Roof Rack Decision (looking at mounting Nissan Frontier racks on Sentra)

[Gone2]

How heavy of a hit do roof racks make on a vehicle if they have an air dam to divert the air over the bars? Is there any impact if the only thing on the vehicle is the side rails, until the rest is needed?

[Gone2]

Let me ask a more pointed question. I am wanting to install the roof rack from my totaled Frontier onto my Sentra. Yes it will take some resizing, anyway, back on point. I am debating on installing it and running some tests to see if I can keep my gas mileage the same (38.5~) and still have the rack. I am debating on moving the cross bars all the way forward so the air diverts over them and down the back of the car. I do have a need for a roof rack, more than less. What are your thoughts?

[3-Wheeler]

A picture of the roof rack would help....

Round tubes create a large wake, almost a large as a flat plate of the same dimension.

If you could shape winglets around each tube, well maybe that could minimize the effect of the rack up there.

There is a lot of high velocity air in that location, so making the tubes into airfoils would be something I would think about.

Jim.

[some_other_dave]

Just having the side rails will hurt your FE. It may not be enough to notice, but I have a gut feeling that it will. Having the cross-bars on will hurt even more. Having the air dam on will probably mitigate that impact to some extent, but likely the MPG will be worse than without any roof rack at all.

The only way to quantify things, though, is to try with and without.

-soD

[Gone2]

The fuel economy is the big deciding factor in this. I have a temporary rack I put on from time to time, but it would be nice to have it readily available. The tubes are very broad, so I probably won't, that and it will take major modifications to get it to fit. Anywho, I will see if I can find a difference of fuel economy for the trucks, and see how big it is. The pick is of the same type rack, not my rack.

[MetroMPG]

Back to the question:

Originally Posted by Sentraguy

to see if I can keep my gas mileage the same (38.5~) and still have the rack.

Short answer: not possible.

A roof rack adds to frontal area and increases the vehicle's drag coefficient. Even side rails alone will do this. So your fuel economy will decrease, and the question is: how much?

Answer is: it depends on the vehicle & the rack design.

FYI, I've "tested" (not vigorously; judge the data for yourself) racks on two vehicles:

Edit: Note the speeds of the tests; the faster you drive, the larger the % change will be.

1998 Pontiac Firefly (Geo Metro - 1.0L 3cyl - 5 spd manual)



@ 88 km/h (54.7 mph)

• 55.2 mpg (US) = no roof racks
• 48.2 mpg (US)(-7.0 mpg / -12.7%) empty roof racks (fixed rectangular cross bars)
• 40.1 mpg (US) (-15.1 mpg / -27.3%) roof racks with mountain bike

Details: Mini-experiment: the wrath of roof racks - MetroMPG.com

2005 Pontiac Montana (3.x L V6 automatic)



@ 88.5 km/h (55 mph)

33.7 mpg (US) - racks off (including side rails off)
32.8 mpg (US) - racks on (-0.9 mpg)

Details in this thread: http://ecomodder.com/forum/showthrea...-6-a-6069.html

---

I think these may represent extreme examples. If I had to guess, I bet your results would fall somewhere between the two.

So instead of modifying the racks to permanently attach them to the car, I would recommend designing some kind of mounting plate/hardware that you can get the racks on & off fairly easily.

[MetroMPG]

I will see if I can find a difference of fuel economy for the trucks, and see how big it is.

As you can see from the above examples, it's not as simple as comparing the truck with/without to your car. The impact of the same/similar racks on very different vehicles will be different.

It's too bad there's no easy answer!

[aerohead]

Originally Posted by Sentraguy

How heavy of a hit do roof racks make on a vehicle if they have an air dam to divert the air over the bars? Is there any impact if the only thing on the vehicle is the side rails, until the rest is needed?

If you had a ScanGauge you might alter the rack as your were thinking about,then attach it temporarily,test it,then decide.
I'm not sure how to think about the air dam itself.It would mean quite a frontal area increase.And that would be permanent.
I've seen many racks in which the crossbars were the lowest drag item on the vehicle.They're operating in the fastest air around the body due to the way the windshield divides the airstream,so their local drag is deceptively high,but might be lower than with the airdam.Maybe you can get 'results' without setting anything into cement.