Skid plate/belly pan

[Billett]

You guys are mistaking my reasoning! I realize I own a vehicle that weighs 5000lbs and has the current aerodynamics of a barn door. I want to do it because no one has tried it.

Think of it this way. If you use that logic why do people do LS swaps? Or lower/raise the truck? Or hell, why bother putting in a nice stereo? We do it because we enjoy it! If I enjoy trying something new, why am I any different than someone that wants to bag a truck?

[Keith Seymore]

Originally Posted by Billett

When you mention using a speedo as the measuring stick, would you judge it based on speed vs rpm? If there is less drag then the truck should be faster given the same rpm correct?

I was being facetious about the gas gage/speed relationship. The point is that there a relationship exists but it is clouded with a ton of other variables.

Similar with speed vs rpm. If it was direct connected then your answer would be "no" - because the rpm would be "geared" directly to provide the same speed regardless of drag. Less drag would mean less power required to maintain speed so the speed and rpm would increase proportionally.

But - there's typically a torque convertor in there, which (by being a fluid coupling) is going to introduce more variation than what you are trying to measure. So your answer becomes - "yes - intuitively directionally correct but not measureable".

K

[Keith Seymore]

Originally Posted by Billett

I had anticipated using a go pro and tape to judge air flow while driving. Any thoughts on that?

Yep - that would work.

Originally Posted by Billett

Please don't think I've ignored what you said, I realize that results will be minimal. Now I'm just curious for the sake of being curious.

No prob - knock yerself out!

I'm not too proud to steal a good idea, regardless of where it comes from



K

[Fitz]

Originally Posted by Billett

I'm seeing an awful lot of nay-sayers, but not a lot of proof. I'm not saying I'll suddenly get 30mpg, but if you could improve your mileage by even 2-3 mpg with a bolt on wouldn't you at least be interested?

When drag count is low, a skid plate helps because the contribution of the rough underside is a significant part of the total drag. When the drag counts are enormous, as in one of our trucks the undersurface is such a small percentage of the total that it is insignificant.

My corvette benefitted greatly when the Engineers at GM gave it a belly pan. The Cd dropped from around 0.33 to about 0.286.
Compare that to the "typical" 66-72 pickup of 0.7 and you can see the differences but that isn't the total story. To get a valid comparison of drag between the two you must use Cda, which is drag coefficient multiplied by frontal area.

My Corvette has a very small frontal area and your truck has the frontal area of a Seven-Eleven. I'm not being negative about your idea; I am laughing so hard it is difficult to type. They just don't look the same, do they?
My 3100 pound banana is sleek and low, my 67 stepper is huge, square and aerodynamically resembeles the third picture more than the first.

[Fitz]

Originally Posted by 68gmsee

Sacrilegious!

Joking aside and not knocking anyone that wants to make changes to their trucks -I've done the same in the past. However, I always stop to consider that the reason I bought it was because it was a 1968 -trucks I grew up with. I drove one in my teens and after I got out of the service. Also, my dad owned a 1970. I figure if I absolutely have to have better mpg's I'll get me a smaller newer vehicle and leave this old truck like it was intended to be.

I'll vote for that! My truck is a TOY. I use it to play not work so the MPG, drag, reliability, towing ability, payload and all the rest of the stuff related to using that truck to do a job simply do not matter.

[Billett]

Front pan could go from lower bumper bolts to sway bar bracket. Potentially over or under the sway bar to meet up with the front cross member.



A roll pan could cure this problem, but I may just try to make a small deflector to direct the air up and over the corner instead of funneling it behind the bumper



Something can definitely be done when it comes to the rear of the bed.



Might be worthwhile to funnel the air from the grille in towards the radiator instead of swirling around in that empty space between the core support and grille. (Oh, and I'm covered on the sticker front haha)





Last place I thought about was covering between the two cross members but I'd have to have some way to dissipate the heat depending on where the exhaust is run. Don't want it to all stay trapped and boil the trans!

What do you think Keith?

[Madtat]

Owning a 70 c/10 "brick"...I don't see how these can improve mpg
(Thats NOT me.)

[Fitz]

Originally Posted by Madtat

Owning a 70 c/10 "brick"...I don't see how these can improve mpg
(Thats NOT me.)

That may not be you but the pic sure is funny!

[Fitz]

Originally Posted by Billett

Front pan could go from lower bumper bolts to sway bar bracket. Potentially over or under the sway bar to meet up with the front cross member.

A roll pan could cure this problem, but I may just try to make a small deflector to direct the air up and over the corner instead of funneling it behind the bumper

Something can definitely be done when it comes to the rear of the bed.

Might be worthwhile to funnel the air from the grille in towards the radiator instead of swirling around in that empty space between the core support and grille. (Oh, and I'm covered on the sticker front haha)

Last place I thought about was covering between the two cross members but I'd have to have some way to dissipate the heat depending on where the exhaust is run. Don't want it to all stay trapped and boil the trans!

What do you think Keith?

The Drag force assuming incompressible flow is:
Fd= FD = 1/2 CD *ApV^2

where:
FD = Drag Force. Lbs Force
CD = Drag Coefficient. Dimensionless (Typical Values)
Ap = Cross-sectional Area perpendicular to the flow. ft sq
r = Density of the medium. Lbs/ft sq
Vs= Velocity of the body relative to the medium. Ft/sec


Let V be 60 MPH or 88 ft/sec
Density r at sea level and 70 degrees f = 0.074887lbm/ft3.
the cross sectional area of a 67 C10 = 24.75 ft sq
BUT you must account for the tailgate which is 51 in by 18 in or 6.375 ft sq
which makes the total frontal area 31.13 sq ft
The GM historical center estimates the Cd (drag coefficient) to be 0.68
V^2 in the equation means V squared or V times V
Fd= (0.5)*(0.68)*(31.13)*( 0.074887)*(88)*(88)=6138 Lbs force
Let's assume that through your belly pan efforts you are 5 Times more successful than all of those silly GM engineers who worked on my corvette and dropped the Cd from 0.33 to 0.286
(0.33-.286)*5= 0.220 and your new Cd = 6.8-0.220=0.6580
Making the new Fd for your truck with belly pan:
Fd =(0.5)*(0.6580)*(31.13)*(0.074887)(*88*88= 5939 Lbs Force
Just covering your bed would give:
Fd=0.5*0.68*24.758*0.74887*88*88= 4880 Lbs Force.
Which says you would get a greater return by installing a bed cover and using the time you would have wasted building your belly pan by taking a physics course instead.

[Billett]

Wow, that's an awful lot of math. I try not to do that on my days off. Now do me a favor and read post number 7 on this thread.

Great, now that we've got that situation covered here's one more thing. The engineers at gm who designed this truck did not design your corvette. They didn't care about drag coefficients because it wasn't a priority. What that means is that the drag sucks as you've pointed out. It also means there is a lot of room for improvement. What you have to realize is that I don't plan on putting a big charger Daytona nose cone on the damn thing and expect the car to suddenly hit 200mph. What I want is to experiment. If you think its a waste of time great, reading a thread on it is probably a bigger waste of time then wouldn't you say?

For the record I'm currently doing research on MXenes as supercapacitors in flexible applications, so the physics lesson isn't necessary. I like experiments. If it doesn't work then everyone knows what not to try. If no one tries, no one can succeed. Frankly, I drive 100 miles a day on the highway in my truck. It isn't a toy to me, its my DD. I don't care how little it will improve, but it certainly can't get worse!

[Madtat]

^^^^ ok guys....solution to the problem. - Drive in front of a Corvette all the time.
and it's Gozinta's! 2 gozinta 4 twice, 4 gozinta 8 twice...
let's move on with a more efficent way to fab the plates in hopes to squeeze a few more MPG's while adding alot more weight to our already heavy trucks which defeats the aro-dynamic issue in the first place. I'm jus say'in.

[Billett]

You're right, that was rude. I'm not arguing the "arithmetic" you presented. What I'm saying is that I'm going to try it either way. My resume was to tell you that I'm not some drop out Walmart bagger. I'm sure you've seen the world and I don't know jack. That's great. With all of your degrees you seem to still be missing the point of this even though I've explained it in clear black and white. It'll take me a while to get a baseline for mileage and then a second set of data, so once that's taken care of you can tell me how big of a waste of time it was. Until then, I'm going to keep on keepin on.

[68shortfleet]

Billett
I like your idea of testing this out, even if the results are not scientific or tested in a wind tunnel.
What kind of mileage are you currently trying to improve upon with the LS and OD in the truck already? Is the truck lowered at all? 1/2 or 3/4 ton?

I am curious to see any gains also.

[Keith Seymore]

Originally Posted by Fitz

the cross sectional area of a 67 C10 = 24.75 ft sq
BUT you must account for the tailgate which is 51 in by 18 in or 6.375 ft sq
which makes the total frontal area 31.13 sq ft

Originally Posted by Fitz

So your counter argument is a resume?

Listen, Slick, I have a Bachelors degree in Aerospace Engineering, from the University of Florida,
A Masters Degree in Aerospace Engineering from the University Of Southern California
A Doctoral Degree in Aerospace Engineering form the University Of Southern California
And Post Doc Research in Hypersonic Flight at Stanford University.
Not to mention 39 years of experience in the aircraft industry in adaptive flight controls, Aerodynamics, Aero modeling and field testing of 7 different aircraft.

It would seem to me that you have calculated the frontal area incorrectly.

The frontal area would be the "silhouette" produced by the overall section from the top of the cab down to the ground. The tailgate has nothing to do with frontal area.

K

[JointTech]

well i for one am getting popcorn. this is the closest thing to a flame war ive ever seen on this forum and im all for poking a sleeping dog.

[BrianG]

I gotta say. This is getting to be one of the most out of control threads I've read in a while. A guy thought he'd try an idea and ran it by us. He's going on with it despite a lot of negativity by many and I applaud him for that. I'm sure he doesn't tear other's down when they want to drop one to a half inch off the ground or raise it two feet in the air, put in a diesel, or paint it purple. Why the bit pi$$ing match???? I've seen a lot better on this site that's for sure! Too many big ego's in my opinion. After the fourth or fifth guy had to tell him how these trucks are not aerodynamic by nature and he said "you're all missing the point, I'm still going to do this, I'm just looking for some thoughts", why keep piling on. "Walk Away" if you don't agree or like the idea of what he's doing.

[Billett]

Originally Posted by 68shortfleet

Billett
I like your idea of testing this out, even if the results are not scientific or tested in a wind tunnel.
What kind of mileage are you currently trying to improve upon with the LS and OD in the truck already? Is the truck lowered at all? 1/2 or 3/4 ton?

I am curious to see any gains also.

Test truck is a '68 LWB with 6.0l lq4 and 4l80, 3.73 rear, 1/2 ton, lowered 2", with bedcover, drip rails intact and essentially west coast Jr mirrors

Before the swap it was a 350/NP435 and I was getting about 13 mpg pretty consistently. Based on a few lsx threads I'm hoping for 20 mpg after the swap but I would be OK with 18.

Right now I'm waiting to get it back from the exhaust shop, then I'll run it over for a tune and then I can start checking mileage for a control.

[allons]

Go for it, Billett! We all want to hear how well you're doing with the experiment.

[allons]

I used to drive BMWs. BMW did an aero study awhile back with all of the usual criteria, frontal area, belly pan, etc. and they found that as the vehicle was lowered with a belly pan the 'lift' issue became a concern. They could get the car to flow through the air, including the surface tension, but the lift got high enough to de-stabilize the car. By adding features to increase downforce, they also increased drag. In other words, at some point they were approaching a point of diminishing returns that to keep the car down on the road required more downforce, which required more horsepower to push the car through the air. More horsepower required more fuel, and so on, and so on.

Having said all of that, I think the experiment is a great idea and some improvements can only be of benefit. One never knows what one can learn, until one tries.

[Billett]

The same thing happened with the 240z and 510 sedan. Both had the same drive train, but the sedan was actually faster around the track because the aero pulled the 240z too hard.

[Fitz]

Originally Posted by Keith Seymore

It would seem to me that you have calculated the frontal area incorrectly.

The frontal area would be the "silhouette" produced by the overall section from the top of the cab down to the ground. The tailgate has nothing to do with frontal area.

K

Not so , Keith! In an incompressible flow model the swirl formed by the trailing edge of the cab roof is trapped by the tail gate when no bed cover is installed forcing the inner vertical dimension of the tailgate to further disrupt airflow in direct proportion to the area of the tailgate. That's why the NASCAR truck series use bed covers. The air dams over what would have been the tailgates of their trucks generate downforce from the turbulent flow caused by separation of the boundary layer at the trailing edge of the cab roof.

[Billett]

I'm honestly not trying to be rude, but I have no reliable way of measuring the frontal area anyway. With or without the belly pan. I'm just going to get a baseline, try adding a few things and see if I notice any improvement in mileage. I'm expecting the baseline to vary by 2 mpg so; say I get 18-20mpg average over 20 tanks worth of tests before I do any mods, if I get 19-21 after I do some of these mods it means that I'm at least on the right track and then I could start looking into it more in depth. If I get 16-18 I definitely need to look into what I'm doing wrong and potentially quit. But I won't know until I try. Is there any better way you would suggest testing this? What I mean is, aside from a wind tunnel or the go-pro and streamers idea is there anything you would try to record and keep track of throughout this?

Edit: Forgot to mention, because i want to direct the excess air into the radiator from around the grille I'm going to record under hood and coolant temps as well. If it doesn't improve the aerodynamics it should at least cool the temperatures slightly. Naturally I would record the ambient temp as well.

[Fitz]

Originally Posted by Billett

I'm honestly not trying to be rude, but I have no reliable way of measuring the frontal area anyway. With or without the belly pan. I'm just going to get a baseline, try adding a few things and see if I notice any improvement in mileage. I'm expecting the baseline to vary by 2 mpg so; say I get 18-20mpg average over 20 tanks worth of tests before I do any mods, if I get 19-21 after I do some of these mods it means that I'm at least on the right track and then I could start looking into it more in depth. If I get 16-18 I definitely need to look into what I'm doing wrong and potentially quit. But I won't know until I try. Is there any better way you would suggest testing this? What I mean is, aside from a wind tunnel or the go-pro and streamers idea is there anything you would try to record and keep track of throughout this?

Edit: Forgot to mention, because i want to direct the excess air into the radiator from around the grille I'm going to record under hood and coolant temps as well. If it doesn't improve the aerodynamics it should at least cool the temperatures slightly. Naturally I would record the ambient temp as well.

"I'm honestly not trying to be rude, but I have no reliable way of measuring the frontal area anyway."

Of course you do. You have a tape measure, don't you? It's a cross sectional area so the width and height do not have to be measured at the same place. The cross section is determined by the largest vertical and horizontal dimension. Measuring the height of the cab from drip rail to lower edge of the sill in the middle of the door gives the vertical dimension. The horizontal dimension is the width of the front fenders at the level of the headlights. The error in height due to the peaked roof will be offset by the error in width due to the trapezoidal shape of the windshield. The resulting measurement will be within 5% error. If you want to reduce that error measure the width and height of the hood then add the area of the trapezoid formed by the upper and lower edges of the windshield using the height of the window glass in the center of the door as the vertical dimension.

The real issue in making any useful measurements will be the bubble formed by the trailing edge of the cab roof and the tailgate. Ever see a piece of trash in the bed float forward in back when you're going down the road? Here's why. A low pressure area forms at the cab vertical rear surface and a high pressure area at the tailgate. Air flows FORWARD from the high pressure area along the bed to that low pressure region, up the rear wall of the cab then back to the tailgate. The amount of drag generated is proportional to the square of the velocity so...that means if you are going to get an accurate baseline you need to keep speeds varying between normal city driving and freeway speeds in equal proportions. Or, fit a bed cover before starting your experiment. The bed cover quenches the formation of the high pressure region at the tailgate and reduces turbulent flow over the rear of the truck making your MPG measurements more consistent.

You are correct that you also need to keep track of ambient temperature. Here in the desert I have to re-jet the Carb on my truck four times a year to keep consistent times at the drags as density altitude here in the desert goes from sea level in winter to 6800 feet in summer! That's a difference in time through the 1/4 mile of almost a full second from 10.2 to 11.0 if I don't change jets. Put in more cogent terms, it's equivalent to a 20% loss in horsepower in summer.

The best way to direct air through the rad without adding drag is to take advantage of the high pressure region in front of the radiator. By putting a metal cover from the top of the grille (with the hood open) to the top of the rad support blocks the air from rising up the face of the radiator and over the top of the core support below the hood. LMC Truck sells what they call a "core support filler panel" for cosmetics alone but the real contribution is improved air flow through the radiator. There's no magic to it, just two bent pieces of sheet metal (in the photo). Controlling the flow through the radiator this way won't add to the drag you already have.

If you do these things you "MIGHT" get some results for comparison but I still believe that the reduction in drag due to a belly pan is such a small part of the total that it will be lost in the measurement errors.

[Keith Seymore]

This might be of some interest:

http://media.gm.com/content/media/us...ckup-aero.html

(I worked with Diane Bloch extensively in the past; Nina Tortosa was credited with most of the work on the Volt program).

K

[Billett]

That's an awesome read thanks!! A least I'm on the right track with the grille, but I hadn't considered ducting... I love new ideas!