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It would be nice if a few people would do some baseline MPG before adding a rack and then repeat after adding
So, not truly scientific, but here is my data from the book I write my expenses in, which includes gas as metered by the pump, and miles and mph recorded by the PM trip computer. (If for some reason I do not fill the tank full, I do not reset the computer, so some of the data points are for more than a tank)

The "before" data is from New York to Arizona via Florida.

The "after" data is from Arizona to Washington and back to Arizona via numerous National Parks.

I almost always avoid Interstates and major highways wherever possible, so most of the time I'll be below 60mph.

There are two panels, each approx 65"x39"x1.5", mounted across on 2" square aluminum tubes attached to DIY mounts also made of 2" aluminum tube. So the surface facing the wind is 65"x1.5" that is about 3"-4" above the roof.

Sky Cloud Vehicle Hood Motor vehicle


Sky Cloud Window Material property Composite material


Overall, for my travel lifestyle, any MPG 'penalty" from the solar panels is lost in the noise of the data.

In fact I actually had slightly better mileage with the panels!

"Before" I traveled 9429 miles on 560 gallons for 16.83 MPG
"After" I traveled 13708 miles on 807 gallons for 16.98 MPG !!!

Since I also recorded the MPH as reported by the trip computer, I can filter to only include data where the average speed was higher than 25 MPH. I really expected the "after" to be worse at higher average speeds, but in fact it did even better ... 17.70 vs 17.21 "before" :unsure:o_O

Obviously, there must actually be some MPG penalty from the panels, but the design I have combined with my travel style, I'm not seeing it.

Before
Datemigmphmpg
2/21/202118312.8631614.23
11/7/2020361.220.2831817.81
11/15/2020131.88.2151916.04
12/8/202025516.6471915.32
12/15/2020165.19.2111917.92
11/27/202034721.7732115.94
1/10/2021232.114.3932116.13
1/13/2021152.29.5362215.96
2/6/2021360.620.4422317.64
11/12/2020166.611.3752414.65
12/3/2020269.617.1772415.70
1/27/2021376.121.9522417.13
2/16/202132121.4852414.94
11/11/2020376.519.0572519.76
11/14/202032820.4472516.04
11/29/2020353.820.8452516.97
12/5/2020418.225.2662516.55
12/10/2020362.721.0672517.22
12/1/2020214.413.8212615.51
1/16/202131818.6182817.08
12/13/2020371.120.0372918.52
1/6/2021493.932.3542915.27
1/19/2021321.417.7362918.12
12/20/202029917.1763017.41
12/17/2020157.59.8573315.98
12/25/2020199.112.33316.19
12/30/2020430.624.033417.92
12/22/2020336.319.3413517.39
12/16/202027013.2823620.33
12/18/2020365.121.4293617.04
12/24/2020131.28.1044516.19
12/23/2020360.920.0664617.99
ALL9429560.18516.83
25+6107.7354.83317.21



After
Datemigmphmpg
10/28/20211118.3951613.22
4/25/202121713.8951715.62
8/21/202123616.4271714.37
4/2/2021216.515.6851813.80
6/4/202125616.3091815.70
6/27/2021240.115.6051815.39
7/4/2021376.821.9891917.14
9/5/202124915.1392016.45
3/28/2021289.518.012116.07
4/12/2021104.76.892115.20
5/6/2021321.518.562117.32
5/14/2021108.56.4282116.88
6/13/2021422.823.9512117.65
6/22/2021359.920.0232117.97
10/6/2021379.124.7642115.31
3/10/2021233.314.0722216.58
9/12/2021345.821.3462216.20
9/30/2021290.917.6472216.48
10/12/202131819.7952216.06
4/16/2021142.58.4822316.80
4/30/2021251.714.4092317.47
5/16/2021190.911.9022316.04
6/1/2021382.122.0882317.30
7/18/202138921.7782317.86
3/18/202133518.1232418.48
4/9/2021304.117.0562417.83
7/29/2021342.119.5882417.46
8/29/2021391.623.2132416.87
10/17/2021365.922.1242416.54
11/3/2021421.424.5832417.14
7/9/2021387.121.3212518.16
9/19/2021344.120.5872516.71
2/25/2021274.316.1862616.95
3/6/2021366.621.8712616.76
4/20/202121712.4512617.43
5/12/202130716.8932618.17
7/23/2021385.421.6262617.82
8/16/202157932.6652617.73
9/24/202142223.562617.91
5/22/2021399.921.9422718.23
8/7/2021302.317.0342717.75
10/24/2021364.621.7442916.77
8/2/2021350.317.9413119.53
11/7/2021312.918.1873217.20
5/17/2021103.44.9734420.79
ALL13708.6807.25716.98
25+5115.9288.98117.70
 

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MMXVI - L2H2 in Indiana
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It will be interesting to see the mpg results from the '23 SHR vs HR, without roof rack.
 

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It will be interesting to see the mpg results from the '23 SHR vs HR, without roof rack.
We can get a “rough” idea from Ford E-Transit range estimates which have low roof at 126, mid roof 116, and high roof 108 miles. I expect going from H2 to H3 roof a ProMaster gasoline van would take a similar hit, and probably more at steady highway speeds.

Applied to a van in range of +/- 17 MPG, that same energy increase per mile would suggest between 1 and 1-1/2 MPG less. It’s not exact but in similar range to difference between H1 and H2 ProMaster (Low versus High Roof).

The H3 will certainly have much more room, but will come at a price — not only fuel economy but also initial cost, handling, etc. It will be a tough choice if needing extra room.
 

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2014, 138WB, High Roof, Gas, SW MT
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Discussion Starter · #44 ·
The super high roof does offer an option to people who need a big roof rack to store stuff - they could put the stuff that would go on the roof rack inside the van, say in a space under the floor or a shelfish thing under the ceiling. No loss in headroom, but everything inside the van.
This might be aerodynamically better in that the SHR increases frontal area, but probably does not effect drag coefficient, while a big roof rack increases both drag and frontal area.

Unfortunately won't work for solar panels :)

I'm more tempted by the low roof than the super high roof. Maybe with an area of lowered floor for full head room like some of the Roadtreks. I wonder what MPG a low roof PM with no external clap trap gets on the highway?

Gary
 

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2019 118" Silver
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The super high roof does offer an option to people who need a big roof rack to store stuff - they could put the stuff that would go on the roof rack inside the van, say in a space under the floor or a shelfish thing under the ceiling. No loss in headroom, but everything inside the van.
This might be aerodynamically better in that the SHR increases frontal area, but probably does not effect drag coefficient, while a big roof rack increases both drag and frontal area.

Unfortunately won't work for solar panels :)

I'm more tempted by the low roof than the super high roof. Maybe with an area of lowered floor for full head room like some of the Roadtreks. I wonder what MPG a low roof PM with no external clap trap gets on the highway?

Gary
Internal ceiling storage makes a lot of sense for a really tall van but the height would limit clearance way too much for my use anyway.

When headwinds are not a factor my 'un-clap-trapped' and lightly loaded 118 low top gets 16 mpg around town, 18-19 mpg combined, and 19-21 mpg @70-75 mph highway. Down in the flatland of Florida, out through the Keys and back, I got 24 mpg doing 45-60 mph on back roads and local highways. When I towed a 5'x8' enclosed U-Haul cargo trailer 1000 miles with loaded weight over 2500 lbs, plus 1000+ lbs in the van (camping gear + cargo) I got 14-15 mpg from the extra weight and wind drag (headwinds over 1/2 the way). Maybe someone with a lightly loaded 136 with a clean top can chime in.
 

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I'm more tempted by the low roof than the super high roof. Maybe with an area of lowered floor for full head room like some of the Roadtreks. I wonder what MPG a low roof PM with no external clap trap gets on the highway?
Is your primary interest in a low roof van to improve fuel economy? I have been thinking a lot about the cost of added space, and have mixed feelings on whether it’s worth worrying excessively about it. For example, if I assume H1 vs H2 vs H3 over 100,000 miles at $4.00 per gallon, assuming 20 MPG vs 18 MPG vs 16 MPG, fuel cost would be $20,000 vs $22,200 vs $25,000 respectively.

Given that an Extended 159” WB Super High Roof is almost twice the volume of a 136” Low Roof, is added $5,000 in fuel over the life of the motorhome worth downsizing? Not to me when it represents much less than 10% of total ownership cost.

I dropped floor in my van quite a bit like classic TransVans of the past, and could do it even easier on a LR ProMaster, but I did not do it to save fuel, though it definitely helps. I did it so I could park van at home without HOA considering it a camper.

A low-roof PM with about 6-feet of headroom due to dropped floor would be pretty cool in that it could pass under most drive thrus and park in 8-foot garages, but fuel savings of +/- $2,200 would largely be spent on dropping the floor, so I would want such a van only if for a different reason than cost savings.

Going from high roof to super high roof is a tougher choice because we gain less and overall difference costs more. Anyway, was just curious about how serious you were about LR with lowered floor.
 

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2014, 138WB, High Roof, Gas, SW MT
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Discussion Starter · #47 ·
Is your primary interest in a low roof van to improve fuel economy? I have been thinking a lot about the cost of added space, and have mixed feelings on whether it’s worth worrying excessively about it. For example, if I assume H1 vs H2 vs H3 over 100,000 miles at $4.00 per gallon, assuming 20 MPG vs 18 MPG vs 16 MPG, fuel cost would be $20,000 vs $22,200 vs $25,000 respectively.

Given that an Extended 159” WB Super High Roof is almost twice the volume of a 136” Low Roof, is added $5,000 in fuel over the life of the motorhome worth downsizing? Not to me when it represents much less than 10% of total ownership cost.

I dropped floor in my van quite a bit like classic TransVans of the past, and could do it even easier on a LR ProMaster, but I did not do it to save fuel, though it definitely helps. I did it so I could park van at home without HOA considering it a camper.

A low-roof PM with about 6-feet of headroom due to dropped floor would be pretty cool in that it could pass under most drive thrus and park in 8-foot garages, but fuel savings of +/- $2,200 would largely be spent on dropping the floor, so I would want such a van only if for a different reason than cost savings.

Going from high roof to super high roof is a tougher choice because we gain less and overall difference costs more. Anyway, was just curious about how serious you were about LR with lowered floor.
Hi Chance,
The low roof appeals to me a lot aesthetically and also in just being able to get it into my garage and into underground parking places and other limited clearance places. I've always liked the look of those lowered floor Roadtreks. I like small, simple solutions for almost anything.

But, the main thing to me would be less gas and less carbon. We got back into RVing with the PM because we could do it for a reasonable 20 MPG rather than the 10 MPG of the one we had before.
Can't wait for an electric solution and a better charging grid.

Gary
 

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But, the main thing to me would be less gas and less carbon. We got back into RVing with the PM because we could do it for a reasonable 20 MPG rather than the 10 MPG of the one we had before.
Thanks Gary. I’m surprised how many here report getting +/- 20 MPG on the highway at 65 MPH with high roof PM. Considering new PM should get at least 1 MPG higher due to upgraded engine, 9-speed, electric power steering, and perhaps lower Cd, it shouldn’t be too difficult to squeeze 20 MPG out of H2 on long road trips.

We just got back from 2,800-mile road trip where we averaged 15 MPG with old Ford V10 by limiting speed to 70 MPH (our previous RV was 8 MPG). Like you, I’d like to do better, and 20 is a nice round number as a goal given van would be much more spacious.

Because of how we travel, solar on roof makes little sense; and if I keep air conditioner off roof again, it should help with aerodynamic drag.

Anyway, if trying to push fuel economy with low roof PM and dropped floor, my thought is that FWD ProMaster lends itself to a lowered aisle down the middle. It should be relatively easy to modify a van as shown in picture below except ending drop floor inside rear doors. Picture shows a full ramp for wheelchairs.

Again, from a cost standpoint, I’m not sure it can ever be justified, but it does have advantages. I have parked in 8-foot garages at airports and university, and never had issue with fast food drive thrus.

Vehicle Car Motor vehicle White Automotive tire
 

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2019 Promaster 3500 Silver high top 159"
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I finally did my test of roof rack penalty. My rack is quite dirty
My test was mainly to see if I could detect a difference. So I used the instantaneous MPG while traversing a 1-1./2 mile floating bridge deck (we have two in Seattle) late at night with low/no winds (forecast was 3-5mph from the south west). I left the van idling while taking the canopy off so the cruise control setpoint was identical for all runs (~60mph). The indicated MPG was noted over 1/2 mile after it stabilized on the bridge deck. The high/low average matched the eyeball integrated average and that was what was written down.

I did two runs each east/west with and without the canopy and cross bars (everything else left the same) and, although the data was noisy there was an average 1-1/2mpg difference @ 60 mph (17.85 before, 19.52 after) Since my typical road trip is around 3000-4000 miles and I have been getting a consistent 16.6-17.1 mpg I would expect to burn ~15-20 gallons less without the roof rack.

I have some ideas for streamlining the roof: raise the solar panels and add a deck so there is a smooth surface from cross bar to cross bar, then add a nose and tail. But, for me, the ~20 gallons/trip is in the budgetary noise so not much incentive to improve things further.

Perhaps I'll just drive a little slower and enjoy the trip :)
 

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2014, 138WB, High Roof, Gas, SW MT
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Discussion Starter · #51 ·
Hi Larry,
Thanks for doing the test!

Aero is hard to make good guesses, but I think those two tall cross bars might be the biggest drag contributors -- lots of frontal area and poor drag coefficient.

The floating bridge was a nice idea - I've been over them about a billion times (lived in Bellevue).

Do you have any more pictures of the rack from different angles?

Gary
 

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Here are some detailed pictures of my roof assembly. In short, I have two 1x3" bars that I am driving into free air. Probably the worst possible configuration.

My "streamline" plan (should I get suitably inspired) would be to
  1. Replace the cross bars with 1x1 (the current scheme is overbuilt)
  2. Raise the solar panels to be flush with the top of the bars and move them against the rear bar
  3. Fill in the remaining space, and around the vent, with a "deck".
  4. I would accomplish 2 & 3 with some sort of side channel the block airflow under the panels. Then some sort of nose to block air from flowing under the forward bar
  5. Streamlining the nose seems pointless given that I have flood lights there. Maybe I should move them to the grill?
Anyway, with those changes the van would be presenting a slightly higher profile to the existing airstream rather than all those turbulators on top. Maybe,
 

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Discussion Starter · #53 ·
Hi,

Some random thoughts and alternatives...

As you know the basic drag formula is...

Drag = 1/2 * (air density) * (Velocity ^2) * (Frontal Area) * (Drag Coefficient)

Not much you can do about air density, and while you can reduce Velocity to good effect, it takes longer to get there :)
Although, the velocity varies all around the van, and moving stuff out of the highest velocity parts of the flow could yeild some benefit.

So, that leaves you with Frontal Area and Drag Coefficient.

I always look at potential changes in light of how the change effects these two. You have to reduce the combination to make any progress, and changes that reduce both are a win-win.
This is a bit of an over simplification in that total drag changes as objects are brought together (interference drag) - but, its a place to start.

So, based on the above logic, here is one alternative (which may not be structurally acceptable)...
Change to cross bars that match the height of the PV panels frames (or close).

Attach the PV panel frame to the cross bars in line - that is, the front end of the PV panel attaches to the aft end of the forward cross bar. The total frontal area then is just the forward facing area of the forward cross bar.
This is a major reduction in frontal area.

If possible, skin the bottom of the PV panel with light sheet metal, so it presents a smooth surface to the air flow instead of a turbulence causing open box.

Let the air flow between the PV panel and the van roof in as clean a path as possible.

If possible butt the aft end of the PV panel against the front face of the aft cross bar, and move the front cross bar back to to the forward face of the PV panel.
The airflow from the windshield onto the roof is accelerating and has a higher velocity, it is also already close to separating, so anything you put in this part of the flow will have higher drag due to the higher velocity and may well cause separation of airflow, which will be a major drag increase.

Use an elliptical fairing on all of the exposed forward facing cross bar surfaces.
Use a long taper boat tail fairing on each of the aft facing cross bar faces (about a 15 deg closure angle)


Obviously this makes the added lights harder - not sure how attached you are to them, or if there is a lower drag version, like some of the ones you see on some police cars.

Its really hard to do good aero predictions (just ask Boeing), but I think that the large reductions in frontal area coupled with big improvements in drag coefficient coupled with keeping stuff out of that windshield to roof transition there is a good chance that drag would be substantially reduced.
For example, if you cut frontal area by a factor of 3, and you cut drag coef from about 1.0 for a box down to under 0.25 or less for a reasonably faired shape, then the drag should go down by a factor of 12.

Just some thoughts - I could be totally off base.

Gary
 

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2019 Promaster 3500 Silver high top 159"
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I can't move the crossbars as they pick up the moment arms of the canopy. I was thinking that a deck to fill in would eliminate the interference drag. The cross bars could be modified to match the existing PV, or, more practically: bring the PV up a bit (the 3" bar is overkill) and use a 1" tube for the bar. I'm guessing you suggest skinning the bottoms of the PV vs. damming the airflow because that presents less frontal area. The floods are nice on back-roads at night. Maybe I can mount them on the hood or grill.

All ideas to think about.
 

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I can't move the crossbars as they pick up the moment arms of the canopy. I was thinking that a deck to fill in would eliminate the interference drag. The cross bars could be modified to match the existing PV, or, more practically: bring the PV up a bit (the 3" bar is overkill) and use a 1" tube for the bar. I'm guessing you suggest skinning the bottoms of the PV vs. damming the airflow because that presents less frontal area. The floods are nice on back-roads at night. Maybe I can mount them on the hood or grill.

All ideas to think about.
Looking @ your photos, the floods (front & rear) stuck out for me as draggy for the smallness of them.

The elliptical “airfoil” leading edge on the cross bars I think would help as well as the skins @GaryBIS suggested. Or any frontal area.

Anything to keep the airstream “laminar” would help including trailing edges, but the biggest bang for buck is on the leading edge.

Anything that creates lift creates drag (even negative lift ).

Anything you can do to create less frontal & less disturbance will help.

Mrs RV8R is demanding an awning & resistance is futile for me, but the frontal on it is not too big 🙄.
 

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How would a low profile rack like Flatline’s or Orion’s design with the swooped rear do with these? Our, hopeful, build is going to be rack and awning less but have 2 solar, a/c, and 1 max fan so I’m hoping the general drag would be less over putting a rack on it.
 

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@GaryBIS
If possible, skin the bottom of the PV panel with light sheet metal, so it presents a smooth surface to the air flow instead of a turbulence causing open box.

I am just about to install solar onto my roof and it is a good time to add a skin underneath.... I wonder if anyone has done this and whether it would cause any overheating issues? One reason I elected to have the panels mounted on a rack was to allow air underneath for cooling. A gap of an inch or so under the panels seems generally to be recommended, though I know many van builders have panels stuck directly to roof using L brackets and VHS tape. Also, I believe that when installing panels on a house roof the output is derated if the panel is close attached.
 

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2019 Promaster 3500 Silver high top 159"
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Some random thoughts and alternatives...

As you know the basic drag formula is...

Drag = 1/2 * (air density) * (Velocity ^2) * (Frontal Area) * (Drag Coefficient)

Not much you can do about air density, and while you can reduce Velocity to good effect, it takes longer to get there :)
So, I thought about it a bit. I'm pretty dense, but I finally understand that minimizing frontal area, with streamlined nose and tail is much better than a front slope/dam to keep air from getting underneath the roof clutter. To that end I am
  1. proposing to take my trusty cordless DeWalt circular saw to my cross bars and reduce the rear from 3" to 1.4" to match the solar panels and the front to 1". I cannot shift the x-bars as they directly transfer the moment load from the canopy arms,
  2. Relocate the PV panels on top of the uni-strut and snug them up to the rear x-bar
  3. Obtain some sheet aluminum and fabricate an aerodynamic shell for the front bar and a nose for the rear PV panels leading edge. I'll use construction adhesive to glue the sheets to the bottom of the xbar, then hand form the remaining shape, perhaps use pop-rivets for the tails, or some vhb tape. There will be some experimenting before the final assembly.
  4. I dunno what I'll do with the flood lights. If I move them, I'll need to patch the roof. Should be easy enough with some RTV and a metal disk on either side of the hole.
  5. Maybe skin the bottoms of the PV. Awaiting a response to #58's question!
The question I have ( @GaryBIS and @RV8R?) is what weight and alloy aluminum? I don't have sheet metal tools so I'll probably get on-line metals to cut the strips, then do the initial fold on my universal metal brake (the table saw edge) and hand form the rest. So, .020? .040? I think the latter might be too stiff for my old hands, the former too delicate for all the tree branches that seem to regularly swoop down and attack my roof.
Handwriting Computer keyboard Font Line Space bar

I suppose I could just rip some 1" and 1-1/2" PVC pipe to make a semi-aerodynamic nose/tail and glue them down.
 

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2021 Silver ProMaster 159
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iI did it like this.
Your setup is using a number of the principles that Gary talked about. As far as the other items, adding a sheet metal bottom to the solar panels would be a big job, but removing the front fairing would be simple change. You could test your mpg before removing the fairing and then do it after the removal for comparison.
 
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