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Old 05-15-2004, 08:38 AM   #1
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Aerodynamics / MPG increase - up 16%

Here are some posts copied from The Other Website. We got into a discussion about wind drag, and I made a plywood air dam for the front of my Jeep Wrangler. Wranglers have a lot of similarity to busses when it comes to aerodynamic performance -- neither one performs really well!

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.

*****************************

> 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.....
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Old 05-15-2004, 09:34 AM   #2
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The air dam is an interesting idea. As for speed and fuel economy.........I tested this theory in a very practicle manor. My bus usually gets 8.5 mpg. Top speed with the pedal to the metal is 57-61 mph. It doesn't matter if the hot tub is full or empty, mileage stays the same. That's a difference of approximately 4,000 pounds. I decided to drive for an entire tank of fuel at 50 mph or less (and the hot tub completely empty) to see how my fuel economy would be affected.

Result: After burning about 35 gallons of diesel, i checked my mileage. I was very dissapointed to find the bus got exactly 8.5 mpg. There was no noticable difference.

Has anyone else tried such an expirement with their skoolie?

It was mentioned that fuel economy is significantly decreased above 60mph. that must be the deciding factor.....above 60 mph.



Below 60 mph didn't seem to make much difference according to my limited testing. I don't plan to repeat the experiment like a good scientist...It was really difficult for me to drive under the speed limit for a couple of weeks knowing that i had the ability to go faster.

When it comes to normal driving, if i didn't have to stop, i could just put a brick on the gas pedal. The engine speed govenor keeps the bus between 57 and 61 on flat ground. I asked the mechanic at the bus garage about running the engine that fast all the time. He said that's exactly how the bus drivers do it and the engine is still running after 265,000 miles.
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Old 05-15-2004, 11:02 AM   #3
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Variables

Maybe it really doesn't matter under 60 mph.

Maybe it doesn't matter if the vehicle has approached some kind of maximum potential fuel economy based on the kid of massive loading a bus engine has versus a the loading on a smaller vehicle. If the engine is operating at maximum RPM at those speeds, reducing wind resistance that is only beginning to have an effect on the overall engine loading might not make any difference.

Maybe it has to do with gearing, too. From the top end you describe, it sounds as if your bus's final ratio is at the upper end of the range.

In 5th gear, my 2.5 liter gas engine is turning about

1800 revs at 45

2000 revs at 50

2250 revs at 55

2500 revs at 60

2750 revs at 65

3000 revs at 70.

Pretty close to 250 revs per 5 mph increase. The difference in revs alone from 45 to 55 is about 16%, and from 55 to 70 about 30%.

My air dam might not be having as *much* effect at lower speeds as I thought....except that my mileage *has* gone up significantly.

More experimentation necessary.
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Old 05-15-2004, 11:17 AM   #4
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Keep going Eric! (this is selfish ) 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!
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Old 05-15-2004, 09:32 PM   #5
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Future experimentation

The next big aerodynamics project is constructing a boattail structure to go on the back of the Jeep. I'm going to try to make it so that it functions as a trunk attached to the spare tire mounts (the spare tire would be inside it.) That way I won't get the rolling eyes from people when I say, "It's a cargo carrier", the way I would if I said, "It's something I'm experimenting with to get better fuel mileage."

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Old 05-15-2004, 09:40 PM   #6
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what an excellent experiment! keep us posted. I'm very intruiged. More miles per gallon of veggie oil would be good.
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Old 05-18-2004, 02:47 PM   #7
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Its very interesting how much of the drag force comes from the underbody. It made me wonder how much power and speed you are losing from adding basement storage compartments to the bus! It would be really interesting to calculate the effects.
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Old 05-18-2004, 03:57 PM   #8
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Clean up that underbody!

One thing that can be done is to smooth out the underbody as much as possible. Galvanized roofing material or corrugated fiberglass panels pop-riveted along the underbody to make the whole thing smooth might be an approach that would work. You'd still have to work around the running gear and the drive shaft, but you could put a flat surface over much of the rest of the bottom.

Of course, drainage would be a concern. Fiberglass panels might be the best choice, both from a drainage standpoint and a noise standpoint: a sheet metal sheathing onthe bottom of the bus might just be one big artificial thunder sound effects machine. Yikes!
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Old 05-18-2004, 04:06 PM   #9
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I am going to be putting sheetmetal over the rest of my unused windows in perparation for a new paint job, I was also considering the smooth the underbody path. I don't really want to add anymore weight, a thick vinyl might do the trick.
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Old 05-18-2004, 09:38 PM   #10
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Quote:
Originally Posted by Tullamore
It made me wonder how much power and speed you are losing from adding basement storage compartments to the bus!
This is just a WAG, but semi-informed. I would think because the bottoms of the storage is not only lower to the road surface, but flat and relatively smooth that they should aid in reducing the drag. It's really all the projecting brackets, tanks, cross members and such that disrupt the air flow. Hmmm.....I wonder how aero I can make my 66 GMC/Wayne ?

Snickering over that thought -

Stego
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Old 05-21-2004, 07:26 PM   #11
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WOO HOO!! WOO HOO!!!



WOO HOO!! WOO HOO!!

I broke 30 mpg and left it by the wayside!!!





I started the day with about 8.5 gallons in the tank, and I have about 3 or 3.5 left, meaning I used either 5 or 5.5 gallons to go 185 miles.

If I used 5.5 gallons, I got 33.63 mpg!!!

If I used 5 gallons, I got 37 mpg!!!!!!!

I made one Special Modification under the hood last night that cost me just under $10.00 and one hour of work.

One of the components of the Special Modification is America's, nay, the world's, favorite home repair item, and the other is something you see by the hundreds in mall parking lots every day during the summer!

I think this would possibly be applicable to busses, too. I'm not that familiar with what things are like under the hood of a bus (gas or diesel), but it might help.

Anybody wanna guess?
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Old 05-21-2004, 07:39 PM   #12
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Somebody please give it a shot!

Quote:
Originally Posted by stego
It made me wonder how much power and speed you are losing from adding basement storage compartments to the bus!

This is just a WAG, but semi-informed. I would think because the bottoms of the storage is not only lower to the road surface, but flat and relatively smooth that they should aid in reducing the drag. It's really all the projecting brackets, tanks, cross members and such that disrupt the air flow.
They would certainly give you a good start -- a good surface to build off of if you were going to cover the rest of the underside. I think though that a good air dam in front would be a more useful step. It would required far less material, and it blocks most of the air from going under the bus to begin with. Possibly some side air dams would be worthwhile, too.


Quote:
Originally Posted by stego
Hmmm.....I wonder how aero I can make my 66 GMC/Wayne ?

Snickering over that thought -

Stego
Don't laugh! If you drive much at highway speeds, I bet a sheet of plywood, three 8 foot 2x4s, and some plastic lawn edging from Wal-Mart would increase your fuel economy by 10%, and a 15% increase is not unlikely. It doesn't even have to *look* aerodynamic. The thing on the front of my J**p looks like some redneck bolted a plywood packing crate on the vehicle.....but it *works*.

It'll take you one Saturday or maybe a couple of evenings to measure, design, build, and paint the thing, but you will *very* likely get a 10% increase at highway speeds. Just one long weekend trip could pay for the air dam at today's prices.

Then when people say, "how can you afford to put gas in that monster?", you can laugh and say, "well, it has special aerodynamic devices that make the cost more bearable."
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Old 07-15-2004, 03:23 PM   #13
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So I was thinking about making the underbody smooth the other day and I think a good cheap way to do it would be to use the $5/sheet foam insualtion that I have been using lately.

I think if I find a good adheasive (such as a strong liquid nails) that bonds to this foam and metal, used a few srews or bolts and went around the edges with some 200mph or high quality duct tape it would hold very good.

It would help insulate the floor in the winter, keep down road noise, AND hopefully improve gas mileage by creating a smooth surface, not to mention protect the underbody from dirt, salt, and debris.

I figure I can run a line of sheets on each side of the drive sheet, probably only needing six-eight sheets ($30-$40) total.
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Old 07-17-2004, 04:35 PM   #14
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...no one has posted any pictures of anything they have tried. I would sure like to see what kinda ideas are brewing or just even a photoshop mock up so I can see what youre talking bout.

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Old 09-28-2004, 09:36 AM   #15
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Quote:
Originally Posted by abdabbs
...no one has posted any pictures of anything they have tried. I would sure like to see what kinda ideas are brewing or just even a photoshop mock up so I can see what youre talking bout.

TonyC in Iowa
yes PLEASE. i need visual aids
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Old 09-29-2004, 03:33 AM   #16
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Okay, here's a picture.

Well, it's not a skoolie, but please don't kick me out of the group....

http://www.skoolie.net/gallery2/Air-Dam-Design

It's also not a real picture, but a mock-up that resembles (very exactly) the real thing.

First I built a 2x2 ladder structure that was as wide as the front bumper. Then I sized a piece of 3/8" plywood to cover the ladder structure and extend above it by the height of the bumper at the top. Then I bolted hte ladder structure to the bottom of the bumper.



The most important dimension in this design is the distance from the hard structure (plywood) at the bottom of the airdam to the ground: 8".

The little yellow strip at the bottom of the bumper is a strip of plastic lawn edging ($10.00 for 25 feet at Wal-Mart) that is screwed to the bottom of the frame of the airdam, and then painted. This is stiff enough to stay vertical at highway speeds, but flexible enough that I don't have to worry about ripping the airdam off if I run into an extra high curb while parking or an steep approach to a driveway.

I think you could probably just bolt a sheet of 3/4" plywood directly to the front bumper of a bus instead of building the frame that I describe. I think 3/4" ply would be stiff enough not to need any framing. If it turned out to have some flex....bolt on another 3/4" thickness! It's not as if the bus will notice the weight!
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Old 12-01-2004, 10:36 PM   #17
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aeodynamics

Hi,

I'm new but have been following skoolie.net for several months. Of particular interest to me is the discussion about aerodynamics. I followed one of the links which mentioned that there was a significant loss of mileage above 60 mph. The question I have is this: Are the losses mentioned for cars (I think this article was about cars) the same as a brick-shaped school bus? I am a rider of recumbent bicyles. My fastest bike is a Reynolds T-Bone which puts me, the rider, in a very laid-back position. The frontal area of this bike is about as small as it is going to get for a bike. As a result, the bike is very, very fast. And, we usually don't cruise faster than 19 mph or somewhere in that range. It isn't that I am an aging Lance Armstrong--it is the aerodynamics. Now, there is a lot more similarity between a Jeep and a dog-nosed bus than my bike; but, I suspect that aerodynamics come into play a lower speeds than 60 mph.

Here is what I am hoping to do--time permitting. I still have the extra seats that I pulled out. I want to put all the tanks and so on in place on the bottom of the bus. I will then cut the backs off of the seats. Then I want to mount the bottom portion of the seat up-side-down so that I have a frame-work to mount treated plywood on. With a little shimming with 2 x 4's I think I can have a fairly smooth belly pan. I also want to make an air dam out of conveyor belt material.

Herb in Utah

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Old 12-06-2004, 10:20 AM   #18
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Aerodynamic Drag Losses for Cars vs. Brick Walls

Here are some CD (coefficient of drag) numbers for cars and for large vehicles:

http://aerodyn.org/Drag/tables.html#cars

Vehicle CD CD x Area (Meters squared)

VW Polo (class A) 0.37 0.636
Ford Escort (class B) 0.36 0.662
Open Vectra (class C) 0.29 0.547
BMW 520i (class D) 0.31 0.649
Mercedes 300SE (class E) 0.36 0.785

Sports Car 0.3 -0.4
Ecomony Car 0.4 -0.5
Pickup Truck 0.5
Tractor-Trailer, with fairings 0.6-0.7
Tractor-Trailer 0.7-0.9
Trailer alone 0.9

The table did not extend the CD for trucks, etc., to the CD x Area, but you can estimate that the frontal area of a bus is 9 feet by 8 feet or about 7 meters squared (1 meter squared = approx 10 square feet.) If a bus has the CD of a Tractor-Trailer, 0.7-0.9, then the CD x Area would be around 4.9 to 6.3, or something like 6 to 8 times that of a Mercedes 300SE.

I can't cite the source, but I do recall reading that below speeds of 45 to 50 mph wind resistance is not a significant factor in energy consumption in motor vehicles ; however, I've done enough cycling to know that in human powered vehicles even the difference between wearing tightly fitted clothes or rain gear while cycling makes a significant difference in energy consumption at cycling speeds. I think that the larger the frontal area, as with a bus, the more impact drag has on the vehicle from 45 to 50 mph up.

A bit more than a third of that drag comes from the air flowing under the bus, so excluding that air has a big impact on the drag. I think that an air dam and sidewalls would have as much impact as sheeting over the bottom of the bus, and sidewalls would be much easier to construct out of a flexible material (conveyer belting) than it would be to bolt a bunch of plywood and frames beneath the bus. I have considered under belly skins, but one problem that I forsee with them is moisture accumulation (and road gunk). Sidewalls would keep air from going under the bus, but would not accumulate any moisture.

If you really wanted to get fancy, you could mount the sidewalls so that they could be raised or lowered with pnematic cylinders, hinged at the bottom of the sheet metal so that they would tuck up flat under the bus when not in use, but swing down once you got on the highway.
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Old 02-05-2005, 12:59 PM   #19
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Join Date: Feb 2005
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Now that the experimentation has been done, I think a trip to Pick n Pull is in order. My Chevy pick-up has an air dam with quick release fittings for off-roading. Even has fog lights built in!! Should be very easy to adapt to a variety of vehicles, and look good too! Once I get my conversion done, an air dam will definately be on my list.
Your newest member (since yesterdsay!), Elkoskoolie, 1990 Chevy/Carpenter/8.2l Detroit/Allison Auto
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Old 04-08-2006, 05:53 PM   #20
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Hello, i'm Augie in Salt Lake City. all new to this forum and been listening in{well reading anyway}on this is very interesting subject. I was thinking of a spoiler under the front bumper would help? something like the old locomotives used! Well just a thought, and this site has been so very helpful to me on ideas to my up-comeing work on my school bus...
Thanks to you all.
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Looking for general ideas on a coversion on 85 BlueBird INTERNATIONAL 66 pass, school Bus. Thanks Augster59.
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