SUV Kammback

[UnicycleDan]

Hi,
I'm new to this site, but I've been considering aeromods to my SUV for a while and researching quite a bit. I drive a 2003 Enovy XL, 14 mpg city, 19 highway. I've changed my driving habits and best I've hit so far is 23mpg. I consider this a success, but of course now it's time to make it better. I know you're thinking: ditch the guzzler and get a car. Sorry, I'm in college, with limited (but thankfully existent) funds. Also, I've used the full capacity of the vehicle moving apartment stuff and people countless times, so I'm keeping it for now.

I've been recording my driving habits with each tank to see how they affect FE, all recorded for my vehicle, BigBlue, in the EcoModder Garage, with notes next to each tank entry. I plan to read through your 100+ Hypermiling tips and improve my driving further, but as an engineer, I want to build stuff, too, so I'm looking into ecomods. I'm kind of sad, though, to find hardly anything on kammbacks and other aeromods for SUVs, just a few for trucks, and one for a van.

To start off, a self-imposed limitation: I want all of my modifications to be non-invasive to the stock car, no holes drilled, etc. I attempted to attach a picture of my car...we'll see if it works.

I have a handful of minor aeromods planned: front grill partial block, gap and headlight recess filler pieces in front, front/side airdams or underbody panels, roof rack, mirror mods, etc. But, I'd like to start the designs for a more major mod: a Kammback attached to the hatchback. My thought for sturdiness and being non-invasive is to hook a bike rack in place and mount the kammback's frame to the bike rack as if it were a bike (only hopefully a bit more aerodynamic).

The attached picture/template shows a basic view of what I'd like to do. I live at an apartment with very limited tools, so the kammback will likely have flat panels (5 panels: 1 top, 2 on each side, since the side of the car is roughly vertical on the bottom half and changes to angle inward toward the roof on the top half-1 panel for top angle, one for bottom). I also plan to used a 90-degree wedge of roughly 4-in PVC for each of the top corners where the side panels meet the top panels so that I get large-radius corners on top. I plan to make the kammback 2.5-3 feet long for biggest aero effect, while trying to keep a reasonable-length car (already 17ft long to start).

Okay...lots of background...now for my first question. My attached picture shows roughly a 9-degree angle slope for my kammback (top and sides each 9-deg inward). I found pictures and comments on your threads and elsewhere online with "best angle" claims for flat-panel kammbacks ranging from 7-12 degrees. Also, Trailer Tail kammbacks for semis are roughly 10 degrees. I know tuft testing will probably give me the best answer, but is 9 degrees a reasonable place to start? During tuft testing, am I essentially looking for the steepest angle that will retain laminar flow?

Thanks.

[whatmaycome14]

Welcome! 9 degrees is probably a good starting point. I would A-B-A test to determine the best results.

[UnicycleDan]

Another question for the thread first: Do my thoughts on construction and design of the Kammback seem reasonable so far?

Also, maybe I'm misunderstanding the concept of A-B-A testing, but wouldn't that be a rather long process to use in testing several different angles/configurations of kammbacks? Wouldn't that be better as a final design testing method, or if you're down to just two or so options?

Also, I have an automatic, so does that mean that I cannot do coast-down testing for the Kammback if I would need to hit the brakes to shift to neutral? (I apologize for my naivete and many questions. I really am very new to these car test methods, so I'm trying to understand the concept and usefulness of each.)

[Baltothewolf]

I can't say anything about the kammback, I'm to stupid for that, but an underbelly pan would be worth looking into as well. I would say an air-dam instead of underbelly but that would require drilling which you don't want to do.

About the automatic, you can just coast-down by popping it into neutral can't you? I would advise against engine off coasting with an automatic because it causes major damage.

Oh also, get an ultra-gauge, this will be invaluable to improving your driving habbits, and will discourage "road rage" the first time you do it and watch your mpg go from normal to ~5 :P.

http://www.ultra-gauge.com/ultragauge/

[aerohead]

Originally Posted by UnicycleDan

Hi,
I'm new to this site, but I've been considering aeromods to my SUV for a while and researching quite a bit. I drive a 2003 Enovy XL, 14 mpg city, 19 highway. I've changed my driving habits and best I've hit so far is 23mpg. I consider this a success, but of course now it's time to make it better. I know you're thinking: ditch the guzzler and get a car. Sorry, I'm in college, with limited (but thankfully existent) funds. Also, I've used the full capacity of the vehicle moving apartment stuff and people countless times, so I'm keeping it for now.

I've been recording my driving habits with each tank to see how they affect FE, all recorded for my vehicle, BigBlue, in the EcoModder Garage, with notes next to each tank entry. I plan to read through your 100+ Hypermiling tips and improve my driving further, but as an engineer, I want to build stuff, too, so I'm looking into ecomods. I'm kind of sad, though, to find hardly anything on kammbacks and other aeromods for SUVs, just a few for trucks, and one for a van.

To start off, a self-imposed limitation: I want all of my modifications to be non-invasive to the stock car, no holes drilled, etc. I attempted to attach a picture of my car...we'll see if it works.

I have a handful of minor aeromods planned: front grill partial block, gap and headlight recess filler pieces in front, front/side airdams or underbody panels, roof rack, mirror mods, etc. But, I'd like to start the designs for a more major mod: a Kammback attached to the hatchback. My thought for sturdiness and being non-invasive is to hook a bike rack in place and mount the kammback's frame to the bike rack as if it were a bike (only hopefully a bit more aerodynamic).

The attached picture/template shows a basic view of what I'd like to do. I live at an apartment with very limited tools, so the kammback will likely have flat panels (5 panels: 1 top, 2 on each side, since the side of the car is roughly vertical on the bottom half and changes to angle inward toward the roof on the top half-1 panel for top angle, one for bottom). I also plan to used a 90-degree wedge of roughly 4-in PVC for each of the top corners where the side panels meet the top panels so that I get large-radius corners on top. I plan to make the kammback 2.5-3 feet long for biggest aero effect, while trying to keep a reasonable-length car (already 17ft long to start).

Okay...lots of background...now for my first question. My attached picture shows roughly a 9-degree angle slope for my kammback (top and sides each 9-deg inward). I found pictures and comments on your threads and elsewhere online with "best angle" claims for flat-panel kammbacks ranging from 7-12 degrees. Also, Trailer Tail kammbacks for semis are roughly 10 degrees. I know tuft testing will probably give me the best answer, but is 9 degrees a reasonable place to start? During tuft testing, am I essentially looking for the steepest angle that will retain laminar flow?

Thanks.

For a simple flat panel slope,the best angle is a function of the aft portion's length as compared to the vehicles overall length.
Here's a drag table to illustrate

[UnicycleDan]

Aerohead, I think I understand the concept of what you're saying. A question, though...I see that the Cd is decreasing as the tail of the wake is brought in closer toward the back of the car, but how do those angle drops in the second picture not produce a lot of turbulence, being so far away from the template? In your first picture, the template trajectories in the right column appear to be the same as their adjacent left column diagram. Do you have an explanation for why the steep angles in the left column produce a better Cd than their corresponding right column angles that follow the template more closely?

If my vehicle is 17ft long and I make a 2.5ft extension, then the kammback accounts for 13% of the total length, indicating I should use roughly a 14-degree decline according to the left column, which is not at all close to the template you advocate people superimposing over their car picture and following...I might be misunderstanding something.

[wmjinman]

That was confusing to me, too.

[backpacker3]

Originally Posted by UnicycleDan

Also, I have an automatic, so does that mean that I cannot do coast-down testing for the Kammback if I would need to hit the brakes to shift to neutral? (I apologize for my naivete and many questions. I really am very new to these car test methods, so I'm trying to understand the concept and usefulness of each.)

You should be able to shift from drive to neutral without hitting the brakes. I do it all the time.

[aerohead]

Originally Posted by UnicycleDan

Aerohead, I think I understand the concept of what you're saying. A question, though...I see that the Cd is decreasing as the tail of the wake is brought in closer toward the back of the car, but how do those angle drops in the second picture not produce a lot of turbulence, being so far away from the template? In your first picture, the template trajectories in the right column appear to be the same as their adjacent left column diagram. Do you have an explanation for why the steep angles in the left column produce a better Cd than their corresponding right column angles that follow the template more closely?

If my vehicle is 17ft long and I make a 2.5ft extension, then the kammback accounts for 13% of the total length, indicating I should use roughly a 14-degree decline according to the left column, which is not at all close to the template you advocate people superimposing over their car picture and following...I might be misunderstanding something.

The point I wanted to make,is that for simplistic,sharp-edged slopes,the best angle would be dictated by how much of a rear end the car had.No cars are constructed this way.And you can see that the 'lowest' drag is quite high.
In this following example,you can see how the 1st-gen Golf/Rabbit achieves lowest drag when the roof is pushed up to the 'Template'

For all I've seen,we can't beat it.

[UnicycleDan]

Thanks backpacker3. Do you know, when coasting to a red light or stop sign, would it be more fuel efficient for me to flip to neutral? Or would that not help much in an automatic while I'm still moving? I'm comparing this in my mind to a lesser version of EOC, which I shouldn't do with my automatic.

...And thanks aerohead. I'm back to the template, but I guess that's good, because the template is straightforward. I still find it very interesting, however, that even in the picture of the Golf/Rabbit, the Cd increases, then decreases again beyond a point, where a certain range of kammback angles would actually worsen aero drag compared to my chopped SUV back.