Roof ski boxes, wind drag and mileage

[OldJeep]

2009 Ram 1500 Hemi Crewcab, haven't noticed any mileage difference with the box on the ladder racks or without - wouldn't expect to on a nearly 5500lb brick shaped vehicle with 600lbs of sand in the bed

 

[tch]

Question as I depart for a 5-week tour with a new box: does it matter where you mount it? I just stuck it up there where I had put the bars and now have about 10" of leeway before I interfere with my hatch in the back. It will always hang over the windshield a bit, but I could slide it backwards. Does mounting it forward/back change mpg?

 

[LegacyGT]

Does mounting it forward/back change mpg?

Don't know. But I mount it as far back as possible so I can access more of the box while standing on one of the rear tires. Also, maybe this moves any added wind noise further from the driver's ears.

 

[Plai]

I also mount the yakima skybox as far back as possible on my 4Runner. When my rear is open, the spoiler at the top of the rear door almost touches the back of the skybox. I wanted to keep the shape of the vehicle as close to original/streamline as possible. To me, any overhang will introduce more drag.

 

[cantunamunch]

Question as I depart for a 5-week tour with a new box: does it matter where you mount it? I just stuck it up there where I had put the bars and now have about 10" of leeway before I interfere with my hatch in the back. It will always hang over the windshield a bit, but I could slide it backwards. Does mounting it forward/back change mpg?

IME it changes road feel. The further forward the more sensitive the vehicle becomes to clean air flow especially behind trucks and buses. On the previous vehicle, getting rid of the air deflector entirely helped reduce the steering feel but the noise came back.

Jeep Renegade 2.5 liter engine 27. 6 bare bars (vehicle determined)
26.4 (by vehicle) Thule Pulse Alpine, no wind deflector, will measure overhang.

 

[David Chaus]

I’ve got a Yakima Skybox 12 on my 2005 Forester. I might get about 1 to 1/2 MPG difference between driving with or without the roof box. That said, when the box is on, it’s in the winter, and weather conditions can play a factor (more wind and precipitation which increases air resistance as well as impacting visibility).

Another consideration is where one drives and therefore the average speed of travel when a roofbox is attached. The first dozen miles I’m on freeway, the rest of the way it’s a mostly 2 lane highway (with some passing lanes in places) with speeds of 50-60MPH but sections where it’s 30-40MPH. Only above 40MPH is where you have wind resistance that’s more than negligible.

FWIW, I’ve been averaging just under 27MPG for combined skiing and daily commute driving, with the roof box on.

 

[Wilhelmson]

How about the $100 spoilers. If they save 1 mile per gallon (which is generous), then on a 30 mpg vehicle at $2.50/gallon, the break even is at about 35,000 miles.

 

[cantunamunch]

How about the $100 spoilers. If they save 1 mile per gallon (which is generous), then on a 30 mpg vehicle at $2.50/gallon, the break even is at about 35,000 miles.

But if your bars start singing to you at anything over 45 mph, the breakeven is a lot quicker, no matter what the mpg indicator says.

 

[tch]

Ok, now I'm officially bummed. Drove 2,000 miles from CT to CO in my VW 4-Motion wagon (2017). Got 27.4 with Yakima Rocketbox mounted rearward. When we drove to FLA this fall, we got almost 35 mpg without a box. That's 7- 7.5 mpg difference. Big.

 

[cantunamunch]

Ok, now I'm officially bummed. Drove 2,000 miles from CT to CO in my VW 4-Motion wagon (2017). Got 27.4 with Yakima Rocketbox mounted rearward. When we drove to FLA this fall, we got almost 35 mpg without a box. That's 7- 7.5 mpg difference. Big.

How much front overhang, how much roof-box distance, and what is your wind deflector set -up like? The box overhang meeting the wind deflector and creating a parachute was the biggest culprit with our VW.

Obviously, faster driving (70mph+) will make a bigger difference than =<65mph

 

[ScotsSkier]

and how much elevation change and average speed difference CT to CO.....after all, CT to FLA is downhill all the way....

 

[LegacyGT]

The wind deflector or fairing gets the wind to pass up and over the crossbars to improve airflow and reduce the whistling noises. This may work when you have a ski or bike rack up there. But when there's a cargo box, I'm not sure the deflector does anything for you. I find that the presence of the cargo box deflects the air away from the crossbars so the deflector isn't really necessary. The questions would be whether it increases or decreases your efficiency. My bet would be that it makes the setup less efficient but by a negligible amount.

 

[cantunamunch]

The wind deflector or fairing gets the wind to pass up and over the crossbars to improve airflow and reduce the whistling noises. This may work when you have a ski or bike rack up there. But when there's a cargo box, I'm not sure the deflector does anything for you. I find that the presence of the cargo box deflects the air away from the crossbars so the deflector isn't really necessary. The questions would be whether it increases or decreases your efficiency. My bet would be that it makes the setup less efficient but by a negligible amount.

I think it depends on the angle where the deflector meets the box; if the deflector angle (from horizontal) matches that of the windscreen and there is some sort of lateral air displacement at the deflector/box interface, like a kite-shaped wedge, it could be made significantly better than stock.

For deflector angles that are less than the windscreen angle, I completely agree. The only thing the deflector is doing is (maybe) reducing hum noise. But it's definitely cupping the air at the box/deflector interface.

 

[Eleeski]

@cantunamunch Did you get your fancy low drag roof box built up?

Eric

 

[cantunamunch]

@cantunamunch Did you get your fancy low drag roof box built up?

Eric

Yes...and 2 days later the car got deered.

The new setup is a lot lower to the roof, so the parachute effect isn't as great, plus the windscreen is a lot steeper. The new deflector setup is going to look a lot more like the Armstrong-era support car deflectors - C-shaped with an extension fence around the side, all the way to the first crossbar.

 

[DanoT]

My old Toyota Celica Alltrac would burn an extra mpg with the box on but the worst part was how it impacted the handling. It turned a light weight low centre of gravity full time AWD car that was amazing on compacted snow on the highway into something that was white knuckle scary.

I went with a scissor type ski rack on a Yakima round tube crossbar.that allowed the scissor rack to be flipped down into a horizontal position when not in use.

Nowadays I drive a Volvo XC70 wagon, so the car is the ski box.

 

[Eleeski]

Yes...and 2 days later the car got deered.

The new setup is a lot lower to the roof, so the parachute effect isn't as great, plus the windscreen is a lot steeper. The new deflector setup is going to look a lot more like the Armstrong-era support car deflectors - C-shaped with an extension fence around the side, all the way to the first crossbar.

Oh deer. Hope you learned and enjoyed the project. Better luck with the next one. Keep us posted.

Eric

 

[graham418]

Didn't I see this in the medal winning bobsled run?

 

[John Webb]

The wind deflector or fairing gets the wind to pass up and over the crossbars to improve airflow and reduce the whistling noises. This may work when you have a ski or bike rack up there. But when there's a cargo box, I'm not sure the deflector does anything for you. I find that the presence of the cargo box deflects the air away from the crossbars so the deflector isn't really necessary. The questions would be whether it increases or decreases your efficiency. My bet would be that it makes the setup less efficient but by a negligible amount.

Interesting-you may be right . . I haven't tried my Yakima box yet to reduce rack noise.,

I find the whistling noise over the round front Yakima rack bar to be totally intolerable at 50mph (even to my weak hearing)

Can't see dropping $70 or more for their simple plate wind deflector so I've tried a few things with out luck. Next I may try a length of foam pipe insulation .
A guy at my work did wind tunnel acoustic experiments found oscillation patterns moving back and forth along a round cylinder - very similar to this problem.
An airfoil shape like the premium Yakima rack or a fairing like an airplane wing solves the problem.

So meanwhile I'm not using the roof box or my 4 newest ski racks. Only the 40 year old one that works on vehicles without rain gutters.

 

[cantunamunch]

Interesting-you may be right . . I haven't tried my Yakima box yet to reduce rack noise.,

I find the whistling noise over the round front Yakima rack bar to be totally intolerable at 50mph (even to my weak hearing)

Can't see dropping $70 or more for their simple plate wind deflector so I've tried a few things with out luck. Next I may try a length of foam pipe insulation .

Have you tried taking pieces of foam sheet and folding them over the bar, then joining the ends on the back side? It will give you a pseudo-airfoil shape.

Roofing felt, EVA, Coroplast... could all work to create:

An airfoil shape like the premium Yakima rack or a fairing like an airplane wing solves the problem..