How much will my MPG be reduced with plywood on a roof rack?

[One Awesome Inch]

I like the idea of putting a fulllsized 4x8 plywood on to my roof attached via roof rack. Painted white it will reduce the heat in the van in the one month of hot we get here in the pnw. It will also hide things on the roof such as a vent and solar panel... if or when i choose to go that route.

My main concern is that while driving, and especially on the highway, the plywood will cause the van's roof to be less aerodynamic thus resulting in poorer gas mileage.

The question how worse will the MPG be?

 

[mockturtle]

I think I'd be more concerned about handling being affected driving in the wind.

 

[flying kurbmaster]

You could possibly improve your mpg. The flat roofs, square sides and square backs are an aero hindrance, more so when things stick out like mirrors, vents etc. Start looking at the shapes on modern high mileage cars like the Ford Focus, Prius or others like these. You will notice that the roofs are rounded the sides curve in at the back, even the bottom of these cars turn up. The optimum turning in angle is between 12 and 15 degrees, if you were to continue those lines  30 feet back  they eventually come to a point releasing the air that you are dragging smoothly, of course having a car that had a 30 foot tail would be impractical so they try to follow those lines as much as they can. If you used two sheets of 1/4 inch ply secured one to your rack bent down the front to touch your roof then did the same at the back extending past your drip rail about 6 to 8 inches rounding the corners, ( notice the cambacks on modern cars) keeping in mind the 12 to 15 degree optimum. you would have to figure out a way to hold it in place perhaps with brackets to your drip rails, layer a second sheet of 1/4 ply over this one, gluing to and securing it with the same brackets, crudely replicating the round roofs of these modern cars, ideally you would fill in the sides with a rounded filler that would go to your gutters, you could use coroplast to this, when installing your solar panels it would be best to cut out the holes in the plywood and install them flush, this could??? improve your mpg not by much but much better then making it worse. If you are driving in the city this will do nothing, keeping your weight down is better then aero at low speeds. An air dam under your front bumper would also help. Here is a picture of my Toyota with aero work wich improve the mpg by more then 10%. blocked grill, belly pan , fender skirts, and camback

 

[akrvbob]

I'm sorry but I never did a before and after comparison, so I can't really help. But I can say I'm unaware of any negative handling problems with either my trailer or van. The roof rack and plywood on the trailer are a full foot above the roof and my trailer is NEVER a problem to tow. Although to be fair I don't tow it a lot, no more than a 1000 miles a year.

I don't have plywood on the van but it does have a 240 and a 140 watt panel so it's covered a lot. Never noticed any handling problems from it.
Bob

 

[GotSmart]

I have done a fair amount of construction... which means wood on the rack.

First. The wood high enough to allow for a vent will mean it must be fairly high. That changes everything with aerodynamics of the brick. Just a piece of painted plywood on the rack will draw attention. It also has less rigidity than it it has panels bolted to it. It will give you a boat rocking feeling in higher winds.

Next, I cut my wood down to 4 x 6, so it does not stick out as far and bent up when going down the road. Also if I add more panels, I can put up to 3 100w units on that size.

Try to center it, as any weight off center magnifies itself at greater heights. (Fulcrum principle)

When I installed a vent, my interior temperature dropped a lot. When I put up the wood, it dropped again. Would it be worth it for you in your situation? in my opinion not unless you will be spending much more time where it would make a difference for a longer period of time.

I believe my gas mileage dropped about 7% when I put up the rack. It did not change when I put up the wood and panel. The wood is only about 10 inches past the rack front and back.

From my experience driving with wood on racks, the longer it is makes a major difference in handling. If it is not centered also makes a difference.

Good luck.

 

[One Awesome Inch]

Very interesting!

I would cut a hole thru the ply so the vent is able to open.

If I were to pursue this I would likely see if I could build my own rack. Perhaps buy the rail clamps off ebay etc, then just bolt together those aluminum "tubes" you can get at home depot. Racks are expensive so this would likely be alot cheaper.

In the end, I am not sure if the whole endeavour is worth it since its only hot here a month or so out of the year. I could always aim to park underground during the day as that would likely help significantly.

I guess I'll know more once I move into the van in July.

 

[MikeRuth]

FYI a decent two bar rack can be had for less than a $100.00. Mine was $75.00.
I did notice that with the rack up by it's self that it made a lot of noise. Now that I put the panels up the noise is gone!I have about 3" clearance now between panels and roof.
Have to say I was blown away that it got quieter. I haven't had a chance to check MPG and frankly I'm not all that concerned as the MPG is gonna be what its gonna be. I may add a fairing.

 

[c_hasbeen]

One company I worked for gave me a company pickup, it had a rack on the back of it to carry ladders and such, once it was not needed it was taken off and put on a differeent rig.
I was amazed that after removing the rack it's self that I was getting 1.5 mpg better than with the rack on it.
I never dd the actual mpg measure by counting miles and gallons, but based off the onboard computer that showed the average MPG.
I have to think the addition of his ply wood would cut things even more, I never had anything on my rack to compare the MPG loded or not so I can only make a best guess.

 

[karl]

The worst things I ever carried on my racks were ladders. They were would whine and scream at certain speeds, and multiple ladders affected handling. Heavier loads of framing stock was quieter and handled as well if not better. Never carried plywood on the racks, but would guess that one sheet might even reduce the turbulence caused by the two or three racks. Doubt that it would be any worse than one or more panels, especially if it is back a few feet from the windshield so that the air stream evens out ahead of the plywood.

Gotta add- I think buying racks will turn out cheaper than creating your own if you have to buy the stock.

 

[Matt71]

Based on my limited physics knowledge I'd say a flat board is not likely to have nearly a negative effect as something oddly shaped as a ladder which is going to throw the air everywhere. I'm also not convinced that curving the plywood will have much effect unless you can make it perfectly flush with the roof of the van. In fact, I'd be worried that the wind would catch on the arch like a hook and rip the hole thing off the roof. I think a flat piece of plywood on the rails would be better as the wind could got straight through under and over the board like a spoiler on the back of a car. A better option might be to add a point to the front of the board to cut through the winds like this ======> rather than this =======\

But my physics knowledge is limited, so what do I know?

 

[Seeker]

Still waiting to find out how my highway MPG with the added racks and solar panel has dropped.  Adding the panel did quite my cargo racks somewhat.  I may have to silicone where racks slide together per RealTruck.com to waterproof and quite racks further.  On occasion at around 20 MPH I get some kind of harmonic vibration on front rack that vanishes at higher or slower speeds.

I bought 2 Cross Tread racks on Craigslist for $50.00 bucks.  I wanted a matching 3rd rack and had to pay over double that for the matching rack.
A SEEKER

 

[Janet100]

I've been wondering about this very thing. I definitely need a sheet of plywood on a rack to keep the van cool enough to house my dog in during the day - I'm in No Cal. And then I would mount solar panels on the top.

But I wondered if driving down the freeway the wind would not lift that plywood off the van. Those clamps don't look very strong to me. I could see where the wind would get into the 3 inch space and pry up the front of the board (and warp the solar panel). I tend to drive at 70 mph.

Do you guys feel the lifting when you drive? Or are you driving at 45-50 mph?

 

[Janet100]

I've been wondering about this very thing. I definitely need a sheet of plywood on a rack to keep the van cool enough to house my dog in during the day - I'm in No Cal. And then I would mount solar panels on the top.

But I wondered if driving down the freeway the wind would not lift that plywood off the van. Those clamps don't look very strong to me. I could see where the wind would get into the 3 inch space and pry up the front of the board (and warp the solar panel). I tend to drive at 70 mph.

Do you guys feel the lifting when you drive? Or are you driving at 45-50 mph?

 

[Optimistic Paranoid]

Personally, I would mount two or three long 2X4s lengthwise on the racks.  The plywood would be screwed down solidly to the 2X4s for the entire length.  The 2X4s would be spaced so the solar panel mounts screwed into them, and not just the plywood.

Regards
John

 

[Almost There]

I've been wondering about this very thing.  I definitely need a sheet of plywood on a rack to keep the van cool enough to house my dog in during the day - I'm in No Cal.  And then I would mount solar panels on the top.

But I wondered if driving down the freeway the wind would not lift that plywood off the van.  Those clamps don't look very strong to me.  I could see where the wind would get into the 3 inch space and pry up the front of the board (and warp the solar panel).  I tend to drive at 70 mph.

Do you guys feel the lifting when you drive?  Or are you driving at 45-50 mph?

A piece of plywood won't do a whole lot to keep a van cool enough to house a dog.

There's a lot of information here on this site about keeping a van cool, but short of being plugged in to shore power with an A/C unit running, there is no way to keep a closed van cool enough for several hours during a sunny day.

There's also lots of information out there on the web about the effects of radiant heating on a vehicle and what the temperature rise is in a closed vehicle.

IIRC you're contemplating the move to stealth vanning to shorten your commute during the week. If that also includes bringing a dog along with you, please, please study up on the whole heat/van/dog situation before you make up your mind.

 

[highdesertranger]

well I have plywood on my rack. can't really say about mpg never measured without the rack. it does make my truck a lot cooler, you see I don't have ac and am regularly in the desert in the summer. with out the plywood I don't think it would be possible. highdesertranger

 

[Almost There]

well I have plywood on my rack.  can't really say about mpg never measured without the rack.  it does make my truck a lot cooler,  you see I don't have ac and am regularly in the desert in the summer.  with out the plywood I don't think it would be possible.  highdesertranger

Yes, but are you sitting inside the van during the heat of the day even with the windows cracked, the roof vent open and the vent fan going?

 

[akrvbob]

I have a ladder rack and plywood on my cargo trailer and a ladder rack with two solar panels and a 8 foot step ladder on my extended van.

I've never noticed any negative affects on the van or trailer in any way from the ladder racks. Except noise, they do create some noise.

Let me repeat, no negative affects in my real world experience.

Bob

 

[akrvbob]

So here is a photo of my van-trailer and shade-cloth. You can see the ladder rack-plywood on the trailer and a little but of it on the van. 

The shade cloth  works extremely well to cool the van-trailer. It's 8x16 and the trailer is 6x10 so it wraps around 3 feet of the front and back. I also hung a 10x16 between the trailer and van and it's always shaded cooling both the trailer and van. 

I know it's counter-intuitive but black shade-cloth works better than light shade-cloth. There is a second piece under the black one on the trailer. The wind still blows right through it giving me a draft through the windows but no heat comes in from the windows.

The Adco van cover on the windshield is 100 times better than Reflectix, It keeps it much cooler.

The bottom line is it stays outside temperature on the inside and with a fan that's quite comfortable. 

 

[GrantRobertson]

I know it's counter-intuitive but black shade-cloth works better than light shade-cloth.

Bob, I'm sure you know this, but for the edification of future readers: This is because:

A) The black material absorbs more of the sunlight, so less ends up striking the vehicle itself.

B) The black material, therefore, gets more hot, which causes the air around it to rise, creating a convection current that draws cool air up from the ground and takes hot air up and away.

Pro Tip: Keep the top of the black shade cloth a few inches away from the vehicle at the top. This lets that column of hot air escape through the gap. If the shade cloth "seals" across the top by laying directly on the top of the vehicle, then it will trap at least some of the hot air against the vehicle with expected results. Yes, this applies even if that shade cloth is mesh. Mesh doesn't let air flow through it freely, as can be observed by just watching it being blown around in the wind.