How much do Roof rack baskets affect MPG

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Tozan

Tozan

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As long as the leading edge of what's stored on the roof rack isn't a square box shape, the MPG will not be much of a difference.

Get some peel -n- stick vortex generators along the front roof edge may "spill" the air better. If you worry about aero- caused MPG.

View attachment 228631

I will have to check into the vortex generators... My aerodynamic design experience is in motorcycles not automotive but, some things do relate... lol I am running three custom made wing shaped cormoly cross bars. From what I understand they were wing spars in their first life.

I am getting ready to install a 270 degree awning and was thinking of putting a nose cone in front of it but I don't think it will actually make a difference since the laminar flow is most likely gone by the time it gets there.
 

mountie

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I will have to check into the vortex generators... My aerodynamic design experience is in motorcycles not automotive but, some things do relate... lol I am running three custom made wing shaped cormoly cross bars. From what I understand they were wing spars in their first life.

I am getting ready to install a 270 degree awning and was thinking of putting a nose cone in front of it but I don't think it will actually make a difference since the laminar flow is most likely gone by the time it gets there.

Aero is effective, from what happens to the air flow AFTER the 'front-most' leading edge. I remember those stainless wings on top of cab roofs, in front of a box compartment that is a flat front surface...... They actually were useless. The air fell down behind the wing. But they looked cool.
( example photo to show wing on my truck...... but most wings were more ahead of box)

Wendt truck cab.jpg
 
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Tozan

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Aero is effective, from what happens to the air flow AFTER the 'front-most' leading edge.
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I agree, one of the cycle magazines did an article on what I was doing with aerodynamics in 1997 we set 4 land speed records and proved cleaning up the airflow in the back is very important. We improved the top end of a bike by 19 mph by changing just aerodynamics.

Problem I see with roof racks is they dirty up the air so badly it may not be possible to clean it up again in such a short area. If you moved the rack to the front and left the last two or three feet without a rack and added the VG's it might work better. But with a stock set up the rack would actually be covering the VG.

I think it would help on a Tahoe without a rack.
 

wjburken

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I don’t think the vortex generators will provide much benefit if you have the rack on as the rack will already shift any laminar flow to turbulent flow with the rack itself. Turbulent flow is able to “handle” bigger flow change gradients than laminar flow. That’s why they put them on the rear of cars. They reduce the suction drag by letting the turbulent air roll down the back better than laminar flow. Same reason they put retractable vortex generators on the leading edges of wings so at high angles of attack, when laminar flow would separate and cause a stall condition, the turbulent flow follows and still provides lift.

I think a deflector mounted on the rear of the rack to pull some air down the back window would be more effective than the vortex generators. At the end of the day, you have a vehicle that has a large frontal area with a lot of drag points so unless you put a fully contoured faring over the rack to provide a clean top surface, there is not much to gain.

I would hazard a bet that if the roof rack had solid sides instead of bars, you might see an improvement as well, when it’s empty. The solid sides will allow a zone, or balloon, of turbulent air to be created within the rack providing a sort of buffer for the air coming over the windshield and act as sort of invisible faring and minimizing the drag caused by air hitting the bars.
 

mountie

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If a roof rack is needed...... I'd ignore the effect. Unless you drive all around the country on a budget.

Tozan...... I did the Bonneville Salt Flats a few times...... Crew for motorcycles & blown gas streamliner.

( The bike gathered over 20 records ) A lady rider!!
 

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