What is the best angle for kammback?
 

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.

Here you go: http://ecomodder.com/forum/showthrea...tml#post167760 These charts helped me in my own design.

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

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.

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+-.

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

Bicycle Bob -

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

CarloSW2

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

http://ecomodder.com/forum/attachmen...1&d=1274976048


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.


http://ecomodder.com/forum/attachmen...1&d=1274977045

Hope this helps.

wildfire8 and ChazInMT -

I applied the streamline template to a 2006 Hyundai Elantra :

http://ecomodder.com/forum/member-cf...arbon-blue.jpg

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

CarloSW2

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.

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.

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.

http://ecomodder.com/forum/attachmen...1&d=1274992884

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.

wildfire8 -

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

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

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.

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.

estimate
 

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.