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2014, 138WB, High Roof, Gas, SW MT
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Discussion Starter · #1 · (Edited)
Hi,
I finally got around to do doing an MPG test for my roof mounted PV panel, and also threw in an MPG test of a full size roof rack.
The details from my site are pasted in below -- the web page is...

Just so you don't have to read the whole thing - here are the results...
The average MPG with full roof rack was18.75 MPG
The average MPG with PV panel was 21.1 MPG
The average MPG for the bare roof van was 21.2 MPG

Van Setup
Simulated Roof Rack
I had planned to just run the MPG test with my current large PV panel against the van without PV panel. But, I figured since I was going to this much effort, might as well test a larger roof rack as well. So, I made a roof rack out of scrap lumber so it could be compared to the PV panel on roof. In all cases my Maxfan was on the roof.
  • Simulated Roof Rack

  • Rack over Maxfan

  • the door slab simulates a PV panel

I know this looks kind of goofy, but its what I could do with scrap lumber I had in hand to simulate a roof rack. The idea is to get about the same frontal area and shape of some existing commercial racks I’ve seen. The door slab mounted on the rack simulates a PV panel that might be mounted on the rack. The rack is 4.5 inches tall by 61 inches wide by 118 inches long. Its held down to the van by ropes at each corner for easy removal with no holes.
The roof rack increases the frontal area of the van by about 4.5%, but maybe more importantly, it increases the drag coefficient of the van — both contribute to increased drag and lower MPG.


The highly sophisticated roof rack mounting method (rope).

PV Panel
This the the large single PV panel that I’ve had mounted on my van since 2014.

The panel is 78 inches long by 39 inches wide by 1 3/4 inches thick. I tried to mount it for low aero drag — that is, lengthwise behind the fan and nearly touching the van roof. Details here…

Bare Roof
The final configuration tested was the van with just the Maxfan installed, and the roof bare otherwise. It would have been nice to remove the Maxfan, but too much work for this time.

Test Procedure
All of the testing was done over about three hours on a low wind, sunny day.
Tires were inflated to 65 psi cold – tire pressure was up to 70 psi for actual test. The van and tires were warmed with an I90 run of 20+ miles before the start of testing. Wind speed was down around 2 mph, and since each run had an east to west followed by a west to east segment, wind effects should be minimal.
Each run consisted of a 65.2 MPH run west on I90 between two mile markers, followed by a run east between the same mile markers at the same speed. The PM Trip Computer was used to record the MPG between the mile markers. The length of the run in each direction was 12 miles. The cruise control was used during the full run.
The I90 section used had some flat areas and some modest hills. The van shifted down from 6th gear to 5th gear for short periods on a couple of the small hills. Pretty typical interstate highway driving.
A full run consisted of these steps…
  • Enter I90 at interchange 288 (Manhattan, MT) going west
  • “resume” cruise control at 65.2 miles per hour (or set at 65.2 if old setting was lost)
  • As mile marker 288 is passed, reset Trip A computer.
  • Drive with as little interference as possible to end mile marker (276) – 12 miles in all.
  • Note Average MPG on Trip A while passing mile marker 276
  • Exit at interchange 274 (hwy 287)
  • Go across I90 and enter I90 going east
  • “resume” cruise control at 65.2 miles per hour
  • As mile marker 275 is passed, reset Trip A computer.
  • Drive with as little interference as possible — 12 miles in all.
  • Note Average MPG on Trip A at mile marker 288.
  • Exit I90 at interchange 288.
Each of the three configurations were tested with two of these runs on I90 – six runs total for 144 miles total.
After the first two runs with the full roof rack, I pulled off the road at exit 288, pulled the roof rack off, and stowed it in the van. This left the van with the roof fan and my regular PV panel (39 by 78 inches). I then did two identical runs with the PV panel. I pulled off the road again and removed the PV panel, so that the only thing left on the roof was the roof fan. I then did two more identical runs in this configuration. About 3 hours total. I only had to reset the cruise control once during the whole process, and got it back to 65.2 MPH using the digital readout on my OBD app.
Since all the racks were stowed in the van, there was no change in van weight over the test except for a few pounds of gasoline used. In reality, the large roof rack would also suffer a small hit in MPG due to the weight of the rack – maybe about 0.1 MPG for a 100 lb rack.
I think this works out pretty well as a test method. Resetting the Trip Meter at the start mile marker while already having established stable cruise control speed, and then noting the Trip Average MPG while passing the end mile marker takes away the inconsistencies that would come from including the the less consistent acceleration and deceleration at the start and end of each run. On the previous MPG test for the vortex generators, I also did very careful gas fillups after each two runs as a check on the trip computer, but the two agreed well, so I did not do the fillups this time.
Of course the trip computer on most ProMasters tends to be a bit optimistic, but this does not matter when we are only comparing runs with and without the racks. In the past, when I’ve tested my trip computer, it has been about 0.5 MPG optimistic.

Results
The average MPG with full roof rack was18.75 MPG
The average MPG with PV panel was 21.1 MPG
The average MPG with no paels was 21.2 MPG

So, the full roof rack was 2.5 MPG worse than the roof with only the Maxfan on it.
Over a 200,000 mile van life, this is 1233 gallons extra gas, or at $3 per gallon $3700 in gas, and 12 tons more of CO2 emissions.
About a 12% decrease of MPG compared to the roof with only the Maxfan on it.

The PV panel was 0.1 MPG worse than the roof with only the fan on it.
Over 200,000 mile van life, this is 45 gallons extra gas, or at $3 per gallon $135 in gas, and about half a ton more of CO2 emissions.


The drag penalty for the PV panel over the roof with just the fan seems small to me. I guess its possible that mounting it low to the roof, far back on the roof, with long axis parallel to the van, and in the drag shadow of the fan all worked splendidly and the drag increment is really only 0.1 MPG. Or, maybe the random variations from run to run added up to favor the PV panel. Or, maybe some of each?
It would have been nice to test the bare roof with no fan — maybe some day

These are the run by run MPG numbers…
Roof Rack 1Roof Rack 2PV 1PV 2Bare 1Bare 2
West run18.319.722.621.923.421.8
East Run18.518.519.620.219.420.1
Miles Per Gallon readings for all 6 West runs and all 6 East runs

I could not find a lot out there on the internet about previous drag tests of roof racks — if you know of any material, please let me know.

This paper: Impact of vehicle add-ons on energy consumption and greenhouse gas emissions Harun Chowdhury*, Firoz Alam, Iftekhar Khan, Victor Djamovski, Simon Watkins
Shows wind tunnel results for various roof mounted stuff, including roof racks. It shows a 20% increase in vehicle drag for one roof rack configuration — substantially more than our test.

Another consideration is the effect of speed. These tests were done at 65.2 MPH. If you speed up to 80 MPH, the drag force goes up by the square of velocity, so it would be up by a factor of (80/65.2)^2 = 1.5. If you assume that at 65 MPH, the aero drag makes up 70% of the total resistance (tire rolling resistance being most of the rest), then you would expect total resistance on speeding up to 80 MPH to go up by a factor of 1.35 and the MPG to go down by the same factor – maybe 14 MPG. On the other hand, if you slow down, MPG improves just as rapidly.

Thoughts on Roof Rack Drag
It seems that just from this little test you could conclude that …
  • Roof racks can have a significant impact on van MPG
  • The details of how you build a rack and how you mount stuff to the roof can make a large difference in the MPG penalty.
For mounting PV panels, clearly the lowest drag way would be to use one of the flexible PV panels attached to the van roof with adhesive. The MPG penalty for this would likely be near zero.

It appears from the test that the mounting of my single large panel close to the van roof, far back on the roof, behind the roof fan, and lengthwise rather than width wise achieved a pretty low MPG penalty.

If going for a full size roof rack, I think the details of how the rack is built could make a significant difference. Tubular racks with lots of open area should help. For the tubes that face the airstream, using elliptical tubing could help, as cylinders have a much high drag coefficient than elliptical tubes or airfoil shaped tubes. Some of the car manufacturers have picked up on this for car rack cross bars. See this video on the Yakima Whispbar.

It would be nice if one of the commercial roof rack producers would get into this, do some testing, and come out with low drag racks for the ProMaster and other like vans along with some test data to support how good it is.

Its tempting to think that doing without solar and just letting the van alternator charge the battery would save gasoline and energy, but you burn extra gasoline to generate energy with the alternator. VERY roughly: If you assume 1) 60 days of van use per year, 2) a need to generate 100 amp-hrs of battery charging per day, 3) an overall efficiency of 20% for the engine charging system, this leads to using using an extra 0.17 gallons of gasoline per day of use, or 10 gallons a year for 60 days of use, or 150 gallons for the 15 year life of the van. This is probably more than the extra gasoline used by a carefully mounted PV panel capable of generating the same 100 amp-hrs per day — and, you have all the other advantages of solar.

It seems like the energy use and carbon emissions winner is the flexible PV panel glued to the van roof.

Any thoughts?


Gary
 

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Nice Test. My experience is far less mpg penalty for a roof rack with solar panels. I have posted the data before and you have discussed my data. Mine was not a scientific test, but involved many thousands of miles with just a maxxfan and then many thousands more miles with a rack and solar in addition to the maxxfan. I have the vantech H3 mounts and crossbars. These crossbars are somewhat aerodynamic similar to the Yakima Whispbar you cite above. I also have the crossbars mounted in the back 1/3 of the roof of my 159 EXT. with only the maxxfan up front. My MPG penalty has been consistently around .5mpg, about 17.8 vs 17.3, calculated (not using the trip meter). Even though not scientifically performed, because I had about 19,000 miles before and another 20,000 after the install of the rack, I think it has some validity. My driving style and conditions don't change much and my results are very consistent over these many miles.


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Thank You Very Much @GaryBIS

I found your report on your testing well done & very well explained.

It certainly makes a great case for a low profile (or flex solar panel glued to the roof) install for solar & details the mileage penalty for a roof rack 12% is a significant fuel efficiency loss.

If glueing flex panels on the roof would it be best to block the valleys of the corrugated roof to force the air flow over top, or leave them open to allow the relative air flow under the panels. Would you be worried about “flutter” of these flex panels?

Other than reduced milage, my only other major concern with roof mount solar is the potential of heat gain in the interior of the van.

The alternator fuel burn vs the roof mounted solar panel extra fuel burn is a bit of a tricky in “real world” scenarios. In the case of the alternator, one only burns additional gas when using the alternator to charge, and in the case of hard mounted solar panels, one is always burning extra fuel with every mile driven. So if the common trip is to drive little and camp for days without moving the solar panels (even rack mounted) would be a winner when the sun is shining. In a scenario where one drives many many miles and rarely sees the sun, then I would see the alternator as the winner.

A big advantage solar has over the alternator (if there is sun), is in these newer solar chargers that are very programable. 3 or 4 phase chargers that can be setup with proper charge profiles custom programed to look after the batteries.

Thanks again Gary - Much Appreciate !!
 

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2014, 138WB, High Roof, Gas, SW MT
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Discussion Starter · #4 ·
Nice Test. My experience is far less mpg penalty for a roof rack with solar panels. I have posted the data before and you have discussed my data. Mine was not a scientific test, but involved many thousands of miles with just a maxxfan and then many thousands more miles with a rack and solar in addition to the maxxfan. I have the vantech H3 mounts and crossbars. These crossbars are somewhat aerodynamic similar to the Yakima Whispbar you cite above. I also have the crossbars mounted in the back 1/3 of the roof of my 159 EXT. with only the maxxfan up front. My MPG penalty has been consistently around .5mpg, about 17.8 vs 17.3, calculated (not using the trip meter). Even though not scientifically performed, because I had about 19,000 miles before and another 20,000 after the install of the rack, I think it has some validity. My driving style and conditions don't change much and my results are very consistent over these many miles.


View attachment 74445
Hi tg,
That does seem like another good data point that solar panels mounted carefully don't have to have a large to have a large MPG penalty.
On my panel mounting, I tried to get them down low with mounts that have minimal frontal area. Yours use more aerodynamic shaped rails and that appear to work well too.

I do think that having our panels toward the back and behind the roof fan also helps.

Gary
 

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I agree with Gary. My ‘14 had the panels mounted quite low to the room and I typically got 16.5/17 mpg. On my ‘20 they are significantly higher and I typically get 15/15.5 mpg but I do tend to drive 75/80 on the highway most of the time. My panels are located far back behind my roof vent but I’ve been thinking of lowering them a bit, but then again…🤔
 

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My panels are actually pretty high. The mount and crossbars are about 5-6 inches above the roof. My panels are mounted on top of the crossbars. I tend to go about 65-70mph, and occasionally 75mph. So the speed appears to be a major factor in the mpg penalty. The crossbar shape probably is a factor as well.


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Thanks for another informative experiment, Gary! As you noted, fore-aft placement is important, too. I've always wondered how much the airflow (wind tunnel) pattern along the roof affects drag from solar panels, etc. Although my panels are mounted pretty low, the leading edge could not be further forward. I've been getting about 21mpg on highway trips, so it doesn't seems to be too detrimental.
 

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Thanks for another informative experiment, Gary! As you noted, fore-aft placement is important, too. I've always wondered how much the airflow (wind tunnel) pattern along the roof affects drag from solar panels, etc. Although my panels are mounted pretty low, the leading edge could not be further forward. I've been getting about 21mpg on highway trips, so it doesn't seems to be too detrimental.
21mpg seems pretty good. Since you're in Canada, I have to ask if that is US gallons or Imperial gallons?
 

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Dennis: are you saying you have a regular hit van that you put a top on it that cranks up to give you additional inside height. If so,
What are the pop up sides made out of and what is the heat loss from those sides.. I poly get about 19 mpg on my 2019 136" wheel base high top with fan housing.
 

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If no one reports their mph then the mpg quote is useless.

Down hill doesn't count :)
 

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I've mentioned a couple of times that there is something of a low pressure area in the center of the roof. This explains why convertible roofs tend to balloon upwards at high speeds. And of course why airplanes fly, although airfoils are a bit more refined aerodynamically than vans. :) My point is that I'm not the slightest bit surprised about the result with the solar panel, as it sits in the low pressure area.

I just did a similar roof rack test on my lifted Scion xB, as I'm going to be taking it on an long overlanding trip in Nevada, and need to carry a full size spare. I felt the loss of power at highway speeds quite dramatically (it has all of 103 hp), so I did a similar freeway comparison, and found that the rack was reducing mileage between 4-6 mpg. At 75, it dropped from 28 to 22. At 65, it was about 3-4 mpg, meaning almost the exact same percentage drop (some 12%). Not a coincidence, I have to assume.
74617
 

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I had a Yakama 21 c ft car top carrier which is areodynamically designed on a Merceds Benz and noticed a less than 1 mile per gallon fuel drop. I averaged from the low to mid-twenties on expresway driving.
 

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Dennis: are you saying you have a regular hit van that you put a top on it that cranks up to give you additional inside height. If so,
What are the pop up sides made out of and what is the heat loss from those sides.. I poly get about 19 mpg on my 2019 136" wheel base high top with fan housing.
I purchased a low top 136 and had Sportsmobile in Fresno California add the pop top. It goes up about 30 inches. The top has some heat loss, so it's really for warmer weather (above 50 F.) I did use it one time with the top up in really cold weather. My heater was able to keep it at 50 F inside when it was 7 degrees outside. The top is made of the same fabric that they use for convertible car tops. Here is a video tour I made for the sale:
 

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Thanks, Gary, for another great experiment and interesting read!

Your results "feel right" to me. I think the location of your solar panel more towards the rear helps keep the "just solar panel" scenario closer to the original (no rack, no solar panel) scenario. The solar panel there would fall in the "shadow" of the airstream as some (much?) of it would get deflected up and over the rear of the van (this is just a guess).

I'm saddened that your rack and panel scenario had such a large decrease in fuel economy. Again, your results "feel right" because I have a large and ugly ("ugly" from both an esthetics and aerodynamics point of view...) solar array up top that has at least a 1-inch clearance from the roof so that air can circulate and keep the hot solar panels from heating up the van as much. So I have the worst-case scenario--very UNaerodynamic solar panels right up to the front of the van (not quite right up front but pretty close (3 x 200W panels, mounted as far back as I could, but with 3 panels, it gets very far up towards the front)), mounted somewhat high up and going nearly all the way across side to side.

I am stunned at the great fuel economy you all get. My fuelly(dot com) tracker says I get sub 15 mpg (US gallons) despite driving not that hard. Though in fairness to my solar array, part of the problem comes from 2 other things (I think):

1) desire to get a smooth ride so tire pressures were reduced to 60 psi on all 4 tires. I think this is a somewhat big contributor; I might increase to 65 all around.
2) lots of city driving sprinkled in with the majority that is highway. I travel long distances so my driving is technically mostly highway, but when I get to a city or town, I look around a fair bit so city speeds for sometimes days. Maybe some of you with great gas mileage go highway to campsite and then back home? (I'm just guessing and am curious).

Fairing. An even sadder point is your angled piece of wood not seemingly able to help. I was going to make something like that in hopes it would make a noticeable effect on my mileage. Maybe it will. Maybe my combination of higher tire pressures and the fairing might bump up my mpg to 17 or so, in which case I'd be happy. (After all, nothing in this world is for free...especially 600W of solar...)
 
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Discussion Starter · #20 ·
Thanks, Gary, for another great experiment and interesting read!

Your results "feel right" to me. I think the location of your solar panel more towards the rear helps keep the "just solar panel" scenario closer to the original (no rack, no solar panel) scenario. The solar panel there would fall in the "shadow" of the airstream as some (much?) of it would get deflected up and over the rear of the van (this is just a guess).

I'm saddened that your rack and panel scenario had such a large decrease in fuel economy. Again, your results "feel right" because I have a large and ugly ("ugly" from both an esthetics and aerodynamics point of view...) solar array up top that has at least a 1-inch clearance from the roof so that air can circulate and keep the hot solar panels from heating up the van as much. So I have the worst-case scenario--very UNaerodynamic solar panels right up to the front of the van (not quite right up front but pretty close (3 x 200W panels, mounted as far back as I could, but with 3 panels, it gets very far up towards the front)), mounted somewhat high up and going nearly all the way across side to side.

I am stunned at the great fuel economy you all get. My fuelly(dot com) tracker says I get sub 15 mpg (US gallons) despite driving not that hard. Though in fairness to my solar array, part of the problem comes from 2 other things (I think):

1) desire to get a smooth ride so tire pressures were reduced to 60 psi on all 4 tires. I think this is a somewhat big contributor; I might increase to 65 all around.
2) lots of city driving sprinkled in with the majority that is highway. I travel long distances so my driving is technically mostly highway, but when I get to a city or town, I look around a fair bit so city speeds for sometimes days. Maybe some of you with great gas mileage go highway to campsite and then back home? (I'm just guessing and am curious).

Fairing. An even sadder point is your angled piece of wood not seemingly able to help. I was going to make something like that in hopes it would make a noticeable effect on my mileage. Maybe it will. Maybe my combination of higher tire pressures and the fairing might bump up my mpg to 17 or so, in which case I'd be happy. (After all, nothing in this world is for free...especially 600W of solar...)
Hi Travelvanvan,
When you look at the MPG on the test, bear in mind that these are roughly 10 mile stretches on a part of I90 that has gentle up/down grades mixed with flats done at 65 MPH and 65 psi tires, so, pretty easy as MPG goes. They are done by resetting Trip A at the start mile marker and reading the average MPG at the end mile marker, to the optimizium of the MPG computer is included in the numbers (about 0.5 MPG on my van).
On real trips we get around 19.5 MPG based on real fill-ups.

It would be nice if a few people would do some baseline MPG before adding a rack and then repeat after adding -- we might get some idea which kind of racks are best MPG wise.

The slopped fairing at the front may indeed help some -- The paper I mention above reports one rack wind tunnel test that showed a 20% hit!

It would be nice to test a rack made from tubes where all the tubes that face the airflow are elliptical. This would minimize frontal area increase, and the drag coefficient of the streamlined tubes might be lower than the van Cd.
Maybe @RV8R could shed some light on this from streamlined wing struts?
It might even be worth putting a bullnose on the leading edge of the PV panels.
 
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