Report: Improving the Aero Characteristics of a Dodge Ram Pickup Truck

[MetroMPG]

After reading the ToyotaNation truck forum freakout, I went searching for some solid info about truck bed aerodynamics. (This was when I thought I was going to sign up & edumacate the lot of 'em .)

• There's this somewhat decent, real world Test: F-150 pickup "cap on" vs. "cap off" @ 60 mph

• And... SAE report: Pickup Truck tailgate aerodynamics... better up, down, or off?

If you search the web for info about tonneau covers vs. efficiency, you find stuff like this:

• Do Tonneau Covers Really Improve Gas Mileage?

That link (and many like it) refer to a report done by senior engineering students @ Western New England College, which not surprisingly shows that a tonneau cover improves efficiency.

But in the list of configurations tested for the report, the best results weren't for the tonneau, but a "new cap design", which wasn't explained any further (at least on those pages that are mostly trying to sell tonneau covers ).

Long story short: intrigued, I wrote to an engineering professor @ the college. He knew of the report, and kindly made & sent me a copy.

NOTE: the students' report is online here Improving Aerodynamic Characteristics of a Dodge Ram Pickup Truck - including a photo of the aerodynamic "new cap design":






The observations/conclusions (from the above web link):

I was not surprised when I opened the report to see an image of the "new cap design" that looks an awful lot like the ones Phil, Brett & big_dave have made for their trucks. (Why this cap style isn't mentioned on all the web pages that reference this report, I don't know.)

I'll post more tomorrow, but that's the crux of it right there.

[MetroMPG]

The report examined a model like this 1995 Dodge Ram 1500 regular cab, with 8 foot bed


One cap style tested. The light area between the cab and the cap is not a gap, it's material
applied to fair the cab to the bed cover. (Apologies for the poor image quality - it's a a
photgraph of a photcopy of a printout!)

The cover page of the report:

Western New England College
1215 Wilbraham Road
Springfield, Massachusetts

School of Engineering

Senior Project

Spring 1997

Improving the Aerodynamic Characteristics of a Dodge Ram Pickup Truck

By: Todd J. Ortolani

Faculty Advisor: Dr. Alan K. Karplus

Excerpt from the Introduction (p.1) describing the objectives and in general the style of bed cap being investigated:

The main objective of this Senior Design Project is to study the aerodynamic characteristics of a late-model Dodge Ram pickup truck, paying close attention to the rear bed, and to reduce the overall drag coefficient of the truck. This loss in drag will directly effect the rate of fuel consumption of the truck. Chrysler Corporation claims a baseline Cd of 0.4 for the Dodge Ram [Katz], a fairly high number by today's standards. A major implemented modification to reduce this number is a type of cap with an aerodynamically advantageous profile for the rear bed that closes off the large opening in front of the tailgate, increases the payload capacity in terms of covered volume, and, at the same time, maintains the functionality of the Ram as a pickup truck. My cooperating colleague, Vanwijak (Khem) Eowsakul has the responsibility in order to attain complete access to the bed cavity.

Excerpt from Construction of Test Apparatus (p.5) describing the wind & water tunnels used to gather data:

The aerodynamic characteristics of a Dodge Ram pickup truck were analyzed by utilizing the Western New England College wind and water tunnel facilities in conjunction with [1/18th and 1/25th] scale models of this vehicle. The water tunnel allows for visualization of the flow as it reaches and flows over the pickup truck. Points of separation and turbulence were to be recognized. The wind tunnel allows for a resultant drag force on the Ram to be obtained, which is entered into equation (1) to calculate the drag coefficient (Cd).

Note:

The WNEC professor who forwarded the report to me, Dr. Said Dini (currently a Professor of Mechanical Engineering who is acknowledged in the report as having assisted the project), offered this disclaimer: "I am not certain of the accuracy of the results."

The student author also discussed some of the factors which may have affected his calculations/conclusions. I'll get to them as well.

More to come...

[Glee17]

I actually graduated from WNEC in Mechanical Engineering the fall before these guys did that test. While I don't remember those guys personally, I do remember a white plastic model being prepped for the wind tunnel. I'm not sure if that is what was used or how they were holding it.

The wind and water tunnels are not that dinky. If the students new their stuff they could have good data. The force data from the wind tunnel may be suspect if the measuring instruments we're not calibrated well, or if the truck was tilted at all, or if they were measuring strain on the post holding the truck. All of which were possible.

When I was there, there were numerous tests being done for optimizing the exhaust for jet engines for military aircraft. Some of which have made their way onto real planes so not all of the tunnel data can be bad.

All that said, I would be highly suspect of the data without seeing the test methodology. I would love to see that report.

Geoff

[MetroMPG]

Note that the water apparatus was used for visualization purposes only, not measurement -

The water tunnel allows for visualization of the flow as it reaches and flows over the pickup truck. Points of separation and turbulence were to be recognized.

The wind tunnel was used to gather data: (p.7)

The wind tunnel consists of a nozzle inlet, a constant area test section (plexiglass), and a diffuser exit. At the exit is located a high-powered DC motor (one speed) and a fan to draw the air through the test section. A model is secured in the test section to a scale balance located below which reads draf force (lbs-f) and lift force (lb-f)

The Scale Balance was calibrated prior to testing, to "allow measurement from the wind tunnel to be converted to an actual drag force value."

Wind tunnel balance scale calibration curve.

[MetroMPG]

Originally Posted by Glee17

I do remember a white plastic model being prepped for the wind tunnel.

Hi Geoff - Could be. The report states that the 1/18th die cast model was used in the water tank, but...

A 1/25th scale model Dodge Ram ... (plastic) was purchased for use in the wind tunnel. The smaller size of this model was desired since the larger 1/18th scale model is too large and heavy... to be used efficiently. It sits too close to the wall of the test section, causing the boundary layer of the wall to impinge on the flow over the truck, thus creating less accurate results. (p. 8)

(Quoting Geoff again...)

Originally Posted by Glee17

The force data from the wind tunnel may be suspect if the measuring instruments we're not calibrated well, or if the truck was tilted at all, or if they were measuring strain on the post holding the truck. All of which were possible.

I'm not clear on how the scale balance is set up. Can you explain it?

I'm guessing there's an actual scale to measure lift (which the student wasn't measuring for this test). For measuring drag force, is there a pivot on the rod holding the model (at the "floor" of the test area), and drag force is measured on the other side of the pivot point?

I would love to see that report.

As it turns out, I tracked down the author and he got back to me yesterday. I'll ask, and If he gives permission to distribute the report, I'll make a digital copy (photos).

[MetroMPG]

Here's the author's disclaimer about the wind tunnel data, from Construction of Test Apparatus - Wind Tunnel (p.8):

Upon calculating Reynold's number with respect to the length of the model truck, the flow was found to be laminar. In the real world, flow over a vehicle traveling at highwa speeds (around 55 mph) is turbulent. Therefore, turbulent flow was desired. However, after many failed attempts to change the cross-sectional area of the wind tunnel inlet, roughing up the surface of the model... and even attempting to speed up the single-speed motor, and consulting Dr. Said Dini of the Mechanical Engineering Department, it was concluded that this could not be done with this particular wind tunnel. Therefore, all drag coefficient results are for laminar flow.

[MetroMPG]

Getting to the meat, now.

The aero cap construction differs from the style Phil Knox and Brett Herndon produced for their trucks. Where they set up either a continuous straight line, or gentle arc from the top of the cab to tailgate level, the report author used a "compound" design to maximize storage space under the cap:

From Construction of Test Apparatus - Miscellaneous (p.11)

Two main alternative cap profiles were established.

Design #1 is a design with two main angles, alpha & beta: alpha remains constant at 15 degrees with the horizontal, since covered volume was desired to be a maximum. Thes particular angle was chosen because it is the largest turning angle that can exists without [flow] separation. This is as stated by Dr. Walter M. Presz Jr., Western New England College.

Beta angle is the controlling factor in Design #1. This angle can vary between 0 and 12 degrees with the horizontal.

Design 1, fixed 15 degree alpha angle, variable beta angles




Design 2, fixed 8 degree beta angle, rounded termination

[aerohead]

In 1981,Dr.Timothy Maxwell et.al.,with Texas Tech Aero Lab,Dept.of Mechanical Engineering,published a SAE report on windtunnel tests conducted on pickup models,with subsequent tests conducted in full-scale with a F-150 at Lockheed,Marrietta,GA., tunnel.The graphical table appeared in Sport Truck Magazine,Dr. Maxwell and Pat Nixon( Sr.Advisor to graduate studies,Texas Tech Aero Lab),toured the truck show circuit in the U.S. in the 1980s.It was their "aerocap" which was the inspiration for my cap on the Dodge and Toyota.I guess people can research this stuff to death if they want to,but if universities are duplicating effort,its a waste of precious resources.I think pickup airflow is now firmly understood and people ought to just build what's been known to work for a long time now.There aren't going to be any "breakthroughs",Earthlings need to move on to new challenges.I think I posted a photo of the Texas Tech graphic table at maxmpg in the misc.photo section.You'll have to look for it,as there are over 250 photos there.

[MetroMPG]

Originally Posted by aerohead

I guess people can research this stuff to death if they want to,but if universities are duplicating effort,its a waste of precious resources.I think pickup airflow is now firmly understood and people ought to just build what's been known to work for a long time now.

Agreed, Phil.

The motivation for posting this thread is to create an online reference to some reasonably good data supporting the aero cap idea. (I'll go have a look over at MaxMPG for that supporting image from Texas Tech.)

While it may be clear to US what works & what doesn't, recent events (see the ToyotaNation fracas over Brett's cap) suggest that there may be some use in posting some documentation online. Thus this thread.

[Glee17]

Originally Posted by MetroMPG

I'm not clear on how the scale balance is set up. Can you explain it?

I'm guessing there's an actual scale to measure lift (which the student wasn't measuring for this test). For measuring drag force, is there a pivot on the rod holding the model (at the "floor" of the test area), and drag force is measured on the other side of the pivot point?

The best i can remember (I never used the tunnels) is the center of the model is attached to a vertical rod that is hinged at its base. Then another rod is attached perpendicular to that and creates a lever arm. The lever arm creates a downward force on a scale.

I was hoping these guys may have come up with a better method because as force (wind) is applied to the model the deflection of the lever/scale might tilt the model thus throwing every thing off.

Geoff