I've had mine in one already. There's a wind tunnel at the college at home. Spent the better part of two days in there playing with it. Well, I mostly watched, they played.
new Airtabs
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- Start date
danthewolf00
Veteran Expediter
i'm here jack lol and yes i got a .3 mpg increse by puting airtabs on the hood and yes it has helpped with bugs ALOT!
gotta go
Veteran Expediter
Turtle, was this before or after you put the Airtabs on your Sprinter? You said it was at a college, was your Sprinter an assignment for a class? Would like to hear more about the experience. Please?
Actually, it was immediately before I put the Airtabs on. I was having my Sprinter checked out for my annual DOT inspection by my regular mechanic, not my Sprinter tech, but my regular mechanic. My mechanic is actually 4 brothers and their dad, and a few other hired hands. It's a small town and everyone knows them, many grew up and went to school with them. They're very good, very honest. They do a lot of things, One is the electronics expert who does a lot of electronics work at the airport. You should see the car he worked up for one of his daughters. Keyless entry taken to another level. It's voice entry and voice start. Radio and windows are voice activated. All kinds of stuff. Very kewl. Another one is the racing expert, having spent time with a couple of NASCAR teams and doing some work in the local race scene. They all have their little areas of expertise.
Anyway, one was doing my DOT inpection and I mentioned that the bug shield gave me quite a hit on fuel mileage, and that I was expecting the airtabs to arrive the next day in the mail. Larry says that Randy (electronics expert) has a kid at the college and they're working in the tunnel, and called him over. Randy says they were looking to get a Sprinter in there, and that the Office Depot (who has one for their deliveries) turned them down unless it was over a weekend. They already had had a short wheelbase in there, but were looking to get a long one in to compare data. He said if you're looking to see just what kind of impact the bug shield has, as well as the Airtabs (which they'd love to take a look at, too), then it might be worth letting them have it for a day or two.
The tunnel is a cooperative effort between the local college and the University of Kentucky's engineering school, and is largely funded by a military contractor in Louisville (who makes stuff for tanks and armoured personnel carriers, so I don't get why they'd help fund a wind tunnel, but what do I know). Anyway, we got to see with and without data on the bug shield, and got to see that without the bug shield that the airflow was nice and easy along the hood, up the windshield and over the top of the roof, and that bugs would slam into the hood and the windshield without the bug shield. With the bug shield in place the airflow was lifted just a tad so that is floated above the hood and landed much farther up on the windshield. It was shown that a bug shield that is about 1.5 inches taller would deflect most or all of the wind up and over the windshield, but would also create even more wind resistance and drag immediately behind the shield, giving the trailing edge of the bug shield just enough room between that and the hood to create quite a little low pressure area. The air is trying to be pushed around the front of the vehicle, and every time you add something, like a bug shield, the area that the air must flow around is increased, thereby increasing resistance. If your bug shield is tall enough, and flat enough, Airtabs on them would almost certainly result in less drag and less bugs.
But it also showed just how dramatic speed can be a factor on the bug shield's effectiveness, something I already knew. It's like windshield wipers at 40 MPH will move back and forth freely, but at 65 MPH they really have to work hard when they sweep into the wind. It's the same with the bug shield, where it is relatively effective at speeds between 35 and 55 MPH, below 35 and it's not a factor and there isn't enough pressure to push the wind upwards, and the farther above 55 you get the less effective the shield is due to the increased wind coming at you. Much above 65, at 70 and above for sure, the bug shield might as well not even be there for bug relief, and only adds to wind resistance. Something that perfectly correlates to my experiences driving down I-35 to Laredo. It is a direct confirmation of Bernouli's Law of Fluid Dynamics.
I then took the van home and cleaned it like crazy and added the Airtabs, then they had it for another day in the tunnel. The overall wind resistance factors of adding the bug shield was completely gone after adding the Airtabs. Well, the bug shield wind resistance was still there, but the overall wind resistance of the vehicle was back to what it was before the bug shield was added because of the reduced drag.
The bug shield added about a 4% increase in wind resistance. The side window rain guards added another 2% (which really surprised me). Both together ended up being about a 7% wind resistance addition, which translates mathematically to about a 4% hit on fuel economy, which is what I experienced with my fuel mileage when I calculated it. At 20 MPG, that's .8 mile per gallon, which is quite a hit.
We're dealing with Fluid Mechanics, and the subset of Fluid Dynamics and Aerodynamics, as well as the Laws of Conservation of Mass and the Laws Conservation of Linear Motion. Basically, the less aerodynamic you are up front, the more air you have to push out of the way and around you. As you become more aerodynamic in front, the air behind you becomes more focused and low pressure drag increases (jet wash, the wake of a boat), even though the mass is the same (Conservation of Mass/Matter). The wind tunnel data showed unambiguously that there was more drag behind the Sprinter without the big shield installed, and less with it installed. But it also showed an increase in drag over the roof at the rear, which is to be expected since more air was flowing over the roof with the bug shied installed (and is what told me to install Airtabs at the trailing edge of the roof).
With the Airtabs installed on the trailing edges of the van, the drag was dramatically reduced to the point of a wind resistance reduction of a little more than 10%, which translates into a fuel economy savings of about 5% or 6%, which also correlates with my MPG filgures. But keep in mind that my figures are with the bug shield handicap, and without the bug shield installed my initial resistance would be less, and therefore the Airtabs wouldn't result in as much of a difference. The bug shield added resistance, which the Airtabs then mitigated, but also reduced whatever drag would be there without the bug shield. Without the bug shield I could expect about a 5% gain in fuel economy, so the wind tunnel math and the claims made by Airtabs are the same (actually, I think the Airtabs claim is 2-4%, but that's for a big truck, and with the Sprinter being more aerodynamic up front, and thus more concentrated low pressure behind it, that would also fit the math that the Sprinter would see a slightly better increase in fuel economy over that of the less aerodynamic tractor trailer). With the Airtbas and no bug shield, I'd be getting 22-23 MPG instead of the 21-22 that I currently get. (I need to dump a couple hundred pounds of junk that have accumulated inside the van, too).
Anyway, it was interesting, and mainly just confirmed stuff I already had a pretty good idea about. One thing it showed that I haven't done aything about is, it wouldn't hurt to place a row of Airtabs underneath the rear step bumper, as that bumper causes quite a bit of drag on its own. Most of it is pulled out by the other Airtabs, but it certainly appeared that there could be some benefit from adding them there, even if it's just a small benefit. I'm just not sure they would hold up under the stresses of the road grime.
It was fun watching it all. I didn't get to go into the tunnel and play, cause the instructor was very anal about that, but it was interesting just the same. I was surprised at how little of an impact that the QC bubble and mount had on wind resistance. Also surprised at how little of an impact that the Sprinter side mirrors had on resistance. Apparently that bracket that holds them out there also acts as sort of a vortex generator on its own and mitigates drag behind the mirrors. Apparently that's not the case with most side mounted mirrors. I was also surprised as how much of an impact that an open Fan-Tastic air vent has on resistance (it was more than 10% at 60 MPH and went up 1% for every 2 MPH above that). If I ever get a Maxx Air vent cover, Airtabs are going on that puppy, for sure.
They said in test after test, a dirty vehicle, especially one with a bug covered windshield, would show a 3% hit in wind resistance at speeds above 55 MPH, which translates into a 1-2% hit in fuel economy. That's way more than I would have guessed. They also confirmed the Mythbuster partially busted myth of windows down versus air conditioning, in that at speeds of 55 MPH or greater, the wind resistance is so high that running the air conditioner is more fuel efficient (10% hit) than with the windows down (12-15% hit depending on speed). But at speeds below 55 MPH, running with your windows down saves fuel.
Anyway, one was doing my DOT inpection and I mentioned that the bug shield gave me quite a hit on fuel mileage, and that I was expecting the airtabs to arrive the next day in the mail. Larry says that Randy (electronics expert) has a kid at the college and they're working in the tunnel, and called him over. Randy says they were looking to get a Sprinter in there, and that the Office Depot (who has one for their deliveries) turned them down unless it was over a weekend. They already had had a short wheelbase in there, but were looking to get a long one in to compare data. He said if you're looking to see just what kind of impact the bug shield has, as well as the Airtabs (which they'd love to take a look at, too), then it might be worth letting them have it for a day or two.
The tunnel is a cooperative effort between the local college and the University of Kentucky's engineering school, and is largely funded by a military contractor in Louisville (who makes stuff for tanks and armoured personnel carriers, so I don't get why they'd help fund a wind tunnel, but what do I know). Anyway, we got to see with and without data on the bug shield, and got to see that without the bug shield that the airflow was nice and easy along the hood, up the windshield and over the top of the roof, and that bugs would slam into the hood and the windshield without the bug shield. With the bug shield in place the airflow was lifted just a tad so that is floated above the hood and landed much farther up on the windshield. It was shown that a bug shield that is about 1.5 inches taller would deflect most or all of the wind up and over the windshield, but would also create even more wind resistance and drag immediately behind the shield, giving the trailing edge of the bug shield just enough room between that and the hood to create quite a little low pressure area. The air is trying to be pushed around the front of the vehicle, and every time you add something, like a bug shield, the area that the air must flow around is increased, thereby increasing resistance. If your bug shield is tall enough, and flat enough, Airtabs on them would almost certainly result in less drag and less bugs.
But it also showed just how dramatic speed can be a factor on the bug shield's effectiveness, something I already knew. It's like windshield wipers at 40 MPH will move back and forth freely, but at 65 MPH they really have to work hard when they sweep into the wind. It's the same with the bug shield, where it is relatively effective at speeds between 35 and 55 MPH, below 35 and it's not a factor and there isn't enough pressure to push the wind upwards, and the farther above 55 you get the less effective the shield is due to the increased wind coming at you. Much above 65, at 70 and above for sure, the bug shield might as well not even be there for bug relief, and only adds to wind resistance. Something that perfectly correlates to my experiences driving down I-35 to Laredo. It is a direct confirmation of Bernouli's Law of Fluid Dynamics.
I then took the van home and cleaned it like crazy and added the Airtabs, then they had it for another day in the tunnel. The overall wind resistance factors of adding the bug shield was completely gone after adding the Airtabs. Well, the bug shield wind resistance was still there, but the overall wind resistance of the vehicle was back to what it was before the bug shield was added because of the reduced drag.
The bug shield added about a 4% increase in wind resistance. The side window rain guards added another 2% (which really surprised me). Both together ended up being about a 7% wind resistance addition, which translates mathematically to about a 4% hit on fuel economy, which is what I experienced with my fuel mileage when I calculated it. At 20 MPG, that's .8 mile per gallon, which is quite a hit.
We're dealing with Fluid Mechanics, and the subset of Fluid Dynamics and Aerodynamics, as well as the Laws of Conservation of Mass and the Laws Conservation of Linear Motion. Basically, the less aerodynamic you are up front, the more air you have to push out of the way and around you. As you become more aerodynamic in front, the air behind you becomes more focused and low pressure drag increases (jet wash, the wake of a boat), even though the mass is the same (Conservation of Mass/Matter). The wind tunnel data showed unambiguously that there was more drag behind the Sprinter without the big shield installed, and less with it installed. But it also showed an increase in drag over the roof at the rear, which is to be expected since more air was flowing over the roof with the bug shied installed (and is what told me to install Airtabs at the trailing edge of the roof).
With the Airtabs installed on the trailing edges of the van, the drag was dramatically reduced to the point of a wind resistance reduction of a little more than 10%, which translates into a fuel economy savings of about 5% or 6%, which also correlates with my MPG filgures. But keep in mind that my figures are with the bug shield handicap, and without the bug shield installed my initial resistance would be less, and therefore the Airtabs wouldn't result in as much of a difference. The bug shield added resistance, which the Airtabs then mitigated, but also reduced whatever drag would be there without the bug shield. Without the bug shield I could expect about a 5% gain in fuel economy, so the wind tunnel math and the claims made by Airtabs are the same (actually, I think the Airtabs claim is 2-4%, but that's for a big truck, and with the Sprinter being more aerodynamic up front, and thus more concentrated low pressure behind it, that would also fit the math that the Sprinter would see a slightly better increase in fuel economy over that of the less aerodynamic tractor trailer). With the Airtbas and no bug shield, I'd be getting 22-23 MPG instead of the 21-22 that I currently get. (I need to dump a couple hundred pounds of junk that have accumulated inside the van, too).
Anyway, it was interesting, and mainly just confirmed stuff I already had a pretty good idea about. One thing it showed that I haven't done aything about is, it wouldn't hurt to place a row of Airtabs underneath the rear step bumper, as that bumper causes quite a bit of drag on its own. Most of it is pulled out by the other Airtabs, but it certainly appeared that there could be some benefit from adding them there, even if it's just a small benefit. I'm just not sure they would hold up under the stresses of the road grime.
It was fun watching it all. I didn't get to go into the tunnel and play, cause the instructor was very anal about that, but it was interesting just the same. I was surprised at how little of an impact that the QC bubble and mount had on wind resistance. Also surprised at how little of an impact that the Sprinter side mirrors had on resistance. Apparently that bracket that holds them out there also acts as sort of a vortex generator on its own and mitigates drag behind the mirrors. Apparently that's not the case with most side mounted mirrors. I was also surprised as how much of an impact that an open Fan-Tastic air vent has on resistance (it was more than 10% at 60 MPH and went up 1% for every 2 MPH above that). If I ever get a Maxx Air vent cover, Airtabs are going on that puppy, for sure.
They said in test after test, a dirty vehicle, especially one with a bug covered windshield, would show a 3% hit in wind resistance at speeds above 55 MPH, which translates into a 1-2% hit in fuel economy. That's way more than I would have guessed. They also confirmed the Mythbuster partially busted myth of windows down versus air conditioning, in that at speeds of 55 MPH or greater, the wind resistance is so high that running the air conditioner is more fuel efficient (10% hit) than with the windows down (12-15% hit depending on speed). But at speeds below 55 MPH, running with your windows down saves fuel.
The windows opened or closed factor is real interesting as a factor...I track my mpgs and think what factors are going or did effect my mileage...on days when all factors were relatively the same...voila windows...I am getting between 24-25 mpgs regularly...sometimes 26 to a high of the 27 range....then with a head wind it drops expodentially....21-22....going home out west 30-40 mph head wind..18-19 mpg....if its a tailwind 26-27.8 mpgs..can't seem to crack 28!