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Old 11-13-2008, 12:03 PM   This thread is in the EcoModder Project Library | #1 (permalink)
Join Date: Dec 2007
Location: Bay Area
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The Miata - '01 Mazda MX-5 Miata
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Trebuchet03's JettKamm (2000 VW Jetta)

I've had this on the back burner for awhile now. The key issue being, how do I mount this thing?

Well, I've had the idea breakthrough I've been waiting for, so I'm now moving on to the next phase of designing/building a kamm back for my Jetta

The design will extend from my roof until the projected line extending from my trunk back. The sides will follow the side of my car - potentially pulling towards the center line of my car. My initial CAD model suggests that the top panel curvature should be progressive to keep an aesthetically pleasing shape (and it even matches a nice foil). I'm not completely against other design features, such as vents to allow some flow over the rear glass (to clear off dew and such).

I'll be using CFD to barometrically compare designs. Basically, run CFD and then watch flow trajectories and local fluid velocities. Each CFD run takes my machine ~12 hours, so this will take some time Hopefully, I can get meaningful results from this that will equate to a more optimized design.

Things to test
  • Slope/Progressive Slope - try a variety
  • Benefits/Drawbacks of vents to allow some flow over rear glass for morning dew purposes
  • extending the trailing edge slightly further and
  • Rounding off trailing edge?
  • Zig Zag trailing edge (skeptical, but I'll follow the evidence here)
  • Flow Fence(s) - to keep left/right flow from interacting
  • Spoiler? (I'm curious)
  • ??? Throw me some ideas


How will I mount? The plan is to conformal mold the fiberglass sides to slip into my trunk. When the trunk closes, it should press the structure between the trunk lid and body of the car - I'll add rubber feet to fill in the gap and allow for a nice grip.

The top will use a material called VHB. VHB stands for "Very High Bond" and is used in the automotive industry (among many others) for things like attaching emblems, rub strips, etc. Transportation: Trailer, Truck Body & Emergency Vehicle, Bus & Rail, Marine, Aerospace. It's super strong; I have no doubt that it can handle the forces at highway speeds AND, with a little effort, can be removed (pdf).


The top leading edge will be mounted with VHB and will have a hinge mounted to it. The rest of the kamm will be mounted to the hinge. The hing will allow full use of the trunk which, when open is higher than the roof

The plan, as it stands now is to construct the kamm from cardboard paper (not corrugated), luan plywood (unless I can find corrugated plastic) and fiberglass. I'll need too find a stainless steel hinge which will be pop riveted on the "permanent side" and screwed on the removable side with thread bosses.

Potentially considering a Lexan section on the top panel to increase rear visibility.

As cheap as possible, of course I'm looking to spend about the same as Metro, circa $100.

Some pretty pictures from an invalid run (forgot to scale up, so fluid flow is actually something like 2mph instead of 25)

Thoughts, comments, etc.?

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Cars have not created a new problem. They merely made more urgent the necessity to solve existing ones.
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