Aero RV (custom boat tail for '95 Ford E-350 Class C motorhome)

[skyking]

The only way I know is to fuel slowly. I use the slowest latch ( we can still latch a pump here and there) and wash the windows. I don't fill often nor do I fill at a busy busy station, so I don't fret about tying up a pump.
When I get back, I keep topping till the fuel just stands there.

[wyatt]

Originally Posted by orbywan

That was before I dropped the floor of the tail down about a foot to accommodate the extended hitch. In the process I'm sure the air flow is better down there. I haven't retested the bottom yet. It's as good as it's going to get. I can't drop it any lower without getting into my departure angle which has to be high enough to accommodate frequent off road forays.

Adding some strakes made out of lawn edging may be advantageous. It will help channel more of the air from underneath in and over the underside of the tail, instead of letting it rush out in any direction it wants. The lawn edging is pretty tuff stuff, and several on here have used it for air dams and the like with very good success. You could use the same stuff for your front air dam unless you have something else in mind.

...Depending on the results, I may go with a lower ratio tail gear. These 4:10's can't be helping, but I'll lose some towing grunt in the process. 1800 rpm is supposedly ideal for mpg on this 7.3 diesel, that shows up at about 52 mph, that's too low for optimum mpg, but it's doing so well even with that handicap I'm not sure I want to jeopardize the towing ability.

Basjoos has the same problem. On his AeroCivic, he dropped the drag so much that he could REALLY see MPG gains by upping the gearing, but he doesn't because having power in some situations is more important (to him) than having better MPG all around. With that being said, re-gearing based on the RVs new drag could significantly improve it's efficiency. You could also downsize the engine .

[skyking]

which transmission do you have in that?

[slowmover]

On a couple of points: I experimented with my old Chrysler sedan with FF/RR rake (torsion bar front suspension). Bringing the car "level" had a quickly deleterious effect on mpg (even when accomodating nose lift at higher speeds than the usual travel speed). Now, were that Class C mine and I were to use airbags to maintain a particular FF/RR rake or bias I would also use public scale weights to have a "look" at Front Axle weight changes (not just fender measurements), not just tire pressure rise (against load and RMA tables).

I would vote steering corrections per 100-miles traveled as the logical place to bring another "margin" into compliance iwth higher mpg as per the now-famous Cummins White Paper. With the oil field runs we make, the difference between the older CAT powered tractors and the brand new CUMMINS powered ones is likely more about overall front end wear than engine spec . . steering.

As to fueling: Same station, same pump is about as good as it gets. When I desire tank-to-tank "repeatability" I place the nozzle in the same position in the tank opening for auto shutoff that occurs at the same time: first clickoff. Yes, I can stand there and try to dribble in more . . not at all worth the time or energy (with a stock configure fuel tank. Quite a few DODGE owners modify the tank vent location to a higher point in order to take on another 1-3 gallons or so and avoid the "foam" problem. Involves dropping tank or removing truck bed). Can't say I'd go to the trouble on this even with the frustration of white noise on tank-to-tank.

Thanks for the pics of the vehicle in use. Looks "better" than I recalled; not "weird" or ungainly per what an older conservative RV crowd might consider . . no tradeoffs in that aspect!! I've linked back to this several times but no takers on those threads (unfortunately).

.

[orbywan]

Originally Posted by skyking

The only way I know is to fuel slowly. I use the slowest latch ( we can still latch a pump here and there) and wash the windows. I don't fill often nor do I fill at a busy busy station, so I don't fret about tying up a pump.
When I get back, I keep topping till the fuel just stands there.

I do it similar to slowmover, I go to the same station, same pump, same direction, when possible, try to insert the nozzle the same distance every time and let it turn off according to back pressure. I'd love to have Mythbusters check that out and see how consistant that is.

I tried filling it until it just stands there and on my rig, I gave up after 10 minutes . No patience this one.

[orbywan]

Originally Posted by skyking

which transmission do you have in that?

It has the A4ODE trans on it. Four speed overdrive. The shift points on it are not quite right but other than that it's solid. I just found a new ECM for this vehicle at the local auto parts store for $139.00. I'm going to change that out and see if that improves the shift points.

I see you've got 3:55 gears in you Dodge. I need to go to the Powerstroke forums and get some opinions on how much towing power I'd lose going to a lower (numerical) ratio. Diesels have so much low end grunt maybe it wouldn't be that much. Straight and level, no load, that should have a noticeable impact on mileage.

Actually with a lower numerical ratio, I'd just have to drop it out of overdrive a little sooner and downshift sooner on the hills. We have some of those around here.

[skyking]

If I find the right adapter and US gear Dual range, want me to let you know? That opens up a whole realm of gears for you.

[orbywan]

Originally Posted by slowmover

On a couple of points: I experimented with my old Chrysler sedan with FF/RR rake (torsion bar front suspension). Bringing the car "level" had a quickly deleterious effect on mpg (even when accomodating nose lift at higher speeds than the usual travel speed). Now, were that Class C mine and I were to use airbags to maintain a particular FF/RR rake or bias I would also use public scale weights to have a "look" at Front Axle weight changes (not just fender measurements), not just tire pressure rise (against load and RMA tables).

I would vote steering corrections per 100-miles traveled as the logical place to bring another "margin" into compliance iwth higher mpg as per the now-famous Cummins White Paper. With the oil field runs we make, the difference between the older CAT powered tractors and the brand new CUMMINS powered ones is likely more about overall front end wear than engine spec . . steering.

As to fueling: Same station, same pump is about as good as it gets. When I desire tank-to-tank "repeatability" I place the nozzle in the same position in the tank opening for auto shutoff that occurs at the same time: first clickoff. Yes, I can stand there and try to dribble in more . . not at all worth the time or energy (with a stock configure fuel tank. Quite a few DODGE owners modify the tank vent location to a higher point in order to take on another 1-3 gallons or so and avoid the "foam" problem. Involves dropping tank or removing truck bed). Can't say I'd go to the trouble on this even with the frustration of white noise on tank-to-tank.

Thanks for the pics of the vehicle in use. Looks "better" than I recalled; not "weird" or ungainly per what an older conservative RV crowd might consider . . no tradeoffs in that aspect!! I've linked back to this several times but no takers on those threads (unfortunately).

.

Maybe I'm just used to it but I kind of like how it looks. Next winter I hope to get my A&P guy to skin it with aluminum and then paint the whole rig to match. I'm not sure if that will help or not, depends on the paint job probably.
I'm going to have to ask him if the aluminum on a steel frame will cause dissimilar metal corrosion. I'm pretty sure it will, will have to find out what to do about that.

Slowmover am I reading the specs right, doe your Dodge have 3:05 gears in it? I know those 6 bangers are torquey but wholly canole. I must be reading that wrong. How much does your trailer weigh?

[orbywan]

Originally Posted by skyking

If I find the right adapter and US gear Dual range, want me to let you know? That opens up a whole realm of gears for you.

Of course, I'll consider anything at this point. Do the US gear units have syncro's on them? I installed a dual range box on a Chevy truck once a long time ago, when it shifted it was like a boulder fell on the cab, BAM! When I called the manufacturer and said what's up with that, they said, yes, they shift rather robustly. Ha! I can't remember who made it though.

[orbywan]

Originally Posted by wyatt

Adding some strakes made out of lawn edging may be advantageous. It will help channel more of the air from underneath in and over the underside of the tail, instead of letting it rush out in any direction it wants. The lawn edging is pretty tuff stuff, and several on here have used it for air dams and the like with very good success. You could use the same stuff for your front air dam unless you have something else in mind.


Basjoos has the same problem. On his AeroCivic, he dropped the drag so much that he could REALLY see MPG gains by upping the gearing, but he doesn't because having power in some situations is more important (to him) than having better MPG all around. With that being said, re-gearing based on the RVs new drag could significantly improve it's efficiency. You could also downsize the engine .

I'm going to make the first air dam out of coroplast so I can trim it easily if needed. Once I think I'm at the right height then I'll use the trim pieces you refer to.

Downsize the engine? Yeah, OK, I can do that.