On the whole, based on my limited, not extremely scientific test data, I'd say that if you drive your bus at highway speeds, you really ought to put an air dam on the front of it. I'm seeing as much as a 16% increase in mileage, and that's pretty significant, especially since the improvement costs extremely little and is technologically something that *anyone* can manage to do.
The discussion says aerodynamic drag is insignifican until you approach 60 mph. I think the threshhold of noticeable effect on performance is significantly lower, at 45 mph or 50 mph. Stick your arm out the window at those speeds and find out! (Be careful.) Air drag may not be the major fuel consumer at those speeds, but, especially on a big flat bus, it has to be somewhat significant.
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> aerodynamic drag is insignificant until you approach 60 mph. most
drag is on the UNDERSIDE of the vehicle. see my airdam.
I recall reading that your airdam increases your mileage substantially
at higher speeds. I have wondered what side air dams would do.
Hmmm...I have been thinking about an airdam for the Wrangler....Yes...
Where did you get your used conveyer belt material?
Now I have been compelled by this subject to look up info on vehicular
aerodynamics on the net.
Found out that my Wrangler has a CD of 0.55. A Honda Accord has a CD
of about 0.34 and a frontal area of 20.61 square feet (which, overall,
is better than the Lamborghini Countach numbers, at 0.40 and 19.21,
respectively.) I'm sure the Jeep has a larger frontal area than an
Accord (or a Lamboughini), but I am having trouble finding the exact
figures....the best reference material I can find says, "see
aerodynamic information for OUTHOUSES."
Very interesting information here (
http://aerodyn.org/Drag/tables-cd-level-2.html ), and on some of the
other links on that page. I'll have to read through it.
You are absolutely right about the under-body having the most drag
(not sure what "base" drag is.)
Table 1: Drag levels for large commercial vehicles
component level
skin friction 5 %
fore-body 20 %
under-body 50 %
base 25 %
Table 2: Drag levels for large buses
component level
skin friction 7 %
fore-body 25 %
under-body 30 %
base 38 %
REALLY cool report here:
http://eed.llnl.gov/aerodrag/pdf/ucrlid151812.pdf
They are using giant "flaps" on the back of a trailer (model) to
reduce drag by 20%.
A similar study here: http://www.osti.gov/fcvt/2000-01-2209.pdf
(With really cool computer graphics pictures.)
Article here about saving gas by reducing speed (topical news story,
not a "scientific" publication):
http://www.kcbd.com/Global/story.asp?S=1174225
"Over 50% of the energy required to move a vehicle down the road is
spent overcoming aerodynamic drag (pushing air out of the way). When
driving faster, the aerodynamic drag and rolling resistance increase.
Consequently, the fuel economy decreases rapidly at speeds above 60
mph. Each 5 mph over 60 mph is like paying an additional $0.10 per
gallon for gas."
Very interesting topic.
*****************************
I spent the better part of the day building an Air Dam for my
Wrangler. Okay, it's not a bus, but it is shaped something like a
bus, so that's "vaguely Skoolie related".
And the results of the
experiment should be indicative of what an air dam on a bus would do.
The vehicle is 68 inches wide across the fender flares. I cut a 68" x
13 3/4" piece of 3/8" CDX sheathing for the dam. About 5" below the
top of the sheathing I mounted a 2x4 that is bolted to the bottom of
the bumper. The sheathing covers the front of the bumper, and is
fixed by 1 1/4" bolts AND 2 1/2" bolts through the front of the bumper
on each side. The 1/4" bolt has about a 3" circle of 3/8" CDX under
it for extra support. The 1/2" bolts hold on some tow brackets that
have a base of 3/8" x 1 1/2" x 7 " metal, which acts like a giant
washer, too. There are cut-outs at the top of the sheathing for the
two little bumper pads.
Below the 2x4 four 1 1/2" x 1 1/2" supports (2 at the ends, and 2
spaced between the ends) run vertically and intersect with a 1 1/2" x
1 1/2" board along the bottom.
Basically it's kind of like stud construction on a house. It's big,
it's FLAT, and it's UGLY.
The bottom of the dam is 8 3/4" from the ground. I wanted about 9" of
clearance between the ground and anything hard attached to the
vehicle, but I wanted less overall clearance between the hard air dam
and the ground. I went to Wal-Mart and bought 20 feet of plastic lawn
edging. This stuff is basically a 3/4" tube of some kind of plastic
(HDPE?) at the top, with a 4 1/2" flange that is supposed to go into
the ground. About 3/4" from the bottom there are some smaller, 1/2"
flanges that stick out to each side. They are supposed to hold the
plastic in the ground when you push the edging down into the ground.
I just cut that bottom 3/4" of plastic off, leaving me with a flat
piece of plastic with a tubular edging along one side.
Originally I was going to just screw through the tubular portion of
the edging material and mount the plastic to the bottom of the air
dam, but when I cut off the flange, the bottom edge of the plastic
wasn't very straight. So I turned it over. At the bottom, back side
of the air dam I screwed a 1" wide strip of CDX as wide as the air dam
over the ragged edge of the plastic, leaving the smooth tubular edge
pointing toward the ground. That probably gives the edge some
stiffness that a flat plastic edge would not have had.
Net result is that the air dam, plus the flexible plastic section
leaves me with 4 3/4 inches of clearance. A little more than I
wanted, but I can live with it. The tallest thing I drive over is
roadkill, and medians in town when I want to make a U-turn.
I painted the back of it black, and the front yellow -- with a
smiley face type smile on it (ever seen the Jeep ads - that is what I
was going for.) The yellow spray paint appears to be sticking to the
plastic lawn edging, which I did not expect. The paint may not stay
on there long, but for a total investment of about $15.00, I don't
really care.
If I wanted a pretty vehicle, I'd have bought a Lamborghini. (Not
really, because I couldn't afford it.)
The thing appears to be rock solid. It is screwed together with about
50 or 60 1" and 2 1/2" galvanized screws. I drove it around a bit but
only got up to 50 mph in town, so I don't know how it will behave at
high speeds.
Without the dam and at a steady 60 MPH I get about 23 mpg. I am
estimating that the dam may give me 10% or 15% better mileage at that
speed. That means that at 60 MPH I should get about 25 or 26 MPG,
which will save me about $40 per month on gas at current prices.
We'll see....
************************************
Well the first set of results are in on the air dam experiment.
I am still testing and gathering data, including a baseline set that I
will begin gathering today.
Initially my mileage was 25mpg. I was driving 45 MPH for 35 miles,
and then 55 MPH for 59 miles. That is an average speed of 51 MPH.
For the first set of test results I increased the speed on the first
leg of my commute to 55 MPH and on the second leg to 65 MPH. That is
an average speed of 58 MPH. Fuel consumption was at the rate of 22.5 mpg.
Average speed up 13.8%
Fuel consumption up 10%
I need to get some baseline comparison data at the original speeds to
see what effect the air dam has there. Since the original speeds are
at the bottom end of the threshhold of where wind resistance begins to
be a significant factor in fuel economy, I am curious to see what the
effect of the air dam is at those low speeds.
One side effect of the air dam is that it seems to make the car feel
more stable on the road.
More to come....
The above is the speed that I have been running today and yesterday
afternoon. Over 280 miles I have used 10 or 10.5 gallons. I can't be
more precise than that because I am going by the gas guage and a count
of the gallons of fuel that I put into the tank.
On the baseline run, according to these results, I am getting
somewhere between 27 and 28 miles per gallon, which is, at a minimum,
a 2 mpg increase in fuel mileage, and perhaps as much as a 3 mpg
increase, or somewhere between 8 and 12 percent, call it 10 percent.
I will make two more runs of 94 miles at that speed tomorrow to
collect more data.
*********************************************
> For the first set of test results I increased the speed on the first
leg of my commute to 55 MPH and on the second leg to 65 MPH. That is
an average speed of 58 MPH. Fuel consumption was at the rate of 22.5 mpg.
> Average speed up 13.8%
> Fuel consumption up 10%
I'll have to take the air dam off and run a couple of trips at the
above speeds without it to see what the results are.
> I need to get some baseline comparison data at the original speeds
to see what effect the air dam has there. Since the original speeds
are at the bottom end of the threshhold of where wind resistance
begins to be a significant factor in fuel economy, I am curious to see
what the effect of the air dam is at those low speeds.
Appears significant.
> One side effect of the air dam is that it seems to make the car feel
more stable on the road.
The feel of the vehicle on the road is a pretty subjective thing, but
I am noticing that there is a dramatic reduction in the effect that
passing trucks has on the handling of the vehicle.
Also there are a couple of modifications that I want to make to the
air dam. 1 is to put some kind of sidewalls on it. Right now it is
essentially a flat sheet stuck on the bumper. I think there is
inefficiency at the edges where the air dam ends and air starts to go
around the edges and it runs into the area in front of the wheels (## is wheel, "ooo" is open space, | is air dam.)
| ooooo###
| open ####
| area ####
| ooooo###
|xxxxxx ###
|xendxx#####
|xplatex#####
|xxxxxx ###
> More to come....
Later!
***************************************
Okay, one whole week at 45 & 55 mph with the air dam. It looks as if
I'm got close to 30 MPG on this last run if not a bit more. I went
475 miles on right at 15 1/2 or 16 gallons of gas. Combined with the
280 from earlier in the week, that's a total of about 755 miles on
about 26 gallons of gas, or an average of right at 29 MPG. That's a 4
mpg increase, or 16%.
To use Greg's recent mileage figures for the 9 liter IHC engine on the
bus he drove, about 13 MPG, a 16% increase would amount to 2 MPG more,
which would put the bus at 15 MPG at speeds between 50 and 55 mph.
Taking a figure of 8.5 MPG that I have seen used, a 16% increase would
amount to 1.36 extra MPG, and put the bus at just under 10 MPG.
On a gasser getting 5 MPG, the mileage would increase to 5.8 MPG.
Anyway you cut it, a 16% increase in mileage is worth getting,
especially since the total investment in material would be around $25.00.
I think I'm going to tinker with the air dam design some before I try
anything else.
Although, maybe I should try a high speed run.....
) I have a 2.5-liter '98 Jeep Wrangler my wife commutes in; with the gas prices creeping ever upward I'm loving your experiments! 
