Rear aero project - Civic notchback coupe

[California98Civic]

Originally Posted by brucepick

I can also reduce and then eliminate the fins. That would make it pretty much as if it were made with a taller trunk, with the trunk trailing edge at the template point.

That would make your design a vague reference to the Civic Del Sol's window/trunk transition:



I wonder if you could pick any design features from the compasrison that would help you shaping the final version.

[brucepick]

Hmmm... ... ... ...

[euromodder]

Originally Posted by brucepick

Yes, the side panels are closer together at the rear than they are up by the glass. I think that's what you're suggesting. Another way of saying it is, the spoiler panel is tapered - it's about 5" narrower at the back than it is up front where it meets the glass.

Take a look at this Volvo concept:

You'll see this kind of tie-in on every recent Volvo - just not as agressive.
This one actually has a double horizontal tie-in (see lights + upper body).


Tie in the sides of the Kammback over the trunk - while keeping all the other angles.

The bottom left corner is now at the side of the trunk, but it doesn't reduce its wake area. While it angles in vertically, it doesn't do so horizontally, so the wake remains as wide as it was.
Nor does it reduce the wake of the "windowed" top part of the car much - its width stays essentially the same - only the roofline is lowered.

[brucepick]

Today's work was to stiffen up the center panel with three supports. Hopefully this will give better test results next time I get to do coastdown tests.



Vertical support is coroplast with 2.7 mm thick lauan plywood strip inside. Cross support is 1.25 inch wide lauan plywood strip held in by duct tape. Longitudinal suport is 1"x1/8" aluminum stock held in place by duct tape.

Here's how it looks in three quarter view from the rear:


In the pic above, you can see that the left side panel continues the body line almost perfectly straight rearward. Possibly it bulges outward a bit. My intention was to taper that line inward slightly, in both side panels. They have a good inward lean (tumblehome) but the whole panel needs to taper towards center a bit. This is one aspect I'll put some attention on the next time I do some panel reshaping, hopefully after I do some coastdown testing on this setup.

Driving it before installing these supports - but with the D-shaped bent coroplast center support that is visible in an earlier photo (see here)- I saw the center panel flexing considerably at highway speed. It bowed up a good 2 or 2.5 inches at the rear, and a matching amount down at the mid point. Plus, the rear edge was fluttering up and down with the air flow. My original design intention of being able to see out the rear view mirror was completely snafu'd by the flexing center panel.

Not to mention that the distorting and vibrating center panel must have wreaked havoc with the intended aerodynamic design.

The three supports work together to give a reasonably stiff panel, though it still does vibrate some at highway speeds. I didn't get to do any formal tests on this version yet but the "butt dyno" at least felt decent.

And, here's a shot of the view out the rear view mirror from the driver's position. The thin curved line across the middle is the spoiler center panel. You can see the vertical support, behind the center brake light housing. This nearly uninterrupted view is one of my design targets. It's a large spoiler, and you STILL can see out the back. Not bad, if I say so myself. The blurred blob at lower left is my hand holding my little Kodak.


Considering that it still has noticeable vibration which would need additional stiffeners if I build the final in coroplast - I'm seriously considering building the final in lauan plywood. That's the 2.7 mm. (just under 1/8") plywood I used for the vertical support and the crosswise stiffener today. It is outdoor rated but there can be voids and also glue voids between the layers. It's very easy to work; it cuts with a "box cutter" knife or a fine saw. Can be sanded, primed and painted. I know someone is going to say to do it in fiberglass but I don't have any fiberglass skills and at this point I don't want to learn a whole new skill area.

[California98Civic]

Originally Posted by brucepick

And, here's a shot of the view out the rear view mirror from the driver's position.

Nice update. And this is a great pic. Question: is this really the template angle from the profile? Your spoiler seems so high at its tail end. How many degrees? How many inches above the trunk lid?

I'd say don't ABA test it until you have the vibrations solved. A premature test would be unnecessarily frustrating. I saw the vibrations you describe in my tests with coroplast too. That too was a reason to quit until I could improve my skills. I want to build fiberglass, though I understand your reluctance. My method to build some skills: finish my mirror delete in fiberglass, then finish the larger lower/upper grill block project, then finish the wheel-well skirts, which I want to make look really good... then maybe I'll be ready for fiberglass side-panels on a lexan-windowed kamm back.

Thanks for the update.

[brucepick]

Originally Posted by California98Civic

Nice update. And this is a great pic. Question: is this really the template angle from the profile? Your spoiler seems so high at its tail end. How many degrees? How many inches above the trunk lid?

I'd say don't ABA test it until you have the vibrations solved. A premature test would be unnecessarily frustrating. I saw the vibrations you describe in my tests with coroplast too. That too was a reason to quit until I could improve my skills. I want to build fiberglass, though I understand your reluctance. My method to build some skills: finish my mirror delete in fiberglass, then finish the larger lower/upper grill block project, then finish the wheel-well skirts, which I want to make look really good... then maybe I'll be ready for fiberglass side-panels on a lexan-windowed kamm back.

Thanks for the update.

I didn't measure angles. The spoiler is about 6.2-6.5 inches above the rear deck level. That's what I measured on the template drawings I've been using, so I HOPE my build is close to the template target. I admit it's tough measuring this stuff on the car, all surfaces are curved and sloped, and so is my driveway.

Right now I'm too impatient to learn to build in giberflass. Your plan makes sense, more power to you if you pull it off!

I'm temporarily off work and will likely be back working in 1-2 weeks, so my hope is to get something built by then that will get improved mpg over what I was getting before my hiatus began a week ago. Even if I only have a nicely working coroplast version, that would have me better off as far as FE than before the hiatus.

[Sven7]

Have you considered getting some flat metal rods/stakes to attach to the underside of the kammback? They would be flat, almost unnoticeable and would stiffen the thing more than enough.

OR you could bend the corners down a little bit, with V creases in the top surface. This would strengthen it a lot as well as help it fit template.



I'd be very interested in a tuft test of this to see if the airflow is attached, and what the fins are doing.

[brucepick]

Originally Posted by Sven7

Have you considered getting some flat metal rods/stakes to attach to the underside of the kammback? They would be flat, almost unnoticeable and would stiffen the thing more than enough.

OR you could bend the corners down a little bit, with V creases in the top surface. This would strengthen it a lot as well as help it fit template.

I'd be very interested in a tuft test of this to see if the airflow is attached, and what the fins are doing.

My notes above indicate I installed a 1/8" x 1" aluminum bar stock longitudinally. I only had the one piece handy so I did the other stiffeners in plywood, which is far stiffer than the coroplast.

I didn't mention it in the text, but there is also a 1/8" diam steel rod slipped into the coroplast groove. You can see its shadow going across the photo, and at the left end it's just about obstructed by the trunk's left rear corner. Its been there since the start of the project and allowed the 2.5" flexing I described two posts earlier.



However since it's still vibrating some, it would need more reinforcement to eliminate the vibration. All those stiffeners would need to be riveted in a permanent setup, not duct taped. And the point-contact nature of the rivet fasteners would not give as thorough a contact bond as the duct tape does, so the materials would flex and separate. That is why I think I'm better off with a stiffer base material.

Sorry - no angled folds in the center panel. I want to leave it as clean as possible on the top side to help the aero flow patterns. I don't really like the way coroplast folds, especially if you try folding diagonally across the flutes.

Yes, a tuft test would be good to do. Any ecomodders in southeastern New England, if you're interested we can get together for that. Especially if you have a pickup truck to shoot pics from.

[California98Civic]

Originally Posted by Sven7

I'd be very interested in a tuft test of this to see if the airflow is attached, and what the fins are doing.

Originally Posted by brucepick

Yes, a tuft test would be good to do. Any ecomodders in southeastern New England, if you're interested we can get together for that. Especially if you have a pickup truck to shoot pics from.

I think this is important. Before you invest in funds and time for the final build. You might learn things that save you effort in the end, and give you information for tweaking the design before you fully commit with materials that can't be as easily altered. I just remember my own disappointment.

Post a dedicated thread for someone's assistance?

[PaleMelanesian]

Originally Posted by brucepick

Oh yes - I have another idea, completely different. A span of clear flexible plastic, the heavy duty stuff for temporary storm windows, stretched across the whole area from roof rear to the current spoiler rear edge. Maybe some bow strips going across to keep it from sagging in the wrong places. Detachable at the sides (velcro??) so you can lift the trunk. But as I said, that's a completely different concept.

I think this might be a better option. Take the line out to the trunk edge, but only in the middle. Taper the sides in so it's narrower than the actual trunk lid when it reaches the rear edge.

Originally Posted by euromodder

Take a look at this Volvo concept:

Tie in the sides of the Kammback over the trunk - while keeping all the other angles.

The bottom left corner is now at the side of the trunk, but it doesn't reduce its wake area. While it angles in vertically, it doesn't do so horizontally, so the wake remains as wide as it was.
Nor does it reduce the wake of the "windowed" top part of the car much - its width stays essentially the same - only the roofline is lowered.

Tapered in from the sides like a Corvette.


Phil's boat-tailed truck tapers in as well as down.