Aero RV Belly Pan(s)

[orbywan]

Originally Posted by grins2go_brett

I've been lurking and watching this build thread with great interest. I have a 2007 Dodge Sprinter for my business. A boat tail is not an option for me since I have to open the back doors all the time. However, a belly pan is something I'm seriously considering. I've learned a lot from your trials, pains, descriptions, and pictures and I'm very appreciative of not only your effort to do the project, but document it so thoroughly for everybody here.

Now, my question is: what kind of MPG's were you getting before you made any aero mods? I'm just trying to put 15-17 mpg into perspective. I'm in the 16-18 mpg range myself. Thanks!

The first run to Phoenix after the mods showed 17.26 mpg, verses original mileage of 12.5 to 13.2 for the same trip. The second trip with the belly pans yielded 16.64, but that was not comparable as there were several events going on that diminished the overall result.

The return trip is what convinced me the pans are not helping at this point, but knowing how rough it was under there to begin with makes me think there are some things going on that I don't understand yet. Time and testing will tell.

The results with the boat tail make it a no brainer to me that this is where serious gains are to be made, pans or not. Is there any chance of making a boat tail for your Sprinter that would split in the middle and fold outwards like the big rig trailer tails? A tail that swings upwards out of the way comes to mind but the weight and the 'sail' factor when it was up there might be problematic.

I have a local RV dealer that sells a lot of your type of vehicle and they are very interested in this build. I'm taking the vehicle over for them to see next week. I'll take a look at one of their vans and see if anything comes to mind. Thanks for the input.

[slowmover]

After three weeks of serious work, the result was zilch. No gain whatsoever, and I wouldn't be surprised if I lost a tiny bit.



I'm sitting here debating . . do I slap the boy, or hand him a cold beer? Maybe both unless he's bigger than me and depending how spry I'm feeling that day. Buddy, you're not only on track, you're succeeding!

Did I miss it or were temps, altitude and winds reported?

All play a role in highway FE. The last, I strongly feel, will -- in the form of crosswinds -- prove the effort of the bellypan a success in the miles to come:

1] by keeping the average mpg higher than otherwise; "smoothing" the airflow from multiple directions.

2] by maintaining easier steering control (the reduction of steering input per 100 miles [cit. ref. Cummins and Kenworth].

3] an alert and rested driver being more sensitive over a longer period of time/miles; being able to maintain a higher level of fine inputs versus gross corrections over the course of the day, thus keeping or further accumulating those elusive tenths and hundredths.

And I'm tempted to post this to one of the RV boards where the iron hand of conventionality rules. None will believe 16.5 mpg . . an error in thinking, calculating, out-and-out lying, etc will be cited as "reasons". Or, photos criticized, etc.

Orbywan, I'm a third generation owner of all-aluminum aerodynamic trailers and I can tell you without doubt that anyone who can achieve 16+ mpg with a turbodiesel tow vehicle and this trailer type is at the near upper end of the scale for a "big" RV, single or combination. I suppose we can compare weights, square footage, storage, equipment, etc, to arrive at some rough comparison, but the fact remains that you've done a heck of job (liberal use of emphatic profanities, here). Motorhomes, moho's, just don't enter into the FE equations for low cost, independent travel (for the most part). They stink.

One has to move to Euro TD SUV's with sub-27' aero trailers to see 18+ averages (highs may be 20+, but averages tell the story for coast-to-coast travel). Or teeny moho's, barely converted vans with no space for extended travel supplies.

Alternately, you go out there and find me the IFTA reports of guys running otherwise comparable expeditor rigs (even smaller than yours) with that kind of mileage. Their experience, their skill, and their determination not only eclipses yours, it makes a mockery of what you bring along when behind the wheel. (I may be stretching, but not to hyperbole.)

Back to RV's, as you know, there is not nearly the "jump" in live-ability from your size rig to a 45' Newell as there was from one smaller up to your size (for extended travel in four seasons over thousands of annual miles); as soon as one has the supplies/clothes for year-round travel, more space is redundant. You've staked out an "important" position, and beaten expectations.

So, altitude, temps, and winds? Please expand your reporting to accommodate extended continent-wide travel under these headings. While you may only travel the desert SW and the Inter-Mountain West, comparisons by others can be made (why else do we post except that our experience/examples may be relevant to others?) for their rigs.

I'd say that not only is the experiment doing well -- the hoped for results -- but that 850-miles is proving the work done to be correct (and cheap).

Bring the driver skill up some more and you'll find the consistency that "proves" efficacy.

And let's "cheat" a little. Many commercial drivers see the same routes over years and decades. You can use one of the "tricks" I used as a new big truck driver under pressure to show low fuel use: don't fill the fuel tanks until past 30-miles of warm-up, and refill at least 30-miles out from a major metro area having already covered 200-miles plus between those points on the open road. (my distances were greater; I had to use a bit of subterfuge and some cash of my own to keep idle time costs lower: the quarterly bonus more than paid it. I never won, but I never got reprimanded either; none of which are your worry).

You can find a 400+ mile roundtrip that can be taken over a days length with appropriate fuel stops, and, over a familiar highway, learn what your highest possible mpg may be (given road, load, conditions and weather) that will consistently report the "racetrack numbers" (no warm-up or metro traffic involved). The more you use the route -- the more easily the mental map comes to mind -- the better will be your judgment of your efforts.

And here's beer numero tres.

.

[Otto]

So, your fuel economy has improved by 33% already, yet not a happy camper?

What do the tuft tests or oil streak tests show for flow around the wheels?

Next stop: Get some oval shaped rubber trash cans at WalMart, cut diagonally to make fairings for the wheels, per Hucho pic of Calibra wind tunnel and your tuft tests, to properly orient the fairings into the relative wind as seen by the wheels.

Got pizza pan aluminum wheel covers yet? I'm told these run ~$3 each.

Got smaller, more streamlined motorcycle mirrors yet, to replace the Dumbo the Elephant ears outside mirrors? ~$30 on eBay.

You've made remarkable improvements already. 20 mpg at steady highway cruise can happen, with the existing belly pan + mods noted above.

[orbywan]

Did I miss it or were temps, altitude and winds reported?

As I recall on the first run to Phoenix and Apache Junction the conditions were as follows;
No load except full fuel and the Mrs.
No dash or roof A/C used.
Ran as close to 56-57mph as I could maintain, no cruise.
We left at 4AM in the morning, about 80 degrees F.
Aviation winds forecast called for 2mph left quartering tailwind.
Tucson is 2389 ft elevation at TIA, Phoenix is 1124 ft at Sky Harbor.
Apache Junction elevation is listed at 1722 feet.
We went approx 100 miles north northwest to Phoenix on I-10, right and due east on hwy 60, 25.6 miles to Shell station in Apache Junction. I-10 and 60 are constantly up and down, up and down. Not a lot in elevation but enough to be a pain if you’re paying attention.
The first leg yielded 17.26 mpg. (!) 126.7 miles, 7.34 gallons.

The return leg two hours later, after having breakfast with my mom and sister, went like this;
Winds forecast was right on the hump between 4 and 7 mph, blowing south, southeast. For the Apache Junction to Phoenix leg (heading due west) it was a right quartering headwind, for the Phoenix to Tucson leg (south, southeast) it was all over the map, mostly an annoying left crosswind.
By then the air temp was right at 100 degrees and we used the A/C the entire trip back, same speed as first leg. Result for this leg was 14.65 mpg. 130.3 miles, 8.89 gallons. The extra 3.6 miles was driving to my mom’s house and back from where we filled it up. Come to think of it, the drive train was also cooling off for two hours as we had breakfast and visited with my mom and sister, which means getting back to the freeway the drive train was getting up to operating temperature.

For the second trip, conditions were as follows;
RV fully loaded with 30 gallons of water, full fuel, food, wine and provisions for 4 days in the mountains, and the Mrs. No toad, don’t have the hitch extended yet.
Winds forcast for first leg of the trip, which was identical to leg one of the first test above, was 4 mph tailwind (from the south).
We left at 5AM in the morning, temp about 85. Intended to not use the A/C but it was just too damn hot. Used the A/C just enough to keep us from dying, was on probably 50% of the time, 56 -57mph. Pain in the butt segment. Had to pull off the freeway about 2 miles to meet friends, stopped a second time halfway to Phoenix to tape the back door on my boat tail shut (had all my big light stuff back there), and worst of all had about 20 miles of 10 mph crawl stop and go when we got into rush hour traffic in Phoenix.
That leg yielded 16.65 mpg. 128.2 miles, 7.7 gallons. Not bad considering.

Sunday afternoon on the way back from the Mogollon Rim, we were able to get back to the same station in Apache Junction on the way back. Same pump, pointing same direction, you know the drill. The same return leg was forecast on Thurs morning for 8 mph, right quartering tail wind (south, southeast). How accurate that was I don’t know, forecasts change a lot in four days around here.
We were about 15 gallons of water and a litre or two of wine and some food lighter.
I can tell you two things about the return segment. The tail wind was slightly stronger, and the cross winds were considerably stronger.
It was about 115 degrees in Apache Junction that day, we left the station at about 3:30 PM, and used the A/C all the way home. Results were 14.96 mpg. 126.6 miles, 8.46 gallons.
I believe the increase over the first return leg was due to the stronger tail wind but maybe the pans played a role in that too.

All I know is after achieving the gains with the boat tail, and spending three weeks on my back in a 100 plus degree shop with crap falling in my eyes, yes I was disappointed with not getting a no-brainer, hell yes it made a difference observation that I could attribute to the belly pans. Hopefully that will change with some more work.

This miniature novel is why I didn’t report all the details before, but there it is for people who want to read it. I’ll answer the other questions in another post so this one doesn’t go on forever.

[orbywan]

Orbywan, I'm a third generation owner of all-aluminum aerodynamic trailers and I can tell you without doubt that anyone who can achieve 16+ mpg with a turbodiesel tow vehicle and this trailer type is at the near upper end of the scale for a "big" RV, single or combination.

.[/QUOTE]

Actually it is a 25 foot (not including the tail) E-350 Ford 7.3 turbo diesel RV. No tow vehicle or trailer involved, yet.

[grins2go_brett]

Originally Posted by orbywan

The first run to Phoenix after the mods showed 17.26 mpg, verses original mileage of 12.5 to 13.2 for the same trip. The second trip with the belly pans yielded 16.64, but that was not comparable as there were several events going on that diminished the overall result.

The return trip is what convinced me the pans are not helping at this point, but knowing how rough it was under there to begin with makes me think there are some things going on that I don't understand yet. Time and testing will tell.

The results with the boat tail make it a no brainer to me that this is where serious gains are to be made, pans or not. Is there any chance of making a boat tail for your Sprinter that would split in the middle and fold outwards like the big rig trailer tails? A tail that swings upwards out of the way comes to mind but the weight and the 'sail' factor when it was up there might be problematic.

I have a local RV dealer that sells a lot of your type of vehicle and they are very interested in this build. I'm taking the vehicle over for them to see next week. I'll take a look at one of their vans and see if anything comes to mind. Thanks for the input.

I'm sure with enough time, money, and energy, I could create a fold away boat tail. However other factors make a boat tail problematic, including daily parking. The van is already as long or longer than most parking spaces here in the Phoenix area. So unless I can create something that opens and closes automatically or semi-automatically (push button on dash maybe), I just don't see it happening.

Back in the spring, I made a full grill block for my van. A lot of it has had to come off this summer of course. But between what I've been able to keep on the grill, and taping up the headlights and bumper to body gap, I've managed to gain about 5% over last years summer mpg's. Hopefully as it cools down and I can put more of the grill block back on, I'll gain a few more percentage points. If a belly pan gave me another 5-10% gain, I would be happy with that. Given the time and cost though, I think smooth wheel covers and rear wheel skirts may come first.

Good to hear you have caught the attention of the local RV dealer. It might be hard to convince customers to do a boat tail on their rig. But if they could do a belly pan on their rigs (out of sight, out of mind for the customers), and prove a 5-10% or more gain in fuel efficiency, I can see customers purchasing that as a dealer add on for a reasonable price. Let us know what they say and how they react. Thanks!

[orbywan]

Originally Posted by grins2go_brett

Back in the spring, I made a full grill block for my van. A lot of it has had to come off this summer of course. But between what I've been able to keep on the grill, and taping up the headlights and bumper to body gap, I've managed to gain about 5% over last years summer mpg's. Hopefully as it cools down and I can put more of the grill block back on, I'll gain a few more percentage points. If a belly pan gave me another 5-10% gain, I would be happy with that. Given the time and cost though, I think smooth wheel covers and rear wheel skirts may come first.
:

I agree, do the easier stuff first, that gets you in the groove so to speak and makes you motivated to take it to the next level.

[orbywan]

Originally Posted by Otto

So, your fuel economy has improved by 33% already, yet not a happy camper?

What do the tuft tests or oil streak tests show for flow around the wheels?

Next stop: Get some oval shaped rubber trash cans at WalMart, cut diagonally to make fairings for the wheels, per Hucho pic of Calibra wind tunnel and your tuft tests, to properly orient the fairings into the relative wind as seen by the wheels.

Got pizza pan aluminum wheel covers yet? I'm told these run ~$3 each.

Got smaller, more streamlined motorcycle mirrors yet, to replace the Dumbo the Elephant ears outside mirrors? ~$30 on eBay.

You've made remarkable improvements already. 20 mpg at steady highway cruise can happen, with the existing belly pan + mods noted above.

I'm ecstatic with the 33% per cent improvement. But after that kind of gain I expected to discern some kind of improvement from the pans after that much work and aggravation. I'm getting over that, partly thanks to everyone's encouraging comments and the realization that somewhere along the line I violated a law or two of aerodynamics and I just have to figure out what it is.

The only testing I've done so far is the road test as far as the pans are concerned. They seem to be durable as hell, which is a great relief. I thought for sure I'd come back with some seriously melted plastic. Hats off to three-wheeler Jim for telling people about using fiberglass insulation for isolating exhaust heat. The vehicle smelled a little the first two times I shut it off after climbing hills, but after that, nothing. We went from Apache Junction, AZ, elevation 1722 feet, I think, to the top of the rim at 7,000 feet, over a distance of about 120 miles. That wasn't extreme climbing in my experience, but it was substantial. I wasn't towing anything but that exhaust system is completely covered from the down pipe all the way back to the rear axle, with a 12X12 vent just below the cat, and a small NACA shaped opening just below the muffler and it didn't deform a single inch of coroplast. I'm amazed.

I've taken it this far so next I'm going to order a smoke generator, hook up a mobile DVR to my camera system and do some testing. Tally ho.

[orbywan]

Originally Posted by slowmover

[B]
And let's "cheat" a little. Many commercial drivers see the same routes over years and decades. You can use one of the "tricks" I used as a new big truck driver under pressure to show low fuel use: don't fill the fuel tanks until past 30-miles of warm-up, and refill at least 30-miles out from a major metro area having already covered 200-miles plus between those points on the open road. (my distances were greater; I had to use a bit of subterfuge and some cash of my own to keep idle time costs lower: the quarterly bonus more than paid it. I never won, but I never got reprimanded either; none of which are your worry).

You can find a 400+ mile roundtrip that can be taken over a days length with appropriate fuel stops, and, over a familiar highway, learn what your highest possible mpg may be (given road, load, conditions and weather) that will consistently report the "racetrack numbers" (no warm-up or metro traffic involved). The more you use the route -- the more easily the mental map comes to mind -- the better will be your judgment of your efforts.

And here's beer numero tres.

.

I didn't follow that til I read it again, you make a good point. This rig has a big, very efficient radiator that takes forever to hit operating temp. From now I'll be sure it's up to temp before I fill it and hit the road. The station on Ironwood in AP is easily 5 miles from the freeway with at least 5 or 6 lights and I always get caught at least twice, one way. I need to find a station right off of 60 for this to be more accurate. Thanks. Oh yeah, I'll take you up on the cerveza also.

[slowmover]

Originally Posted by orbywan

Orbywan, I'm a third generation owner of all-aluminum aerodynamic trailers and I can tell you without doubt that anyone who can achieve 16+ mpg with a turbodiesel tow vehicle and this trailer type is at the near upper end of the scale for a "big" RV, single or combination.

.

Actually it is a 25 foot (not including the tail) E-350 Ford 7.3 turbo diesel RV. No tow vehicle or trailer involved, yet.[/QUOTE]

I should have written: Even with a rig better suited to higher mpg than a motorhome, nevertheless, . . . . (it's outstanding for a big TD/Aero trailer rig, it's unreal for a moho of your Class.)