What is the best angle for kammback?

[ChazInMT]

How long to be effective??? Look at the Lexus RX350 for inspiration, the Kamm spoiler on it is 8" back max and maybe 3" from the window surface. The "Angle" NEEDS TO FOLLOW THE ROOF! Just extend the roof. If you drop the kamm angle, you'll create lift, if you go up with it, you'll create downforce drag. Place a long flexible strip of wood on your roof, holding it in the center of the roof and the strip extending back over your rear window, the Kamm should be built to match the curve created when you push down on the strip of wood over your hatch and it just makes contact with the back part of your roof.

Don't get caught up into thinking you need to cover the whole rear hatch or extend the Kamm even much beyond even 1 foot. The air simply needs to make a clean departure from your roof to do some good.

[Bicycle Bob]

When I put a 19' kayak on top of my car, I have almost no extra trouble parking. You might want to try driving around with a temporary extension to probe your limits. Going forward, a tail needs no more width on the road until the bodywork is centered at the rear axle.

[cfg83]

wildfire8 -

Originally Posted by wildfire8

So how long should I make it to be effective?

Thanks for the teardrop pictures. I have wanted to know what my car look like in relation to the teardrop shape. I would love to make a full boat tail but i think it would make my car too long to park anywhere.

The longer the better, but I think you could make the absolute minimum be the rear bumper of the car. In a few states it's illegal to have any part of the car extend beyond the rear bumper. Sooooo, if you take that as a minimum, you will be in compliance with all state regs.

CarloSW2

[Piwoslaw]

And any extra skin drag from the length will be countered by reducing the wake area behind the car.

[wildfire8]

I just wish there were more ecomodders here in utah. So I could get some help on this. Cuz now I am having to many ideas going threw my head on what to make. Long short higher on the hatch or lower on the hatch.

[ChazInMT]

Originally Posted by Piwoslaw

In this case, the sooner the airflow detaches the better.

EDIT: The following in red isn't exactly true, see post #10 for details.

I saw a chart somewhere (can't find it right now, grrrr). Pretty much it showed that as the rear angle gets steeper (starting from 0°=flat) lift decreases until 12°-14°, then it increases again until 26°-28° when airflow detaches and lift goes down again.

So, if you can find the spot where the tangent is 12° and make a small "step", then the airflow will detach there, reducing lift at the best point. Many new cars already have this small "step" around the top of the rear window.

I have also seen somewhere, just cant find it, where if you take an ideal aerodynamic shape, and start chopping off the back end, bit by bit, you won't seriously affect the cd of it until you chop it near it's widest point.

This is why I say a short Kamm spoiler will do some serious good. I'm not just pulling this idea out of my butt. I have put a lot of serious thought into this. Look at current automotive designs for aero cues. They put a Kamm spoiler on them where the roof line starts to break and transition over the rear hatch. (RX 350) This is because this point is where the flow separation will occur, or, where the shape of the car seriously deviates from the "Ideal" aero shape which had been maintained to that point.

And hey, worst case, the materials and time used to make what I propose are tiny compared to trying to cover the whole back end of your car with coroplast. If it does not work, not much lost.

[aerohead]

wildfire,if you'll go to page six here at the aero forum and look at "BOAT TAIL Drag Reduction Estimates",you can get a feel for what any given length of tail would mean to drag.