Ram Promaster Forum banner
21 - 40 of 73 Posts

·
Premium Member
2014, 138WB, High Roof, Gas, SW MT
Joined
·
2,954 Posts
Discussion Starter · #21 ·
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:
Hi Dennis,
Nice video on the van.

Have you ever done any MPG testing on the poptop van?

Gary
 

·
Premium Member
2014, 138WB, High Roof, Gas, SW MT
Joined
·
2,954 Posts
Discussion Starter · #22 ·
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.

Gary
 

·
Registered
2014 136” HR
Joined
·
7,277 Posts
I’d like to see a test before and after MaxAir install in the normal front position. When we first bought the van, it was not difficult to get 18-18.5mpg. At some point, it became 17mpg, but unfortunately, I didn’t pay enough attention.
 

·
Premium Member
2018 159 High Roof gas, BC, Canada
Joined
·
1,831 Posts
It might even be worth putting a bullnose on the leading edge of the PV panels.

Gary
Thanks, Gary. You now have me figuring out how to make a fairing just for the leading edge of the PV panels. I'm thinking it might be better if it's more pointy than bullnose. Maybe something like this:

74757


I'm wondering if something with that kind of profile is available in a 5-foot piece. Otherwise, I'm thinking of taking a piece of 2x3 and lopping off the front 2 corners with a table saw and then smoothing it out a bit with a belt sander.
 
  • Like
Reactions: 83Grumman

·
Premium Member
2018 136 HR Ont.
Joined
·
1,482 Posts
Thanks, Gary. You now have me figuring out how to make a fairing just for the leading edge of the PV panels. I'm thinking it might be better if it's more pointy than bullnose. Maybe something like this:
I have considered squashing a piece of aluminum thin wall tubing to create an oval, cutting it in half and seeing if that makes a good leading edge. Similar to your illustration but maybe with the edges of the tubing overlapping the solar panel frame edge.
 

·
Premium Member
2018 159 High Roof gas, BC, Canada
Joined
·
1,831 Posts
I have considered squashing a piece of aluminum thin wall tubing to create an oval, cutting it in half and seeing if that makes a good leading edge. Similar to your illustration but maybe with the edges of the tubing overlapping the solar panel frame edge.
Interesting idea! Waterproof too! (Wood would need painting.) You have me thinking about alum or some kind of tubing now.

My panels are quite thick though so I'm not sure what diameter tube to use. Maybe 3-inches? 4-inches? So I wonder if that will have to be something else other than aluminum.

Could it be PVC? It's already white and I was envisioning my fairing to be white (like a nose cone on a missile! It would also match my white PM).

Looks like PVC can be heat formed:

Thanks for your thought.

Edit to add: Maybe heat it up to the point of flexibility and then squish it evenly by using a board with weight on top but with spacers under it to make it "clamp down" to a uniform size. Then run it through a table saw.
 

·
Premium Member
2018 159 High Roof gas, BC, Canada
Joined
·
1,831 Posts
I’d like to see a test before and after MaxAir install in the normal front position. When we first bought the van, it was not difficult to get 18-18.5mpg. At some point, it became 17mpg, but unfortunately, I didn’t pay enough attention.
Did you change your tire brand? People weren't super enamoured with the OEM Pirelli's that came with the early PMs (or were they Continentals?) But they apparently lasted forever and that could be because they had a hard compound--which often yields better fuel economy.

I quite like my Nokians which seem to have come with 2018+ PMs. But users who switched to them from the earlier tire, while happier with the Nokian's quieter ride and better grip, noticed a decrease in fuel economy.
 

·
Premium Member
2014, 138WB, High Roof, Gas, SW MT
Joined
·
2,954 Posts
Discussion Starter · #29 ·
Hi,
I think that the biggest benefit of some kind of bullnose fairing on the PV panels would be preventing separation of airflow just aft of the leading edge of the PV panel, so the more gradual the gradient where the bullnose meets the PV panel the better (I think :)

Being a wood worker, I think you were on track with sanding or spoke shaving a rectangular wood shape into an ellipse could be pretty fast and effective.

So, can you figure out a way to test this? Maybe bond the bullnose onto a PV panel thick piece of wood and then hold it out the window at 70 MPH and measure the force on it compared to a square edge? Maybe a pivot on the window sill and use a spring scale to measure the force on the part that extends into the van? Science Rules!
This might take two people for safety :)

Gary
 

·
Premium Member
2018 159 High Roof gas, BC, Canada
Joined
·
1,831 Posts
Hi,
I think that the biggest benefit of some kind of bullnose fairing on the PV panels would be preventing separation of airflow just aft of the leading edge of the PV panel, so the more gradual the gradient where the bullnose meets the PV panel the better (I think :)

Being a wood worker, I think you were on track with sanding or spoke shaving a rectangular wood shape into an ellipse could be pretty fast and effective.

So, can you figure out a way to test this? Maybe bond the bullnose onto a PV panel thick piece of wood and then hold it out the window at 70 MPH and measure the force on it compared to a square edge? Maybe a pivot on the window sill and use a spring scale to measure the force on the part that extends into the van? Science Rules!
This might take two people for safety :)

Gary
Thanks, Gary, I may do the paring down of a 2x3 just to be quick.

I love science! So I'll see what I can do about doing a test. :)

I could also envision making 2 of these fairings (if easy) and putting them on the opposite sides of a 2x3 and then spinning them with a motor. Theoretically, the more aerodynamic setup would spin faster(?) To avoid waste, the extra fairing could go on the trailing edge of my solar panel array as I believe that could help as well. :D
 
  • Like
Reactions: GaryBIS

·
Premium Member
2014, 138WB, High Roof, Gas, SW MT
Joined
·
2,954 Posts
Discussion Starter · #31 ·
For sure a boat tail fairing on the back of the PV panel should help -- I think conventional aero would say its likely to reduce drag more than the bullnose on the front. I'm not so sure given that there is probably a lot of separation going on back there, but testing would be the way to find out.

For boat tail fairing designed to keep the airflow attached, the rule of thumb is a 15 deg closure angle. Carry the 15 deg angle as far as possible and then cut it off vertical -- like a Kammback car.

Gary
 

·
Registered
Van #2 2021 EXT
Joined
·
4,830 Posts
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.
Thanks @GaryBIS

So 1st I will state I believe Ram has done a very good job of producing a largish cargo van that in my mind is very aerodynamic for its utility. In other words I think they have made it as slippery as practical all while trying to make a profit in light of factory production. I am actually amazed at the PM's slippery aerodynamic body (for what it is).

Big ticket items for fuel economy would be things such as;
  1. Drive Slower
  2. Replace the side mirrors with another method like small cameras and a video feed
  3. tire pressure & tire choice
  4. possible cleaning up the under chassis
  5. weight

You tried the rear vortex generators, but the low pressure suction created by the displaced air at the rear I believe to be a bigger issue. The problem is how do you exit the relative airflow to not create a void? Pretty difficult if you want those wide tall doors at the back (and I do).

Now to your question of "streamlining" like wing struts; I spoke of being in awe of the BMW F1 racecar (a word longer than radar - both of which I am capable of spelt backwards), having carbon fiber shaped wishbone suspension. The shape was just like a symmetrical airfoil ending at a sharp trailing edge. This is the shape you are referring to and the one you know is "streamlined". Like a laminar wing it disrupts the leading edge and the trailing edge as little as possible. Further to that shape, one of my experimental planes has what is referred to as "Pressure Recovery Wheel Pants" please see photos below. The round bar landing gear and the tire out in the airstream are not in the shape of what you know is efficient - the symmetrical laminar airfoil. So the Kit Manufacturer created "gear leg fairings and "pressure recovery wheel pants" to gain "slipperiness". The gear leg fairings are in the same shape as the BMW F1 racecar wishbone suspension. Pressure recovery as the fat tires are too wide for a proper airfoil, so a "fat" one has to be designed with lots of leading edge drag, but the shape creates low pressure right behind the widest portion and the air rushes against the wheel pants and reduces to a fairly sharp point (all sides closing in on the back). I assume you see how that is working.

Anyway, when I first flew my Vans RV-7 I did not have gear leg fairings or wheel pants. When I landed after performing air testing and cruise speed power setting tests my buddy could see something on my face and he asked me "already knowing" what was up. I told him my plane is 20 knots slower then spec. He reminded me that the fairings and wheel pants were not yet on. I said ya but 20 knots? He just gave me an all knowing smile haven been there before many times. So when I got the landing gear cleaned up the cruise speed came back up to spec (170 knots or 200mph).

74786


74787




Just as important as the leading edge & might be more important is the trailing edge.

I like the idea of flexible panels glued down (this comes with it's own set of cons). I would like to understand what Ram has done (things like the long thin roof corrugations in line with the slipstream). But I am left to look at it and ponder. I have my own theories, but the only thing I have on my PM roof is a needed roof fan.

It is pretty easy to theorize what will cost you more gas with your PM - anything you hang outside in the slipstream will be a penalty. It is just a matter of how much a penalty.

There are Pros & Cons to Solar or a Roof Rack or anything we DIYers do. I am still of mind that we should build these vans however they work for us. If you want/need solar & roof rack put it on. Everyone use and utility is a bit different.

And All of Our Mileages Will Vary.




















without a wind tunnel we are in the same situation as "Experimental Aircraft" where we can take theory and if we are careful to dial out the variables we can experiment and test.
 

·
Registered
2018 Silver 159 HT
Joined
·
317 Posts
When we bought our PM 2500 HT 159, we drove it up from GA to VA, going into a big head wind. The van had 365 miles on it. We got 20.8 mpg, using gas pumped. We added ac, rack w/solar and Maxx vent. Our build is about 7,000 lbs before gear. We also dropped tire pressure to 65 psig. Last time out we got 18.7 mpg. If I use Lucus fuel additive I get another .6 mpg. Here's our..
Grille Hood Automotive design Bumper Automotive lighting
 

·
Registered
2017 - 2500 159
Joined
·
1,865 Posts
I agree trailing edge is probably more important. Thinking of any "aero" profile for bikes - aero frames, time trial helmets, aerobars - they are all teardrop shaped. Rounded front edge, pointy or semi-pointy trailing edge.
 

·
Registered
Joined
·
964 Posts
Ok Ok Ok . Here's my 2 pennies : My rack is primarily 80/20 longitudinals , with Yakima JetStream crossbars made adjustable at each end . The rear bar was adjusted to make yarn "dance" on the rear window ( bringing boundary layer to the doors) . The front bar was intended to break up airflow in front of the solar panels . But it makes a lot of noise , noise = drag = less MPG , back to the drawing board on that part .
In front and back of the panels are 1517-ls 80/20 with the curve downward intending to speed up airflow under the panels (?)
Now I'm slowly working on an upgrade to use 2 1517 shapes to make a half round in front of the panels , maybe 1 in the rear .
This upgrade also will include my world famous telescoping leg shade awning . (everyone that has seen the shade setup likes it).
The next part gets expensive . Looking at Strut 3.5625" x 1.3215 x 10' | De-aircraft.com struts with telescoping tubing inside replacing the JetStream end bars .
NASA has a great program (for the kids) breaking down aerodynamics . Worst aero shape = square , then round and near best is those spars .
There are plenty of pics on this forum of my rack (roof) and the shade , search is your friend .

PS , If anyone knows where I can get the strut material CHEEP out west here , thanks in advance .
PSS , putting a simple rack on the daughters Transit Connect weekender cost us about 2 MPG hiway , 31/28
 

·
Premium Member
2018 159 High Roof gas, BC, Canada
Joined
·
1,831 Posts
I wish I had before/after comparison data for my solar panels vis-a-vis fuel economy. But I bought my van new, drove it home, built it out in the yard, including its solar array, and its first real drive anywhere was its maiden road trip.
 

·
Premium Member
2021 Silver ProMaster 159
Joined
·
292 Posts
Hi Dennis,
Nice video on the van.

Have you ever done any MPG testing on the poptop van?

Gary
When I'm on the highway and not going over mountain passes, I've gotten an average of 21 to 23 mpg. Two weeks ago was an especially nice day and a four hour drive from Roseburg to Portland on 1-5 I got 25 mpg. If I'm doing short errands in town, then it can drop down to 16 or 17. Dennis
 

·
Premium Member
2021 Silver ProMaster 159
Joined
·
292 Posts
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.
Sorry for my slow response. I had a low top ProMaster 136. I had Sportsmobile in Fresno California add the Penthouse (pop top). The sides are heavy canvas. It’s the same material used for convertible. There is some heat loss, but my 16k btu propane heater does fine. I’ve tested it down to 10 degrees F. The 25 mpg was on a trip from Roseburg to Portland Oregon. I was trying to get good mileage and drove about 65 mph with the cruise control on.
 
21 - 40 of 73 Posts
Top