Airtabs, 14% MPG gain and (mostly) eliminates trucks pushing you around

[Chance]

Originally Posted by JamieGeek

The thing here is that: If you have to be this precise on the experiments to measure an affect beyond "noise" there must not be that much of an affect at all.

In our experience the mpg difference between bringing a toad along or not gets lost in the "noise". I would suspect that the airtabs would have a smaller affect than an extra two tons or not.

Great point.

When aerodynamic improvements are significant, well above data collection noise, they are quickly accepted indisputably. One example that comes to mind is drafting of one vehicle behind another. We see it in car racing, but can also feel the difference when riding a bike in a pace line, where aero drag can be reduced by 30%. When pedaling at 70% versus 100%, the perceived difference is unquestionable. I’ve never met a single bike rider that is skeptical about benefits of drafting.

Regarding air tabs, I suggest that a simple test can be set up to measure their effect. If the drag difference is even as high as 10%, it should be measurable by instantaneous fuel consumption.

[blw2]

there are so many variables. I'd guess about the only thing you could do to pin down the fuel economy impact is back to back tests of significant length...as in running several tanks of gas before installing them then immediately several tanks after installation.

Round trip out and back runs cancel out the headwind affect. (50% of the run is headwind, 50% is tail wind)
you mentioned variable in traffic, truck slip streams, etc...
and you mentioned tire pressure
other variables from the top of my head that could affect 1MPG rounded variation
terrain
different formulations in gasoline
temperature
number of slow downs for curves
stop light numbers
lengths of waits at stop lights...
different road surface conditions

....It's crazy when you think about it.....

Originally Posted by Chance

I can see how a teardrop shape, like those of small teardrop trailers that resemble an airplane wing, could benefit from controlling the air flow. However, at the back of a truncated motorhome body, I personally can’t see how air tabs can make much of a difference. As a motorhome travels down the road at 60+ MPH, the squared-off body at rear will leave a large low-pressure zone (like a partial vacuum). Controlling air flow with air tabs to make it either more or less turbulent isn’t going to suddenly make the air flow want to make a 90 degree turn at back of motorhome. Hence negative pressure should remain about the same at rear of motorhome.

Just my guess, but i think the idea is that the air tabs add energy to the air flow, so that instead of making that 90 degree turn at the back, it flows more or less straight back to make that teardrop shape "virtually". There a little vortex that rolls off of each tip....so two little horizontal tornadoes for each tab. These tornadoes don't make that turn and instead trail back behind the rig like ropes.... does that make sense?

[Chance]

Originally Posted by blw2

there are so many variables. I'd guess about the only thing you could do to pin down the fuel economy impact is back to back tests of significant length...as in running several tanks of gas before installing them then immediately several tanks after installation.

Round trip out and back runs cancel out the headwind affect. (50% of the run is headwind, 50% is tail wind)
you mentioned variable in traffic, truck slip streams, etc...
and you mentioned tire pressure
other variables from the top of my head that could affect 1MPG rounded variation
terrain
different formulations in gasoline
temperature
number of slow downs for curves
stop light numbers
lengths of waits at stop lights...
different road surface conditions

....It's crazy when you think about it.....


......cut.....

It would not be a “perfect” test, but the following would be easy and good enough for me.

I’d find a stretch of road without too much traffic, preferably Interstate in middle of night. I would then travel at steady 65 MPH (or whatever your normal speed) on cruise control and note fuel economy readings on level road as I passed mile markers. Five or ten readings should do it.

I’d then U-turn and go back to same exit for same starting point, and in same direction run the same test. Same speed, same tire air pressure, same temperature, same load, same pretty much everything.

Only difference is that on first run I’d have the air tabs covered up ahead of time, and on second run I’d peel off the covers for quick turnaround. Unless there was an obvious change in weather, a test duration of 30 minuets or so should give me good-enough indication as to whether fuel economy potential is real or not.

[Bob Denman]

That might work... assuming that your covered-up air tabs aren't creating a worse than normal aero-package...
If you could monitor and note tire pressures and temperatures: that'd be cool...

[blw2]

yeah, probably as good as it would get.....
but how to cover them without making a negative impact?

[Chance]

Originally Posted by Bob Denman

That might work... assuming that your covered-up air tabs aren't creating a worse than normal aero-package...
....cut....

Originally Posted by blw2

yeah, probably as good as it would get.....
but how to cover them without making a negative impact?

The beauty of what I propose is that a “negative” impact on aerodynamics, compared to a baseline of standard motorhome, would make air tabs look even better. If we accidentally made the motorhome aerodynamically dirtier by covering the air tabs, then that part of the fuel economy test would be even lower, making air tab improvement seem higher. We’d be giving the air tabs all the realistic benefits of the test. If under those conditions they still don’t work to improve MPG significantly, then I’d write them off.


Actually, the worst case test scenario would be if covered-up air tabs reduced drag compared to baseline of a plain motorhome. Depending on how they are covered over, the odds of this happening are next to zero, otherwise we’d have to believe that any appendage attached to back sides of MH would also reduce drag. Theoretically possible, but not likely at all.


By the way, I’d try finding tape that sticks well that wouldn’t damage the MH’s finish. I’d them cover the row of air tabs on each side with vertical thick vinyl strip or similar material, tapped at front and back.

The only other option I can think of would be to run test with air tabs first, then pull them off quickly and repeat test without them. I was just trying to come up with test that wouldn’t require modifying MH semi-permanently.

[Joe-FL]

Jim, you said in your original post:

"First, is the fact that we are no longer blown off the road as much as we were before the Airtabs. Do we know a truck is passing at high speed? Yes, but the push/pull on steering is very mild. However, we also installed a Roadmaster steering stabilizer, and that also helps with better handling so I can't entirely chalk up the improvement to the Airtabs. But they certainly feel like they help."

I think if anything made a difference when getting passed by big rigs it was that steering stabilizer and not your plastic gismos. How a F-53 suspension responds in those situations depends on two things, not counting suspension modifications in my experiences. How much faster is that big rig going than you, and what are the other wind conditions.

If the larger truck is passing you at 75mph and you are going 60, then in an unmodified F-53 suspension you had better be ready for it. If you are heading into a strong wind or there is a strong cross wind then you had REALLY better get ready for it.

I think you put the Airtabs on plus the steering stabilizer then experienced a couple good road trips with low to moderate winds. IMHO you would have gotten the same results from just doing the stabilizer.

As far as the mpg increase goes there are just too many factors involved plus your sample period was probably too short. There are tons of variables that can effect mpg, wind being about the biggest when it comes to most RVs. Did your tire psi's change, was the outside temps the exact same, etc. Heck, even other vehicles in front of you can effect your mpg if you stay behind them long enough.

These things remind me of deer whistles. Do they work? I have no idea but I can tell you that whenever I was issued another squad car the first thing I would do is stick 4 of them on the front. One of the fastest ways to get killed was to hit a 200 pound deer at 3am responding to a call at 125mph. I never hit a deer but they made me feel better.

[lildel]

We installed them on our previous motor home (a class A diesel) and the handling for both buffeting from big rigs and nature’s wind was an amazing difference. We honestly couldn’t verify any change in MPG, the handling improvement alone made them well worth the cost.

[brucev]

Auto manufacturers spend a great deal of time trying to improve airflow over and around vehicles so the need for something like this isn't needed. We, on the other hand are driving vehicles that never saw the inside of a wind chamber and are shaped like a brick.

There have been studies about fuel savings and they report max increase of 8%, the manufacturer reports 5% max. I am interested in possible wind buffeting improvements from vehicles passing me from behind. If they came in light brown I would probably try them.

here is the link to a study if you are interested:

http://airtab.com/Site/documents/cas...s-on-truck.pdf