Aero RV Belly Pan(s)

[orbywan]

Originally Posted by slowmover

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.)

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Thanks for the input slowmover, you are passionate about this and I appreciate your input. You make a lot of good points. Am I a racer or a trucker? Neither, I’m an ecomodder. My definition of that is someone who is in the middle of the two. I am interested in getting from point A to point B in the shortest amount of time possible (that would be my definition of a racer, which by the way, doesn’t fit aerohead at all in my opinion), but doing it in the most efficient way possible fuel-wise, and, this is important in my opinion, enjoying the process.

I will readily admit I want to go as fast as possible, but when I’m in my RV I’m not interested in scrutinizing every last nuance of performance to squeeze every last mile out of a gallon. There are two reasons for that. One, when I’m in my RV, it usually means I’m going some place fun, and when you get into every layer of minutia concerning fuel economy at some point it can become more work than fun. I am, however, willing to bust my chops modifying the aerodynamics of my vehicle to make it the most efficient form I can make it, and, to do 80 - 90% of the things you describe.

I think this whole process of discovery concerning aerodynamics and it’s effects, is about an ever increasing interest in those fine points as you go down the road, pun intended. I think we all get more technical and into the finer ’minutia’ as we go, so never mind what I said about not willing to scrutinize every detail.

[slowmover]

I always assume that others will follow these posts today, next year, etc. But I also had to make leaps and leave out examples galore in the above post to keep it a reasonable length. (And no more long ones after this).

Yeah, we scrutinize the details to make the best choices. Or we miss opportunities.

in the shortest amount of time possible

I should have been more explicit: a racer is someone unconcerned by the "cost" (however determined) to reach a particular goal. Travel speed is irrelevant. The FLOSCAN is a racer tool by this definition as it won't ever pay itself off in a private vehicle.

when I’m in my RV I’m not interested in scrutinizing every last nuance of performance to squeeze every last mile out of a gallon.

Neither am I (see below).

at some point it can become more work than fun.

I'll say that we should define work and fun:

Work is divided visual attention: what takes my eyes away from the road (steering & traffic management observation/scans); and Fun is skill application or grace; fun is lowering the number of driver inputs and attenuating gross actions. (the lesser attention needed for audio/verbal processing also benefitting: the wife).

Thereby, them that have not planned two-hour rest breaks and four-hour food/fuel breaks have increased their workload and stress level by now having to direct visual attention away from driving to find the next place to pull over to meet those necessities. Capice? A trip-plan reduces stress, relieves the workload and, one is only driving when driving.

Known stops, right down to Exit Number and physical address, in advance. Leg lengths of time, not just distance. Being fresh at the end of the drive is the goal. We work backwards to construct the days drive.

To wit:

I used to run my 16,000-lb combination rig at 62-63 mph. I gained mpg by dropping to 58-mph in the same gear, but not enough to matter (.5 - 1.0; from 15 to 16+) What changed is that instead of running at the top of my range (barely past peak BSFC) I found that by running towards the lower end of the optimal 58-62 mph towing range was that the EGT and MAP rise was nearly insignificant while on cruise control (closer to the Cummins 666 rule of 6-psi, 600F and 60 mph for best FE).

I have an "aggressive" CC. Where before I might "cancel" the cruise temporarily for rises and gentler hills to manually control speed, I now over-rode it slightly, instead, to speed up in advance of the grade, allowing the CC to "catch" and take over throttle work at a different point. In all events this made driving easier on me, and if I decided to drop to Direct out of OD the rpm rise was more favorable for economy-under-power. I was rocking along now . . . at the other side of the engine sweet spot (about a 75-100 rpm difference). A two-for-one win.

58-mph made for lower engine work: the rig could take the small Interstate rises on it's own versus 63-mph, and my having to monitor gauges and make calculations for virtually no mpg penalty.

Having the CC on 100% is a proven heavy vehicle strategy. Mine is on before the entrance ramp ends.

This is what I was trying to get to in delineating the difference between an O/O and a company driver. Or, a racer and a commercial driver: The OODA Loop.

Just that much less air resistance made a difference for the driver since the motor was now underneath the curve . . which translated into fewer vehicle inputs and unbroken attention to traffic and terrain maintained. Fewer diversions = more fun.

You reasonably feel that your belly-pan may have been a lot of effort for naught. I'm arguing that as skills increase and finer sensitivities come to the fore that your average mpg will benefit as you make the kind of discoveries as above. I strongly feel, as also above, that crosswind buffeting will be "reduced" in re steering inputs. With no reduction in fun, however defined, will your numbers increase over time and some miles. Your work is inspiring, Orbywan, so keep at it.

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[orbywan]

Originally Posted by slowmover

Just that much less air resistance made a difference for the driver since the motor was now underneath the curve . . which translated into fewer vehicle inputs and unbroken attention to traffic and terrain maintained. Fewer diversions = more fun.

You reasonably feel that your belly-pan may have been a lot of effort for naught. I'm arguing that as skills increase and finer sensitivities come to the fore that your average mpg will benefit as you make the kind of discoveries as above. I strongly feel, as also above, that crosswind buffeting will be "reduced" in re steering inputs. With no reduction in fun, however defined, will your numbers increase over time and some miles. Your work is inspiring, Orbywan, so keep at it.

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Thanks once again for the input slowmover. After this trip I admit I felt like all the belly pan(s) work was for naught, but having arrived at that point and letting the disappointment dissipate somewhat, what's left is waiting for it to cool off a bit, and going from here.

I need to extend my hitch so I can tow various things with it, and I need to lower the tail considerably, and I need to finish blocking off the wheel wells (which seemed to make a noticeable difference with graydoneengineering's Frontier) and build some fairings for the duallies and the front tires.

I also need to add some vents to the front pans. Every time I look under there I am shocked to see how much better it looks like the airflow should be, so I know there is something holding back the benefits of that work. I'll do all of that and see how it does and if it still doesn't show improvements, then I'll rip it all out and throw it on a bonfire.

[orbywan]

Originally Posted by orbywan

I need to extend my hitch so I can tow various things with it, and I need to lower the tail considerably, and I need to finish blocking off the wheel wells (which seemed to make a noticeable difference with graydoneengineering's Frontier) and build some fairings for the duallies and the front tires.

Anyone have any guild lines or suggestions for how much I should drop the lower part of the boat tail to achieve the right airflow down there?

[wyatt]

Originally Posted by orbywan

Anyone have any guild lines or suggestions for how much I should drop the lower part of the boat tail to achieve the right airflow down there?

Hucho's book says maximum gains for a "short" diffuser are achieved at 3.66 degrees upward slope, and for a "long" diffuser it's 2.66 degrees. The long one has positive benefits (compared to a zero degree diffuser) up to about 6 degrees, and a short one sees benefits up to about 11 degrees. The 6 and 11 numbers may go further, but that's where the graphs stop.
I am guessing your diffuser qualifies as a "long" diffuser. If I were you, I would take a straight edge and try to not exceed the taper from your back wheel to your back bumper, that way you will not drag your diffuser on a steep driveway. On the other hand, your RV may be tall enough that this is of little concern, and you could get away with a more shallow angle.

[orbywan]

Originally Posted by wyatt

Hucho's book says maximum gains for a "short" diffuser are achieved at 3.66 degrees upward slope, and for a "long" diffuser it's 2.66 degrees. The long one has positive benefits (compared to a zero degree diffuser) up to about 6 degrees, and a short one sees benefits up to about 11 degrees. The 6 and 11 numbers may go further, but that's where the graphs stop.
I am guessing your diffuser qualifies as a "long" diffuser. If I were you, I would take a straight edge and try to not exceed the taper from your back wheel to your back bumper, that way you will not drag your diffuser on a steep driveway. On the other hand, your RV may be tall enough that this is of little concern, and you could get away with a more shallow angle.

I don't know if I can get away with 6 degrees back there without dragging it once in a while but I'll take it as low as possible. Thanks.

[slowmover]

As to the hitch, it will depend on what you are towing. Some measurements of the trailer tongue are in order, as is clearance through more than 90-degrees of swing.

The TORKLIFT SuperHitch Supertruss Extension may be workable. It's overkill for capacity, but the design is meant to clear a big truck [bed] camper with from two to five feet of extension. I'd contact them. Their reputation is high among truck camper enthusiasts.

You might also read Diesel Dave's explanation to me of BSFC maps . . similar to what I was trying to say in re road, load, etc., on this thread. The Ford guys may well have dug up a better map for you. The more baseline work (mechanical condition and driver skill), the better the aero result, IMO.

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[orbywan]

Originally Posted by slowmover

As to the hitch, it will depend on what you are towing. Some measurements of the trailer tongue are in order, as is clearance through more than 90-degrees of swing.

The TORKLIFT SuperHitch Supertruss Extension may be workable. It's overkill for capacity, but the design is meant to clear a big truck [bed] camper with from two to five feet of extension. I'd contact them. Their reputation is high among truck camper enthusiasts.

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I'll take a look, thanks.

[orbywan]

Thanks for the links to various RV discussions and data slowmover. Doing a quick and dirty tally, out of all the class C results I read from all those sources, the average mpg for this particular sample of class C RV's, is about 8.5 mpg so I guess I should appreciate the results from my aeromods more.
The Sprinter/Mercedes vans are reporting 16 to 18 on average and they are mostly shorter and lighter with engines less than half my displacement and horsepower. That's pretty good.
One guy with a 27 footer and a Cummins 6 banger says he gets 15 to 17mpg, which is the highest I've seen so far, which reinforces my own opinion of Cummins drive trains. They may not be as strong under a big load as this 7.3, but they are killer dependable and generally get great mileage. As soon as it cools off a little I'll continue my efforts. 20 mpg straight and level is my goal. Aim High and reduce the Air Force. We'll see.

[Sven7]

Wow, cool project. I'm tempted to put some belly pans on my own Twin I Beam now!

One question: what did you bolt the belly pans to?