How much do Roof rack baskets affect MPG

[Tozan]

I am considering building a full roof rack basket on my Tahoe anyone have data on how many MPG I might lose from this?

I keep try to keep my MPG reasonable but I keep adding more stuff to it... lol Right now with all my modifications I only average between 14.5 to 19 My rig and two people weighs in at 6800 pounds right now.

 

[wjburken]

My rig and two people weighs in at 6800 pounds right now.

Is that before or after you clean all the mud off?

I would guess depending on what you do on the leading edge and how tall you make it, you shouldn’t be surprised to lose 1-2 MPG.

 

[swathdiver]

I think it's more like 4-5 mpg lost, a basket on the hitch will cost less, if any, loss in mpg. Won't look as cool and gets in the way of the back hatch. Too much stuff up top raises the center of gravity and will make her tipsy. Roof weight limit is about 200 pounds if memory serves.

 

[Tozan]

The hitch already has a spare tire on it. I don't really want any more stuff back there plus a rack on back would get knocked off, smashed or possibly get me hung up or stuck... I am mounting a 270 degree awning on it and was going to put a rack on so I could mount the awning to it... The other option is just weld up a mounting arm for it.

This is a daily driver but on weekends it is an Overlander for camping and exploring. Two weeks ago we did 3 hours off road at night with shifting into 4x4 low more than a few times before getting to our campsite. The next day we did another 50 miles to our next camp site.

 

[Tozan]

I am thinking of building the bottom of the basket so it is 1/2 off the roof unloaded and if it sits on it when loaded it should be no big deal. The side rails I am thinking will actually be sitting on the factory side rails to keep the overall profile real low. The front and rear bars will at best be about 4 or 5 inches above the roof surface maybe slightly higher than my custom cross bars I made. Hopefully this will keep my MPG close to what I was before...

As a side note I went from 315/70x17 tires to 35x12.5x17 weight difference between them was only 5 lbs per tire and width is actually a 1/4 inch less and the tire is only .2 in taller but, my MPG on the highway dropped by almost 2 MPG on the average... Funny how some things can make such a difference...

On the up side when off road most of this makes very little difference in MPG..

 

[swathdiver]

On the up side when off road most of this makes very little difference in MPG..

Same will be true for the roof top basket and all the stuff in it, just watch the weight so she don't keel over.

 

[Tozan]

I was walking by my Tahoe the other day and it dawned on me that the top of my hood is exactly shoulder high... lol It's not to top heavy with all the gear in it.

 

[Neill Campbell]

I am considering building a full roof rack basket on my Tahoe anyone have data on how many MPG I might lose from this?

I keep try to keep my MPG reasonable but I keep adding more stuff to it... lol Right now with all my modifications I only average between 14.5 to 19 My rig and two people weighs in at 6800 pounds right now.

View attachment 228327

I run a roof rack and often have up to 3 bikes on it. It makes nearly no difference in MPG. And those, while not heavy, produce way more drag than a basket with or without fairing.

As for wheel weight, rotational weight is far more impactful than non-rotational rate.

And while your new wheels may be more narrow they likely have a larger contact patch on the ground. That is where friction is created and rolling resistance.

So add the roof rack and go nuts with it.

Great looking rig. BTW, what else have you done to it? How much is it lifted?

 

[mountie]

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.

 

[Tozan]

I run a roof rack and often have up to 3 bikes on it. It makes nearly no difference in MPG. And those, while not heavy, produce way more drag than a basket with or without fairing.

As for wheel weight, rotational weight is far more impactful than non-rotational rate.

And while your new wheels may be more narrow they likely have a larger contact patch on the ground. That is where friction is created and rolling resistance.

So add the roof rack and go nuts with it.

Great looking rig. BTW, what else have you done to it? How much is it lifted?

Actually the new tires measure out to be less that the old ones but they are supposed to be 6# heavier. I will need to actually weigh a new tire to see what the real difference is. I think I am going to check into a set of racing rims to see if I can loose a few pounds there.

I am running a 4 in lift.

 

[Tozan]

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]

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)

 

[Tozan]

Aero is effective, from what happens to the air flow AFTER the 'front-most' leading edge.
View attachment 228653

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]

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]

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!!