Aero RV Belly Pan(s)

[orbywan]

Originally Posted by grins2go_brett

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!

Thank you.
I realize a boat tail would be difficult to pull off on your vehicle but I would be willing to bet with an engine that small that you would see a tremendous gain with that vehicle, in the order of 25 to 30% +. Overall it's almost identical to mine, short in length (compared to the big guys), and chopped off square in the back.

The main difference is yours is much lighter than mine, quite a bit shorter but with almost the same square footage of your rear 'footprint' (creating the same or similar drag), with an engine that is less the half the displacement of mine and built for economy. That makes for maximum effect. You'd be grinning all the way to the pump.

On my way back from the mountains I saw several of the vans like you have on the road and thought that would be the perfect vehicle to get maximum effect with a boat tail. When I go to Sandy's I'll look at what they have and see if any eureka's come to mind.

[slowmover]

I think as you confine testing to a particular stretch of road, and that your notes include conditions present, that your test record over a years time will show an average that can be maintained without always running slowly.

So, to take your mind off aero for a moment:

Another suggestion is an airspeed indicator: Yes, it's nice to have a tailwind to get a one-tank (meaningless) high mpg. But it's the average that counts (and experience in operating under defined -- and differing -- conditions). If the average winds up being 14.75 mpg on an annual basis, then an airspeed indicator allows you (with Scangauge) to, say, speed up a little on the favorable portions of road, and then drop back on the return trip; the goal being to maintain the average rather than a one-tank record.

The airspeed indicator will tell you a great deal about winds on a two-laner with opposing traffic. Finding the correct travel speed is what I am trying to say is not always obvious. The old MOBIL ECONOMY RUN teams would haul ass under favorable conditions . . but it took plenty of work and understanding to know how & when!

Warmups can be minimized with block/pan heaters, etc. If the initial fill is 30+ miles down the road, and the next is prior to entering metro traffic (preferably more than 100 miles), I think you've "captured" the high mpg point.

As opposed to a car, handling a truck ideally means much longer distances for acceleration, braking, etc. And a huge emphasis on mirrors (which is why I'd never go down in size, but maybe in shape). Thus any untoward motions from pure lane-centered-ness carry a heavy penalty at the end of the run as all actions cannot be recalled (so to speak), unlike a car where the penalty is meaningless (almost).

The RV.net crowd probably has an opinion on which of these is best:

Safe T Plus

Steer Safe

The Kevin Rutherford/Bruce Mallinson Class 8 Glider Kit tractor has rack & pinion steering.

.

[Otto]

Could be, the now smooth belly is allowing more and faster air to hit the wheels, which unobstructed have greater drag than before.

But, with properly oriented wheel wheel fairings angled to the relative wind, this problem will largely go away.

[orbywan]

Originally Posted by slowmover

I think as you confine testing to a particular stretch of road, and that your notes include conditions present, that your test record over a years time will show an average that can be maintained without always running slowly.

So, to take your mind off aero for a moment:

Another suggestion is an airspeed indicator: Yes, it's nice to have a tailwind to get a one-tank (meaningless) high mpg. But it's the average that counts (and experience in operating under defined -- and differing -- conditions). If the average winds up being 14.75 mpg on an annual basis, then an airspeed indicator allows you (with Scangauge) to, say, speed up a little on the favorable portions of road, and then drop back on the return trip; the goal being to maintain the average rather than a one-tank record.

The airspeed indicator will tell you a great deal about winds on a two-laner with opposing traffic. Finding the correct travel speed is what I am trying to say is not always obvious. The old MOBIL ECONOMY RUN teams would haul ass under favorable conditions . . but it took plenty of work and understanding to know how & when!

Warmups can be minimized with block/pan heaters, etc. If the initial fill is 30+ miles down the road, and the next is prior to entering metro traffic (preferably more than 100 miles), I think you've "captured" the high mpg point.

As opposed to a car, handling a truck ideally means much longer distances for acceleration, braking, etc. And a huge emphasis on mirrors (which is why I'd never go down in size, but maybe in shape). Thus any untoward motions from pure lane-centered-ness carry a heavy penalty at the end of the run as all actions cannot be recalled (so to speak), unlike a car where the penalty is meaningless (almost).

The RV.net crowd probably has an opinion on which of these is best:

Safe T Plus

Steer Safe

The Kevin Rutherford/Bruce Mallinson Class 8 Glider Kit tractor has rack & pinion steering.

.

So, slowmover, let me ask you some questions, and thanks for the input by the way. You’ve got time in the saddle with big diesels, mine is small by comparison but some things are similar. I like the idea of the airspeed indicator. I’m assuming what that would do would be something like this; if you’re driving 60 mph indicated on the speedo and the air speed indicator says 45, you’ve got a 15 mph tailwind, correct?

Assuming that to be right, at that point I would also assume that would indicate it’s time to take advantage of the reduced drag and speed up a little. That brings up a question. My 7.3L Ford turbo diesel supposedly gets its best fuel economy at 1800 rpms. On my rig with a 4.10 rear end and an E4OD trans, that equates to about 54 mph. Call me Sammy Hagar but I can’t drive 55 for long, I get too impatient, but that’s beside the point. The question is, how much do you speed up without having the increased rpm level cancel out your advantage of the tail wind, any idea?

What I really need is a GearVendor or other gear splitter on the back of the trans so I can bring the speed up to 60 or 65 and the rpms down a little, or a lot. I would replace the 4.10 with a 3.83 but I’m not sure how much that would effect my hill climbing.

[orbywan]

Originally Posted by Otto

Could be, the now smooth belly is allowing more and faster air to hit the wheels, which unobstructed have greater drag than before.

But, with properly oriented wheel wheel fairings angled to the relative wind, this problem will largely go away.

I hope that's true Otto, I'm going to find out I guess. Do you think it's possible there is not enough vent area on the pans to let the frontal air out? Thanks.

[skyking]

Gear vendors is still in business. Us Gear is not. I still prefer the US gear if one can be had reasonably.
Here is one in Dallas with the proper adapter for your transmission.
Overdrive unit $800.00

That is a far cry from the ~$3000 for a new gear vendors.

[Otto]

Originally Posted by orbywan

I hope that's true Otto, I'm going to find out I guess. Do you think it's possible there is not enough vent area on the pans to let the frontal air out? Thanks.

Well, if the frontal air cannot get out, it won't go in, so I'm not too worried about that. Still, if you want good exit geometry and flow-through synergy, study tuna or shark gills, or Google pics of the Arnold AR5 sportplane cowling, best in the business.

Before you did all that beautiful belly pan work, the airflow under the car had lots of obstructions and turbulence, so a chunk of it it piled up in front and spilled around the sides rather than hit the wheels at, per Hucho's Calibra picture, a blunt angle. Now that the belly is smooth, you have a net reduction in overall belly drag, but I suspect perhaps an increase in wheel drag, due to more air going past and for lack of wheel fairings. Not to worry, this too shall pass.

Also, brake cooling worries aside, why not try the pizza pans, at least on the open road, if not on mountain roads?

[orbywan]

Originally Posted by skyking

Gear vendors is still in business. Us Gear is not. I still prefer the US gear if one can be had reasonably.
Here is one in Dallas with the proper adapter for your transmission.
Overdrive unit $800.00

That is a far cry from the ~$3000 for a new gear vendors.

I installed one or the other in a Chevy truck for a customer once, the guy was not happy. When he shifted it, it felt like you got hit by a Mack truck from behind. I think it was a US Gear unit but I can't remember. I called the company, they said, "Hey, it shifts hard."

Anyway, that's not a bad price even if it shifts hard, I suspect his technique was off a bit, but I have too many irons in the fire right now and not enough cash flow to spring for that right now but thanks for the heads up.

[orbywan]

Originally Posted by Otto

Well, if the frontal air cannot get out, it won't go in, so I'm not too worried about that. Still, if you want good exit geometry and flow-through synergy, study tuna or shark gills, or Google pics of the Arnold AR5 sportplane cowling, best in the business.

Before you did all that beautiful belly pan work, the airflow under the car had lots of obstructions and turbulence, so a chunk of it it piled up in front and spilled around the sides rather than hit the wheels at, per Hucho's Calibra picture, a blunt angle. Now that the belly is smooth, you have a net reduction in overall belly drag, but I suspect perhaps an increase in wheel drag, due to more air going past and for lack of wheel fairings. Not to worry, this too shall pass.

Also, brake cooling worries aside, why not try the pizza pans, at least on the open road, if not on mountain roads?

I can't do the pizza pans on the front Otto, they're dually type wheels where the center of the wheel sticks out at the hub. I'm reluctant to do that in the back because of brake overheating but I suppose I could take my infrared thermometer gun, make a run without them in the mountains pulling the boat, check the inside wheel temp, and do it again with the pizza pan and check it again.

Come to think of it, my boat has surge brakes on it that work really well, there's not a lot of extra braking going on even with the boat hooked up, maybe I'll give that a try. Thanks.

I need to get in touch with graydoneengineering and talk to him about his Frontier. He said his belly pan didn't seem to do much good until he blocked off the wheel wells and vented the pan up by the engine. Sounds familiar. I also like what he did with the diff, very creative.

I agree, I think the wheels, especially the duallys', are creating considerable amounts of turbulence. It's going to be a week or two before I can get back on this but I'm going to try all the above. I'll make this work or rip it out and toss it on a bonfire.

[slowmover]

Originally Posted by orbywan

So, slowmover, let me ask you some questions, . . Call me Sammy Hagar but I can’t drive 55 for long, I get too impatient, but that’s beside the point.

The question is, how much do you speed up without having the increased rpm level cancel out your advantage of the tail wind, any idea?

You’ve got time in the saddle with big diesels, mine is small by comparison but some things are similar.

Here we divert into philosophy: The Truck Is The Job. The truck is in charge. The driver is only the eyes the truck lacks. The driver is the employee, the indentured servant. Period. Sounds simple but isn't as experience counts. It's a wholly different approach. One might appreciate that a company driver is pretty good at making all his transmission shifts and can get across the continent without incident (not easy at first). The difference between him and an experienced O/O is like this: on a steep grade one knows one will downshift. A heavy truck will quite literally come a stop between shifts. Not, sort of. It will flat stop and roll backwards. Just between shifts. A decent company driver will make appropriate headway and lose no more speed than necessary. The O/O will make the split-second shift in such a way that

- speed
- temperatures
- drivetrain longevity
- fuel economy

are all optimized. It is not just the difference between a 13 or 18 speed transmission and a 9 or 10 in a company truck (it is partly) but taking in all the information of road type and surface condition, load, weather, altitude, traffic, etc. You may not have these options in your Tonka toy, but understand that each aspect or component bearing on efficient operation is exhaustively studied in this industry. The O/O has only himself to rely upon, not the subsidies and cheaper financing available to fleets.

The above is better stated for a commercial driver, and who's to say he won't come around here and correct me? But the RV owner ought to be able to come to his own understanding of this minutiae. What's a matter of near-irrelevancy in a car is a distinct percentage loss in a heavy vehicle.

The aids to determining best adaptations to present conditions are the well-known Scangauge (if available; big trucks have their engine manufacturer versions), and an EGT and MAP gauges for starters. As well, a fuel flow meter (but these are expensive by comparison), such as FLOSCAN. As with the above O/O example, comprehensive temperature and pressure monitoring of all systems/components (is also expensive).

As I see it the point -- as before -- is to not fall below a favored fuel burn rate: where speed/time is not the determinant of headway (after safety/reliability/longevity concerns are met). Gauges other than airspeed are to help grease the skids towards highest numbers when desired, but to absolutely (for that day's road/load/conditions) not fall below the desired mpg. Their expense makes them unlikely for an RV, but tracking (logging) what is available in some readable format is the homework I think you are looking for. The number crunching needs more than just the basic inputs.

The "desired mpg" goal is lower than the highest number achieved (racetrack numbers), but it is proven viable over a calendar year of driving (time/temperatures/altitudes/scale weights/towing). You need first to determine what it will be. And how much is skill versus rig condition (aero/rolling problems).

The question is, how much do you speed up without having the increased rpm level cancel out your advantage of the tail wind, any idea?

Rolling resistance is the most important consideration up to 55-mph: no steering slop (poly bushings on anti-roll bars, etc), perfect alignment, Centramatic wheel balancers; best shock absorbers; no brake drag, best tire choice and pressures (TPMS system; numbers per builder and scale weights). One wants the vehicle to be loose and tight as appropriate. Sensitivity is vital, IMO.

Aerodynamic: Coolant system like new (thermostat not only opens, but opens fully; no mineral build); exhaust system has minimized backpressure (tested); if turbocharged, CAC system verified. The motor has a nice and wide envelope so that any speed changes don't start to track on the appropriate gauges as quickly.

It is all about predicting fuel use.

There's no way around the 55 thing. I like 58 'cause it just feels that much faster. 62 might be your upper limit. The calculation of winds might allow a scooch more.

As to gear splitters, what I read on Gear Vendor and USGear is that the latter is "better". I'd snap up that $800 piece if at all possible. What I otherwise "know" about them both is that they are not standing up to use by the hotshot crowd when deadheading (empty miles); or have found to not add a sufficient increase to warrant initial and ongoing expenditures.

Racers care about that aspect not at all. So, shall you be a racer or a trucker? It's a different philosophical approach. One is to be the best, damn the cost; the other is to have the lowest overall cost of ownership & operation. Both travel the same road a long distance together, but make a serious route change from the other at a particular point.

For example, Aerohead is a racer with that cool rig of his. Once proven, who knows what step he'll take next? But he won't be carrying multiple 3,500-lb loads in that Bonnevile-bound truck and trailer (is the salient point.)

.