, after endless debate, I finally landed on a Alu-Cab expedition RTT. I decided to mount it as far forward as possible to use enable bikes to be draped over the tailgate, as well as use the a cabinet I have mounted in the bed as a step. This also puts the weight super far forward over the rear axle (drives crazy well actually). BUT, what I've found is I now have horrible air reverberations between the tent and roof. It sounds and feels like one window is open -- pressure whomping. I started w/ a 4" gap over the roof, now it's 5".

My question to you all is:
- What is the ideal roof/cab over gap?
- Are there other solutions to mitigate this air reverberation effect?
- I have manually measured the bar height (despite the readouts on the Yakima racks) and the front is 1/2" lower. I plan on leveling that. Would this make a difference?

And yes, I expect some wind noise, just not pressure whomping. Here's the setup as is.

 

[SDDiver5]

Hmmm... The only thing I can think of is that it is so far over the cab...I'd try moving it more towards the rear as a first test. I had my RTT mounted more forward originally as well but when I centered it more there was a decrease in wind noise. I never had what you're explaining however.

I'd try getting rid of the shovel and traction pad bar, move the rear rack back and slide the entire RTT back at least a few feet.

Plenty of guys have this set up with no issue...your tent is just mounted in a different location I think.

 

[fisher205]

You may want to put this under RTT section, it may get more responses.

 

[ThundahBeagle]

In automotive aerodynamics there is down force where the hood meets the bottom of the windshield, but along most of the hood and roof, there is lift. The placement of your RTT is displacing that lift and back flow.

Did you ever see some raised ram induction hoods in muscle cars, and see how some of them have the intake raised an inch or so before the opening? Your RTT is exactly in the way, and air is getting spoiled under and around it. The air is moving backward and upward and hitting the bottom of the RTT. Getting baffled in that no man's land bounced up and down in that gap between the RTT and roof.

One thing I can think of is a slanted valence like on the roof rack of the Nissan XTerra, like you find on some roof baskets. That will guide back flowing air up to the RTT and help it over it, so it doesnt get trapped in that gap.

Either that or move the RTT back and let the air move as it should.

 

[NatersXJ6]

I had a similar issue when the RTT was all the way forward on my Jeep. You need to move it back or put some sort of wind deflector/fairing in front of it to direct the air up and over. There is a slight chance that you can make it better by moving forward too, getting the tent “under” the air as it separates coming off the windshield. @ThundahBeagle is spot on. Sometimes you can find aerodynamic model diagrams for specific vehicles on the web that can help identify the best point to put stuff. Otherwise, CAD (cardboard aided design) is your friend. Watch Ford vs Ferrari for examples.

 

[Herbie]

One thing I can think of is a slanted valence like on the roof rack of the Nissan XTerra, like you find on some roof baskets. That will guide back flowing air up to the RTT and help it over it, so it doesnt get trapped in that gap.

Yeah, +1 on some sort of wind deflector. The thrumming is almost certainly the repeated pressurization of that area between the cab roof and RTT. As the pressure builds up, it exhausts back into the bed, which lowers the pressure enough that airflow over the windscreen again finds its way back under the leading edge of the RTT. If nothing else, a single crossbar (Yakima, etc.) with an ABS deflector on it would probably work wonders.

 

[ChasingOurTrunks]

I would add that wind deflectors don't need to be fancy - it doesn't take much to dramatically alter the way wind flows over a moving vehicle (at least, dramatically enough to change the impact of wind buffeting). Some folks experience buffeting because of roof loads even with wind deflectors - the angle isn't right or whatever -- but simply by wrapping a rope or bungee around the deflector ( i.e. just wrapping it 5-6 times around the length of the deflector and securing it) can create enough disruption of airflow to make noise and buffeting go away.

Moving it further back is the cheapest option to see if it works, but you still have the need to transport bikes, so if moving it back is a deal breaker for the bikes here are some ideas you might want to try.

- Stick a rubber strip around the front bottom edge of the tent. You can probably get a cheap 4"-6" piece of rubber trim and stick it on with tape, and go for a drive to test to see if that has altered the air flow enough to reduce buffeting. If it works you can go for a more permanent mounting solution.
- Slightly more expensive might be a Putco-style "Eyebrow" on your truck that mounts over the windshield like what you see commonly on 70-series (pic below). These "eyebrows" are also nice for sun glare too, and will change how wind flows over your hood/roof and SHOULD reduce buffeting (but there is a risk this might introduce other problems/wind noise depending on the aerodynamics of the vehicle).

 

[ramblinChet]

My guess is that when you open a window this pulsing is reduced?

If so, your vehicle interior is acting like a Helmholtz Resonator which tells me there is an opening somewhere where you have air passing over it very close and very fast. Maybe there is a cabin vent somewhere in the back of the cab up top or even down below the window? You can experiment and cover the cabin vent to verify where your problem originates. Once you do that you must now disrupt the airflow immediately around the vent or possibly upstream.

Good luck and keep us updated.