1989 Chevy Celebrity 4 door - kammback ideas

[wagonman76]

Thanks metrompg. We have had a few nice days this year. Every other day it just wants to stay 30F all day and rain/snow/wind, but nicer weather has gotta show up sooner or later. I had to go downstate yesterday so I took off the bars after taking the pic. Gotta go back tomorrow, then I'll be home for almost 2 weeks again.

About the flow, azraelswrd was kind enough to do this for me back in the flow illustrator thread.

http://ecomodder.com/forum/44755-post49.html

Which makes me think that the flow over the trunk is really not that bad, and the most benefit would be a stub tail that starts at the end of the trunk, sorta like #2 in my original sketches. It certainly would be a lot easier to make.

But I'm not so sure about the accuracy of the flow illustrator, especially when everyone said that the angle from the top of the back window to the tip of the trunk looks too steep for attached flow. I would think the people here (who are going to have first hand experience) would give better advice.

[TestDrive]

Originally Posted by wagonman76

About the flow, azraelswrd was kind enough to do this for me back in the flow illustrator thread.

http://ecomodder.com/forum/44755-post49.html

Which makes me think that the flow over the trunk is really not that bad

2D model of a 3D world.
Look at the curved sides of the rear window.

There must be flow from the sides which will combine with flow across the roof and I'd guess (complete guess - not amateur let alone an expert) disrupt what looks like a more or less static bubble in flow illustrator.

Might be worth tuft testing with and without simple side fins and possibly a small truck deck lid extension.


The kammback wouldn't necessarily have to be hinged.
It could be mounded on rails.
Slide it back to open truck.

[MetroMPG]

Testdrive raises some valid points. I'd also be reluctant to conclude anything from the flow illustrator demo because for another thing, the free stream flow seems to be entering from above instead of straight ahead, flowing downward at an angle top left to bottom right. Not sure why. But that would mess things up too.

To confirm, all you need is a single row of tufts taped across the last 4 or 5 inches of the trunk, and some way (or someone, if your mirror doesn't show it and you don't have a vid cam) to watch them from inside while you drive. If they're streaming back, you've got attachment. If they're swirling around randomly, you don't, and your Kammback is going to help even more.

[wagonman76]

Today after work I did a tuft test. I removed the kamm base rods, then taped on 7 pieces of white yarn along the last 5 inches of the trunk lid. I could just barely see them out my rearview mirror so I took the wife with me about a mile down the road and back. Both there and back, the strands were wagging back and forth like a dog's tail.

So after we got back I got to do more on the kammback for a few hours. The pics show what I accomplished till I could not see anymore. I ended up just leaving the back bar at 6 1/2" high, it should still be a lot better than the current airflow. It is just tack welded but it is pretty rigid so far and is nice and light, weighs maybe 10 lbs. The only thing I might change is knock out the frontmost diagonal braces and replace them with pieces bent at the top so it more closely follows the c-pillar.

I am actually thinking more and more about leaving the extensions on the back and completing the boattail, that's why I haven't cut them off yet. It measures about 2 feet, which is just about the same as another sketch I drew (just a little bit shorter version of the one posted by testdrive). The bottom of my steel bumper has holes that I can use to bolt some angle iron to support the back end, and I can hinge it there. (I really would rather just hinge it on the rearmost anchor bolt rather than try to make it slide.) Over the 2 ft boattail the width of the car would taper in to meet the bars that currently extend out, which comes out to about 15 degrees per side.

Hope the weather stays decent, I want to keep working on this.

[TestDrive]

Nice work. Keep it up.

[wagonman76]

Here is tonight's progress. The bottom beams will be angle iron from a scrapped bed frame. I just have them as wood right now because wood is a lot easier to make adjustments to. They are bolted to the bumper the same as the angle iron will be. There are actually 7 holes in the bottom of the bumper so I can attach some triangular bracing down there. The angle in the top view is a bit steep where the boards are, so I will probably make the back end of the frame a little wider at the bottom.

Then my welder quit working. The fan runs, the wire feeds, there is continuity through the gun and wire, but it just won't arc. Hope to have it fixed really soon.

[TestDrive]

The transition from the sides of the car to the sides of the tail appear to be quite abrupt.
I wonder if this wouldn't be an instance of less is more?

[wagonman76]

At trunk lid level, the transitions are very gradual. Down at the bottom the car is a little wider, plus the tail does not go back as far at the bottom. So the angle of the boards does make for a more abrupt transition than I would like at the bottom. That was what I was saying about widening the tail at the bottom where the boards are. It's not nearly as bad as it looks in the pics, the bottom looks very abrupt in some pics because the boards go up as well as inward. But the bottom is still more abrupt than I would like at the moment.

Also once I can weld again, it will have more triangular side bracing that comes close to the taillights and the bracing will be a bit convex to better match the car profile. Same with the short bars that go from the trunk lid to the back bar, they are currently straight but will be convex a bit when done. They are actually not attached at the trunk lid end, that is where my welder quit. Gotta have lots of triangular bracing to keep it light and rigid.

Hopefully this is what you are referring to. With a smoother transition, does it look like it should be beneficial so far?

[TestDrive]

Originally Posted by wagonman76

It's not nearly as bad as it looks in the pics, the bottom looks very abrupt in some pics because the boards go up as well as inward. But the bottom is still more abrupt than I would like at the moment

My eyes weren't processing that and I managed to skim over the "widening the bottom" part. Sounds like you'll do fine once the welder is up and running again. Hope that turns out to be a cheap an easy fix.

[KJSatz]

I've wanted to see someone put a boattail or partial boattail on a sedan for a while. Good luck! It's looking great so far.