What is the best angle for kammback?

[wildfire8]

I am going to make a kammback for my Elantra hatchback, and needing to know what is the best angle is? And what is the leaway on the angle?

Any help would be appreciated.

[AeroModder]

Here you go: New bug, .38 CD why? These charts helped me in my own design.

Between 12 and 13 degrees is the optimum angle for drag reduction.

[wyatt]

I guess it depends on how long you want to make it. If you get a gentle enough slope (and good enough conditions), you can have up to a 22degree angle before there is separation. People generally say 15degrees is a good number to stick to. Mine is between 10 and 13 (10 on top, 13 on bottom) and there is good attached flow. I would like to build a boat tail, and would try to make it steeper, like 15degrees. This would help keep the tail shorter.

[wildfire8]

Cool thanks. This gives me a good place to start. I have 2 sheets of coroplast on the way. They where only $11.30 las time I bought it was $15+-.

[Bicycle Bob]

The part of the shape that is attributed to Dr. Kamm is close to vertical.

[cfg83]

Bicycle Bob -

Originally Posted by Bicycle Bob

The part of the shape that is attributed to Dr. Kamm is close to vertical.

Would it be better to ask what the best "teardrop-extension" angle is for the Elantra hatchback?

CarloSW2

[ChazInMT]

I agree with cfg, angle schmangle, you only need to extend the roofline back following the ideal aero template to achieve the best result.




I'm pretty certain you have attached flow on the back of your windshield now all the way to the spoiler. The Kamm would only add a small amount of efficiency by making your car closer to the ideal aero shape, essentially eliminating a lift creating area on your back window which is also creating drag.

The kamm should only need to come back a foot or so to achieve the desired effect. If you go back further, it will be more difficult to build, become counter productive due to more skin drag being created, and I think it would be ugly.




Hope this helps.

[cfg83]

wildfire8 and ChazInMT -

I applied the streamline template to a 2006 Hyundai Elantra :



The vertical red line is where I think the "Max Roof Camber" is.

CarloSW2

[Bicycle Bob]

A good way to start might be by taping a few tell-tales on the rear window, to see if you do happen to have attached flow there now. If so, an opaque tail extension is relatively easy and worthwhile. The classic 1st guess angles are 15 top and sides, and 4 on the bottom, but there are a lot of considerations that can raise or lower those estimates. If you do have attached flow on the rear window now, it may depend on the high pressure zone ahead of the spoiler, so you can't just continue the same line with confidence.
Avoiding lift usually costs a bit in drag, and is of importance depending on weight, speed, and wind, etc.

[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.